Micro Light Emitting Diode: Fabrication and Devices: Micro-LED Technology 9811655049, 9789811655043

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Series in Display Science and Technology

Jong-Hyun Ahn Jae-Hyun Kim   Editors

Micro Light Emitting Diode: Fabrication and Devices Micro-LED Technology

Series in Display Science and Technology Series Editors Karlheinz Blankenbach, FH für Gestaltung, Technik, Hochschule Pforzheim FH für Gestaltung, Technik, Pforzheim, Germany Fang-Chen Luo, Hsinchu Science Park, AU Optronics Hsinchu Science Park, Hsinchu, Taiwan Barry Blundell, University of Derby, Derby, UK Robert Earl Patterson, Human Analyst Augmentation Branch, Air Force Research Laboratory Human Analyst Augmentation Branch, Wright-Patterson AFB, OH, USA Jin-Seong Park, Division of Materials Science and Engineering, Hanyang University, Seoul, Korea (Republic of)

The Series in Display Science and Technology provides a forum for research monographs and professional titles in the displays area, covering subjects including the following: • • • • • • • • • • • • •

optics, vision, color science and human factors relevant to display performance electronic imaging, image storage and manipulation display driving and power systems display materials and processing (substrates, TFTs, transparent conductors) flexible, bendable, foldable and rollable displays LCDs (fundamentals, materials, devices, fabrication) emissive displays including OLEDs low power and reflective displays (e-paper) 3D display technologies mobile displays, projection displays and headworn technologies display metrology, standards, characterisation display interaction, touchscreens and haptics energy usage, recycling and green issues

More information about this series at https://link.springer.com/bookseries/15379

Jong-Hyun Ahn · Jae-Hyun Kim Editors

Micro Light Emitting Diode: Fabrication and Devices Micro-LED Technology

Editors Jong-Hyun Ahn Electrical and Electronic Engineering Yonsei University Seoul, Korea (Republic of)

Jae-Hyun Kim Korea Institute of Machinery and Materials Daejeon, Korea (Republic of)

ISSN 2509-5900 ISSN 2509-5919 (electronic) Series in Display Science and Technology ISBN 978-981-16-5504-3 ISBN 978-981-16-5505-0 (eBook) https://doi.org/10.1007/978-981-16-5505-0 © Springer Nature Singapore Pte Ltd. 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Contents

Part I

Materials and Processes

1 Epi-Growth and Chip Fabrication Process for Micro-LEDs . . . . . . . . Yun-Li Charles Li, Tzu-Yang Jovi Lin, and Yen-Lin Alfred Lai

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2 Quantum Dot Color Filter and Micro LED . . . . . . . . . . . . . . . . . . . . . . . Kyoungwon Park, Yeongbeom Lee, Jeongno Lee, and Chul Jong Han

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3 Laser Lift-Off (LLO) Process for Micro-LED Fabrication . . . . . . . . . . Jaegu Kim and Jae-Hyun Kim

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4 Transfer Technology of Micro-LEDs for Display Applications . . . . . . Jae-Hyun Kim, Bongkyun Jang, Kwang-Seop Kim, and Hak-Joo Lee

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Part II

Device Applications

5 Advanced Epitaxial Growth of LEDs on Van Der Waals Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hyunseok Kim, Wei Kong, and Jeehwan Kim

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6 Implantable LED for Optogenetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Yiyuan Yang, Abraham Vázquez-Guardado, and John A. Rogers 7 Flexible and Stretchable Micro-LED Display . . . . . . . . . . . . . . . . . . . . . 141 Luhing Hu and Jong-Hyun Ahn

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Part I

Materials and Processes

Chapter 1

Epi-Growth and Chip Fabrication Process for Micro-LEDs Yun-Li Charles Li, Tzu-Yang Jovi Lin, and Yen-Lin Alfred Lai

1.1 Introduction Light emitting diode (LED) is an excellent light source thanks to its high luminous efficiency and low electricity consumption. LEDs have been widely applied in the categories of lighting, display, biosensor, disinfection, and light communication. When the industries of mobile phone, wearable device, high-end TV, and transparent display were thriving, the revolution of display technology started. The requirements of the next-generation display can be roughly divided into two classes. One is the ultra-high resolution, and another is the large area. For both classes, the characteristics of high brightness, high bright-dark contrast ratio, great color rendering index (CRI), short response time, and long operation lifetime are necessary. The examples are shown as follows.

1.1.1 Mobile Phone The mobile phone display had been upgraded from the ~2 in 48 × 84 pixels (65 PPI) monochrome LCD display to the 6.2 in 2400 × 1080 pixels (421 PPI) fullcolor AMOLED display over the past 20 years. There are sevenfold and 640-fold increasements in the area and pixel number, respectively. If we keep the screen size enlarging, the mobile phone will not be easily carried. A foldable display is one the way to enlarge the “effective” screen size. Many mobile phones using the foldable display have been presented up to date. For future mobile phone display, the high

Y.-L. C. Li (B) · T.-Y. J. Lin · Y.-L. A. Lai PlayNitride Inc., Kezhong Rd., Zhunan Township, Miaoli County 35053, Taiwan e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2021 J.-H. Ahn and J.-H. Kim (eds.), Micro Light Emitting Diode: Fabrication and Devices, Series in Display Science and Technology, https://doi.org/10.1007/978-981-16-5505-0_1

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resolution, great CRI, sufficient brightness (in sunlight), foldable screen, and low power consumption are the targets.

1.1.2 Large-Area Display “The wall”, a modular display developed by Samsung Co., with an area of 292 in 8K display was exhibited in CES 2020. The largest area of “The wall” can be extended to 583 in. Other size options of “The wall” include 75, 88, 93, 110, 146, 150, 219 in. For example, the 110 in 4K micro-LED “The wall” display has 24,000,000 selfilluminating LEDs. “The wall” provides the perfect black and solves the problem of OLED screen burn-in. “Real Micro LED display”, developed by LG Co., with an area of 145 in was also demoed in CES 2020. The micro-LEDs having the size less than 50um is applied in the “Real Micro LED display”.

1.1.3 Ultra-High Resolution in a Small Display The 4 K and 8 K micro-display, which owns an ultra-high resolution (15,000 PPI), was demonstrated by AUO. The full-color 1.6 and 1.8 in micro-LED displays were launched in CES 2020 by Glo. Their PPIs are ranging from 200 to 300 PPI. Jade Bird Display (JBD) presented the micro-LED displays, which have the resolution ranging from 400 DPI to 10,000 DPI. The development of micro display technology is important to the AR, VR and other wearable device applications. Fine pitch and miniatured pixel size are the key factors to realize the highresolution display. Recently, many efforts have been put on how to miniaturize each size of a single pixel composed of three-primary colors, red (R), green (G) and blue (B). Up to now, the inorganic and organic micro-LEDs (micro-LEDs and micro-OLEDs) are two excellent candidates for novel micro display technology. The inorganic micro-LEDs have the advantage of the robust and safe operation. The inorganic micro-LEDs meet all the requirements of the novel display. One challenge of inorganic micro-LEDs is its high cost. Therefore, the improvement of the production yield of micro-LED is critical to make the micro-LED be commercialized.

1.2 Considerations in Vertical and Horizontal MOCVDs Epitaxial growth for micro-LEDs as well as conventional LEDs uses metal–organic chemical vapor deposition (MOCVD) as the main production method. In the MOCVD process, substrates are placed on a wafer carrier and heated to process temperature in a reactor. Controlled amounts of hydrides and MO precursors with carrier gas are passed into the reactor over the substrates, pyrolyzing to form desired

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compound on the wafer surface. Since micro-LED chips need to be mass-transferred without binning, the key requirements of the epitaxy are high wavelength uniformity, which is facilitated to a great extent by appropriate configuration of a MOCVD reactor system and flow field tuning. This section gives a discussion on the two major types of MOCVD reactors for LED manufacturing, the vertical rotating disc reactor and the horizontal planetary reactor, characterized by their respective gas flow and wafer carrier configurations.

1.2.1 Vertical Rotating Disc Reactors Structure of the vertical rotating disc type reactor developed by Veeco is shown in Fig. 1.1 [1]. In the reactor, substrates stand on pockets of a wafer carrier. Hydrides, MO precursors, and carrier gas are injected vertically from the flow flange at the top down towards the wafer carrier and then pumped out through carrier edge. During process, the wafer carrier is heated by a resistive heater below it, spinning at a high rate up to 1000 rpm at the same time. The centrifugal and viscous forces drive the above gases to form a high-velocity gas stream through the heated wafer surface for deposition. To achieve uniform growth, parameters such as reactor pressure, temperature, flow rate, and carrier rotation speed need to be optimized to establish a steady laminar flow field. High capacity and low cost of consumable parts are the main advantages of this vertical rotating disc system due to its simplicity of reactor structure. During continuous production, the one-piece wafer carrier is the only part to exchange for off-line baking. The highest capacity at present can reach 12 × 6 wafer for LED. In addition, flexibility in switching wafer size is another benefit. Nevertheless, shortcomings from this structure exist. No sub-rotating parts are available to average out the radial deviation, as shown by resulting wavelength in Fig. 1.1 Schematic cross-section of a vertical rotating disc reactor chamber [1]. Copyright (2021) Veeco

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(a)

(b)

Fig. 1.2 PL wavelength mapping of some 4 green LED epi-wafers grown in a vertical rotating disc reactor: a 14 × 4 batch, b single 4 wafer showing typical uniformity level with Wd_std = 0.82 nm

Fig. 1.2a. Besides, while the wafer carrier is spinning at high speed, windward side of wafer would turn up slightly because of buoyancy, which causes localized variations of flow, temperature, and resulting wavelength within the wafer, as evidenced by the red tails appearing on the inner side of the outer-ring wafers in Fig. 1.2a. Moreover, the gas flow through the wafers located at upstream also affect the neighboring wafers at downstream. This two windward effects can be mitigated by adjusting carrier design, producing an average performance of Wd_std at about 0.8 nm on a 4 green LED wafer under optimal conditions, as shown in Fig. 1.2b.

1.2.2 Horizontal Planetary Reactors Schematic of the horizontal planetary reactor developed by Aixtron is shown in Fig. 1.3 [2]. The MO precursors and hydrides flow horizontally and radially from the injector at the center of the reactor out to the exhaust on the reactor’s edge. Each wafer is loaded onto a separate disk holder called satellite. All satellites are arranged in a circle located on a larger round carrier called susceptor. The latest model has a capacity of 8 × 6 wafer. During growth, the susceptor rotates at several rpm and is inductively heated and transfers the heat to satellite disks and wafers upon it. Temperature of ceiling at the top of the reactor is adjusted by purging gas to control the vertical thermal gradient, which is important for achieving a stable flow field. The growth rate decreases gradually along the radius of susceptor with depletion of reactants. Thus, it is necessary that the satellite disks spin in order to average out the radial deviation. This is realized by means of injecting gas flow below the satellite disks. Each satellite disk is connected to susceptor with a pin at disk bottom. Gas such as N2 passes through a channel inside the susceptor and comes out from an opening on the susceptor surface forming gas foil below the disk. Viscosity of this gas foil then drives the disk to spin.

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Fig. 1.3 Schematic of a horizontal planetary reactor configuration [2]. Copyright (2014) Elsevier

With the help of spinning disks, average level of Wd_std at 0.77 nm can be achieved on a 6 green LED wafer, as shown in Fig. 1.4. On the other hand, cost of consumable parts is higher because of sophisticated parts composition and machining. To Summarize, the horizontal planetary reactor is capable of higher uniformity of wavelength as well as thickness for now thanks to its rotating satellite disk. While sacrificing some within-wafer uniformity due to the windward effects and lack of wafer spinning due to high-speed rotation, the vertical rotating disc reactor possesses a more competitive cost of ownership for consumable parts. Fig. 1.4 PL Wd mapping of a 6 green LED epi-wafer grown in a horizontal planetary reactor with Wd_std = 0.77 nm

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1.3 The Capability of Improving Peak Efficiency 1.3.1 Peak Efficiency Affected by Defect-Related Mechanism In the past decade, one of the most significant challenges for high power GaN-based LEDs is the efficiency droop. The decrease in external quantum efficiency (EQE) of an LED is as increasing drive current. Typically, InGaN-based LEDs have a peak in efficiency at current densities less than 10 A/cm2 and gradually decreases afterwards. For micro-LED application, the desired operating current is ranged from 0.5 to 10 A/cm2 , which region is away from the onset of efficiency droop. Upon this current density, the increase of the absolute peak efficiency and saving energy consumption are the crucial considerations. LEDs with low peak efficiency is due to high defect density, which causes some non-radiative recombination, such as defect-related Shockley–Read–Hall (SRH) recombination. This SRH inside the InGaN MQWs has strong influences on the maximum efficiency [3]. Schubert et al. [4] evaluated InGaN LEDs grown on templates with low and high threading dislocation densities, as shown in Fig. 1.5. For low defect density (sample A), a high peak efficiency at low current and followed by droop as current increases. On the contrary, high defect density (sample B) show low peak efficiency and slight droop. It is found that the efficiency between two samples was quite difference at low current region and revealed the same behavior of droop as increasing current. The authors concluded that the dislocation density is responsible for the maximum of peak efficiency rather than the efficiency droop. In other words, the mitigation of defect density within the MQWs potentially is one of the solutions to improve the maximum efficiency at low current density, which is normally operated current for micro-LEDs. Fig. 1.5 Efficiency as a function of current for the two samples [4]. Used with permission. Copyright (2007) AIP Publishing

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1.3.2 Effects of Sidewall Damage on the Peak Efficiency The performance of peak efficiency also depends on the chip size, which had been widely studied for development of micro-LEDs. The maximum efficiency will be reduced as reduction of LED chip size, resulting from the non-radiative recombination at the etched sidewall. The ratio of sidewall perimeter to the total area increases as the chip size shrinks. For example, the ratio of perimeter/area of the 10um x10um is about 0.4 which is 10 times higher than that of 100um x100um. Therefore, the higher perimeter/area ratio indicates more opportunities to exposure the edge surface of sidewall damage, which causes more non-radiative recombination, such as SRH. Hwang et al. [5] had fabricated the micro-LEDs with areas from 10–4 to 0.01 mm2 to systematically evaluate the relationship between the chip size and EQE. Figure 1.6 showed the peak EQEs of the smallest and largest micro-LED were 40.2 and 48.6%, respectively. The difference in efficiency was from nonradiative recombination originating from etching damage of sidewall. They pointed out that the lower peak efficiency of the smaller micro-LEDs is a result of lowered internal quantum efficiency, which can be attributed to sidewall damage from dry etching. Additionally, the surface recombination velocity of InGaN is on the order of 104 cm/s, compared to 105 cm/s for AlInGaP [6]. AlInGaP-based micro-LEDs may experience sever drops in peak efficiency as the size diminishes compared with InGaN materials. To address this issue, sidewall treatment such as atomic-layer deposition (ALD) passivation method, had been utilized for the both AlInGaP and InGaN-based micro-LEDs [7, 8]. Fig. 1.6 Dependence of EQE on current injection (legend describes the mesa edge length) [5]. Copyright (2017) The Japan Society of Applied Physics

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1.3.3 Improvement of the Peak Efficiency by Epitaxy Structure It is a possibility to improve the peak efficiency through the modification of epitaxy structure. Suk Choi et al. [9] had reported that the LED without an electron-blocking layer (EBL) shows a sharp peak efficiency at J = 3 A/cm2 , which is extremely different from the LEDs with an AlGaN EBL and an InAlN EBL show peak efficiencies at J = 6 A/cm2 and 11.5 A/cm2 , respectively. Figure 1.7 revealed the quantum efficiency of the LEDs with and without an EBL versus the injection current. One can clearly see that the peak efficiency of the LED without an EBL is much higher than other LEDs with EBLs and at relatively lower current density. They explained the EBL creates barriers in the valence band that may act as a hole-blocking barrier. At low current densities, this barrier may limit the hole transport into the active region and may result in nonuniform of hole distribution, leading to lower quantum efficiencies for LEDs with an EBL than for LEDs without an EBL. Other efficiency behavior also had been investigated by varying the quantum well thickness. Li et al. [10] observed that the thinner well gives relative higher internal quantum efficiency due to the radiative recombination rate raised. By thinner well thickness, the reduction of the internal field will be created to enhance the spatial overlap of electron and holes within the MQWs. Moreover, the thinner wells allow growth of strained InGaN, whereas the thick wells promote the growth of relaxed InGaN. Thus, the higher crystal quality could be obtained by thinner wells that have less defects formation within the wells. Since defect density, operating current and micro-LEDs device architectures are different, we can’t just adopt the conventional concept that used for general lighting. The epitaxy structure and process technique must be optimized a lot to maximize the peak efficiency. Fig. 1.7 Quantum efficiency versus current density for the LEDs without an EBL, with an AlGaN EBL and an InAlN EBL [9]. Used with permission. Copyright (2010) AIP Publishing

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1.4 Epitaxy Growth Technique Between 4 and 6 Sapphire 1.4.1 Large-Sized Wafer Substrate The LED sapphire market had moved up from 2 wafers to 4 wafers. 4 wafers already occupied the majority of sapphire wafer area today. For the further enlarged diameter, 6 , which is not in most of the sapphire wafer market. However, the challenge of fabrication has been overcome by sapphire makers and the usability has been proven by the LED chip manufacturers at 6 diameter. Up to now, the 6 sapphire is ready to penetrate into the market instead of 4 as a commodity product. For years, 4 sapphire wafers have been the standard for LED production. In terms of the quantity of LED chips, 6 diameter wafers can provide more chips from each wafer processed. Current size of LED power chip for lighting is about 1 mm, so we will roughly receive 6,100 die (each 1 mm × 1 mm) on our 4 diameter wafer if wafer edge excluded by 5 mm. On the other hand, a 6 diameter wafer will produce 15,000 die, which is approximately 2.45 times as many chips as compared to a 4 . The usable area for chip is 9% more although geometrical area increase is 2.25 × from 4 to 6 diameter, as shown in Fig. 1.8. This is called “edge effect” due to the decreased curvature of the larger-sized wafer allows more LED chips to fit along the outer perimeter [11]. Additional to the high chip yielding, mass transfer also requires the larger wafer size because of the use of transfer stamps. In the same way, more usable area can be allocated for any given stamp size with larger diameter wafers. On the other hand, more wafer surface area can be utilized for bigger transfer stamps and thus less transfer frequency. The bigger stamp with more chips can be successfully transferred at a time, hence, the number of total transfer event can be reduced for cost savings. Moreover, the less transfer event of bigger stamp might alleviate the mura defect between the boundary of each stamp to minimize the blemishes on panel.

9% more chips or stamps per area

4” (100 mm)

6” (150 mm)

Fig. 1.8 The 6 wafer yields 9% more usable chips for the same cost

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In short conclusion, the large wafer size enables the economic use of high yielding by single-wafer processes like in the IC industry in terms of chip volume or transfer stamp. Other types of micro-LED grown on silicon wafers use 8 and even 12 —an indication this is the path to efficiency and productivity. Indeed, micro-LED chip manufacturers are moving to larger wafers because they are handling fewer units. Each time the robot arm picks up an 8 wafer it moves more chip area by an order of magnitude compared with a 4 wafer [11].

1.4.2 Relationship Between Wafer Bow and Wafer Diameter Reducing production cost of micro-LED is essential for mass production. The larger wafer diameter will be the first tier for cost reduction in terms of wafer edge effect and mass transfer efficiency. Micro-LED epitaxy on larger diameter wafers such as 6 sapphire is one approach for reducing the manufacturing costs. The growth on 6 sapphire has proven to be more difficult, mainly due to the large thermal and lattice mismatches between the sapphire and GaN layers, which result in large bowing of the wafers during epitaxy process. To compromise the wafer bow, different diameter sapphires have various thicknesses, the 4 substrate had a thickness of 650 um and 900 um, and 6 substrate ranged from 900 to 1300 um. In Fig. 1.9 [12], the wafer bowing of 2 , 4 and 6 sapphire have been compared during LED growth. As expected, the profile of concave bow increased drastically during the high temperature n-GaN growth. At the end of n-GaN layer, the 6 wafer with 1300um has the bow about 260 um, which is much larger than the 430 um-thick 2 and 900 um-thick 4 wafers. Calculating the curvature (K) by using the flowing formulas: K = 1/Rcurve = B ∗ 8/2 Fig. 1.9 Evaluation of concave bow during n-GaN growth process for 2 , 4 and 6 wafers with substrate thickness of 430 um, 900 um and 1300 um, respectively [12]. Copyright (2009) Veeco

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Here, Rcurve is the radius of curvature, B is the bow and  is the substrate diameter. After calculation, the 1300 um-thick 6 wafer concave bow is 260 um at the end of n-GaN, corresponding to the curvature is −90 km−1 . For the 900 um-thick 4 wafer concave bow is 120 um that corresponding to the curvature is −93 km−1 . As a result, 4 and 6 wafers had the equipment curvature although they revealed the large difference of concave bow at the end of n-GaN growth. The huge concave bow occurred at larger diameter wafer has to take into account during LED growth process.

1.4.3 Pocket/disk Design for Wafer Bowing Since the wafer bow is the amount of deflection of the outer radius of the wafer compared to the center, a huge thermal gradient is created. The non-equilibrium heating can have strong effects on the growth rate as well as the thickness uniformity during bulk GaN growth. Thus, the 6 process requires the space between wafer bottom and surface of pocket/disk to accommodate the large concave bow during bulk GaN growth. The so-called rim/bump had been adopted around the edge of pocket/disk inside to support the wafer edge, as shown in Fig. 1.10 [12]. This design will have more space for highly concave bulk GaN growth to minimize thermal gradients across the wafer caused by the large concave bow. In addition, thermal uniformity within a wafer is ultimately important at the InGaN active layer. It is important to modify the surface profile of pocket/disk to match the wafer shape during the MQWs growth. As previous discussion, the affections of concave bow during bulk GaN have been diminished by introduction of the rim/bump, whereas the surface profile of pocket/disk will dominate the temperature distribution within a wafer during MQWs growth. At the InGaN growth stage, the wafer is back to relatively flat compared with n-GaN growth stage. As seen in Fig. 1.9 [12], the 4 wafer remains concavity of 25 um, however, the 6 wafer still shows more concavity of 50 um. Based on our experimental results, a 10 um bow deflection during MQWs Fig. 1.10 Rimmed pocket design geometry [12]. Copyright (2009) Veeco

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growth results in 1.8 nm wavelength shift approximately. Therefore, the surface profile of pocket/disk need to be designed to match the wafer concave bow at the InGaN growth stage. By understanding these dynamics of wafer bow and optimization of the pocket/disk accordingly. Outstanding within-wafer uniformity of bulk thickness and wavelength will be achieved on larger diameter substrate, such as 6 sapphire, which can be utilized as micro-LED epitaxy wafers.

1.5 Device Manufacturing “Micro-LED” means that the dimension of a single LED chip (R, or G, or B) is less than 100 um square. “Micro-LED” also indicates that the thin-film LED chip does not carry any growth or temporary substrate. Micro-LEDs can be briefly classified into two types according to the direction of drift current. One is the vertical type, and another is the lateral type. The two types of micro-LEDs are plotted in Fig. 1.11. To vertical type, the p-electrode and n-electrode are in two opposite sides of the microLED epitaxy. The current is driven along the epitaxial growth direction (z-axis). In contrary, the p-electrode and n-electrode locate in the same side of LED epitaxy in lateral micro-LEDs. The driven current flows in parallel to the plane of epitaxial layer (x–y plane). The advantages of the latter include that (a) it can be easily integrated on the printed circuit board (PCB), thin-film transistor (TFT), and CMOS and (b) it has no shielding effect arising from the metal and light-absorption material in the direction of light output. So far, the lateral micro-LEDs are commonly used. In the next paragraph, we will introduce the lateral micro-LEDs in detail. The illustrations of lateral micro-LEDs are shown in Fig. 1.12. As shown in Fig. 1.12a, the p metal is electrically ohmic connected to the p-semiconductor by the through hole, where the partial of n-semiconductor and the partial of MQWs are removed. The n metal is directly connected to the top n-semiconductor. In hence, the n and p metals locate in the same side of micro-LED epitaxy. Then, the direction of light output is in opposite to the p and n metals. The conversely polarity can be also completed as shown in Fig. 1.12b. To enhance the light extraction efficiency, several methods can be employed. First, the patterned or roughened light-output surface is useful to suppress the totally

Fig. 1.11 a Vertical-type micro-LED and b lateral-type micro-LED

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Fig. 1.12 The illustrations of a n-side up and b p-side up lateral micro-LEDs device

internal reflection (TIR) as shown in Fig. 1.13a. Second, the reflector can be added in the side of n (and p) metal as shown in Fig. 1.13b. The reflector can be the metal reflector, omni-directional reflector, and distributed Bragg reflector (DBR). Third, the trapezoid cross-section shape of chip is benefit to light output as shown in Fig. 1.13c. Fourth, anti-reflector coated on the surface of light output can be also used to reduce TIR as shown in Fig. 1.13d. The conventionally large-size LEDs (>10mil2 ) is usually operated at high current density, that is j higher than 30 A/cm2 . Under high current density, the effects of electrical and thermal droop on internal quantum efficiency (IQE) are easily observed. This is because that the electrons and holes get the excess energy and then escape from the QW. In contrary, the current density, which is used to drive the micro-LEDs, is low. The driven current density is ranging from 0.01 A/cm2 to 5 A/cm2 , which is much lower than the current density of maximal IQE. Therefore, try to increase the “effective current density” might be a good choice to improve the IQE of microLEDs. The illustration of the “effective current density” is shown in Fig. 1.14. To increase the “effective current density”, shrinking the thickness of window layer (as plotted in Fig. 1.15b), reducing the light emitting area (as plotted in Fig. 15c),

Fig. 1.13 The four methods to improve the light extraction efficiency

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Fig. 1.14 The illustration of a relatively low current density and b relatively high current density

Fig. 1.15 The illustration of the methods to achieve the highly effective current density

and decreasing the current area by current confinement are ones of the methods (as plotted in Fig. 1.15d). At the sidewall of through hole, these might be a growing defect density. The defects are mostly generated from the plasma damage in the dry-etching process. These defects might induce the surface recombination when micro-LED is under operation. Because of the small dimension of micro-LED, the exposed surface to

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volume ratio becomes high. The surface recombination would play a critical role on the radiative efficiency and IQE in micro-LED. In the viewpoint of device manufacturing, we can (a) use a moderated plasma or plasma-free etcher to etch the through hole, (b) clean the native oxidation on the surface and passivate the surface, and (c) design a structure for current confinement. The former two items are used to reduce the defect density (Ns), and the latter is used to decrease the carrier density around the defects.

References 1. Veeco, Uniform injector flow-flange TurbodiscTM reactor (K465i) thickness & alloy composition tuning guide. 2. Yang, F. H. (2014). Modern metal- organic chemical vapor deposition (MOCVD) reactors and growing nitride-based materials. Woodhead Publishing. 3. Dai, Q., Shan, Q., Cho, J., Schubert, E. F., Crawford, M. H., Koleske, D. D., Kim, M. H., & Park, Y. (2011) Applied Physics Letters, 98, 033506. 4. Schubert, M. F., Chhajed, S., Kim, J. K., Schubert, E. F., Koleske, D. D., Crawford, M. H., Lee, S. R., Fischer, A. J., Thaler, G., & Banas, M. A. (2007) Applied Physics Letters, 91, 231114. 5. Hwang, D., Mughal, A., Pynn, C. D., Nakamura, S., & DenBaars, S. P. (2017) Applied Physics Express, 10, 032101. 6. Boroditsky, M., Gontijo, I., Jackson, M., Vrijen, R., Yablonovitch, E., Krauss, T., Cheng, C.-C., Scherer, A., Bhat, R., & Krames, M. (2000). Journal of Applied Physics, 87, 3497. 7. Wong, M. S., Hwang, D., Alhassan, A. I., Lee, C., Ley, R., Nakamura, S., & Denbaars, S. P. (2018) Optics Express, 26(16), 21324. 8. Wong, M. S., kearns, J. A., Lee, C., Smith, J. M., Lynsky, C., Lheureux, G., Choi, H., Kim, J., Kim, C., Nakamura, S., Speck, J. S., & Denbaars, S. P. (2020) Optics Express, 28(4), 5787. 9. Choi, S., Kim, H. J., Kim, S. S., Liu, J., Kim, J., Ryou, J. H., Dupuis, R. D., Fischer, A. M., & Ponce, F. A. (2010) Applied Physics Letters, 96, 221105. 10. Li, Y. L., Huang, Y. R., & Lai, Y. H. (2007) Applied Physics Letters, 91, 181113. 11. Nabulsi, F. (2015) Semiconductor today. Compounds & Advanced Silicon, 10(3). 12. Armour, E., Lu, F., Belousov, M., Lee, D., Quinn, W. (2009) Semiconductor today. Compounds & Advanced Silicon, 4(3).

Chapter 2

Quantum Dot Color Filter and Micro LED Kyoungwon Park, Yeongbeom Lee, Jeongno Lee, and Chul Jong Han

2.1 Color Conversion Display Recently, color conversion type displays are emerging. The first generation of this kind is TVs and monitors with quantum dot enhanced films (QDEF) which were successfully commercialized since 2014. This device minimizes alteration from the original LCD structure. By inserting one color conversion layer (CCL) between a liquid crystal and a light guiding plate, it can generate more vivid colors. After advent of this, QD color filter (QD-CF) based color converting displays have been pursued which are regarded as the next generation display. This display consists of a blue light source and a QD-CF CCL. Since color conversion process occurs at the outmost of the panel, one can experience the uniform luminance distribution at any angle. Depending on which blue light sources are implemented, it can be QD-OLED, QDLCD or QD-μLED [1–3]. Since blue is already given, blue to red or blue to green energy downconverter are necessary (Fig. 2.1b). Although QDs are highlighted as an efficient energy downconverter nowadays, other organic emitters are also studied [4]. Or, UV light source based CCL is also studied where RGB QDs are employed to convert UV to each colors [5]. The main huddle for commercialization of μLED displays is its expensive cost and time-consuming integration process. Chip fabrication and wafer dicing are common steps for semiconductor manufacturing. However, μLED chips’ integration into a single panel requires repetitive robot-aided pick-and-place and interconnection jobs. To embed red, green, and blue μLEDs in each sub-pixels in a panel, these pick-andplace and wire-bonding processes become triply tedious work. Accordingly, this RGB μLED display is regarded as mass production unfriendly. In contrast to this, QD-μLED simplifies the manufacturing steps by sole blue color formation (Fig. 2.1). K. Park · Y. Lee · J. Lee · C. J. Han (B) Korea Electronics Technology Institute, Seongnam-si, Gyeonggi-do, Korea e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2021 J.-H. Ahn and J.-H. Kim (eds.), Micro Light Emitting Diode: Fabrication and Devices, Series in Display Science and Technology, https://doi.org/10.1007/978-981-16-5505-0_2

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Fig. 2.1 Conceptual structures of a RGB LED and b QD-CF on B-LED

In this chapter, we discuss QD-μLED display’s structure, manufacturing process, materials especially QD and bank and optical properties including color purity, efficiency and viewing angle in detail.

2.2 Quantum Dot Color Conversion Layer Semiconductor nanocrystals (NC) receive of-great attention due to their unique physio-chemical properties [6]. Among them, quantum confinement effect (QCE) and delta-function like density of states make this nanosized material more fascinating for opto-electronics applications [7–9]. When a semiconductor shrinks to the nanoscale, energy level spacing becomes larger and even resolvable at room temperature (E  kT). If a NC size is smaller than its Bohr radius (rBohr ), size dependent bandgap is observable, called as QCE [7]. In other words, one can simply control its photoluminescence (PL) wavelength by changing its size [10]. For example, the bulk bandgap (Eg ) of binary InP semiconductor is 1.35 eV, but it can increase to 2.6 eV via QCE, emitting from infrared to cyan color [11]. CdSe has 1.75 eV of Eg , but when it becomes nanocrystal, its Eg increase to 2.8 eV, emitting all primary R, G, B colors [12]. Figure 2.2 describes diverse semiconductors’ wavelength coverage from bulk to nanoscale size. For the display purpose, emitting three primary colors (red, green and blue) is mandatory. If blue LEDs are used as an excitation light source in a color converting display, only red and blue emission are required via blue to red and blue to green color converters. Among semiconductors listed in Fig. 2.2, red and green emitters are limited to Cd type and InP materials [13]. Ternary semiconductors such as CuInS2 , AgInS2 used to have wide PL spectrum since they have multiple decaying paths of an exciton including exitonic, trap-mediated and dopant-mediated emissions [14]. Each of them has different energy releasing paths, showing broaden PL. Among the limited choices, Cd-type are restricted to use due to the European environmental regulation, Restriction of Hazardous Substances (RoHS) although they have good optical properties and readily synthesizable [15]. Therefore, InP material turns out to be sole, environmentally benign color converting material, yet [16]. Equally importantly, a zero dimensional NC has narrow PL spectrum due to the discrete energy level near its conduction and valence band-edges, so it is called

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Fig. 2.2 Reported spectral ranges of emission for different semiconductor nanocrystals, reprinted with permission from reference [13]

as a quantum dot (QD) or dubbed as ‘artificial atom’ [9]. A single QD’s emission bandwidth is at around 20 nm which is superior to most other fluorophores [17]. Although inhomogeneous broadening due to heterogeneous size distribution is difficult to suppress completely, the InP QD based display can sufficiently cover the whole DCI-P3 color space, and larger than 90% of BT2020 color space [16]. In contrast to this inherent superiority in spectrum linewidth, PL quantum yield (PL QY) is not inherent nature. Therefore, numerous synthetic routes have been investigated to increase PL efficiency [16, 18–22]. Among them, depositing inorganic protective layer (shell) on top of the emissive InP or CdSe core is one such critical advance [18]. The protective shell material should have larger Eg with lower electron affinity and deeper ionization energy than the core material (Fig. 2.3, Type-I). This type of energy structure can protect an exciton from exciton quenching chemicals in outer environment and tunneling through a shell. ZnSe and ZnS are two well known shell materials that are epitaxially grown on InP or Cd- type cores [23]. Developing reactive precursors is another important breakthrough for uniform InP core synthesis. Recently, highly reactive phosphor precursors, (Tris(trimethylsilyl)phosphine, (TMSP)) are developed and readily adopted in massproduction [18]. Other technical advances for QD synthesis are (i) gradient shell [21] or quantum well [24] type core/shell structures for strain relaxation from the lattice mismatch between a core and a shell material, (ii) annealing at elevated temperature for concentric shell morphology [22], (iii) HF etching for removing oxidized phosphorous in InP core [25]. With these progresses applied, the state of the art InP QDs have nearly 100% of PL QY and 36 nm of full width at half maximum (FWHM) for both green and red colors [17, 22].

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Fig. 2.3 QD energy structures and semiconductor bandgaps. Reprinted with permission from reference [23]

2.3 CCL Patterning Process and Color Resist Composition Defining a absorptive color filter (CF) layer is achievable by the conventional photolithography or recently highlighted ink-jetting process [1, 26]. The former method is widely used in current display industry and quantum dot based color converting color filter patterning via lithography is nothing different from manufacturing CF process except for the color resist composition. Color photoresist (PR) consists of resist and milbase components [26]. Resist serves as a polymerizing agent, composed of photo-initiators, monomers, binders, solvent, and other additives. Milbase provides, on the other hand, wavelength selectivity in transmission via pigments’ or dyes’ absorption. When these colorant (pigment and/or dye) are replaced with QDs, absorptive property is changed to color conversion, forming QD photoresist (QD-PR). In QD-PR, scatterers are necessary for increasing blue light’s absorption [27]. There are several candidates for scattering particles such as TiO2 , ZrO2 , ZnO, BaSO4 whose refractive index (> 2) are larger than the polymer matrix (~1.5). This metal oxide particles should be large enough to scatter blue light and small enough not to be precipitated to have shelf stability. Once scattering particles exist in QD-CF, blue photons’ travel path become longer and QDs have more chances to absorb them. There is no need of color conversion in a blue pixel, but scatterers are still required for generating the same viewing angle as red and blue pixels. Otherwise, the QD-CF panel will show color difference from the side-view. Photolithography process of QD-CF panel is illustrated in Fig. 2.4a, b, c, d and e. First, a black matrix (BM) is formed to separate and define sub-pixels. Next, a conventional CF layer is formed by using the usual selective polymerization and

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Fig. 2.4 QD-CF process of (a–e) photolithography and (i–iv) ink-jet a BM patterning b CF pattern by photolithography c R QD-PR coating d photolithography of R QD-PR e development i bank forming ii metal reflecting layer deposition iii inkjet printing of CF iv inkjet printing of QD-CF

development process. After formation of the CF layer, QD-PR is spin- or slit- coated and soft baked at around 100 °C. Both PR and QD-PR are usually negative type, so they will be polymerized when exposed to UV via mask. (Fig. 2.4d). With a base solution development, followed by a hard bake process at elevated temperature (> 180 °C), one sub-pixel of QD-CF is completed. Normally, QDs are prone to be degraded by heat, blue scattering pixel should be processed first, followed by red or green pixels. Photolithography is mature technology and it requires no new equipment. But slight modification in component from PR to QD-PR is required. However, QDs are very expensive material compared to the other components in QD-PR. Compared to the 1–3 μm of CF height, QD-CF is 6–10 μm of height [1]. Accordingly the cost of forming QD-CF is not negligible and at least two thirds of QD-PR is wasted while developing process (Fig. 2.5). To minimize the cost due to the material waste in photolithography, display industry pursues to adopt ink-jetting process for making OLED and QD-CF panel. For having jetable property, QD-PR should be transformed to QD-ink which satisfy Reynolds and Ohnesorge criteria, by tuning its viscosity, and surface tension. In addition, vapor pressure should be taken into a consideration not to clog the nozzle. Major difference in QD-ink and QD-PR is its solvent content. QD-PR contains roughly 30– 80% of solvent in its weight. However, the solvent should be minimized in QD-ink or solvent-less ink should be developed. The maximum achievable bank height by

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Fig. 2.5 DBR employed QD-μLED structure

photolithography is at around 10–15 μm and which is the marginal thickness of QD-CF for maximizing blue photon absorption, discussed later. These conditions indicate that there is not enough room for solvent in QD-ink. If solvent content is large, QD-ink will easily spill over other pixels. Another feasible design of QD-CF is using distributted Bragg reflector (DBR). By alternating high and low refractive indexed layers, one can reflect only blue photons into QD-CF and trasmit red and green PL photons. This structure is indeed, advantageous in terms of blue photons’ recycle. Possible combinations are HfO2 (n ≈ 2 at 500 nm) / SiO2 (n ≈ 1.5 at 500 nm) and SiN (n ≈ 2 at 500 nm) / SiO2 . After depositing DBR, selective etching in blue pixels should be processed.

2.4 Optical Property 2.4.1 Color Purity The main advantage of using QDs in display application is its high color purity. QD has discrete energy levels near band maxima and minima and excitons mostly decay between these two extremas. Therefore, this single decaying path generates a sharp gaussian-like emission spectrum. The FWHM of a single green InP QD is measured to be 22 nm [17], which is smaller than any other fluorophores used in commercial display. However, inhomogeneous broadening due to the size heterogeneity worsen this value to 36 nm in ensemble which is yet superior to any other current organic emitters [17, 22]. Figure 2.6 compares the spectral bandwidths of RGB LEDs and R, G QDs. It is noted that the peak wavelengths are adjustable for both LED and QD but spectral bandwidths are hard to diminish and important for estimating color purity. Previous study investigated that the RGB LED display has 98.9 and 91.4% agreement with DCI-P3 and BT2020 color space [28]. InP QD’s color coverage is clearly summarized in ref. [16]. Figure 2.7 present traces of x, y coordinate while changing FWHMs of green and red InP QDs. According to this calculation, QDs can sufficiently cover the DCI-P3 color space. BT2020 is a new color standard and 140% larger than the

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Fig. 2.6 Color filter transmission (filled color), measured LED (solid) and QD (dash) emission spectra

Fig. 2.7 The evolution of vertices in the color triangle space depending on the fwhm (from 45 to 20 nm) of the Gaussian spectra with peak emission at 528 and 637 nm (center). Enlarged graphs for each vertex in green (left) and red (right) are shown. Figures are reprinted with permission of reference [16]

DCI-P3. State of the art of QDs, having 36 nm of FWHMs, reach 92.0% of BT2020. If FWHMs of red and green are reduces to 20 nm, the agreement with BT2020 will be 95.7%. If CFs with high color gamut are applied, the color space will agree more with BT2020 standard. To summarize it, this investigation indicates that QD color converting display possesses comparable color purity as RGB LED display having nearly 92% of BT2020 area.

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2.4.2 Efficiency CFs cut the unselected spectrum which reduces the overall photon flux. For example, a red CF only transmits red light, wasting two thirds of blue and green light. In contrast to this, QD-CF can utilize most of the blue backlight. Providing that all blue photons are absorbed, the color converted PL and extracted outward photons are not 100%, though. Photon conversion efficiency (PCE) is defined as the ratio between extracted outward PL photons over generated blue photons by LEDs and it becomes the representative figure of merit of a QD-CF. PCE determines a QD-CF’s brightness but it is not solely determined by QDs’ inherent optical properties. However, it is still manifest that internal PL QY of QDs is the largest factor for determining PCE. Another factors are the degree of absorption and re-absorption. PL QY of QDs can be degraded for several reasons such as mixing with resist components, exposure to UV, soft- / hard- baking and development steps during the patterning process. To remove this loss, screening of unharmed chemicals to QDs especially in dispersants and photoinitiators will be the priority [29]. Secondly, QDs should be synthesized stable enough to endure the multiple patterning steps by adopting core–shell structure or metal doping [30]. Although all these efforts are applied, degradation in PL QY is, to a certain degree, unavoidable. This posterior QY (pQY, ς) of QDs suffering all these stresses, determines the PCE of the QD-CF. The direct measurement of pQY is hard to obtain, but inversely estimable, discussed later. The absorption of the QD-CF can be easily measurable by detecting the difference in summed blue photons with and without QD film. Here, QD film is referred to the spin-coated and baked QD color resist without patterning. If QD weight percentage in QD-PR or QD-ink is high enough (30–40%) and efficient scatterers are included, larger than 90% of absorption will be achieved. Simple calculation that estimating the residual blue photons at certain depths along the film thickness (t) is presented in Fig. 2.8. The 90% of absorption is guided by the red dash line. To achieve this absorbance in 10 μm of the film, at around

Fig. 2.8 Calculated a residual blue photons along the CF depth and b efficiency loss due to the multiple absorption (n)

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4.4 μm of mean free path (l) is realized. Or, 2.6 μm of l is required if one uses 6 μm of QD-CF thickness. From this, we can generalize the relationship of l and t for a certain absorption level. t = log 10 = 2.3 l

(2.1)

For 90% of absorption, the mean free path should be 2.3 times shorter than the total thickness (Eq. 2.1). In other words, blue photons have 2.3 chances to be absorbed by QDs in average until they reach the other side of the QD-CF layer. Generally speaking photons should be absorbed multiple times and re-absorption is unavoidable for sufficient blue photon absorption level. This multiple absorption process, unfortunately, reduces the PCE. n times of absorption results in its efficiency to ςn . Fig. 2.8 shows the rapid efficiency drop with multiple absorption occurrences (n) with different ς. Although 90% pQY is achieved, ςn is estimated to be 77% in 90% of absorption level (where n = 2.3). Unless 100% pQY is guaranteed, multiple absorption will eventually decrease the PCE (Fig. 2.8b). Due to the exponential nature of the absorbance, complete removal of blue leakage is unrealistic (Fig. 2.8a). Adopting a conventional CF or adopting DBR is, thus, recommended for high color purity (Figs. 2.1b and 2.5). Absorbance can be increased with two different approaches. Adding more QDs and scattering particles is the easier solution. This strategy shorten the mean free path, resulting in increased absorption or enabling a thinner QD-CF. This approach is ready to use only if better chemical formulation that dissolving high content of QDs and scattering particles in a color resist or an ink is established. Importantly, this approach may degrade the PCE by increasing re-absorption, if two many absorption occur. QD-CF’s PCE and residual blue photons at certain depth are conceptually described in Fig. 2.9. If a QD-CF is too thick, PCE will be rather lowered due to the multiple re-absorption. Second approach is structuring QDs such that having increased absorption and suppressing re-absorption. Figure 2.10 presents CdSe and InP QDs’ (both red and Fig. 2.9 Conceptual PCE and blue photons in QD-CF

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Fig. 2.10 Absorption spectra of CdSe and InP a green b red. LED and QDs’ emission spectra are also inserted with filled curves

green) absorbance and PL spectra. Blue GaN LED spectrum (440–500 nm) is also presented as the blue filled curve which is the absorption band of QD-CFs. Reabsorption bands are near their first excitononic (1 s) peak which are 500–550 nm and 600–650 nm for green and red, respectively. Therefore, optical density (O.D) ratio between near the absorption band (440–500 nm) and re-absorption (near 1 s peaks) determines the relative strength of absorption over re-absorption. Since ideal QDs for a color converter should have higher absorption and lower re-absorption, O.D at 460 nm is preferably higher than that of 1 s peak. Compared to CdSe QDs and the red InP QD, the green InP QD is expected to be weak at absorption compared to the re-absorption band (Fig. 2.10). The relative strength of absorption and reabsorption are depicted with the magnitude of blue LED and QDs’ PL spectra for easier understanding in Fig. 2.10. The difference in O.D ratio directly influences the PCE. Accordingly it is estimated that the maximum PCE will be higher for red QD-CF than green, although the same pQY is assumed. In addition, the depth showing maximum PCE is shallower for red QD-CF. However, not many choices are given to increase O.D ratio. Depositing a thick ZnSe shell is one such strategy since its bandgap is 2.7 eV (= 460 nm). If adopting this, O.D at wavelength shorter than 460 nm will be increased. Alternatively, adopting alloyed core such as InGaP or InAlP is deserved to try. GaP and AlP have larger bandgap than that of InP. As explained above, CdSe needs smaller QCE than InP for emitting green emission. And, smaller QCE induces dense energy levels. Accordingly, green CdSe QD has more energy (higher O.D) levels to absorb blue photons than green InP QD (Fig. 2.10). Alloying InP with GaP or AlP will bring the similar effect as what CdSe does. To generalize the efficiency, PCE is determinded by pQY, absorption and reabsorption. In addition to these, the portion of the front side emission (towards viewers) among the total emission does matter. Since blue photons are absorbed more at the back side, close to LED, the front side emission portion is not 50%, rather it is close to 45%. To summarize this, PCE follows Eq. 2.2.

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PC E ≈ 0.45 × absor ption × ξ n

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(2.2)

With 90% of pQY (ς) and 90% absorption level and n = 2.3, PCE is approximately 31%. If higher absorption and lower re-absorption such as red InP QD is used, smaller n is expected. When a same calculation with n = 1.8 is performed, PCE will be 34%. It is noted that the loss due to the QD-CF is roughly two thirds which is similar to the normal CF. In order to increase PCE, selective red and green reflecting DBR is worthwhile to try. So far, PCE of pattern-less QD film is discussed. However, a patterned QD-CF has a bank or BM structure. Both pixel defining structures are normally black to block the color mixing. If taken into this structure account, actual PCE will be somewhat lowered than what we calculated above.

2.4.3 Blue Leakage QDs have higher optical density in larger energy than their bandgap. However, this does not guarantee 100% blue absorption. Several recent studies provide the required QD-CF thickness by assessing the blue leakage or prove the need of the blocking layer of blue leakage, paradoxically. Hu et al. investigated the thickness dependent PL and blue leakage of the CdSe QD-CF (Fig. 2.11). Blue leakage is still observable even with 10 μm height and high absorbing CdSe QD-CFs. Yue et al. adopted scattering particles in QD-CF to increase PCE and reduce blue leakage [27]. According to this study, blue absorption is increased at least twice for both colors and PCE is increased × 5 and × 3 for green and red, respectively as well. These results show that the complete removal of blue leakage needs a normal CF.

Fig. 2.11 PL spectra QD-CF at different thicknesses, excited by B-OLEDs. Reprinted with permission of reference [1]

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Fig. 2.12 Relative angular luminance of QD-LCD (blue) and QDCF (red). Reprinted with permission of reference [3]

2.4.4 Viewing Angle Another main advantage of using a QD-CF over conventional displays is its wide viewing angle. Although some LC mode such as In-plane switching (IPS) provides wide viewing angle, (u’v’) and gamma shift ratio (GSR) are not impeccable. Meanwhile, OLED display also utilizes and adopts a resonance structure to extract the light. Consequently, from the off-resonance angle, it suffers from the color shift and weaken luminance. In contrast to this, the QD-CF display does not have angledependent color and intensity change since the QD-CF is located at the out-most layer, underneath the glass and QDs emit omnidirectional. Figure 2.12 compares the angle dependent luminance between QDEF based LCD and QD-CF display [3]. Large difference in intensity is found in QD-LCD over the angle, uniform intensity is drawn from the QD-CF display.

2.5 Conclusion In short, QD CCL on LED simplifies the RGB LED’s highly time-consuming integration process. By considering the material waste, ink-jet printing process of QD-CF is recommended and highlighted nowadays. To do that, high concentrated QD-ink should be prepared with minimizing solvent content. Scattering metal oxide nanoparticles are another important material in QD-ink to suppress the blue leakage and increase the PCE. The color purity of the QD-CF on blue LEDs is calculated to be equal to that of RGB LED, covering > 90% of BT2020. The PCE is estimated to be roughly 30, 35% for green and red QD-CF, respectively which are slightly better or equal to the normal LCD’s transmittance ratio.

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References 1. Hu, Z., et al. (2020). Inkjet printed uniform quantum dots as color conversion layers for fullcolor OLED displays. Nanoscale, 12, 2103–2110. 2. Lin, C. H. et al. Quantum dots based full-color display on MicroLED technology. 3. Han, S., Kiselev, F. D., & Mlejnek, M. (2019) Quantum dots on color filter LCD design study. 4. Kim, W. H. et al. (2020) High-performance color-converted full-color micro-LED arrays. Applied Science, 10. 5. Han, H.-V., et al. (2015). Resonant-enhanced full-color emission of quantum-dot-based micro LED display technology. Optics Express, 23, 32504. 6. Brus, L. (1991). Quantum crystallites and nonlinear optics. Applied Physics A Solids Surfaces, 53, 465–474. 7. Takagahara, T., & Takeda, K. (1992). Theory of the quantum confinement effect on excitons in quantum dots of indirect-gap materials. Physical Review B, 46, 15578–15581. 8. Klimov, V. I. (2000). Optical nonlinearities and ultrafast carrier dynamics in semiconductor nanocrystals. Journal of Physical Chemistry B, 104, 6112–6123. 9. Alivisatos, A. P. (1996). Semiconductor clusters, nanocrystals, and quantum dots. Science, 80(271), 933–937. 10. Shirasaki, Y., Supran, G. J., Bawendi, M. G., & Bulovi´c, V. (2013). Emergence of colloidal quantum-dot light-emitting technologies. Nature Photonics, 7, 13–23. 11. Mi´ci´c, O. I., et al. (1997). Size-dependent spectroscopy of InP quantum dots. Journal of Physical Chemistry B, 101, 4904–4912. 12. Norris, D., & Bawendi, M. (1996). Measurement and assignment of the size-dependent optical spectrum in CdSe quantum dots. Physical Review B—Condensed Matter, 53, 16338–16346. 13. de la Fuente, J. M., & Grazu, V. (2012) Nanobiotechnology inorganic nanoparticles versus organic nanoparticles. Admission Assessment: Exam Review. 14. Viswanatha, R., Brovelli, S., Pandey, A., Crooker, S. A., & Klimov, V. I. (2011). Copperdoped inverted core/shell nanocrystals with ‘permanent’ optically active holes. Nano Letters, 11, 4753–4758. 15. EUR-Lex—Ares (2017) 644052-EN-EUR-Lex. https://eur-lex.europa.eu/legal-content/PL/ ALL/?uri=PI_COM:Ares(2017)644052. 16. Jang, E., Kim, Y., Won, Y. H., Jang, H., & Choi, S. M. (2020). Environmentally friendly InPbased quantum dots for efficient wide color gamut displays. ACS Energy Letters, 5, 1316–1327. 17. Kim, Y., et al. (2019). Bright and uniform green light emitting InP/ZnSe/ZnS quantum dots for wide color gamut displays. ACS Applied Nano Materials, 2, 1496–1504. 18. Haubold, S., Haase, M., Kornowski, A., & Weller, H. (2001). Strongly luminescent InP/ZnS core-shell nanoparticles. ChemPhysChem, 2, 331–334. 19. Li, L., & Reiss, P. (2008). One-pot synthesis of highly luminescent InP/ZnS nanocrystals without precursor injection. Journal of the American Chemical Society, 130, 11588–11589. 20. Xu, S., Ziegler, J., & Nann, T. (2008). Rapid synthesis of highly luminescent InP and InP/ZnS nanocrystals. Journal of Materials Chemistry, 18, 2653–2656. 21. Lim, J., et al. (2011). Inp@znses, core@composition gradient shell quantum dots with enhanced stability. Chemistry of Materials, 23, 4459–4463. 22. Won, Y. H., et al. (2019). Highly efficient and stable InP/ZnSe/ZnS quantum dot light-emitting diodes. Nature, 575, 634–638. 23. Toufanian, R., et al. (2018). Bandgap engineering of indium phosphide-based core/shell heterostructures through shell composition and thickness. Frontiers in Chemistry, 6, 567. 24. Jeong, B. G., et al. (2016). Colloidal spherical quantum wells with near-unity photoluminescence quantum yield and suppressed blinking. ACS Nano, 10, 9297–9305. 25. Kim, T. G., et al. (2018). Trap passivation in indium-based quantum dots through surface fluorination: Mechanism and applications. ACS Nano, 12, 11529–11540. 26. Kwak, M., et al. (2019). Past, present, and future of WCG technology in display. Journal of the Society for Information Display, 27, 691–699.

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27. Yue, S. (2018) P-12.4 quantum dot photoresist for color filter application. SID Symposium Digest of Technical Papers, 49, 724–726 28. Guo, W., et al. (2019). The impact of luminous properties of red, green, and blue mini-LEDs on the color gamut. IEEE Transactions on Electron Devices, 66, 2263–2268. 29. Huang, Y., Zhu, Y., & Egap, E. (2018). Semiconductor quantum dots as photocatalysts for controlled light-mediated radical polymerization. ACS Macro Letters, 7, 184–189. 30. Kim, Y. H., et al. (2020). Photo-patternable quantum dots/siloxane composite with long-term stability for quantum dot color filters. ACS Applied Materials & Interfaces, 12, 3961–3968.

Chapter 3

Laser Lift-Off (LLO) Process for Micro-LED Fabrication Jaegu Kim and Jae-Hyun Kim

3.1 Introduction Laser lift-off (LLO) is a process of separating layers on a substrate. By epitaxially growing GaN on a sapphire substrate, light-emitting diodes (LEDs) that produce UV, blue and green light can be fabricated [1]. Previously, such LEDs were diced together with the sapphire substrate and used in the form of chips. Micro-LEDs, however, allow a smaller chip size by separating the sapphire substrate and LED epilayer. The LLO process is to irradiate a laser beam onto the interface between the sapphire substrate and GaN layer and then separates the GaN layer from the substrate. The laser wavelength for the LLO process is in the UV range, and the laser is irradiated from the sapphire substrate side. Typically, an excimer laser (ArF—193 nm, KrF—248 nm, XeCl—308 nm) or diode-pumped solid-state (DPSS, frequencytripled laser—355 nm, frequency-quadrupled laser—266 nm) laser is used. The photon energy of the wavelengths in the range of 248–266 nm is 3.50–5.0 eV, which is lower than the sapphire bandgap energy of 9.9 eV but higher than that of GaN at 3.44 eV. Therefore, the laser beam passes through the sapphire and is absorbed by the GaN layer. This reaction can be expressed as follows. GaN (s) ↔ Ga (s) + 1/2 N2 (g) Generally, before performing LLO, a temporary substrate in the form of a glass/adhesive layer, metal joint, or polymer protective film is bonded to the surface containing the LED. After laser irradiation, the layers are heated at the Ga melting temperature of 30 °C or higher. Slight force is applied to separate the LED and J. Kim (B) · J.-H. Kim Nano-Mechanical Systems Research Division, Korea Institute of Machinery & Materials, 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon, Korea e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2021 J.-H. Ahn and J.-H. Kim (eds.), Micro Light Emitting Diode: Fabrication and Devices, Series in Display Science and Technology, https://doi.org/10.1007/978-981-16-5505-0_3

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sapphire substrate [2]. Ga or GaO can remain at the GaN interface and can be removed using a diluted acidic solution of HCl or H2 SO4 /H2 O2 . This chapter introduces the LLO mechanism and LLO systems depending on the laser type and describes the various supplemental processes of LLO.

3.2 Laser Lift-Off Mechanism The basic structure of the GaN epilayer on the sapphire is shown in Fig. 3.1. When the interface between GaN and sapphire is exposed to the laser, optical penetration occurs due to the properties of GaN; the heat generated due to the absorption of laser energy breaks the GaN bonds. The laser intensity decreases exponentially as the intensity of incident light (I0 ) gets further from the surface (I = I0 e−αx ). The optical penetration depth (δ) is defined up to the point where the intensity becomes 1/e (approximately 36.8% of the initial intensity) of surface intensity. The optical penetration depth is determined by the absorption coefficient (α) of a substance. The absorption coefficient is proportional to the extinction coefficient, which is the imaginary part of the refractive index. Substances with large absorption coefficients experience a rapid increase in surface temperature as laser intensity drops significantly with distance traveled. – δ: optical penetration depth, (δ = α1 ) ): 2.05 × 105 cm−1 @248 nm, 8.2 × – α: absorption coefficient, (α = 4πκ λ 4 −1 10 cm @355 nm – λ: Wavelength – κ: Extinction coefficient (The imaginary part of the refractive index). While the absorption coefficient of GaN varies by wavelength, the absorption coefficient at 248 nm is reported to be approximately 2.05 × 105 cm−1 (10% uncertainty) [3]. In that case, a laser penetrates up to a depth of 50 nm, and 13.5% of surface intensity remains at the point where penetration depth doubles. The absorbed laser energy causes an increase in carrier concentration within GaN and raises the electron temperature and lattice temperature. When the increased temperature exceeds

Fig. 3.1 Schematic diagram of laser lift-off of GaN-based LED on sapphire and its band diagram

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the GaN binding energy, GaN is decomposed into solid Ga and N2 gas. Assuming the surface is uniformly irradiated by the laser, temperature distribution can be derived from the heat transfer equation and is generally expressed as follows. T(z, t) = Ia (1 − R)

z D ier f c K D

(3.1)

The surface temperature [4] is given by Eq. 3.2. D 1 T(0, t) = Ia (1 − R) √ (ier f c(0) = √ ) K π π

(3.2)

Ia : Absorbed irradiance (W/cm2 ) K : Thermal conductivity (K = 1.3 W/cm K) √ D: Diffusion length (D = 2.1 × 10–4 cm, = 2 kτ ) K ) k: Thermal diffusivity (k = 0.437 cm2 /s, = ρC p c p : Heat capacity (C p = 9.75 cal/mol K (= 0.487 J/g K)) ρ: Density (ρ = 6.11 g/cm3 ) τ : Laser pulse duration (ns) R: Reflectivity 0.3. √ Thermal penetration depth is expressed as kτ and is defined up to the point where the temperature becomes 35% (atz = D/2in Eq. 3.2) of the surface temperature. A shorter laser pulse duration is more effective in the lift-off process as it decreases the thermal penetration depth. The thermal diffusion length is the point at two times the thermal penetration depth; the temperature at this point corresponds to about 9% of the surface temperature. According to Chu et al. [5], for GaN material, the decomposition temperature (T) is about 900 °C–1000 °C, and τ = 25 ns. Using these values, the absorbed laser fluence required for decomposition of GaN is about 0.3 J/cm2 , which agrees with the experiment value. In this case, the thermal penetration depth is calculated and found to be approximately 1 μm. Ueda et al. experimentally verified that decomposition begins at 850 °C; this temperature seems reasonable compared to the theoretically calculated temperatures of 950 °C or 883 °C [6]. From a bond enthalpy perspective, the standard molar enthalpy of formation at 298 K in the reaction of Ga + 1/2 N2 (g)→GaN is known to be 110.5 kJ/mol [7]. This value is converted into 8.05 kJ/cm3 ; the energy density required to break bonds for a thermal penetration depth of 1 μm is calculated and found to be approximately 800 mJ/cm2 . If the surface temperature is 1000 °C, the temperature at a depth of 1 μm is around 350 °C. The distance up to 69% of surface temperature is 0.42 μm based on √ 0.4 kτ . The required energy is 0.337 J/cm2 , and the surface temperature at this point is 950 °C. Most experimental data are reported in the range of 850–1000 °C. Ueda et al. exposed GaN to a 355 nm laser with a pulse width of 60–10 ns at 260 mJ/cm2 ; they found that the thickness of the decomposed GaN layer is approximately 200 nm or less [6].

– – – – – – – –

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Table.3.1 Summary of LLO parameters Laser wavelength (nm) -pulse duration (ns)

Materials

Beam size

Fluence (mJ/cm2 )

Reference

1100

[8]

Excimer laser 193–25

PSS/GaN

193

GaN

193–15

AlN

248–38

GaN

248–25

GaN

248–35 248–25

[9] 1×2

mm2

1000

[10]

400–600

[2, 11]

300

[5]

GaN

800

[12]

GaN

650

[13]

248

GaN

900

[14]

248–25

GaN

544

[15]

800

[16]

750

[17]

254

[18]

1.2 × 1.2 mm2

248–20 0.4 × 0.4 mm2

248-

GaN

248-

GaN

248–25

GaN/CNT

10 mm2

1300

[19]

308-

GaN

3 μm × 50 mm

800

[20]

300

[14]

266–20

GaN

Ø10 ~ 23 μm

~1200

[21]

355–10

GaN

150–320

[22]

355–5

GaN

200

[12]

355–6 ~10

GaN

260

[6]

DPSS Laser 266–5

Table 3.1 summarizes the incident laser energy density and materials without considering beam intensity attenuation at the sapphire or reflection between the interface of sapphire and GaN. While many experiments have different incident energy densities, as shown in Table 3.1, a more critical factor is pulse intensity. A 355 nm DPSS laser with short pulse width and low energy density has a pulse intensity similar to an Excimer laser with long pulse width and high energy density. This implies that if the pulse does not have a sufficient intensity to decompose GaN into metallic gallium, lift-off will not occur even when the accumulated total incident energy becomes larger under multiple pulses. [2, 21] Tavernier et al. stated that the 1 pertinent parameter in selecting a laser for the lift-off process is E p /(d 2p τ 2 ), namely the energy density divided by the square root of the laser pulse length, not the energy density per se [23]. As shown in Fig. 3.2, N2 gas is produced when GaN is decomposed by laser energy; the resulting mechanical pressure acts at the interface. In general, the equilibrium vapor pressure as a function of temperature ps (T) is the saturation pressure at surface temperature T and is related to the following Clausius-Clapeyron relation.

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Fig. 3.2 Schematic diagram of physical state at GaN interfacial layer

 ps (T ) = p0

Hlv RTb

  Tb 1− T

(3.3)

Here, p0 is the ambient pressure, H lv is the heat of vaporization, and T b is the evaporative equilibrium temperature under ambient pressure. However, GaN has an actual melting temperature greater than 2500 °C [7]; its boiling temperature is unknown. That is, Ga and N2 undergo thermal decomposition due to the incoming energy of the laser before the temperature at the surface of GaN reaches the evaporative temperature. Assuming that N2 gas exists at the decomposition temperature (~1000 °C), the pressure p is calculated and found to be 0.442 MPa using the following equation [24]. p=

ρ RT mm

(3.4)

Here, ρ is the molar gas constant, and m m is the molar mass. Since the yield stress of GaN is 100–200 MPa [25], the pressure of N2 gas alone does not result in the separation of the LED. The mechanics of the LLO process was analyzed by Tavernier et al. [23] and Heise et al. [24] The cracks produced during the LLO process were attributed to the pressure resulting from the N2 gas generation and stress waves produced under the rapid cooling process right after laser irradiation. In the actual process, bending deformation can be suppressed using a supporting substrate; laser process conditions can be adjusted to minimize cracks. The GaN deposition process by MOCVD requires a high temperature of 1000 °C. When cooling the epilayer on sapphire down to room temperature, we observe compressive stress in the GaN layer and tensile stress in sapphire due to the thermal expansion coefficient mismatch between sapphire (7.50 × 10–6 K−1 for Al2 O3 ) and GaN (5.45 × 10–6 K−1 for Wurtzite-GaN) [22, 26]. The compressive stress is relaxed by the GaN decomposition during the LLO process, resulting in lattice disorder, the red shift of peak energy due to dislocation, or the formation of cracks [5, 22]. Ueda et al. stated that thinner GaN films are subject to higher compressive stress; epilayers should be over 4 μm in thickness to obtain the lift-off results without cracks [6]. Doan el al. observed a blue shift and reduced intensity after LLO, and attributed these behaviors to the diffusion of indium through LLO-induced defects [15]. They found that InGaN/GaN MQWs can be damaged if the thickness of the GaN layer

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is below 5 μm. In addition, the cathodoluminescence (CL) images on the boundary between the KrF-irradiated and non-irradiated regions suggest that the propagation of the KrF laser beam and an accompanied recombination enhanced defect reaction, rather than the propagation of a thermal shock wave, are the primary origin of the damage effects of the LLO process on the InGaN/GaN MQWs and the n-GaN layer [15]. Other than the structural damage of tens of nanometers on the GaN surface caused by 248 nm KrF excimer laser absorption, Chen et al. revealed that lattice deformation is induced at the point of generation of laser-induced shock waves and due to the multiple stage dissipation [27]. Regarding the laser wavelength dependency on the LED quality, Wu et al. showed that the screw dislocation density of MQWs after the LLO process is higher for a 355 nm laser than for a 248 nm laser due to the difference in absorption coefficients, which increases the leakage current [12]. On the other hand, the longer pulse width (35 ns) of the 248 nm laser brought about plastic and shock waves, which caused dense dislocation and deformed the superficial structure around the GaN/sapphire interface [28]. Moser et al. analyzed the GaN LED mesa-trenches machined with the laser pulses at a wavelength of 355 nm and 248 nm. Note that they studied the trench machining process, not the LLO. Because the 355 nm laser has a deeper penetration depth than the 248 nm, the sidewall leakage current of the LEDs machined by the 355 nm laser is higher than those machined by the 248 nm laser [29].

3.3 Lift-Off Process by Excimer Laser (193 nm, 248 nm, 308 nm) An excimer laser typically combines a noble gas and a reactive gas by electrical discharge; it uses the light produced when the excited state decays to the ground state. The raw output is a quasi-rectangular beam, typically around 10 × 20 mm, with a near-Gaussian profile along the short axis and a “top hat” (super-Gaussian) profile along the long axis. Beam homogenization is required as the output-beam intensity is spatially non-homogeneous, and the non-homogeneous beam cannot be directly utilized for LLO processes. The homogenized beam is adjusted to an appropriate size using a mask and an imaging lens; it is then applied to the lift-off process. Figure 3.3 shows a schematic of the excimer laser lift-off system. A mask and an imaging lens are used for lasers with large beam sizes, like excimer lasers. As shown in Fig. 3.4, the image size (Di ) at a sample position and the distance (S i ) from the lens are determined from a lens focal length and the distance from the imaging lens to a mask. 1 f 1 si − f 1 = =− + , MT = − f so si f so − f MT : Magnification (—means that the image is inverted). Each optical component is characterized as follows.

(3.5)

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Fig. 3.3 Schematic apparatus of excimer laser lift-off system and beam profile

Fig. 3.4 Position and size of mask and image at sample

Attenuator: Error compensation for beam path using a pair of beam attenuators Homogenizer (Freeform-type homogenizer [30]): Spatial homogenization of beam Variable Aperture (or mask): Metal plate with the desired pattern The beam required for the process is made into a square beam using the mask or a line beam using a cylindrical lens. The step and repeat technique can be applied in the square beam, or the beam moving linearly across the sample can be used in the line beam as shown in Fig. 3.5. A hard material like glass or Si is commonly adopted as a supporting substrate for distortion prevention and easy handling, or a flexible polymer is attached for the adhesion control. The LLO process is then carried out by laser irradiation on the sapphire side. Lee et al. performed experiments with various films and supporting substrates (308 nm, XeCl laser) [20]. They found that Thermal Release Tape (TRT) with SU-8 passivation gives the best lift-off quality, 1.2 mm × 1.2 mm square laser beam (Fig. 3.6 a) produces undesirable cracks, while GaN LEDs exposed by 3 μm × 5 cm line shape laser beam makes minor cracks on u-GaN surface. Chu et al. irradiated samples with a laser with a beam size of 1.2 mm × 1.2 mm (KrF excimer laser); they noted a metallic silver color at the interface of GaN and

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Fig. 3.5 Step and repeat technique a and line beam processing method b

Fig. 3.6 Laser lifted-off surface of GaN LED arrays with different XeCl laser beam dimensions a 1.2 mm × 1.2 mm2 laser beam b 3 μm × 5 cm line shape laser beam. Reproduced with permission20 . Copyright 2012, SPIE

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sapphire with the ejection of some dust-like particles from the edge of the samples. The surface roughness before LLO was 0.3 nm; it increased to 12 nm after LLO. A red shift was observed due to localized defects at the interface of sapphire and GaN. When lift-off experiments were conducted at a low pressure of 10–3 Torr and atmospheric conditions, the latter case had a lower etching rate but better surface morphology of GaN [5]. Chun et al. deposited Cr/Au on GaN and performed irradiation using a KrF excimer laser beam (750 mJ/cm2 , beam spot: 400 μm × 400 μm) after contacting PET film coated with conductive epoxy. The epoxy layers were protected by employing Cr/Au as a laser blocking layer; LEDs separated at high yield were transferred onto the PET substrate. The sample was then dipped in diluted HCl, Cr/Au etchants and BOE to remove any residual Ga, Cr, Au, and SiO2 on the surface of the separated GaN LED chips [17].

3.4 Lift-Off Process by DPSS Laser (266 nm, 355 nm) The 266 nm or 355 nm DPSS laser uses a Gaussian beam and performs lift-off by focusing with the f-theta lens and the XY scanning mirror. As the beam size of the DPSS laser is a few mm, which is much smaller than the excimer laser, it makes direct focusing more suitable than imaging. Defocusing may increase the beam size but is restricted by the required output power for the LLO process. Figure 3.7 provides a schematic of the lift-off system with the DPSS UV laser. Since the 355 nm DPSS produces high output power, a homogenizer may be placed in front of the scanning mirror to achieve beam homogenization [6]. The DPSS laser beam has a Gaussian distribution and thus relies on direct focusing. The optical characteristics of the focused laser beam are described in Fig. 3.8. z R (Rayleigh range) is the point at which the radius of curvature of the wavefront is the smallest; 2 z R (Depth of focus (DOF)) expresses the distance from the focal

Fig. 3.7 Schematic of DPSS laser lift-off system and the beam profile of DPSS laser

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Fig. 3.8 Spreading of DPSS laser beam when the beam is passing through a LED sample

√ point to the point at which beam area is doubled ( 2 times the beam radius). For the defocusing method, the beam size after moving a distance z from the focal point is obtained by Eq. 3.6. 



w(z) = w0 1 + zR =

z zR

2

π w02 λ

(3.6)

(3.7)

Here, w0 is the minimum beam waist. Figure 3.9 presents the Gaussian irradiance distribution at the focal position and Rayleigh range position for the focal beam with a radius of 5 and 10 μm, but the same power. The required output power should be maintained considering beam size and pulse width since the peak irradiance plays a more critical role than the accumulated incident energy in the LED LLO process. As shown in Eq. 3.7, a small beam size is infeasible with a long DOF. The total output (Pt ) measured by the power meter is the integral of power density in the radial direction; the average power density (Iave ) of laser irradiated on the sample at a radius of w is as follows.

Pt I0 = Iave w/cm2 = 2 πw 2

(3.8)

I0 is the peak power density at the center of the beam. Since Iave is the average power density of pulse laser, peak irradiance should be evaluated by considering the pulse width. In Table 3.1, when the wavelength is 266 nm and the pulse width is set at 5 and 20 ns, a small fluence of 300 mJ/cm2 is needed for a small pulse width, while a larger value of 1200 mJ/cm2 is needed for large pulse width. However, it is noted that the peak irradiance is maintained at 60 MW/cm2 for both small and large fluence. When the pulse repetition rate is kept constant using the same objective lens, the output required for lift-off changes with the beam size variation caused

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Fig. 3.9 Gaussian irradiance distribution at focal position and Rayleigh range position with different focal beam sizes at the same power

by defocusing. The LED LLO process can be implemented by adjusting the output based on the peak irradiance at which lift-off occurs. As a demonstration, the LED lift-off process was performed using the focused laser beam. Overlapping between pulses is needed because of the smaller beam size than the LED size; the overlapping rate is determined by the pulse repetition rate and the scan speed. Laser beams can be irradiated in a straight line or a zig-zag manner as shown in Fig. 3.10. Since the edge of the LED is more vulnerable to

Fig. 3.10 Focused DPSS laser beam on the LED sample (a), line by line processing by individual driving of scanner and stage (b), and processing by simultaneous driving (c)

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Fig. 3.11 Schematic drawing of selective LLO processing (a), selectively lifted-off LEDs on sapphire (b), and LEDs transferred onto flexible polymer film (c) for the LED size of 100 μm × 100 μm

cracks, the irradiation method should be optimized to minimize crack formation. Individual lift-off can be selectively performed using even smaller focused beam as shown in Fig. 3.11. In this case, there are different accumulated energies at the edge and the LED inner region due to the speed difference caused by accelerating and decelerating movement for the scanning direction change. The advantage of the selective lift-off is that lift-off can be performed for targeted LEDs without a mask, thereby significantly enhancing the LED array design complexity. Kim et al. used a 266 nm laser with an objective lens of NA 0.32 and examined various properties while varying the focusing position [21]. Lift-off at each defocusing position was carried out for low- and high-output regions. Lift-off quality was superior at low-output density and in regions with large defocusing; the irradiance of each pulse was found to be more important than the accumulated fluence. The beam size increased with increasing defocusing position; total accumulated fluence with overlapping grew larger under higher-output irradiation, but the peak irradiation at defocusing position was highly similar to that of the focusing position. The average irradiance was 38 kW/cm2 , and the outstanding lift-off quality was observed for a peak irradiance in the range of 60 to 63 MW/cm2 . Park et al. irradiated laser beams on sapphire and GaN using an XY scanner with a laser wavelength of 266 nm, a repetition rate of 30 kHz, a pulse duration of 20 ns, and a beam diameter of 10 μm, and measured the shearing force required for LED separation as shown in Fig. 3.12 [31]. Lift-off occurred for regions irradiated with a specific output. Figure 3.13 shows that the samples irradiated with low-output beams needed smaller shearing force for separation than those irradiated with high-output beams. This was attributed to the greater friction caused by the higher surface roughness, which the LLO process at the high output produced as shown in Table 3.2. In batch lift-off or selective lift-off, the separation of supporting substrates or films after laser irradiation produces defects in each LED device, affecting the LLO yield. The main failure types are breaks, cracks, chipping, and missing as shown in Fig. 3.14. The causes of failure behind the LED lift-off are related to process variables and hardware performance. The process variables include laser beam power, spacing/overlap length, scanning method, a supporting substrate, adhesive strength

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Fig. 3.12 Schematic diagram of the system used to measure adhesion strength between GaN LED and sapphire substrate. Reproduced with permission [31]. Copyright 2016, Elsevier

and type (UV or thermal release) of film, and contamination of the sapphire substrate. The hardware performance is assessed in terms of pulse power stability, pointing stability, objective lens specification, and stage and scanner position accuracy. For smaller focused beams, the process time is longer, and there is a higher risk of failure. However, the limited laser output restricts the possible increase in beam size. In addition, increasing the pulse repetition rate is desirable for speeding up the process, but this is also restricted by the laser output, which has to meet appropriate irradiance levels.

3.5 Supplemental Process 3.5.1 Temporary Substrate For flexible applications of GaN LEDs, the LED side of a sapphire substrate should be bonded by a TRT, UV-release tape (UV-RT), or PDMS substrate. Lee et al. achieved the best results under TRT with SU-8 passivation [20], Pan et al. demonstrated a liftoff yield close to 100% with UV-RT and TRT. The positional uncertainty was larger for TRT than UV-RT due to heat-induced pores; PDMS had a transfer yield of around 95%. A tape-assisted laser transfer (TALT) process, which involves controlling the adhesive force of UV-RT was proposed as shown in Fig. 3.15 [32]. Choi et al. bonded InGaN epitaxial layer grown on a sapphire substrate to a PI. The other side was bonded to a dummy sapphire deposited with ITO/SiO2 . The LED chip isolation was carried out after the LLO process between the sapphire and epitaxial layer, and finally, the dummy substrate was also separated with a laser to obtain the final flexible substrate

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Fig. 3.13 Measured adhesion strength between GaN LED and sapphire substrate according to laser power densities and its AFM topography. Reproduced with permission [31]. Copyright 2016, Elsevier

Table 3.2 Surface roughness of sapphire substrate after separation of LED by LLO using laser power densities of 83 and 102 kW/cm2 Power density [kW/cm2 ]

Rq [nm] (root mean square)

Ra [nm] (average roughness)

Rz [nm] (peak to peak)

83

53

42

470

102

73

55

618

[19]. Lee et al. electroplated a Cu foil to dissipate the heat produced during lift-off and selectively eliminated the foil by a wet etching [33]. For the improved production yield, LEDs are bonded to a temporary substrate by metal fusion before the LLO process, and additional process steps are performed [13, 34]. Chun et al. proposed creating a pedestal structure between an LED and a temporary substrate to transfer the LEDs to a flexible substrate coated with glue

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Fig. 3.14 Failure LED types by selective lift-off: break/crack (a), chipping (b), and missing (c) with 100 μm × 100 μm LED size

Fig. 3.15 TALT process: a attaching UV tape to LED array, b conducting LLO, and removing sapphire substrate, c attaching another UV tape with stronger adhesion to backside of LED array, d peeling away original tape, and e transferring full-wafer micro-LED array to the second tape. Reproduced with permission [32]. Copyright 2020, WILEY–VCH

[35]. Kim et al. bonded an LED to a temporary substrate using In/Pd and performed pick-up via PDMS [14, 36]. Ezhilarasu et al. used thermoplastic laser debondable polyimide based adhesive (HD3007) and attach the LEDs to a temporary glass carrier before the LLO process [37].

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3.5.2 Underfill and Sacrificial Layer for LED Quality Improvement For preventing the LEDs from breaking during LLO, an underfill or sacrificial layer can be inserted between the LEDs and a carrier substrate. Lee et al. used a SU-8 substance as an underfill [20], while Aoshima et al. inserted an epoxy resin (U84372) between a carrier substrate and LED layer. Tang et al. deposited GaN/InN below a GaN film and separated the sacrificial layer using an IR laser (λ = 533 nm) for liftoff [38]. Kunoh et al. successfully transferred GaN films grown on high crystalline quality GaN substrates onto different substrates. They used an absorption-enhanced InGaN layer as a sacrificial layer for the 532 nm green laser irradiation [39]. Long et al. grew GaN after bonding CNT on a sapphire substrate and showed that residual stress decreased comparing with the case without CNT. They also reported a decrease in lift-off energy due to the higher laser absorption coefficient of CNT, which played as a powerful heating wire [40]. Chun et al. employed Au/Cr layers as laser blocking layers so that the laser beam passed through GaN only, thus enhancing the lift-off efficiency [17]. In the case of DUV-LED, lift-off is required between the AlGaN layer and sapphire substrate. The strong bond between Al remaining after thermal decomposition and the substrate makes the lift-off process difficult. Ueda et al. succeeded in the lift-off of a thin 1.0 μm AlGaN film using a 0.3 μm GaN layer as a sacrificial layer [22]. Takeuchi et al. improved lift-off efficiency by inserting 200-period 2 ML-AlN/2 ML-Al0.22 Ga0.78 N SPSL as a sacrificial layer [41].

3.5.3 Light Extraction Improvement Various methods have been applied to improve the luminous efficiency of LED in lift-off. A light extraction efficiency of 1.7 times higher than that before LLO was achieved through KOH etching at 100 °C, forming a triangular texture [10]. Chen et al. improved efficiency by roughening the n-GaN surface with KOH and depositing Ag as a highly reflective mirror [13], while another study introduced Al mirror coating [42]. Cho et al. formed periodic photonic crystals (PhC) on n-GaN after lift-off and reported a 76% improvement in light output power [9]. Jung et al. formed GaN on a patterned sapphire substrate (PSS) containing hemispheres, obtained nano-sized patterns by employing a photo-electrochemical (PEC) etching method after LLO, and deposited Al/Ti/Au as a reflective layer, thereby increasing PL intensity by 65% as shown in Fig. 3.16 [8].

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Fig. 3.16 Fabrication procedure of freestanding GaN-based LEDs with and without PEC-textured reflector. Reproduced with permission [8]. Copyright 2012, AIP Publishing

3.5.4 Photochemical Process Photochemical lift-off processes without laser have been proposed to minimize the crack-induced decrease in yield during GaN lift-off and to achieve lift-off at a lower cost. Youtsey et al. illuminated broadband UV of an unfiltered Hg arc lamp to a GaN layer for vertical etching. They proposed photoelectrochemical (PEC) methods that apply filtered UV illumination (energies below the bandgap of GaN) to drive lateral etching of InGaN, which was used as a sacrificial layer as shown in Fig. 3.17 [43].

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Fig. 3.17 Epitaxial lift-off process flow: a perforated metal support layer is first applied to the top surface (Step 2), followed by a vertical wet etch of the GaN epitaxial layer through the perforations down to the release layer (Step 3). Bandgap-selective wet etching of the InGaN release layer around the perforations is used to lift off the GaN film (Step 4). The released GaN ELO foil is then bonded to a low-cost carrier (Step 5), followed by wet chemical removal of the metal encapsulation layer (Step 6). Reproduced with permission [43]. Copyright 2017, WILEY–VCH

Park et al. proposed a lift-off method that involves the etching of only the Si-doped n-type GaN using pure 0.3 M oxalic acid at 10 °C, without changes in p-type and undoped GaN [44].

3.6 Summary The two popular lasers for the LLO process of the GaN layer on sapphire substrates are excimer and DPSS UV lasers. They have different optical systems because of their unique beam characteristics. Typically, excimer lasers rely on imaging, while DPSS UV lasers are suitable for the focusing method. When a laser beam is irradiated on the interface of sapphire and GaN, the difference in absorption coefficients of two laser types results in different penetration depths, affecting the quality of LLO. The penetrated beam is converted to heat, and pressure and shock waves produced from

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the GaN decomposition can result in GaN damage. It is essential to optimize the process conditions that minimize the GaN damage; in particular, the peak irradiance should be selected considering the pulse width. Appropriate film types and sacrificial layers should be employed to improve the lift-off yield. Higher light extraction efficiency can be achieved through mirror coating and LED surface texturing. In addition to the LLO, photochemical methods have been studied. Today, many researchers are actively improving the LLO process yield to reduce the process time and cost.

References 1. Nakamura S., Senoh, M., Iwasa, N., & Nagahama, S. N. (1995) High-power InGaN singlequantum-well-structure blue and violet light-emitting diodes. Applied Physics Letters, 67, 1868. 2. Wong, W. S., Sands, T., & Cheung, N. W. (1998). Damage-free separation of GaN thin films from sapphire substrates. Applied Physics Letters, 72, 599–601. 3. Muth, J. F., et al. (1997). Absorption coefficient, energy gap, exciton binding energy, and recombination lifetime of GaN obtained from transmission measurements. Applied Physics Letters, 71, 2572–2574. 4. Von Allmen, M., & Blastter, A. (1994). Laser-Beam Interactions with Materials, Physical Principles and Applications. Springer. 5. Chu, C. F., et al. (2004). Study of GaN light-emitting diodes fabricated by laser lift-off technique. Journal of Applied Physics, 95, 3916–3922. 6. Ueda, T., Ishida, M. & Yuri, M. (2011) Separation of thin GaN from sapphire by laser lift-off technique. Journal of Applied Physics, 50. 7. Lide, D. R. (2004). CRC Handbook of Chemistry and Physics (85th ed.). CRC Press Inc. 8. Jung, Y. et al. (2012) GaN-based light-emitting diodes by laser lift-off with micro- and nanosized reflectors. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 30, 050605. 9. Cho, H. K., et al. (2008). Laser lift-off GaN thin-film photonic crystal GaN-based light-emitting diodes. IEEE Photonics Technology Letters, 20, 2096–2098. 10. Aoshima, H., et al. (2012). Laser lift-off of AlN/sapphire for UV light-emitting diodes. Physica Status Solidi C, 9, 753–756. 11. Wong, W. S., et al. (1999). Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off. Applied Physics Letters, 75, 1360–1362. 12. Wu, Y. S., Cheng, J. H., Peng, W. C. & Ouyang, H. (2007) Effects of laser sources on the reverse-bias leakages of laser lift-off GaN-based light-emitting diodes. 251110. 13. Chen, M., et al. (2012). Fabrication of vertical-structured GaN-based light-emitting diodes using auto-split laser lift-off technique. ECS Solid State Letters, 1, Q26–Q28. 14. Kim, T. I. et al. (2012) High-efficiency, microscale GaN light-emitting diodes and their thermal properties on unusual substrates. Small, 8, 1643–1649. 15. Doan, M. H. et al. (2012) Influence of laser lift-off on optical and structural properties of InGaN/GaN vertical blue light emitting diodes. AIP Advances, 2, 0–8. 16. Goßler, C. et al. (2014) GaN-based micro-LED arrays on flexible substrates for optical cochlear implants. Journal of Physics D: Applied Physics, 47, 205401. 17. Chun, J., et al. (2014). Laser lift-off transfer printing of patterned GaN light-emitting diodes from sapphire to flexible substrates using a Cr/Au laser blocking layer. Scripta Materialia, 77, 13–16. 18. Lee, J. W., Ye, B. U., Wang, Z. L., Lee, J. L., & Baik, J. M. (2018). Highly-sensitive and highlycorrelative flexible motion sensors based on asymmetric piezotronic effect. Nano Energy, 51, 185–191.

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19. Choi, W., Park, H. J., Park, S., & Jeong, T. (2014). Flexible InGaN LEDs on a polyimide substrate fabricated using a simple direct-transfer method. IEEE Photonics Technology Letters, 26, 2115–2117. 20. Lee, S. H., Park, S. Y. & Lee, K. J. (2012) Laser lift-off of GaN thin film and its application to the flexible light emitting diodes. Biosensing Nanomedicine V, 8460, 846011. 21. Kim, J., Kim, J. H., Cho, S. H. & Whang, K. H. (2016) Selective lift-off of GaN light-emitting diode from a sapphire substrate using 266-nm diode-pumped solid-state laser irradiation. Applied Physics A, 122. 22. Ueda, T., Ishida, M., & Yuri, M. (2003). Laser lift-off of very thin AlGaN film from sapphire using selective decomposition of GaN interlayer. Applied Surface Science, 216, 512–518. 23. Tavernier, P. R., & Clarke, D. R. (2001). Mechanics of laser-assisted debonding of films. Journal of Applied Physics, 89, 1527–1536. 24. Heise, G. et al. (2012) Laser lift-off initiated by direct induced ablation of different metal thin films with ultra-short laser pulses. Journal of Physics D: Applied Physics, 45, 315303. 25. Yonenaga, I. (2005). Hardness, yield strength, and dislocation velocity in elemental and compound semiconductors. Materials Transactions, 46, 1979–1985. 26. Miskys, C. R., Kelly, M. K., Ambacher, O. & Stutzmann, M. (2003) Freestanding GaNsubstrates and devices. Physica Status Solidi C, 6, 1627–1650. 27. Chen, W. H. et al. (2007) Study of the structural damage in the (0001) GaN epilayer processed by laser lift-off techniques. Applied Physics Letters, 91. 28. Cheng, J.-H., Wu, Y. S., Peng, W. C., & Ouyang, H. (2009). Effects of laser sources on damage mechanisms and reverse-bias leakages of laser lift-off GaN-based LEDs. Journal of the Electrochemical Society, 156, H640. 29. Moser, R. et al. (2013) Laser direct writing of GaN-based light-emitting diodes-The suitable laser source for mesa definition. Journal of Applied Physics, 113. 30. Jin, Y., Hassan, A., & Jiang, Y. (2016). Freeform microlens array homogenizer for excimer laser beam shaping. Optics Express, 24, 24846. 31. Park, J., Sin, Y. G., Kim, J. H. & Kim, J. (2016) Dependence of adhesion strength between GaN LEDs and sapphire substrate on power density of UV laser irradiation. Applied Surface Science, 384. 32. Pan, Z., et al. (2020). Wafer-scale micro-LEDs transferred onto an adhesive film for planar and flexible displays. Advanced Materials Technologies, 2000549, 1–11. 33. Lee, H. E., et al. (2018). Monolithic flexible vertical GaN light-emitting diodes for a transparent wireless brain optical stimulator. Advanced Materials, 30, 1–10. 34. Um, J. G., et al. (2019). Active-matrix GaN μ-LED display using oxide thin-film transistor backplane and flip chip LED bonding. Advanced Electronic Materials, 5, 1–8. 35. Chun, J., et al. (2012). Transfer of GaN LEDs from sapphire to flexible substrates by laser lift-off and contact printing. IEEE Photonics Technology Letters, 24, 2115–2118. 36. Kim, R. H., et al. (2012). Materials and designs for wirelessly powered implantable lightemitting systems. Small (Weinheim an der Bergstrasse, Germany), 8, 2812–2818. 37. Ezhilarasu, G., Paranjpe, A., Lee, J., Wei, F. & Iyer, S. S. A. (2020) Heterogeneously integrated, high resolution and flexible inorganic μLED display using fan-out wafer-level packaging. Proceedings of the Electronic Components and Technology Conference (pp. 677–684), June 2020. 38. Tang, L., Wang, Y., Cheng, G., Manfra, M. J., & Sands, T. D. (2012). Free standing GaN nano membrane by laser lift-off method. Materials Research Society Symposium Proceedings, 1432, 53–58. 39. Kunoh, Y., et al. (2010). Fabrication of light emitting diodes transferred onto different substrates by GaN substrate separation technique. Physica Status Solidi C, 7, 2091–2093. 40. Long, H., et al. (2017). Carbon nanotube assisted Lift off of GaN layers on sapphire. Applied Surface Science, 394, 598–603. 41. Takeuchi, M., et al. (2009). AlN/AlGaN short-period superlattice sacrificial layers in laser lift-off for vertical-type AlGaN-based deep ultraviolet light emitting diodes. Applied Physics Letters, 94, 1–4.

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42. Zhang, J.-Y., et al. (2012). Performance enhancement of GaN-based light emitting diodes by transfer from sapphire to silicon substrate using double-transfer technique. Nanoscale Research Letters, 7, 244. 43. Youtsey, C. et al. (2017) Wafer-scale epitaxial lift-off of GaN using bandgap-selective photoenhanced wet etching. Physica Status Solidi B, 254. 44. Park, J., Song, K. M., Jeon, S. R., Baek, J. H., & Ryu, S. W. (2009). Doping selective lateral electrochemical etching of GaN for chemical lift-off. Applied Physics Letters, 94, 1–4.

Chapter 4

Transfer Technology of Micro-LEDs for Display Applications Jae-Hyun Kim, Bongkyun Jang, Kwang-Seop Kim, and Hak-Joo Lee

4.1 Introduction The liquid crystal display (LCD) and the organic light-emitting diode (OLED) display contribute to 73.1% and 20.7% of the 2018 global display market, respectively [1]. The share of the OLED display rapidly grows to replace that of the LCD. Display companies start to find future display technologies exceeding the OLED performance. The micro-LED display gets spot-light in this regard [2–6]. It has the highest brightness, durability, power efficiency, the fastest response time, and the most suitable for stretchable and transparent displays over the LCD and OLED displays [3, 4, 7, 8]. Despite the micro-LED display’s outstanding performance, the micro-LED display is not commercially available to consumers yet. Several obstacles prevent micro-LED displays from their widespread utilization in our daily life. The biggest obstacle to the commercialization of micro-LED displays is costcompetitiveness [5, 6]. First, we need to consider the chip cost. A 4 K UHD display requires 25 million micro-LED chips. Assuming that the chip cost is $0.001 per chip, the total LED cost of the 4 K UHD display is $25,000. The chip cost is rapidly decreasing with a decrease in the chip size. Recycling the mother wafers for repetitive growth can reduce the chip cost because epitaxial layer growth for LED requires expensive mother wafers. Another cost factor is the manufacturing cost [5]. A die bonding machine typically transfers 5 ~ 10 chips per second. This speed is so slow that it takes approximately 700 h to transfer 25 million chips for a 4 K UHD display panel. J.-H. Kim (B) · B. Jang · K.-S. Kim · H.-J. Lee Department of Nano-Mechanics, Institute of Machinery & Materials (KIMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea e-mail: [email protected] H.-J. Lee Center for Advanced Meta-Materials (CAMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea © Springer Nature Singapore Pte Ltd. 2021 J.-H. Ahn and J.-H. Kim (eds.), Micro Light Emitting Diode: Fabrication and Devices, Series in Display Science and Technology, https://doi.org/10.1007/978-981-16-5505-0_4

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We need to increase the transfer speed more than 100 times that of the conventional die bonding machine to reduce the manufacturing cost. Transfer technology revolutionizes the die bonding process [7, 9–11]. The transfer speed is as high as 1000 chips per second or more, depending on the transfer type [5– 7]. The transfer technology is indispensable for the smaller LED chips. The conventional die bonder cannot handle them due to their mechanical fragility when their thickness decreases less than 10 µm. In this regard, transfer technology is a critical market driver for the micro-LED industry. Unfortunately, developing transfer technology is not completed yet in 2021, and lots of transfer technologies are vigorously competing with one another [10]. We examine some promising transfer technologies and describe their basic principles. Because each technology has its advantages and disadvantages over other technologies, one transfer technology could not cover all the micro-LED applications. By understanding each transfer technology’s characteristics, we can choose a suitable transfer technology for our target micro-LED application. We prepare this chapter based on our previous Korean articles [12, 13] and revise those articles’ figures and descriptions considering the fast-moving trends of micro-LED display technology. This chapter consists of 6 sections. Section 4.2 describes the manufacturing process of micro-LED displays and emphasizes the transfer step. Section 4.3 explains the fundamental mechanics of the transfer technology, including the essential elements of the transfer and their selection flowchart. Section 4.4 presents some promising technologies applicable to micro-LED transfer, such as electrostatic transfer, laser transfer, rubber stamp transfer, self-assembly transfer, and roll transfer. Section 4.5 provides three practical examples of the roll transfer for micro and mini-LEDs, demonstrating face-up and face-down transfer of micro-LEDs and face-down transfer of mini-LEDs. Finally, Sect. 4.6 summarizes this chapter with some research suggestions for researchers and engineers studying transfer technology.

4.2 Manufacturing Process of Micro-LED Display Panel Figures 4.1 and 4.2 show the manufacturing processes of micro-LED display panels. The manufacturing process can be classified into face-down and face-up processes depending on the LED configuration. In the face-down process, pad metals of a LED chip face down toward electrodes of a backplane circuit board. The positional alignment between the pad metals of the LED and the electrodes of the circuit board should be secured to provide the electrical connection between the LED and the circuit board. On the other hand, in the face-up process, pad metals of a LED chip are facing upward, and the opposite side of the LED chip is bonded on a circuit board using an adhesive as described in Fig. 4.2. These two processes have advantages and disadvantages over each other, which we discuss at the end of this section.

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Fig. 4.1 Face-down manufacturing procedures for mini and micro-LED displays. Face-down manufacturing facilitates direct electrical interconnection between LED pads and circuit electrodes. LEDs should be transferred on a circuit board with the LEDs’ pads facing the circuit electrodes. The LED chip and circuit board fabrications can proceed independently

Fig. 4.2 Face-up manufacturing procedures for micro-LED displays. Face-up manufacturing facilitates the transfer step and leads to a higher transfer yield. After the transfer step, the circuit board fabrication step follows. Thus, the face-up manufacturing procedures proceed sequentially

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4.2.1 Face-Down Manufacturing Process The face-down manufacturing process consists of epitaxial layer growth, chip fabrication, lift-off, circuit board fabrication, face-down transfer, and inspection-repair steps, as shown in Fig. 4.1. The epitaxial layer growth can be performed using liquidphase epitaxy (LPE), vapor phase epitaxy (VPE), molecular beam epitaxy (MBE), or metal–organic chemical vapor deposition (MOCVD) [14]. Depending on the wavelength of a LED, we need to choose a proper combination of the epitaxial layers, constituting a diode structure of p-type and n-type semiconductor materials. The epitaxial layers are grown on a single crystalline wafer, such as sapphire, GaAs, and Si. Using the wafers with the epitaxial layers, we can fabricate LED chips using the conventional semiconductor process. Circuit boards can be prepared in a separate process from chip fabrication. Depending on the addressing scheme, the circuit boards are designed with the passive matrix or the active matrix and are manufactured using the conventional display backplane process. For the electrical interconnection via solders, we may need to deposit under-bump metallurgy on the corresponding electrodes of the circuit board with the p- and n-pads of LED chips. In the transfer step, we pick up the prepared LED chips and place them on the prepared circuit board with the face-down configuration. The electrical connection between the LED chips and the circuit board is formed by interconnection materials such as thin-film solders, anisotropic conductive films, and solder pastes. In most cases, we need additional steps for depositing the interconnection materials and for forming interconnection like reflowing. Finally, we need to inspect any defective LEDs on the circuit board and repair them. For the face-down transfer step, we need to place LEDs on the circuit board covered with interconnection materials. Because the adhesion of the interconnection materials is typically tiny, it is not trivial to place LEDs on the interconnection materials with a high yield. Besides, we need to apply mechanical pressure to secure the mechanical contact between the pads of LEDs and the interconnection materials on a circuit board with precise positional alignment. For example, when the solder is formed on electrodes of a circuit board as an interconnection material, we need to coat flux on the solder and place LEDs on the flux-coated solder with mechanical pressure. This pressure makes contact between the pads of LEDs and the solder by squeezing out the flux. The magnitude of the mechanical pressure should be carefully tuned to obtain the high transfer yield and the high reliability of the joint between the solder and the pads.

4.2.2 Face-Up Manufacturing Process The face-up manufacturing process consists of epitaxial layer growth, chip fabrication, lift-off, face-up transfer, circuit board fabrication, and inspection-repair steps, as shown in Fig. 4.2. Most steps in the face-up manufacturing process are similar to those

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in the face-down process, but the main difference is the process order. The transfer step is completed before the circuit board fabrication in the face-up scheme. LEDs are placed on a display substrate during the face-up transfer with a spacing determined from the pixel size. They are typically bonded with an adhesive of strong adhesion with both LEDs and the display substrate. The use of adhesive significantly improves the transfer yield, and the transfer step becomes trivial. After the transfer step, the interconnection between LEDs and thin-film-transistors (TFTs) on the circuit board is formed by the deposition of conductive electrodes and planarization layers. This circuit board fabrication is nothing but conventional photolithography and thin-film deposition. When we need to repair the LEDs after the circuit board fabrication, it is tricky to remove bad LEDs and replace them with good LEDs. Since LEDs are accessible by removing the circuit electrodes and planarization layer, the repair step requires expensive laser processing or complicated etching. It is critical to minimize the number of repaired LEDs by improving the overall process yield. In terms of the transfer, the face-up transfer is typically more straightforward than the face-down one. The yield of the face-up transfer step can be maintained to be very high by introducing an adhesive between LEDs and a display substrate, but the repairing step for the face-up process is problematic, requiring additional considerations for the circuit electrodes and other devices like TFTs. The redundant circuits [8] are helpful for the face-up process to resolve the difficulty in the repairing step.

4.2.3 Comparison Between Face-Down and Face-Up Processes Table 4.1 shows the comparison between the face-down and the face-up processes. The face-down transfer is more complicated than the face-up one because we need to introduce an interconnection material between LEDs and a circuit board for facedown transfer. The adhesion of the interconnection material is typically tiny before reflowing or curing, and this tiny adhesion makes the face-down transfer difficult. The Table 4.1 Comparison between face-down and face-up manufacturing procedures

Comparison items

Face-down process

Face-up process

Transfer difficulty

Difficult

Easy

Interconnection moment

During transfer

After transfer

Interconnection materials

Solder, ACF, ACP

Adhesive (no electrical connection)

Repair difficulty

Possible (but, not easy)

Very difficult

Main failure step

Transfer step

Metallization step

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face-up transfer can be readily realized via an adhesive, which has strong adhesion with both LEDs and a circuit board. In the face-down process, we need to align the pads of LEDs with the electrodes of a circuit board. For making the electrical connection between the pads and the electrodes, we need an interconnection material between them. The interconnection materials are solder, an anisotropic conductive film (ACF), and an anisotropic conductive paste (ACP). In the face-up process, no electrical connection forms during the transfer step. LEDs are bonded on a display substrate using an adhesive. LEDs are electrically connected with TFTs in the circuit fabrication step of metallization and photolithography after the transfer step. In terms of repairability, the face-down LEDs are more accessible than the face-up ones. It is possible to remove the smallsized bad LEDs and replace them with good LEDs for the face-down cases. For the face-up LEDs, we need to remove several layers deposited on the LEDs. The primary failure step is the transfer step for the face-down process, while it is the metallization step for the face-up process. The face-down transfer requires mechanical pressure to make physical contact between the LED pads and the circuit board electrodes. The yield of the face-down transfer is affected by the transfer conditions such as stamp pressure, stamp speed, alignment accuracy, and pressure uniformity. The yield of the face-up transfer is typically excellent because strong adhesive between LEDs and a circuit board prevents transfer failure. The face-up process suffers from the tricky fabrication of conductive electrodes or TFTs after the LED transfer, which requires planarization layers, multi-layer deposition, and precise overlay alignment. The repairing step is more tricky for the face-up transfer than for the face-down one. Among the process steps for manufacturing micro-LED panels, the transfer step is unique. The transfer is not required for manufacturing other display types such as liquid crystal display (LCD) and organic light-emitting diode (OLED) display. We proceed to understand the mechanics of the transfer step.

4.3 Mechanics of Transfer Process The transfer step is realized by the competition of the adhesion forces among LEDs, a stamp, a carrier film, and a target substrate [15–17]. Understanding the mechanics of the adhesion forces is critical to realizing the transfer step, which consists of two subsequent actions: One is picking, and the other is placing. In the picking action, micro-LEDs on a carrier wafer are picked up using a stamp, as shown in Fig. 4.3a. The placing action moves the micro-LEDs on the stamp to a substrate, as shown in Fig. 4.3b. The adhesion competition between the adjacent layers is critical in these two actions, respectively. For picking LEDs, the adhesion (denoted by W2) between a stamp and LEDs should be larger than that (W1) between a carrier film and the LEDs. For placing LEDs, the adhesion W2 should be smaller than that (W3) between the substrate and the LEDs. When W3 is larger than W2 and W2 larger than W1, the transfer process is straightforward, and this is readily realized for the face-up

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Fig. 4.3 Schematic illustration of transfer mechanics. The transfer step consists of picking and placing actions. a Picking action, where the stamp adhesion (W2) with LEDs is higher than the carrier film adhesion (W1) with LEDs. b Placing action, where the stamp adhesion (W2) with LEDs is lower than the target substrate adhesion (W3) with LEDs

transfer. Unfortunately, in most face-down transfers, W3 tends to be smaller than W1. The key issue of the transfer step is closely related to this small adhesion of W3.

4.3.1 Two Approaches to Solving the Adhesion Issue We have two approaches to solve this adhesion issue in the face-down transfer. One is to change the stamp adhesion W2 as demanded. We need to increase W2 for the picking action and to decrease W2 for the placing action. There are some available technologies for changing the stamp adhesion W2, and they are discussed in Sect. 4.4.4. The other approach is to optimize the material combination producing the adhesion order of W1 < W2 < W3. The optimization of the materials starts with the selection of an interconnection material like solder, ACF, or ACP. This interconnection material electrically interconnects LEDs with a circuit board in the face-down configuration. The adhesion W3 between LEDs and an interconnection material should be considered in advance at the moment of the picking action. When we can use an interconnection material with a large W3, we can extend our selection range for the carrier film and the stamp materials with the constraint of W1 < W2 < W3. The interconnection materials like solder and ACF suffer from low adhesion with LEDs under room temperature before reflowing and curing. The reflowing and curing process requires an elevated temperature, leading to positional uncertainty caused by thermal expansion. When we choose a larger W3 by placing LEDs on a substrate at an

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elevated temperature, we need to consider the thermal expansion. Alternatively, when we choose to place LEDs under room temperature, we need to find an interconnection material with a large W3. The flowchart in Fig. 4.4 summarizes the material selection procedure for an interconnection material, a carrier film, and a stamp. Understanding their mechanical behavior is an elementary step for realizing the transfer process, and adhesion is the most critical parameter. The material selection procedure starts with a selection of interconnection material. The interconnection material has a close relationship with the nature of the target application. To decide the interconnection material, we need to consider the target application’s temperature limitation, transparency, and material cost. After choosing the interconnection material, we need to evaluate W3 using an adhesion test reported in [17, 18]. The next step is to select a stamp material. There are many choices for the stamp material depending on the transfer technology. Here we focus on the stamp-based transfer: rubber stamp transfer and roll transfer. After choosing the stamp material, we evaluate the W2, then compare it with W3. When W2 is stronger than W3, we need to go back to the stamp material selection step. If there is

Fig. 4.4 Material selection flowchart of an interconnection material, a carrier film, and a stamp. The flowchart starts with the selection of an interconnection material. Next, the stamp material selection follows the interconnection material selection. Then, the carrier film selection follows after the stamp material selection

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no remaining choice for the stamp material, we need to go back to the interconnection material selection step. When W2 is less than W3, we can proceed to the next step of carrier film selection. After choosing the carrier film, we need to evaluate W1, then compare it with W2. When the W1 is stronger than W2, we go back to the carrier film selection step. If there is no remaining choice for the carrier film, we need to go back to the stamp material selection step. After following the flowchart procedures, we can determine the material combination with adhesion characteristics of W1 < W2 < W3. We describe the mechanical characteristics of these transfer media in the following subsection for practical understanding.

4.3.2 Mechanical Characteristics of Transfer Media The transfer media are a carrier film, a stamp, and an interconnection material. Because these media directly contact micro-LEDs, their mechanical characteristics are critical for the successful transfer of micro-LEDs. The micro-LED chip makers provide micro-LEDs arrayed on a carrier film, consisting of a base film and an adhesive layer. The base film can be a flexible polymeric film or a rigid substrate such as a glass or silicon wafer, and the adhesive layer provides adhesive force between the carrier film and micro-LEDs. We need to optimize the adhesive layer considering two aspects. One is that the micro-LEDs should be easily detached from the adhesive layer using a stamp. For this easy detachment, the smaller adhesion is beneficial. The other is that the arrayed micro-LEDs can experience positional change during handling the carrier film when the adhesion is too small. The adhesion can change depending on the picking speed, temperature, surface roughness, adhesive layer thickness, and surface functionalization. Besides the adhesion issue, the base film stiffness is critical for minimizing the positional change in the arrayed microLEDs during the transfer process. The transfer process accompanies the mechanical pressing of micro-LEDs during picking and placing steps. This mechanical pressing leads to unintended micro-LED movement, and this movement tends to increase with the decrease of the base film stiffness. When the transfer process requires an elevated temperature, we also need to consider the carrier film’s thermal expansion. We pick up the micro-LEDs on a carrier film using a stamp and then place them on a display substrate. The stamp adhesion is critical and may need a change after the picking step. The stamp material is dependent on the type of transfer technology, as will be discussed in Sect. 4.4.4. A rigid inorganic material is popular for the electrostatic stamp [19, 20], while elastomeric materials are used for the rubber stamp transfer [16]. The dynamic release layer made of polymeric materials is critical for laser transfer [21]. A polymeric layer on a stiff polymeric base film is adopted for roll-based transfer [22]. The stamp adhesion is affected by several parameters such as chemical affinity, surface morphology, surface energy, and mechanical stiffness. The micro-LED delamination from a stamp can be described in terms of surface generation and mechanical deformation energies. The mechanical deformation energy includes both elastic and inelastic deformation energies. The surface generation

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energy is related to the chemical potential energy and the van der Waals interaction between the stamp and micro-LED. The electrostatic transfer uses an additional electrostatic force to control the transfer process, and the laser transfer introduces an additional mechanical pressure induced by the laser irradiation. The van der Waals adhesion increases with the increase in the contacting area and the van der Waals interaction. The van der Waals interaction rapidly increases when the distance between two interacting objects decreases. The conformal contact between a soft polymeric material and a micro-LED increases the van der Waals adhesion. The interconnection materials provide the electrical connection between microLEDs and a circuit board. Widespread interconnection materials are solders and anisotropic conductive film (ACF). An interconnection material is indispensable for the face-down interconnection but not required for the face-up interconnection. Interconnection materials typically have low adhesion with other materials before reflowing or curing, and this low adhesion leads to the yield issue. The solders are metallic alloys. Au-20wt%Sn has a eutectic temperature of 278 °C and is suitable for applications requiring high thermal reliability, and SnAgCu (SAC) alloy has a eutectic temperature of around 220 °C depending on its alloy composition and is suitable for applications requiring low cost and low processing temperature. We can use the solder in the form of paste or thin films. Solder paste is a mixture of flux resin and solder particles, and mini-LED applications have been extensively adopting solder paste for its low material cost and easy processing. There are several grades of solder particle size, and we need to select a proper particle size for LED applications. The flux has a function of the chemical activation of solder particles by removing the oxide layer formed on the particles. Solder thin films are suitable for micro-LED applications. Using vacuum deposition or electroplating, we can deposit solder thin films on the micro-LED P and N pads or the corresponding locations on a display substrate. Before the transfer and reflowing process, we should apply a flux layer to interconnect micro-LEDs with a panel via solder thin films. The ACF is a convenient solution for the micro-LED interconnection with a circuit board [23, 24]. After applying the ACF on a circuit board, we need to place micro-LEDs on the ACF with mechanical pressure under elevated temperatures and precisely control the pressure and temperature according to the ACF specifications. The ACF consists of conductive particles and polymeric resin [25, 26]. The conductive particles should be in an ordered array because randomly arrayed particles can lead to interconnection failure due to the small micro-LED size. The conductive particles in a conventional ACF are polymeric beads coated with a conductive metal, and the mechanical contact with the particles is responsible for the electrical connection. This mechanical contact can change as the polymeric ACF resin ages under harsh temperature and humidity conditions, resulting in a reliability problem. Solder beads can replace polymer beads to solve this issue. The transfer process is dependent on the adhesion competition among the transfer media, including a carrier film, a stamp, and an interconnection material. While these materials’ adhesion characteristics are critical to realizing the transfer process, the transfer technologies provide another means for tuning adhesion. Researchers have

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actively developed technologies for changing adhesion between a stamp and microLEDs. We briefly review and compare those technologies in terms of transfer size, stamp stiffness, transfer precision, stamp cost, and stamp durability.

4.4 Available Transfer Technologies There are several available technologies for transferring micro-LEDs [2, 3, 5–7, 10, 27]. The underlying mechanisms for the transfer technologies are related to controlling the adhesion of a stamp, W2. Some of them are electrostatic transfer, laser transfer, rubber-stamp transfer, self-assembly transfer, and roll transfer, as listed in Fig. 4.5. These transfer technologies have advantages and disadvantages over other technologies concerning transfer type, transfer area, positional accuracy, chip size, stamp cost, stamp durability, and suitable applications. We introduce the basic principles of the listed transfer technologies and discuss them in the following subsections. To demonstrate the practical manufacturing process of micro-LED panels, we choose the roll transfer technology and present the technical details in Sect. 5.5.

Fig. 4.5 Characteristics of available transfer technologies. Transfer technologies have their advantages and disadvantages and should be chosen considering target applications’ technical specifications. This figure’s transfer accuracy and area are estimated based on the authors’ experience and can vary with a transfer machine’s actual design

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4.4.1 Electrostatic Transfer Electrostatic transfer technology uses an electrostatic force to pick up and place micro-LEDs [28, 29]. It is hard to find any academic article for electrostatic transfer besides patents. As shown in Fig. 4.6, it is necessary to fabricate a sophisticated stamp with the controllability of electrostatic force. The electrostatic stamp typically has a function to address and transfer each micro-LED selectively among an array of micro-LEDs. This function helps avoid the transfer of bad micro-LEDs but requires selective light irradiation or electrical voltage application. The main concerns for the electrostatic stamp are its high cost, high stiffness, and low lifetime. The electrically addressable electrostatic stamps are manufactured by the semiconductor fabrication process such as multiple vacuum deposition, optical lithography, and etching, and this semiconductor process makes the stamp cost expensive. The light irradiation can also control the electrostatic force using a photoconductive material, a critical element in laser printer technology [30]. The body of an electrostatic stamp is a transparent wafer such as quartz or sapphire, and this is useful for making optical alignment between micro-LEDs and a circuit board. Because the transparent wafer’s elastic modulus is much higher than that of polymeric materials, the electrostatic stamp’s stiffness is higher than the polymer stamp’s. This high stiffness is responsible for the fracture of fragile micro-LEDs during transfer. As the stamp size increases, the number of fractured micro-LEDs increases because the stamp’s high stiffness prevents the conformal contact between the stamp and micro-LEDs. The difficulty in the conformal contact leads to applying a higher load during transfer, making the stamp lifetime decrease. In the electrostatic transfer, the high stiffness stamp limits the transfer size. When the transfer size increases, making conformal contact becomes difficult, and the typical transfer size is less than 2 in. With a small transfer size and a high stamp stiffness, it is possible to obtain high transfer accuracy and precision. The electrostatic transfer is suitable for high pixel density and small-size micro-LEDs, considering the electrostatic transfer’s characteristics.

Fig. 4.6 Electrostatic transfer technology. The electrostatic force is electrically controllable and is beneficial to the selective transfer of LEDs among high-density LED arrays

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4.4.2 Laser Transfer The laser transfer uses a focused laser beam to release a micro-LED from a temporary substrate [10, 11, 31, 32], as shown in Fig. 4.7. The excimer laser and diode-pumped solid-state (DPSS) laser are prevailing for this technology. The temporary substrate typically contains a light-absorbing layer to react with an incident laser beam, and the structural change of the layer leads to the detachment of micro-LEDs. The structural change can be delamination, decomposition, or volume change. The main concern is the damages induced in micro-LEDs during laser irradiation. The light-absorbing layer needs to block most energy of the incident laser and protects micro-LEDs from laser damages. Another concern is the positional accuracy of the transferred micro-LED. The relative position between the micro-LED and the incident laser beam before transfer affects the transferred micro-LED position. The laser spot’s positional deviation can lead to an amplified deviation of the transferred micro-LED position depending on the distance between the micro-LED and a circuit board. The critical component in the laser transfer is the temporary substrate. The temporary substrate typically consists of a transparent wafer, a light-absorbing layer, and an adhesive layer holding micro-LEDs [31]. The transparent wafer allows the laser light transmission and the optical alignment between the micro-LEDs and a circuit board. Another role of the transparent wafer is to provide resistance to mechanical deformation during the transfer operation. This resistance prevents the change in the spacings between micro-LEDs induced by the mechanical deformation during the transfer operation. The light-absorbing layer has a high absorbance at the laser wavelength and accompanies structural change such as volume expansion, delamination, and decomposition/ablation after absorbing laser light. A soft polymeric material is suitable for an adhesive layer, and its adhesion should be appropriately tuned. The laser transfer can handle both mini and micro-LEDs and has the advantage of avoiding mechanical contact. The transfer precision is around 2 µm depending on the optics, the laser wavelength, and the machine configuration. The temporary substrate holding the LEDs plays a role of a stamp, and its cost-effectiveness is critical because it is disposable. Another concern of the laser transfer is the pressurization step

Fig. 4.7 Laser transfer technology. A focused laser beam selectively transfers micro-LEDs with the aids of a light-absorbing layer

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after the transfer. The pushing force on LED induced by the laser irradiation is tiny and insufficient to secure the electrical interconnection between the LED metallic pads and the circuit electrodes. The laser transfer typically requires an additional pressurization and heating step after the transfer and can suffer from LED drift or movement during this additional step.

4.4.3 Rubber Stamp Transfer Rubber stamp transfer uses the rate-dependent adhesion of a rubber [8–10, 16, 17, 33–36] as shown in Fig. 4.8. The adhesion between a rubber and a device increases with the deformation rate. We increase the rubber stamp’s retraction speed after contacting the device on a temporary substrate for the picking action. This increased speed results in increased stamp adhesion with the device. We need to decrease the rubber stamp’s retraction speed after contacting a circuit board for the placing action. This decreased speed leads to decreased stamp adhesion with the device and leaves the device on the circuit board after the stamp retraction. A rubber stamp is typically made from transparent silicone rubber and plays a critical role in realizing the transfer. The stamp surface contacting with devices should be smooth and cleaned periodically with adhesive tapes or other means. The structural stiffness of a stamp needs careful design considering its area and dimensional stability. A low stiffness is desirable for conformal contact over a large area, but the low stiffness can bring positional uncertainty to the transferred device because of the stamp deformation. Another concern is the stamp durability. A rubber stamp’s lifetime [8] is much longer than that of the rubber blanket used for offset printing technologies. This longer lifetime is because the rubber transfer stamp exposes to no solvent.

Fig. 4.8 Rubber stamp transfer technology. Rate-dependent adhesion of the rubber stamp enables the adhesion change required for the picking and placing actions. Therefore, fast retraction is beneficial to the picking action, while slow retraction is to the placing action

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The rubber stamp transfer is suitable for micro-LEDs, but not suitable for miniLEDs. Due to the rubber stamp’s low stiffness, it is difficult to apply sufficient transfer pressure on the mini-LEDs for electrical interconnection. The transfer precision is around 2 µm depending on the stamp stiffness, stage and optics resolutions, and the transfer area. The transfer area is limited by the planar nature of the rubber stamp. Making conformal contact between micro-LEDs and a circuit board becomes difficult as the transfer area increases. It is not easy to increase the transfer area larger than 3 in. We can realize the selective transfer of micro-LEDs using a patterned stamp. Contrary to the electrostatic transfer, selective transfer based on one-by-one addressing is impossible for the rubber stamp transfer.

4.4.4 Self-Assembly Transfer Self-assembly transfer utilizes small-scale forces such as dielectrophoresis force, surface tension, and capillary force together with shape matching [37–43]. MicroLEDs are small enough to be handled by those small-scale forces. There are many variants for self-assembly transfer, and they share some features like enhanced mobility, site-specific force distribution, and device shape constraint. For example, Park et al. [40] demonstrated that the dielectrophoresis (DEP) force could align and assemble nanorod LEDs, as shown in Fig. 4.9. They introduced a liquid solvent to provide enhanced mobility for the nanorod LEDs, modified circuit board electrodes to activate site-specific DEP force, and fabricated rod-shaped LEDs to enhance alignment with the applied electric field. Because the self-assembly transfer is stochastic, not deterministic, designing the pixel and the corresponding circuit is required considering the self-assembly’s probabilistic nature. Making a LED ink with LEDs and a solvent replaces the picking step’s difficulty. Jetting or dropping the LED ink on a circuit board eliminates the use of a stamp or mechanical contact for the placing step. For aligning or assembling LEDs on a pixel, we need to introduce a small-scale force interacting between the LEDs and the pixel.

Fig. 4.9 Self-assembly transfer technology. Micro-scale forces can handle and align micro-LEDs. Nano-rod LED ink is dispensed on a target substrate, and the nanorod LEDs are aligned along an electrical field due to the dielectrophoretic (DEP) force

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This small-scale force often requires specially designed electrodes [40] and physical structures like dams or wells on a circuit board. In some cases, we need additional metal deposition and patterning steps to make the electrical interconnection between LEDs and circuit board electrodes. The self-assembly transfer has no restriction on the transfer size and is attractive for manufacturing large-area display applications like digital signage. However, it suffers from its stochastic nature because the yield requirement for display applications is very high. Some self-assembly techniques can apply to mini-LEDs [41], but micro-LEDs or nanorod LEDs are suitable for self-assembly transfer, considering the small magnitude of the self-assembly forces. Because the self-assembly transfer precision is typically worse than the other transfer technologies’ precision, the circuit board should be designed with high robustness to the LED positional deviation.

4.4.5 Roll Transfer Roll-transfer technology [22, 26, 36, 44, 45] is not a modified version of the rubber stamp transfer. The roll stamp does not rely on the rubber stamp’s rate-dependent adhesion and high compliance. We need to optimize the roll pressure to realize the micro-LED transfer process. As shown in Fig. 4.10, we pick up micro-LEDs with a roll stamp by applying a roll pressure. This roll pressure ensures the mechanical contact between the micro-LEDs and the roll stamp but should be tuned to avoid the excessive deformation of the transfer film holding the micro-LEDs. We apply a higher roll pressure for the placing action than the roll pressure for the picking action. This increased roll pressure enhances the micro-LED detachment from the roll stamp and increases the contact area between the micro-LEDs and the interconnection materials

Fig. 4.10 Roll transfer technology. A transfer film wrapped around a roller holds micro-LEDs. The micro-LEDs are delaminated from the transfer film and placed on a target substrate under the increased roll pressure

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on the target substrate. The roll transfer is different from the conventional roll-based printing technology, such as offset or reverse-offset printing, in that the transferred object is a solid device, not a liquid ink layer. The roll transfer suffers from electrical interconnection failure, which is not a problem in roll-based printing. The overlay alignment for the roll transfer can benefit from the roll-based printing studies [46–51]. A roll stamp typically consists of a rigid cylindrical roller, polymeric blanket, and a transfer film. The cylindrical roller is made of a high modulus material such as stainless steel, and the roller roundness should be secured along the roller axial direction. The polymeric blanket is made of silicone rubber, and its modulus and thickness should be optimized considering the roll pressure required for the transfer process. The contact area between the roll stamp and a substrate depends on the blanket modulus and thickness and the roll pressing load. We need to consider both the pressing load and the contact area to evaluate the roll pressure. The transfer film wrapped around the roller with a blanket governs the adhesion characteristics between the roll stamp and micro-LEDs. The transfer film consists of a base film and an adhesive layer. The base film should be stiff to prevent circumferential deformation upon roll pressing, and the adhesive layer should be tuned to provide sufficient adhesion with micro-LEDs. The roll transfer is suitable for both micro and mini-LEDs. The main advantage of the roll transfer is its scalability. It can deal with a transfer size of more than 4 in. The roll transfer can handle a smaller transfer size of 2 or 3 in., but the rubber stamp transfer is more suitable for the smaller transfer size. It is possible to increase the roller length up to 20 in. or more, but the transferred LEDs’ positional deviations increase as the roller length and diameter increase. Typically, the positional accuracy of the roll transfer is worse than that of the rubber stamp transfer because the rubber stamp facilitates the overlayed alignment. When a circuit board is optically transparent, the overlayed alignment applies to the roll transfer. This alignment improves the transfer accuracy up to 2 microns. The transfer film for the roll transfer process can be disposable and cheap.

4.5 Some Examples of Roll Transfer This section describes three practical examples of mini and micro-LED transfer, all realized by the roll transfer technology. Figure 4.11 shows the roll-to-plate (R2P) transfer machine developed by the Korea Institute of Machinery and Materials. The roll transfer machine consists of a roll stamp, cameras, loadcells, an X–Y-Theta stage, linear guides, and motors. We can transfer mini- or micro-LEDs from the roll stamp to a target substrate by controlling the roll pressing load, as explained in Sect. 4.4.5. The roll transfer has better controllability of the contact pressure distribution between the stamp and the circuit board than the flat stamp transfer. This advantage enables the scaling-up of the transfer size. On the other hand, the roll transfer has a disadvantage in aligning LEDs on the roll stamp with the circuit board

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Fig. 4.11 Example of the roll transfer machine. a Photograph of the roll transfer machine developed in KIMM. b The roll transfer machine has three sets of microscopes. One set (C1) is for the overlayed alignment, and the other two (C2 and C3) are for the calibrated alignment

electrodes. Figure 4.12 shows two alignment schemes for the roll-based transfer. One is a calibrated alignment, and the other is an overlayed alignment. When we utilize the calibrated alignment scheme, we typically adopt two microscopes, at least. One microscope is for measuring the LED positions on a roll stamp, and the other is for measuring the electrode positions on a circuit board. By calibrating the two microscopes’ positions before the transfer process, we can precisely place the LEDs on the circuit board’s electrodes. After making alignment between the LEDs and the circuit board, we need to approach the roller toward the circuit

Fig. 4.12 Two alignment schemes for the roll transfer technology. a The calibrated alignment requires a set of microscopes (C3) to observe the roll stamp and other microscopes (C2) for observing the circuit board. b The directly overlayed alignment requires a microscope (C1), a transparent stage, and a transparent circuit board

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board to make the actual contact between the LED electrodes and the circuit board electrodes. The typical alignment accuracy for the calibrated alignment is around 5 µm. The alignment accuracy is dependent on roll stamp structure, X–Y-Theta stage performance, stage-roller orthogonality, and roller roundness. On the other hand, the overlayed alignment scheme’s accuracy is much better than that of the calibrated one. The overlayed alignment is feasible when the display panel is transparent. The LEDs are directly overlayed with the display panel without mechanical contact before transfer. In this section, we present three examples of the LED transfer. We adopt the calibrated alignment scheme for the mini-LED transfer and the face-up micro-LED transfer, and we apply the overlayed alignment to the face-down micro-LED transfer for high alignment accuracy.

4.5.1 Roll-To-Plate Transfer of Mini-LEDs A conventional die bonder can handle mini-LEDs with an acceptable alignment accuracy but suffers from a low transfer speed of approximately 10 LEDs per second. To manufacture a digital signage panel of 4 K UHD, we need to transfer 25 million mini-LEDs on printed circuit boards (PCBs), and it takes 695 h when we use one die bonder. We demonstrate roll-based mini-LED transfer for achieving high productivity. A LED chip maker fabricates lots of mini-LEDs of approximately 125 µm × 225 µm × 80 µm in size and arranges them on a carrier film with a prescribed mini-LED pitch. We use solder paste screen-printed on a PCB as an interconnection material. The transfer procedure begins with picking up mini-LEDs by contacting the roll stamp with the mini-LEDs on a carrier film. For the successful picking step, the miniLED and carrier film adhesion should be smaller than that between the mini-LEDs and the roll stamp. After picking the mini-LEDs with the roll stamp, we prepare a PCB with screen-printed solder paste and align the mini-LEDs with the PCB’s electrodes. It is not trivial to make alignment between mini-LEDs on the roll stamp and a flat PCB. The roller’s roundness is critical, and the cylinder axis should be perpendicular to the PCB’s moving direction. Because the adhesion between the solder paste and the mini-LEDs is tiny under room temperature, we need to tune the roll stamp adhesion for high transfer yield. In this demonstration, we utilize a disposable film wrapped around the roller as the roll stamp, and the adhesion of the disposable film can be adjustable by the roll pressure. We apply slight roll pressure to increase the adhesion between the disposable film and the mini-LEDs during the picking step. The slight roll pressure provides conformal contact between the disposable film and the mini-LEDs but should not accompany severe mechanical deformation on the disposable film and the carrier film. We apply an increased roll pressure to delaminate the mini-LEDs from the disposable film during the placing step. This increased roll pressure also leads to an increased adhesion between the mini-LEDs and the solder paste.

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Fig. 4.13 Roll-to-plate transfer of mini-LEDs. a The red, green, and blue mini-LEDs can be transferred on a printed circuit board (PCB) using a roll transfer machine. b Red, green, blue mini-LEDs are arranged on a carrier film with a prescribed pitch. c The mini-LEDs on a carrier film are transferred on a PCB using the same roll transfer machine

Figure 4.13 shows some examples of roll-transferred mini-LEDs. It is possible to sequentially transfer red, green, and blue mini-LEDs on a PCB, as shown in Fig. 4.13a. We need to consider the order for the three-time transfer and typically transfer the thinnest mini-LEDs first. When a chipmaker provides RGB mini-LEDs on a carrier film, as shown in Fig. 4.13b, we can manufacture a mini-LED color module by a one-time transfer, as shown in Fig. 4.13c. The chip thicknesses of RGB mini-LEDs should be similar to one another. Transfer yield and speed are the critical parameters. The transfer yield for roll-based transfer can reach 99–99.9% in a laboratory environment. We need to improve the transfer yield up to five or sixnine in a controlled cleanroom environment to commercialize mini-LED signage applications.

4.5.2 Face-Up Transfer of Micro-LEDs A typical example of face-up transfer can be found in the article [22]. The micro-LEDs are approximately 100 µm × 100 µm × 3 µm in size and formed on a GaAs wafer with an AlAs sacrificial layer. We need to prepare a substrate with transferred microLEDs before circuit metalization for the face-up interconnection. We sequentially transfer thin-film transistors (TFTs) and micro-LEDs using the roll transfer machine to realize an active matrix micro-LED panel, as shown in Fig. 4.14a. The challenge is

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Fig. 4.14 Face-up transfer of micro-LEDs. a Schematic for face-up transfer of TFTs and LEDs, respectively. b Stretchable active-matrix micro-LED display manufactured by the face-up transfer [22]

to control the relative position between the TFTs and micro-LEDs using the transfer process. Because metal interconnection lines form after the transfer process, the relative position error results in interconnection failure. We use a polymeric layer wrapped around a roller as the roll stamp. We deposit a UV adhesive on a temporary glass substrate coated with a sacrificial PMMA layer, then transfer TFTs on the temporary substrate using the UV adhesive. We coat an epoxy planarization layer on the TFTs transferred on the temporary substrate, then deposit UV adhesive again for the micro-LED transfer. We adopt the calibrated overlay scheme to make alignment between the micro-LEDs and the TFTs. The calibrated overlay scheme uses two microscopes installed on the roll transfer machine. One microscope measures the TFT positions on the temporary substrate, and the other microscope does the micro-LED positions on the roll stamp. We transfer the micro-LEDs on the temporary substrate at a relative distance from the TFTs. As mentioned in Sect. 4.2.2, we readily realize the face-up transfer by incorporating an adhesive, and the interconnection between the micro-LEDs and the circuit is

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straightforward without requiring any interconnection materials. Figure 4.14b shows a red micro-LED panel with a TFT active matrix. The metal interconnection between adjacent pixels has a serpentine structure and forms by Au thin-film deposition and photolithography. The transfer yield is 100% because of the use of UV adhesive.

4.5.3 Face-Down Transfer of Micro-LEDs We use flip-chip micro-LEDs for the face-down transfer. The micro-LEDs are approximately 30 µm × 30 µm × 5 µm in size, as shown in Fig. 4.15, and form on a sapphire wafer. We need to lift off the micro-LEDs from the sapphire wafer using a UV laser, as described in Chap. 3, and we have the micro-LEDs on a carrier film after laser lift-off (LLO). We pick up the micro-LEDs from the carrier film using the roll stamp. As the chip size decreases, the alignment margin also decreases. The challenge is to enhance the alignment accuracy using the roll transfer. We use a disposable layer wrapped around a roller as the roll stamp. Similar to the mini-LED transfer, we have two choices for using the disposable layer. We can pick up the micro-LEDs on a carrier film by laminating the disposable layer and then delaminating it from the carrier film. We typically treat the micro-LEDs on the disposable layer with UV or plasma before wrapping the disposable layer on a roller. The other choice is to wrap the disposable layer around a roller before the picking step. By rolling the roller with the disposable layer, we can pick up micro-LEDs, then place them on a circuit board. We can deposit thin-film solder either on the

Fig. 4.15 Face-down transfer of micro-LEDs. a Micro-LEDs on a sapphire wafer. b A transparent glass PCB. c A photograph on the face-down roll transfer process. d Face-down transferred microLEDs on a glass PCB

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micro-LED electrodes or on the circuit board electrodes. In this case, we use microLEDs with thin-film solder on their electrodes. Before placing the micro-LEDs on the circuit board, we need to coat flux on the circuit board to remove the metal oxide on the solder surface. Because the circuit board is transparent, we adopt the directly overlayed alignment scheme and observe the alignment with a microscope underneath the transparent quartz stage. Figure 4.15 shows the transfer results. The transfer yield is 99%, and the alignment accuracy is approximately 1.5 µm.

4.6 Summary and Future Direction This chapter started by describing the manufacturing process of micro-LED display panels, where the transfer process is one of the critical bottlenecks. We introduced transfer mechanics, which relies on adhesion competition between two interfaces contacting LED’s top and bottom surfaces. There are several ways to change the adhesion, and they are realized in transfer technologies, including electrostatic transfer, laser transfer, rubber stamp transfer, self-assembly transfer, and roll transfer. We discussed their advantages and disadvantages in terms of transfer size, transfer accuracy, and stamp cost. To familiarize the practical transfer process, we presented three examples: mini-LED transfer, micro-LED face-up transfer, and micro-LED face-down transfer. Industrial engineers need to select a suitable transfer technology considering target applications, pixel density, panel size, LED types, LED size, and interconnection methods. Figure 4.16 shows some promising applications of mini and micro-LED displays. Manufacturing a mini-LED backlight unit (BLU) is feasible with cost-effectiveness and will impact the LCD market [3, 52–55]. The next application should be miniLED signage or big-sized television, which requires three-color mini-LED chips or a color-conversion layer with blue mini-LEDs [56–61]. Manufacturing the miniLED signage is technologically ready and needs cost reduction by improving the manufacturing equipments and process. The cost-reduction need will transform the mini-LED applications into micro-LED ones. We can reduce the LED chip price by making smaller chips while the difficulties in display manufacturing amplify as the chip size decreases. The mass production of micro-LED display system is under development, and we need to resolve some technological hurdles such as inspection, rework, light extraction efficiency, and interconnection reliability [62– 68]. The significant application of micro-LED could be the augmented reality (AR) [69–78]. We need to develop high-density color micro-LED panels to impact the AR market, which accompanies technological barriers such as color conversion, chip transfer/interconnection, rework, and driving circuits. We expect the microLED display to be perfect for the automobile window display, which requires high transparency, brightness, power efficiency, and reliability. LCD and OLED displays could not meet those requirements sufficiently. The transfer technology is at its introductory period and has not been utilized in any industry yet. It is far from conventional display and semiconductor process

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Fig. 4.16 Promising applications of mini and micro-LEDs. a Schematic and photograph of miniLED. Mini-LED backlight unit (BLU) and signage applications. b Schematic and photograph of micro-LED. Augmented reality glass and television applications

technologies and requires understanding the adhesion mechanics between a stamp and a device. We can control the adhesion by adjusting nano and microscale forces such as electrostatic/electromagnetic and van der Waals forces. We need to devise a novel transfer method using unconventional forces or materials with unconventional mechanical responses depending on our target applications. The transfer technology will extend its applications to other industries like flexible high-efficiency solar cells [79–82], implantable or attachable medical devices [83–89], flexible semiconductor devices [10, 26, 44, 90, 91], quantum dot devices [92–98], and meta-structured sensors [99–102]. Acknowledgements The authors thank Hyosoon Lee of Design and Seung Han of UST for the excellent preparation of the figure illustration. This work was supported by the Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science, ICT, and Future Planning as a Global Frontier Project (CAMM No. 2014063701, 2014063700), and by the Technology Innovation Program (KEIT No. 20000619) funded by the Ministry of Trade, Industry, and Energy (MOTIE, Republic of Korea). This work was also supported by an internal research program of the Korea Institute of Machinery and Materials (NK230D).

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Part II

Device Applications

Chapter 5

Advanced Epitaxial Growth of LEDs on Van Der Waals Materials Hyunseok Kim, Wei Kong, and Jeehwan Kim

5.1 Introduction Light emitting diodes (LEDs) are semiconductor light sources that emit photons by injecting electric current. LEDs are indispensable devices in extremely wide areas in our daily life, and to name a few, visible LEDs are widely used for lighting and displays, infrared LEDs for communications, sensing and monitoring, and ultraviolet LEDs for sensing and biomedical devices. Epitaxy is one of the material growth and deposition techniques employed for the fabrication of LEDs. The crystallographic information of epitaxially grown materials follows that of the substrate, and epitaxy of inorganic materials has been widely used to manufacture LEDs. This chapter focuses on the epitaxy of LEDs on van der Waals (vdW) materials, i.e. materials having weak van der Waals interaction on their surfaces. Two epitaxy methods to grow high-quality materials on vdW surfaces, which are van der Waals epitaxy and remote epitaxy, will be introduced in detail. In specific, the theoretical background of these methods, material systems to utilize these methods, advantages and challenges of each method will be introduced. Inorganic LEDs based on III-N, III-P, and III-As compound semiconductors will be first discussed with focuses on the epitaxy mechanisms, growth tactics, device performance, and functionalities. Next, newly emerging materials for LEDs and optoelectronics, such as transition metal dichalcogenides and perovskites, will be introduced, with the LEDs made of these materials. Recently, there have been rapidly growing interests in developing LEDs with additional functionalities, such as bendability, stretchability, bio-compatibility, and H. Kim · J. Kim (B) Massachusetts Institute of Technology, Cambridge, USA e-mail: [email protected] W. Kong Westlake University, Hangzhou, China © Springer Nature Singapore Pte Ltd. 2021 J.-H. Ahn and J.-H. Kim (eds.), Micro Light Emitting Diode: Fabrication and Devices, Series in Display Science and Technology, https://doi.org/10.1007/978-981-16-5505-0_5

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so on, and LEDs formed on vdW materials can be utilized to fulfill many of these features. Therefore, once matured, advanced epitaxial growth techniques and material systems introduced in this chapter could be the stepping stone for next-generation LEDs.

5.2 Brief Summary of Conventional Epitaxy of LEDs 5.2.1 Epitaxy of III-V LEDs III-V compound semiconductors with direct bandgap have been extensively used for manufacturing inorganic light emitting devices for the last few decades. These materials include III-N (GaN, InGaN and AlGaN), III-As (GaAs, InGaAs and AlGaAs) and III-P (InGaP and AlInGaP), and they are typically formed by epitaxy, which is a special type of crystal growths wherein the crystal structure and crystal orientation of the grown materials follow the substrate lattice information. In general, to form highquality epitaxial films, it is critical to match the lattice constant and thermal expansion coefficient of epitaxially grown materials close to those of substrate materials. Otherwise, mismatches induce strain in epitaxial films and ultimately lead to formation of dislocations above critical thickness, which potentially act as non-radiative recombination centers and seriously deteriorate radiative efficiency [1]. Especially, the optical properties of III-As and III-P materials are more severely affected by these defects than III-N, and to prevent the degradation of material quality by lattice mismatching, ternary and quaternary compounds are widely used to match the lattice constant while acquiring the bandgap of interest simultaneously. On the other hand, III-N materials are less susceptible to the defect density, and III-N LEDs can still operate with reasonably good radiative efficiency even with the presence of fairly high defect densities owing to carrier localization effect [2–4]. Because of the lack of native substrates, III-N LEDs are typically grown on sapphire, SiC, or silicon substrates, even though these substrates are not lattice-matched to III-N. The spectral range of LEDs made by III-V covers the entire visible wavelengths, and also spans to both far shorter and longer wavelengths. In III-N LEDs, the combination of AlN, GaN, and InN to form AlGaN and InGaN alloys has realized LEDs with their emission wavelengths spanning from deep-ultraviolet to green. In IIIAs and III-P LEDs, InGaP and AlInGaP-based emitters on GaAs substrates cover from yellow to red, and InGaAs (P)-based emitters on InP substrates covers nearinfrared wavelengths. Furthermore, by employing III-V superlattices, which are periodic heterostructures of thin layers, interband and intersubband emission from these superlattices have enabled LEDs and lasers to operate at short-, mid-, and long-wave infrared regime, and even at terahertz regime, by growing these materials on III-V substrates such as GaAs and GaSb [5–7]. These inorganic LEDs have advantages over organic LEDs in terms of energy efficiency, lifetime, and durability, and thus are widely used in many real-world

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applications, such as solid-state lighting, displays, imaging, communications, and sensing [8]. Although electroluminescence from III-N materials can cover only up to green wavelengths so far, red and white light can be realized by coating the III-N with phosphor, and thus most of the inorganic solid state visible emitters are manufactured using III-N materials. Still, it should be noted that III-P and III-As can achieve red LEDs with higher quantum efficiency than III-N requiring phosphor coating, and to harness such high energy efficiency, manufacturing RGB pixels by combining III-N and III-P is under investigation despite several challenges such as different lifetimes and luminescence efficiencies [9]. Aside from visible wavelengths, III-N emitters and III-As emitters are extensively used in ultraviolet and infrared regimes, respectively.

5.2.2 Layer Transfer Techniques of LEDs Although III-V compound semiconductor-based solid-state LEDs exhibit excellent energy efficiency, stability and durability, one of the stringent requirements to manufacture these LEDs is that the device needs to be epitaxially grown on lattice-matched or closely-lattice-matched single-crystal substrates. In other words, as-fabricated LEDs cannot be bent or made flexible, which is in stark contrast to organic LEDs that can be readily formed on flexible substrates by various methods including solution processing, spin coating and roll-to-roll printing [10, 11]. There are ever-increasing demands for bendable, flexible, and curved LEDs in numerous applications such as flexible displays, augmented reality, and bio-integrated chips [12, 13], while it is challenging to employ inorganic LEDs for these applications if the LEDs cannot be detached from rigid substrates. To satisfy these market needs, there have been extensive studies to transfer epitaxially grown LEDs from their host substrate onto foreign substrates which could be bendable, stretchable, or compatible with target applications. Conventionally, the most widely used method in transferring III-V LEDs is chemical lift-off process shown in Fig. 5.1a, which is also known as epitaxial lift-off (ELO). In ELO process, a sacrificial layer is grown in between the substrate and LED structure, which can be selectively etched by chemicals [14]. Once the sacrificial layer is removed by immersing the device in chemicals, the LED structure is detached from the substrate and can be transferred onto other substrates by scooping or by using a stamp [15, 16].

Fig. 5.1 Schematic of lift-off techniques to exfoliate and transfer epitaxial layers [1]

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For AlInGaP-based LEDs on GaAs substrates, AlAs or AlGaAs layer is typically used as a sacrificial layer, which can be selectively etched by HF- or HCl-based solutions. For infrared LEDs on InP substrates, InGaAs or InAlAs is used as sacrificial layers which can be etched by FeCl3 [17, 18]. However, this process is extremely timeconsuming process, and roughening and residue formation on the wafers during liftoff are an issue, necessitating additional wafer refurbishing processes for substrate re-use. For III-N LEDs, laser lift-off process is widely used by growing LEDs on sapphire or SiC substrates, because there are no lattice-matched materials with good etching selectivity necessary for chemical lift-off. Sapphire and SiC are transparent to deepultraviolet wavelength lasers, and thus GaN at the substrate/GaN interface side can be heated and melted by applying the laser beam from the backside of the wafers as depicted in Fig. 5.1b, from which grown LEDs can be exfoliated and transferred. This process is a relatively quick process, but the growth wafers roughen during the laser lift-off process, which limits the re-usability of the wafer [19]. Besides these methods that are already mature, a few other techniques are being investigated to transfer III-V LEDs. For example, III-N LEDs grown on silicon wafers can be made free-standing by completely etching away the wafer using KOH solution [20]. Mechanical spalling is another high-throughput process, wherein nickel is deposited on LEDs as a stressor to generate cracks (Fig. 5.1c). The crack propagates underneath the substrate, and LEDs can be exfoliated by spalling the LEDs with a portion of substrate [21]. Lastly, an emerging approach is to utilize van der Waals materials as an interlayer between the substrate and LEDs (Fig. 5.1d). Since van der Waals materials intrinsically have weak chemical bonding to the substrates or overlayers, LEDs grown on top can be easily exfoliated without damaging the substrate. This will be introduced in detail in the following sections.

5.3 III-V LEDs on Van Der Waals Materials 5.3.1 Van Der Waals Epitaxy of Thin Films and Nanostructures Van der Waals (vdW) materials are layered materials with weak van der Waals interaction between each layer. This is because these materials do not have dangling bonds at the surface, which is in stark contrast to many conventional crystalline materials including III-V compound semiconductors that have dangling bonds at the surface. Because of the weak out-of-plane interaction, vdW materials can be made monolayer or a few stacks of monolayers by exfoliation from their bulk forms. The most widely known layered vdW material is graphene, which is a monolayer carbon of hexagonal lattice that was first acquired by exfoliating from graphite using scotch tape [22]. So far, a myriad of new two-dimensional (2D) vdW materials have been reported either

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by top-down or bottom-up fabrication processes, and their unique material properties are extensively studied for many applications. When the epitaxy of 3D materials (such as III-V) is conducted on vdW materials and the nucleation process is governed by the vdW material, then the growth mode is referred to as either vdW epitaxy or quasi-vdW epitaxy. The reason that it is sometimes called quasi-vdW epitaxy is because the growth takes place on the surface of materials having vdW interaction, but the materials grown on top are not vdW material. Here, we will note this growth mode as vdW epitaxy. On the other hand, if the nucleation is governed by the 3D materials (typically semiconductor wafers) beneath the vdW materials, then the growth mode is referred to as remote epitaxy, which will be discussed in the following sections. Compared with conventional epitaxy of III-V on 3D substrates like III-V, sapphire, and SiC, vdW epitaxy of III-V is more challenging, because the surface free energy of vdW materials is much lower than bulk materials. In other words, the chemical interaction between the vdW surface and III-V is much weaker than that between III-V and the conventional substrates [23], and therefore the crystallization of IIIV materials on vdW surfaces is harder to control. In specific, the weak chemical interaction results in lower nucleation density, which promotes island-like growth mode with low density of nuclei and makes the formation of planarized thin films difficult. Therefore, vdW epitaxy of III-V materials has been studied in two distinctive directions, one in utilizing island-like growth mode and forming nanostructures (or microstructures), and the other in promoting the nucleation and forming merged thin films. In nanostructure-based approaches, such as nanowires and nanowalls, there are several unique advantages compared with thin film-based approaches. First, these nanostructures have small interface area with the substrate, and the critical thickness to form dislocations is greatly increased when the interface area is reduced [24, 25], resulting in lower defect density in the as-grown materials compared with planar counterpart. Furthermore, if grown on vdW materials, the nanostructures can expand along in-plane directions, so the strain induced by lattice or thermal mismatch can be laterally relaxed, leading to a better material quality [26]. Also, due to their small sizes, there is a larger degree of freedom in growing lattice-mismatch heterostructures within the nanostructures without generating dislocations, which is very challenging in thin film epitaxy. Besides, it is possible to grow three-dimensional composite heterostructures, such as core/shell/axial structures, that could bring higher device efficiency and additional functionality in device design. Lastly, the high surface-tovolume ratio of nanostructures, their unique geometries, and arrangement of these nanostructures can greatly boost the performance of devices that is not achievable by thin film-based devices, for example by forming photonic cavities, metasurfaces, or nanoantennae. The vdW epitaxy of GaN nanowires is experimentally demonstrated on both crystalline and amorphous substrates, revealing the usefulness of employing vdW materials as an interlayer [27, 28]. On graphitized SiC substrates, the nucleation of GaN nanowires occurs only at the bilayer stripe region, which is similar to the growth of GaN thin films on graphitized SiC [29], and the orientation of nanowires

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Fig. 5.2 a SEM (left) and cross-sectional TEM (right) images of GaN nanowires grown on a graphene/SiC substrate [28]. b Cross-section SEM image of GaN nanowires grown on a graphene/SiO2 /Si substrate [27]

was perpendicular to the step edges. On the other hand, for graphitized SiC with multilayer graphene, GaN nanowires grew mostly vertically on the entire surface area (Fig. 5.2a), showing the effect of the number of graphene layers on the nucleation and growth. On amorphous SiO2 /Si substrates with graphene flakes, the growth of vertical GaN nanowires is observed on the graphene-coated area as shown in the SEM image in Fig. 5.2b, with the epitaxial relationship of the wurtzite lattice parallelly aligned to the zigzag carbon chain in graphene, substantiating that the graphene works as a buffer for GaN nanowire growth [27]. Here, the density of nanowires decreases as the number of graphene layers is increased, which is attributed to the effect of strain. III-N nanowires with advanced heterostructures are also demonstrated, which provides a path toward high-performance electronic and optoelectronic devices. As an example, core/shell heterostructures with InGaN multi-quantum-well (MQW) sandwiched in doped GaN inner and outer shells are grown on graphene-coated SiO2 /Si substrates, using ZnO seeding layer [30]. The successful demonstration of III-N thin films and nanostructures on various substrates via vdW epitaxy not only is promising for realizing high-performance devices, but could bring additional functionalities, such as atomic precision layer transfer and flexible electronics, which will be discussed in detail in the later sections. In a similar manner, the growth of III-As and III-P nanostructures is accomplished by vdW epitaxy. III-As and III-P have even higher tendency to form nanostructures on vdW surfaces than III-N due to the low adsorption energy of adatoms on vdW surfaces [31, 32]. In specific, compared with the adsorption energy of nitrogen on graphene, the adsorption energies of other common group III and V elements like gallium, arsenic, indium, and aluminum are much lower than nitrogen, meaning that these adatoms on the graphene do not easily stick on the graphene surface but easily migrate long distance to form islands and 3D structures. Therefore, the growth of III-As and III-P on vdW surfaces typically results in the formation of nanowires. Interestingly, it has been postulated that although graphene and III-V semiconductor have high lattice mismatch, (111)-oriented III-As/P materials and hexagonal graphene lattice can exhibit domain-aligned relationship [33–35], and thus graphene could work as a perfect buffer layer for (111)-oriented III-As/P material growth. This speculation has been experimentally verified by several material systems, such as

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Fig. 5.3 a SEM images of InGaAs nanowires grown on graphene-coated SiO2 /Si substrate [37]. b Schematic illustration (left) and false-color SEM (right) of InAs nanostructures grown on freestanding monolayer graphene [38]

(111)-oriented vertical InAs nanowire growth on graphene-coated SiO2 /Si substrates [34], GaAs nanowire growth on graphitic substrates [35], InP island growth on epitaxial graphene/Ge substrates and graphene-transferred SiO2 /Si substrates [36], and InGaAs nanowire growth on graphene-coated SiO2 /Si substrates (Fig. 5.3a) [37]. Increasing the number of graphene layers resulted in increased density of nanowires [34], which agrees with the case of III-N on graphene where the nucleation density increases with the number of graphene layers. In the InGaAs growth, indium composition in Inx Ga1-x As was tuned to almost entire range from 20 to 100% showing the possibility for wide range of bandgap tuning. Vertical InAs nanowires are successfully grown even on freestanding graphene (Fig. 5.3b), which directly shows the seeding effect of graphene, because nanowires will grow along random directions if there is no epitaxial relationship between InAs and graphene lattice [38]. The growth of nanowires on TMDs was also feasible, such as InGaAs nanowires on MoS2 -coated SiO2/Si substrates [37]. In vdW epitaxy of III-As/P nanowires, the (111)-oriented nanowires often have stacking disorders or zinc blende/wurtzite polytypism, which is frequently observed in conventional nanowire epitaxy as well [39]. Such polytypism in nanowires deteriorates electrical and optical properties of nanowires, and needs to be reduced or eliminated for improved optoelectronic properties. One of the biggest advantages of nanowire-based approach is that the nanoscale dimension of nanowires enables the growth of lattice-mismatched heterostructures without forming dislocations. Also, the unique geometry of nanowires can be utilized to engineer light-matter interaction, which is of great interest in the field of optoelectronics [40, 41]. To exploit these features, as an example, nanowires composed of InAs/InGaAs/GaAs heterostructures with p–n junction are grown on multi-layer graphene, and solar cells are fabricated using these nanowires [42]. Transparent metal electrode is deposited on top of nanowires, and graphene is used directly as a bottom contact, resulting in conversion efficiency of 2.51% from in-situ passivated nanowire solar cell. These results show that vdW epitaxy of III-V materials is promising as a new approach to fabricate III-V optoelectronic devices. However, controlling the uniformity of the size, material composition, and density or location of nanowires remain as a challenge, which are crucial requirements for better device performances.

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Instead of forming nanostructures, growing merged thin films on vdW surfaces has been also actively investigated as a path toward practical and functional III-V devices on vdW materials. Especially, since nitrogen has much higher adsorption energy on vdW surfaces than other group-III and V elements which promotes higher density of nucleation, III-N materials have been extensively studied to form thin films on vdW surfaces. Graphene and h-BN are widely used as vdW surfaces for vdW epitaxy of III-N, since the hexagonal in-plane lattice of these materials matches to wurtzite crystal structure of III-N materials, providing ideal platforms for vdW epitaxy. Also, these materials have high mechanical, chemical and thermal stability and demonstrate a viable window for processes under the growth conditions of IIIN [43, 44]. To circumvent the challenges caused by low surface free energy of vdW surfaces, various strategies have been proposed to demonstrate III-N thin film growths on conventional substrates coated with graphene or other 2D materials. One of the most widely employed approaches is to utilize vdW materials having defects or wrinkles, either pre-existing or intentionally formed, as the epitaxy template, so that the surface free energy is increased. Treating the surface of vdW materials with plasma is a direct way to deform the structure of vdW materials and increase the surface free energy. Various radicals have been studied to form defective graphene or h-BN by plasma treatment, including Ar, O2 , N2 , and NH3 [45–47]. With these plasma treatments, the 2D layer is partially damaged and decorated with dangling bonds, which can be confirmed by Raman spectroscopy [48], promoting nucleation of III-N. Although the III-N films grown with these plasma pre-treatment have been proven effective to promote nucleation and form high-quality III-N films on various substrates, such as sapphire, Si, and SiC [46, 47, 49], the cause of enhanced nucleation is complex. The damage of vdW materials, generated intentionally by plasma treatment, or unintentionally by transferring and high-temperature epitaxy conditions, can partially expose the substrate by creating nanoscopic holes. The exposed area can act as a nucleation site, which will indeed increase the nucleation density, while this is not vdW epitaxy anymore but selectivearea epitaxy, since the III-N nuclei basically grow from the substrate at the holes and converge laterally into a thin film. Many of the reports on III-N thin film epitaxy on vdW materials fail to provide enough evidence to identify the exact mechanism of thin film formation, which leads to confusions in the community and generates uncertainties for replication. Although a standard for verification is still missing, III-N thin film grown by vdW epitaxy should be easily peeled off from the vdW surface without roughening the substrate surface because of the weak interaction between the two. On the other hand, III-N thin films grown through the holes would be either stuck to the substrate or could be forcefully separated from the substrate but leaving roughened surface, because the III-N thin film and the substrate are basically connected through the damaged sites of vdW materials. Among the widely used substrates for III-N growth, SiC has a unique advantage for vdW epitaxy, in that graphene can be formed by graphitization of SiC substrates and naturally stays on top of the substrate surface without the need for graphene transfer processes [50]. Under high temperature, sublimation of Si occurs at the SiC surface, and this leads to the formation of graphene with controllable number of

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Fig. 5.4 a Low-magnification cross-sectional TEM image of GaN grown on a graphene/SiC substrate (scale bar, 1 nm) [29]. b HRTEM taken at the interface of GaN/graphene/SiC showing well-ordered and aligned crystal lattices of GaN/graphene/SiC (scale bar, 2 nm) [29]. c Crosssectional STEM image GaN grown on a graphene/SiO2 /Si substrate and corresponding HRTEM image showing clear crystal lattice of GaN and Si, respectively [58]

layers given the Si-face surface is used [51]. Compared to graphene on sapphire or Si, which is usually formed by transferring graphene, graphitization process does not induce contamination or transfer residue on the substrate, which is a great advantage to grow high-quality materials on top. However, the low nucleation density poses a great challenge to form converged thin films due to the low surface energy. Jeehwan Kim has demonstrated in his pioneering work on vdW epitaxy of III-N materials that vicinal steps of graphitized SiC facilitate nucleation of III-N [52]. The surface of SiC could be vicinalized at high temperature, forming the steps with enhanced surface energy and promoting nucleation. By carefully tuning the growth conditions for nucleation and growth, extremely smooth single-crystalline GaN film can be formed on graphitized SiC, as shown in Fig. 5.4a, with low density of threading dislocations around 1×109 cm−2 [29]. This process is ideal for practical applications, since the graphene coverage is not limited by transfer process, and the entire process can be performed in wafer-scale. The nucleation at the vicinal steps shows that the surface free energy on vdW surfaces is the key governing factor in vdW epitaxy. Another way of changing the surface free energy is by tuning the thickness of vdW layers, and the effect of the graphene thicknesses on the properties of GaN grown on top has been demonstrated on sapphire substrates with directly grown graphene [53] and transferred graphene [54]. VdW epitaxy of III-N films has also been demonstrated using other vdW materials. As an example, growing GaN films on h-BN has been shown as an effective way to alleviate the lattice mismatch and grow high-quality films, because the lattice mismatch between h-BN and GaN is smaller than that between sapphire and GaN [54, 55]. The growth of single-crystalline GaN and AlN films on TMDs has also been demonstrated possible [56, 57]. Another advantage of vdW epitaxy is that the growth can take place even on amorphous substrates. If sapphire or SiC wafers are replaced with low-cost amorphous substrates such as SiO2 /Si made by oxidizing silicon wafers, this can significantly bring down the cost of wafers for III-N device growth and fabrication. Since graphene lattice is hexagonal, which matches to (0001) plane of wurtzite-phase III-N materials, successful growths of single-crystalline III-N (0001) films are demonstrated as

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shown in Fig. 5.4b, with the material quality comparable to III-N directly grown on sapphire [58]. Therefore, these results suggest that vdW epitaxy is highly promising for bringing down the manufacturing cost of III-N devices with even better device performances, while it has to be emphasized that whether the seeding of III-N is initiated by vdW materials or the substrate materials needs to be carefully studied. As mentioned above, vdW epitaxy of III-As or III-P thin films is more challenging than III-N due to the low adsorption energy. To circumvent this issue, a special prelayer deposition process is devised to grow GaAs thin films by vdW epitaxy [32]. Since gallium has higher adsorption energy and lower migration energy than arsenic, graphene-coated Si (111) substrate is first terminated by gallium prelayer deposition, followed by GaAs growth with slow growth rate, and smooth surface with (111) oriented fiber texture is acquired. The material quality of GaAs layer on graphene assessed by XRD reveals that the crystal quality is better than GaAs directly grown on silicon with the same thickness, suggesting that graphene works as a buffer layer, but inferior to homoepitaxial GaAs film. Besides graphene, it is theoretically predicted that vdW epitaxy of GaAs films on h-BN or TMDs will be more problematic, as the adsorption energy of gallium and arsenic on these surfaces is smaller than that on graphene [59]. In summary, vdW epitaxy of thin films and nanostructures is promising in many aspects, including functionalities, broader choice of substrate materials, and the capability to peel the grown structure. However, controlling the nucleation is challenging primarily due to the low surface energy, and the growth is severely affected by the properties of vdW materials. Especially, the effect of imperfections in vdW materials, either intentionally or unintentionally formed during the preparation of growth platforms or during epitaxy on vdW surfaces, is not very well studied yet. Therefore, for vdW epitaxy to become more practical and mature, it is crucial to investigate further on the interplay between the adatoms and the vdW surfaces under various conditions, so that the growth mode can be better understood both theoretically and experimentally.

5.3.2 Remote Epitaxy of Thin Films and Nanostructures The vdW epitaxy of III-V materials on crystalline and amorphous substrates has shown that vdW materials can work as a seeding template for epitaxy. However, the seeding contributions from the substrate beneath the graphene had been neglected for a long time, while the seeding effect from the substrate can overwhelm that of the vdW materials at extremely close distances. It has been observed that the contact angle of a liquid droplet changes depending on the number of graphene layers coated on a substrate, as if the attraction from the substrate is gradually screened by increased thickness of graphene [60, 61]. These results are direct evidence at macroscopic scale that the interaction between the substrate and overlayer through 2D layers exists and should not be ignored.

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In vdW epitaxy, the growth is mostly governed by the vdW layer, but the effect of substrate on the crystallinity or material quality of as-grown materials was unclear until recently. As graphene has turned out to be partially wetting transparent and only partially screens the interaction from the substrate, one can speculate that the substrate could still affect the growth on top of the graphene layer during vdW epitaxy. If the interaction from the substrate is strong enough, the contribution from the substrate can dominate, and the crystallization of as-grown material may follow the substrate instead of the vdW material as the epitaxial template. This concept was termed “remote epitaxy”, indicating the remote interaction of substrate and epitaxial materials via vdW materials. The remote epitaxial growth was first demonstrated on III-As and III-P materials, and expanded to III-N, II-VI, I-VII, oxides and even metal [62–69]. In remote epitaxy, the atoms in epitaxial layer and the substrate interact through electrostatic potential, and the potential field can penetrate through the vdW layer to a certain degree, because electrostatic interaction is long-range in comparison to direct chemical bonding. Density-functional theory (DFT) calculations of graphenecoated material systems show that the electrostatic potential fluctuation, which is the difference between potential energy minimum and maximum on the surface of graphene above the substrate, varies significantly depending on the polarization of substrate materials. If the fluctuation is large, it means that the atoms arriving on the surface of graphene-coated substrate can be stabilized at energetically favorable potential minimum, while small fluctuation means there is higher chance for the atoms to impinge not on the potential minimum but on random location without following the substrate crystal structure. Therefore, the crystallography of grown materials will follow that of the substrate if the fluctuation is large, but the grown material will be polycrystalline if the fluctuation is small. For elemental materials, such as silicon and germanium, the field penetration is extremely weak and remote epitaxy does not occur, which is verified experimentally in that polycrystalline films are grown on graphene-coated substrates. On the other hand, the fluctuation of substrate field potential through graphene is large enough in III-V, allowing remote interaction through graphene. In 2017, the first demonstration of remote epitaxy was reported by Jeehwan Kim of MIT and his colleagues by showing that single-crystalline GaAs (100) can be grown on graphene-coated GaAs (100) substrate, having the lattice of epitaxial GaAs layer aligned to the substrate via graphene as shown in Fig. 5.5a and b [62]. The DFT calculations have revealed that the weak van der Waals potential of graphene does not completely screen the potential field of GaAs substrates, which enables epitaxial growth through graphene despite the presence of monolayer graphene. It was predicted that such remote epitaxial registry of adatoms can occur with the substrate–epilayer gap of up to 9 Å, which is larger than the thickness of a monolayer graphene. More detailed calculations have revealed that the distance between the GaAs substrate and remote epitaxial layer is 6.6 Å when monolayer graphene is inserted, while the gap exceeds 9 Å in the case of more than one graphene layers [63]. Therefore, remote epitaxy of GaAs can occur only from monolayer graphene, but not from bilayer graphene, and this

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Fig. 5.5 a High-resolution STEM images of GaAs (001) remote epitaxially grown on graphene/GaAs substrate. Convergent-beam electron diffraction patterns from the epilayer (top inset) and the substrate (bottom inset) show identical zinc-blende (001) orientations. b Low-angle annular dark field STEM image showing no dislocations. (c–e) EBSD maps of GaAs grown on 1-ML (c), 2-ML (d), and 4-ML (e) graphene-coated GaAs substrates [62]

is experimentally shown by the formation of polycrystalline GaAs film on GaAs substrates coated with bilayer and tetralayer graphene (Fig. 5.5c, d and e). The remote epitaxy of III-V thin films can be universally applied regardless of the substrate orientation, which is supported by the growth of GaAs (111) thin film on graphene-coated GaAs (111) substrate. Also, the generality of this approach is verified from the demonstration of remote epitaxy using other III-V material systems, for example InP (001) growth on graphene-coated InP (001) substrate and GaP (001) on graphene-coated GaP (001). What is more interesting is that it is possible to grow lattice-mismatched III-V materials on top of graphene-coated III-V substrate by remote heteroepitaxy, wherein the quality of materials grown by remote heteroepitaxy is even better than conventionally grown heteroepitaxial thin film without graphene interlayer [26]. This is because the graphene layer allows spontaneous relaxation of strain on graphene without forming dislocations, owing to the slippery graphene surface. DFT calculations showed that the threshold energy required for interface displacement on graphene is much smaller than the energy to form dislocation, and thus sliding of III-V lattice on graphene is energetically more favorable than misfit formation in the lattice-mismatched remote heteroepitaxy. This was experimentally verified by comparing Raman shifts and cross-sectional STEM of InGaP grown on bare GaAs substrate and graphene-coated GaAs substrate, wherein the degree of strain relaxation of InGaP film is much higher with the presence of graphene. As another example, the threading dislocation density of GaP thin film grown on GaAs is significantly reduced by inserting graphene, thereby suggesting that graphene

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Fig. 5.6 a DFT-modelled atomic structures of 2-ML (upper) and 3-ML (lower) graphene-coated GaN. b Maps of potential fluctuation on the same scale (0–25 meV) for 2-ML (upper) and 3-ML (lower). c EBSD of released GaN film grown on 2-ML (upper) and 3-ML (lower) graphene-coated surface. d SEM of as-grown GaN on 2-ML (upper) and 3-ML (lower) graphene-coated surface [63]

provides an alternative pathway of relaxing the strain and can be utilized for growing high-quality thin films with lattice-mismatched system. Compared with III-As/P materials, III-N is more strongly polarized [70] and theoretically, its substrate field potential can penetrate up to two layers of graphene (Fig. 5.6a and b). Experimentally, single-crystalline GaN film can be remote epitaxially grown on GaN substrates coated with monolayer graphene and bilayer graphene, while three-layer graphene coating resulted in polycrystalline growth of GaN, as shown in Fig. 5.6c and d, in excellent agreement with DFT calculations. It has been also shown that III-N nanostructures and microstructures can be grown by remote epitaxy [71]. Similarly to the case of remote heteroepitaxy of III-As/P, strain relaxation effect by graphene is also experimentally demonstrated by growing GaN on graphene-coated SiC substrates and the mechanism is studied by DFT calculations [72]. Unlike graphene, h-BN is a polar material and therefore the screening effect of substrate field penetration is more severe compared to the case of graphene interlayer. DFT calculations revealed that the substrate field penetration with the presence of monolayer h-BN is significantly weaker than the case of monolayer graphene. In fact, GaN grown on GaN substrate with monolayer h-BN coating resulted in domains corresponding to both the seeding from the GaN substrate and the h-BN, suggesting that the potential field from the substrate is partially screened, overlapping with the contributions from the h-BN. When the number of h-BN layers is increased to three layers, the substrate effect is completely screened and conventional vdW epitaxy

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occurred [63]. This is in contrast to the growth of GaN on three-layer graphenecoated GaN substrate, which resulted in partially (0001)-oriented polycrystalline film formation, because of the relatively weaker screening from graphene. Because the crystallographic information is transferred through layered vdW material, the quality of vdW interlayers is critically important in remote epitaxy. In wet transfer process, which is one of the most widely used processes for graphene transfer using graphene/copper foils, process-induced adsorbates can reside at the graphene surface and at transfer interfaces, which negatively affects the yield of remote epitaxy [73, 74]. On the other hand, it was shown that transferring graphene from graphitized SiC substrate onto GaAs substrate by dry transfer process ensures high-quality single-crystalline growth of GaAs remote epitaxial films, because residue formation is minimized in dry transfer process [62]. Ideally, directly growing graphene on III-V substrates will be the best approach to completely eliminate transfer defects and acquire highest quality of remote epitaxial thin films, similarly to the III-N growth on as-graphitized SiC, but growing graphene on III-V substrates could be challenging. In summary, remote epitaxy is a very powerful approach to grow single-crystalline III-V thin films on vdW materials. Unlike vdW epitaxy with weak adsorption energy between III-V and 2D materials, the fluctuation of substrate field potential that is penetrated through 2D layers provides effective nucleation sites for adatoms. Therefore, the remote epitaxy technique is a new way of forming single-crystalline III-V thin films that can be made freestanding, providing unique opportunities for heterointegration of III-V materials with other material systems. Also, 3D/2D/3D stacks of materials made by remote epitaxy could be a platform to study physical coupling of these material systems and enable completely new types of devices [75]. It has to be noted that the strain relaxation effect on slippery graphene shown in remote heteroepitaxy could be a pathway toward defect-free heterointegration of lattice-mismatched III-V materials as well as other dissimilar materials.

5.3.3 Epitaxy of LEDs on Van Der Waals Materials As vdW materials can help release the strain due to the slippery nature of graphene [26], LEDs grown on vdW materials have a potential to operate with higher efficiency than their conventional counterparts. Furthermore, nanostructure-based epitaxy on vdW materials can be utilized to make LEDs exploiting the unique geometry of those nanostructures. For example, their 3D geometry can greatly increase the light extraction efficiency, and core/multishell structure can realize efficient carrier injection in LEDs. The demonstration of LEDs on vdW materials is achieved by both vdW epitaxy and remote epitaxy. The first demonstration of III-N LEDs on vdW materials is achieved by transferring graphene onto sapphire substrate and treating the surface with oxygen plasma, followed by vdW epitaxy of LED structures [76]. In this approach, single-crystalline GaN film is grown by first epitaxially forming ZnO nanowalls on plasma-treated

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graphene. On top of the GaN buffer, InGaN MQW layer sandwiched in p-GaN and n-GaN layers is grown by MBE, and bright blue emission is observed by electrical injection at room temperature, demonstrating for the first time that vdW epitaxy of III-N can be employed for the fabrication of inorganic thin film LEDs. For practicality, wafer-scale processing is preferred, while this is difficult to achieve by transferring graphene from foreign substrates. Therefore, growing LEDs on wafers with grown graphene is a straightforward and practical approach for waferscale demonstration. In this regard, graphitized SiC substrate is the most ideal platform for wafer-scale epitaxy of III-N on graphene. LEDs on graphitized SiC are fabricated by using two-step growth of GaN on graphitized SiC as a buffer layer using MOCVD. Next, InGaN MQW layer is grown on top with GaN top and bottom cladding, and the device has shown bright blue light electroluminescence with the center wavelength at 440 nm, substantiating the potential for wafer-scale processes [29]. Besides graphene, h-BN is also studied to grow LEDs by vdW epitaxy. h-BN layer on sapphire can act as a buffer layer to grow high-quality GaN on highly latticemismatched sapphire substrate, and nucleation of III-N film on h-BN is relatively easier than on graphene, which could be due to the polarity of h-BN that promotes seeding. For the fabrication of blue LEDs, AlGaN or AlN is typically used as a seeding layer on h-BN, on which InGaN MQW layers are grown [77–79]. Also, the operation wavelengths of LEDs can be tuned to deep-UV regime by using AlGaN MQW as an active layer [80]. This UV-LED did not show any shift of peak wavelength when the injection current is significantly increased, suggesting that the superior thermal conductivity of h-BN works as a heat sink and enables efficient dissipation of heat on sapphire substrate that has very poor thermal conductivity. The same approach of growing LEDs by vdW epitaxy is also realized using TMDs on sapphire, revealing that various vdW materials can be used as an interlayer [81]. As each vdW material has different lattice constants, this could provide large degree of freedom in closely lattice-matching the III-N materials with 2D layers, as III-N materials can exhibit wide range of lattice constants depending on the composition in AlGaN or InGaN. The growth of thin film LEDs by vdW epitaxy is also demonstrated on amorphous substrates, proving its potential for LED fabrication on low-cost substrates. For example, InGaN MQW-based LEDs are grown on SiO2 substrate with multilayer graphene using AlN seeding, by pulsed sputtering deposition [82]. In this approach, by tuning the indium composition of InGaN MQW layer, full-color LEDs with blue, green, and red colors are respectively fabricated. Nanostructure-based III-N LEDs are also fabricated by remote epitaxy and vdW epitaxy, which could be a versatile platform to control the light and carriers in three dimensions. Nanowires, nanorods, or microrods, are typical examples of epitaxially formed III-N structures, and vertical growth of these structures have been demonstrated on various substrates [28, 30, 71, 83, 84]. Since these wires/rods grow both vertically and laterally as they elongate, the heterostructure naturally forms core/shell type of structures, and based on this approach, MQW active area are inserted as multi-shell in these vertical nanostructure arrays. Making top metal contacts is not as straightforward as thin film devices due to their unique geometry, and dielectric

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Fig. 5.7 a Cross-sectional SEM image of heterojunction LEDs. b I-V curves of LEDs grown on graphene-GaAs substrates and directly on GaAs. Inset, emitted red light from the LEDs grown on the graphene-GaAs substrate. c Electroluminescence spectra of the LEDs grown on graphene-GaAs substrates and directly on GaAs, Inset, photographs of functioning LEDs grown on both substrates. d I-V curves of LEDs before and after transfer. e Light emission of LEDs before and after transfer [62]

spacer materials are normally deposited or coated to prevent the top metal contact from touching the substrate when fabricating LEDs. For III-As and III-P materials, the demonstration of LEDs was reported only by remote epitaxy so far. Red LEDs with AlGaInP-GaInP double heterojunction structures are grown on graphene-coated GaAs substrate by remote epitaxy, as shown in Fig. 5.7a [62]. The I-V characteristics of LEDs were comparable to the LEDs directly grown on GaAs without graphene (Fig. 5.7b), showing similar turn-on voltage. Electroluminescence spectra of the remote epitaxially grown LEDs in Fig. 5.7c also showed nearly identical spectral linewidth and emission intensity compared with directly grown LEDs. Besides remote epitaxy-based red LEDs, there is no report on vdW epitaxy-based LEDs employing III-As/P materials, which is attributed to the challenges of growing thin films of III-As/P by vdW epitaxy. Although growing IIIAs/P based thin film LEDs by vdW epitaxy will be challenging, it could be feasible to grow nanostructure-based LEDs by vdW epitaxy. In summary, vdW epitaxy and remote epitaxy have been successful to demonstrate III-V LEDs with their operation wavelengths spanning wide ranges from deep-UV to visible wavelength, on various crystalline and amorphous substrates coated with many kinds of vdW materials. The approach using vdW materials as interlayers could have advantages over conventional approach in terms of better material quality and efficient heat dissipation, not to mention the capability to exfoliate that will be discussed in the next section.

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5.3.4 Layer Transfer of LEDs The demand for advanced functional light sources which are made flexible, stretchable or curved is rapidly increasing in diverse fields, including displays, sensing, military, and automotive. Also, heterointegration of light sources and other (opto)electronic components with soft matters is gaining huge attention in biointegrated chips. As III-V layers grown on vdW materials can be easily exfoliated and transferred due to the weak interface between the vdW material and III-V layer, III-V devices grown by vdW epitaxy or remote epitaxy have huge advantages for heterointegration, since there is no need for complex lift-off processes like chemical lift-off and laser lift-off for layer transfer. The first demonstration of the exfoliation of III-N LEDs was achieved by vdW epitaxy of III-N MQW structures on a graphene-coated sapphire substrate [76]. After deposition of Ni/Au contacts, the blue LEDs are transferred onto foreign substrates such as metal, glass and plastic (Fig. 5.8a). The performance of LEDs did not show degradation after transferring onto foreign substrates, confirming the reliability of layer transfer process utilizing vdW interface. Also, the transfer onto plastic substrates unambiguously shows that this process can be applied for flexible optoelectronics. The layer transfer of LEDs grown from epitaxial graphene was also demonstrated, supporting the scalability for wafer-scale processing [29]. The InGaN MQW LEDs directly operated on the tape after exfoliation, with a bright blue emission at 440 nm. After the layer transfer of LEDs, the SiC substrate was reused multiple cycles for repeated growths of III-N films and exfoliation. Therefore, this approach has a direct impact on bringing down the manufacturing cost of III-N LEDs wherein the substrate cost takes huge portion. Fully wafer-scale processing is achieved by directly growing a few nanometer-thick h-BN layer on a 2-inch sapphire

Fig. 5.8 a Optical images of light emissions from the as-fabricated LED on the original substrate and transferred LEDs on the foreign metal, glass, and plastic substrates [76]. b Schematic of nanowire LED fabrication and transfer onto a flexible plastic substrate (left). Light emission photographs at bending radii of 5.5 and 3.9 mm (right) [30]

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wafer, followed by MOCVD growth of InGaN MQW LEDs and exfoliation using an aluminum or copper foil with an acrylic conductive adhesive layer [79]. However, the EL intensity of blue emission has degraded after exfoliation due to the crack formation, which is attributed to the sensitivity of transfer processing. The III-N LEDs fabricated by growing nano- or microstructures are also an effective platform for flexible LEDs, since light extraction could be made more efficient and the active medium does not easily form cracks. Several types of core/shell structures have been grown on graphene-coated substrates by both remote epitaxy and vdW epitaxy, and transferred onto flexible substrates [30, 71, 83, 85]. Because these structures are not merged, the voids between the structures are filled with dielectric materials to form metal contact and to conduct exfoliation. Due to their unique geometry, 3D structure-based LEDs could be more robust to bending or stretching compared with thin film-based LEDs, especially if the backfilled dielectric material has flexibility. For example, the optical performance of the blue LEDs was unchanged after the transferred LEDs experience repeated bending cycles of hundreds or even thousands, with a very small bending radius of a few millimeters (Fig. 5.8b) [30, 85], providing that this could be a promising platform for flexible optoelectronic applications. Similarly to the case of III-N on 2D materials, the interface between the IIIAs/P and 2D materials form weak bonding, which has enabled mechanical exfoliation of thin films precisely at the interface. The red LEDs based on remote epitaxy of AlGaInP-GaInP on graphene-coated GaAs substrates are exfoliated by first depositing Ni stressor, and then attaching thermal release tape. Simple mechanical exfoliation of LEDs was successful at the graphene interface, and the exfoliated layer is transferred onto silicon substrate, followed by removal of the tape and metal stressor. After transfer onto silicon, the I-V characteristics have shown slight degradation of current, as shown in Fig. 5.7d, while bright electroluminescence of red light is still observed as in Fig. 5.7e, confirming that the transfer processes does not induce significant degradation of the device performance. In GaAs-based IIIV LEDs, epitaxial lift-off using sacrificial AlAs layer has been typically used to make freestanding LEDs and transfer onto foreign substrates [14, 16]. However, these processes are very slow, taking hours or even days [15], and the substrate is roughened and contaminated with byproducts by wet etching, necessitating surface refurbishing process such as chemical–mechanical polishing (CMP) to reuse the substrate. Therefore, remote epitaxy has advantages over conventional layer transfer processes, in terms of high throughput and cost saving by substrate reuse without the need for refurbishing. In organic electronics and optoelectronics, vdW interfaces have been already widely employed for both exfoliation and integration, as many organic materials possess vdW surfaces. Because of this, organic LEDs have been actively investigated for flexible chemical sensing platforms, implantable devices, biointegrated chips, and displays. In this regard, one of the important implications of vdW material-based layer transfer techniques is that III-V LEDs with higher efficiency and reliability can be utilized in these fields, and realize advanced functionalities and performances for future optoelectronic platforms.

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5.4 Emerging Materials-Based LEDs on Van Der Waals Materials 5.4.1 TMDs-Based LEDs III-V families are traditional inorganic semiconductor materials that have been extensively studied for decades in LED industry. Recently, TMDs are emerging as a new choice of material that can be employed as active media for LEDs, which could be the path toward the thinnest possible LEDs with a few-nanometer thickness. TMDs are typically indirect bandgap materials in their bulk forms, but when the thickness is reduced down to monolayer scale, they transition to direct bandgap [86], and thus the radiative quantum efficiency is greatly improved. H-BN, which is an insulating vdW material, can be utilized as a barrier or tunnel junction of TMD-based LEDs, and graphene, which is a vdW material without bandgap, or transition metal monochalcogenides (TMMs), which are metallic, can work as ideal electrical contacts with clean interface. Van der Waals epitaxy of TMDs was first reported in 1991 by growing MoSe2 on vdW surfaces such as SnS2 and MoS2 [87], and soon followed by TMMs [88], which regained attention recently. One of the biggest challenges is that 2D materials typically grow as triangular flakes, and it is difficult to achieve monolayer or a few-layers with uniform thickness over large area, making the formation of vertical heterostructures with large sizes challenging. Also, grain boundaries are formed when these flakes merge during the growth, which degrades electrical and optical properties of 2D layers. Despite such challenges, the growths of vertical heterostructures with different bandgaps and dopings have been demonstrated by careful control of growth conditions and surface treatment [87–89]. For the fabrication of LEDs using 2D materials, layer transfer and stacking of 2D materials from bulk are still dominating compared with vdW epitaxy [90]. This is primarily because the material quality attainable by transferring from bulk is superior to directly grown 2D materials, since the growth of 2D layers typically induces incorporation of impurities and grain boundaries [91]. In transferred 2D layers, on the other hand, the material quality of 2D layers can be degraded during transfer processes due to tearing, folding, or residue formation, and controlling the number of transferred layers is challenging. In 2018, Jeehwan Kim and his colleagues have demonstrated a method for layer-resolved splitting of vdW materials, such as h-BN and TMDs [92]. In this quasi-dry transfer process, by controlling the propagation of crack using a metal stressor and engineered interfacial toughness, wafer-scale transfer of monolayer TMDs is achieved repeatedly to form vertical vdW heterostructures, which can be an ideal approach for wafer-scale fabrication of high-performance 2D material-based devices. 2D vdW materials-based LEDs have been realized in many forms. The simplest form will be a monolayer TMDs with two metal contacts, from which electroluminescence was observed at visible wavelengths (Fig. 5.9a) [93]. For efficient operation of LEDs, the active medium is sandwiched by cladding materials with higher bandgap

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Fig. 5.9 a False-color image of EL emission from MoS2 in the vicinity of a contact edge [93]. b Schematic (left) and optical image (right) of vdW heterostructure LED embedded inside a microcavity. Scale bar, 10 nm [94]

for carrier confinement, such as h-BN, resulting in bright room-temperature operation (Fig. 5.9b) [94, 95]. Furthermore, the large binding energy of excitons, strong spin–orbit interaction and valley degree of freedom in TMDs are ideal for studying valleytronics [96], and these features have enabled unconventional characteristics of LEDs such as electrically switchable valley-polarization [97]. 2D TMDs have also been utilized as quantum emitters, by exploiting intrinsic impurities [98] or by extrinsically applying strain [99], which could be a unique platform for ultrathin and position-controlled quantum emitters. Lasing has been achieved based on monolayer TMDs, which is an evidence of superior optical properties of TMDs. Due to the atomically thin nature of 2D vdW materials and robustness, these 2D layers can be suspended on patterned substrates, and have marginal effect on the optical field profiles of underlying 3D structures, thus making it possible to transfer TMDs on resonators and photonic slab nanocavities to achieve lasing by optical pumping [100, 101]. These results exemplify that vdW materials-based LEDs and emitters could be a versatile and highly functional platform for both practical applications and tools for studying nanophotonic and quantum optical phenomena.

5.4.2 Perovskite LEDs Perovskites are recently emerging as a promising material system, both in 3D and 2D forms, in optoelectronic applications, due to their long carrier diffusion lengths, tunable bandgap, and relatively low-cost production. The general chemical formula for bulk 3D perovskite compounds is ABX3 , where ‘A’ can be inorganic or organic cations, ‘B’ is other metal cations, and ‘X’ is a halide anion, with a covalent bonding [102]. 2D perovskites, on the other hand, generally have a form of A2 BX4 , with a vdW surface [103]. Similarly to III-V compound semiconductors with ionicity, single-crystalline perovskite thin films can be grown on vdW surface by either vdW epitaxy or remote epitaxy, as demonstrated on graphene (Fig. 5.10a and b) [104, 105]. Exfoliation of perovskites thin films on graphene is also shown, by remote epitaxially growing perovskites on graphene-coated NaCl (001) and CaF2 (001) substrates. Compared

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Fig. 5.10 a SEM image of remote epitaxial CsPbBr3 flakes on Gr/NaCl [104]. b Cross-sectional SEM image of the remote epitaxial CsPbBr3 thin film on Gr/NaCl [104]. c TRPL of CsPbBr3 films in remote and ionic epitaxy, showing improved carrier lifetime by remote epitaxy [104]. d EL spectra of NFPI7 and NFPI6 B perovskite MQW LEDs under 3 V. Inset, photograph of a 64 mm2 NFPI6 B device [110]

with ionic epitaxy on these substrates without graphene, the nucleation density in remote epitaxy was lower. By remote epitaxy on graphene, lower dislocation density and improved carrier lifetime of perovskite films are achieved, as shown in Fig. 5.10c, indicating that graphene not only is semi-transparent allowing substrate field penetration but allows strain relaxation, which agrees with the remote heteroepitaxy of III-V compound semiconductor [26, 104]. The first room-temperature LEDs based on bulk perovskite with the form of ABX3 is demonstrated in 2014, operating in infrared, red, and green wavelengths, by solution processing [106]. Since then, the external quantum efficiency of perovskite LEDs has been greatly improved to > 20% by engineering the material characteristics, band alignments, passivation, and metal contacts (Fig. 5.10d) [107, 108]. 2D layered perovskites have vdW surface with no dangling bonds, and thus their optical and electrical properties are not degraded by surface states. The first demonstration of LEDs based on layered perovskites was realized from multilayer forms, operating only at cryogenic temperatures [109], and since then, the efficiency has been greatly enhanced by employing heterostructures of 2D [110] or quasi-2D [111] layered structures. The growth of atomically thin perovskite sheets has also been demonstrated on SiO2 /Si substrate with their bandgap tunable in entire visible wavelengths [112]. Perovskites, in both 3D and 2D forms, are getting huge attention in the community and regarded as one of the most promising materials for future optoelectronic devices, especially for emitters and solar cells. There are still many issues that need to be addressed, such as the stability of materials and toxicity of lead-containing perovskites. Improving the durability of perovskite devices and developing lead-free perovskites are therefore crucial and under active investigation. Perovskites can be grown by vdW epitaxy and remote epitaxy, which provides an additional degree of freedom in improving the material quality, choice of growth substrates, and capability for layer transfer.

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5.5 Conclusion and Perspective In this chapter, the growth techniques for LEDs on vdW materials are extensively discussed, mainly on van der Waals epitaxy and remote epitaxy. Compared with conventionally utilized substrates, the growth on vdW materials poses many advantages but also challenges. In terms of material properties, vdW surfaces can act as a buffer or a slippery plane, which make it possible to grow high-quality materials on lattice-mismatched substrates or even on amorphous substrates. In terms of LEDs grown on vdW materials, vdW materials can work as a heat sink, and more importantly, can work as a release layer since the binding energy between the vdW materials and LEDs is weak. This enables simple mechanical exfoliation of LEDs precisely at the vdW interface, and the capability for layer transfer not only provides additional functionalities of LEDs by integrating onto foreign substrates but facilitates reuse of substrates after the transfer which are often very expensive. Van der epitaxy and remote epitaxy are still in early stages of development and need further progress to be implemented in commercial LEDs, for both conventional III-V materials and emerging materials such as TMDs and perovskites. One of the biggest challenges is the reliability and quality of materials grown on vdW surfaces. Both the growth process and the growth template need to be extensively studied to overcome the challenge. First, for the growth process, vdW surfaces inevitably exhibit low surface free energy, making the nucleation difficult to control. Modifying the surface free energy by surface treatment in vdW epitaxy and promoting the nucleation by substrate field penetration in remote epitaxy are being studied based on many unique and creative approaches, but more fundamental understanding of the interaction between the substrate, vdW materials, and adatoms of growth materials by theoretical study and experimental verification is necessary. Experimentally, the nucleation and growth conditions need to be carefully studied taking into account the reactivity and stability of underlying vdW materials. Second, for the growth template, the quality of vdW materials for vdW epitaxy and remote epitaxy needs to be superior and well-controlled. Currently, transfer processes are dominantly used to form 2D layer-coated substrates, except for specific substrates capable of growing high-quality graphene. However, transfer processes typically lead to the formation of wrinkles and residue, which deteriorates the quality of epitaxially grown materials atop. Also, the size of the vdW materials transferred onto growth platforms from foreign substrates is currently limited to relatively small sizes, which hinders largescale fabrications. In this regard, wafer-scale layer transfer or direct growth of 2D materials will be the viable path for practical applications. In summary, van der Waals epitaxy and remote epitaxy are attractive techniques to form freestanding inorganic LEDs which could be made curved, bendable, stretchable, and integrable to foreign platforms. There have been growing interests for such functionalities in many application fields, such as augmented reality, surveillance, bio-integrated chips, self-driving cars, and biochemical sensing platforms. Therefore, the epitaxy techniques introduced in this chapter for the fabrication of LEDs

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on vdW materials could be the cornerstone for next-generation LEDs and integrated optoelectronic/photonic systems.

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Chapter 6

Implantable LED for Optogenetics Yiyuan Yang, Abraham Vázquez-Guardado, and John A. Rogers

6.1 Introduction Transmembrane ion fluxes represent fundamental cellular activities that establish sustained homeostasis in all living organisms [1]. The biological components that give rise to these fluxes include several classes of functional receptors, anchored at the cell membranes, that serve as the transaction mediator for ions, neurotransmitters, hormones, and a broad range of other biomolecules, all of which participate in dynamic intracellular signaling. For example, the binding of insulin to insulin receptors on the membranes of cells regulates the intake of glucose into the cell cytoplasm [2]. The release and binding of neurotransmitters at the neurons synaptic cleft contributes to downstream action potential propagation that results in muscular motion control, the processing of sensory stimuli or even the consolidation of memory [3, 4]. In this context, artificial methods that trigger the disruption of transmembrane flux activities on-demand and in-vivo represent current efforts in biology research,

Yiyuan Yang and Abraham Vázquez-Guardado contributed equally. Y. Yang · J. A. Rogers (B) Department of Mechanical Engineering, Northwestern University, Evanston, IL, USA e-mail: [email protected] A. Vázquez-Guardado · J. A. Rogers Center for Bio-Integrated Electronics, Northwestern University, Evanston, IL, USA J. A. Rogers Department of Materials Science and Engineering, Northwestern University, Evanston, IL, USA Departments of Biomedical Engineering, Chemistry, Electrical Engineering and Computer Science, Northwestern University, Evanston, IL, USA Department of Neurological Surgery, Feinberg School of Medicine, Querrey Simpson Institute for Bioelectronics, Northwestern University, Chicago, IL, USA © Springer Nature Singapore Pte Ltd. 2021 J.-H. Ahn and J.-H. Kim (eds.), Micro Light Emitting Diode: Fabrication and Devices, Series in Display Science and Technology, https://doi.org/10.1007/978-981-16-5505-0_6

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the consequences of which guide an understanding of basic cellular behaviors as well as the complexity of vital functions across all living organisms [5–7]. Such forms of stimulation can modulate several types of cellular functions, in the form of activation or inhibition of processes to disrupt or stabilize the natural state of the biological system. For instance, the functional control of neurons allows the study of the nervous system, where basic neuronal activity patterns lead to complex behaviors, or abnormal function defines pathologies [8, 9]. Techniques for neuromodulation rely on different aspects of electrical [10, 11], pharmacological [12, 13] and/or optical [14–16] mechanisms. Electrical stimulation enables the modulation of neuronal activity at localized brain regions or across large cortical areas at several depths, with high spatial and temporal resolution [10, 17]. In spite of a long history of use in research and in therapies, such strategies fundamentally lack cell specificity and are unable to differentially apply to co-located genetically heterogeneous cell populations [18, 19]. In contrast, pharmacological stimulation employs the endogenous receptors, such as the G-protein-coupled receptors, on the cells as a way to gain access and control of their intracellular processes [13, 20]. The administration of drugs, however, typically lacks spatiotemporal control due to uncontrolled diffusion into surrounding tissues or into the body as a whole. Recent strategies based on synthetic biochemistry allow for the implementation of light-activated drugs with increased spatiotemporal control [21, 22]. Here, upon light absorption, typically in the ultraviolet spectrum, otherwise inert drugs become active toward the target receptor, such that the illumination volume defines a region of activity. Although these stimulating mechanisms enable cell specificity, with some degree of spatial control, the response times are often insufficient for real time modulation of neuronal patterns. Other means for pharmacological stimulation use designer receptors exclusively activated by designer drugs (DREADDs) to achieve a high degree of cell localization with activation that does not interfere with endogenous biochemical signaling [23, 24]. In this way, a genetic line of cells can be targeted, although at the expense of long activation or deactivation kinetics. These limitations motivate the development of alternative means for neuromodulation that are capable of real time manipulation of cell activities with precise genetic and spatiotemporal control. Optogenetics is a prominent example, which provides optical control at time scales that permit biologically relevant gain or loss of functions that manifest macroscopically in wide ranging types of biological systems [25, 26]. This chapter covers recent optical technologies that form the basis for frontier neuroscience research using optogenetic method in animal models. This chapter starts with the fundamentals and motivations for the use of optogenetics in-vivo. Then, it discusses recently developed tools used for light delivery to target tissues, followed by representative examples in neuroscience. It then introduces wireless battery-free, fully implantable, optogentics devices, in which we discuss wireless power transfer mechanisms that enable battery-free operation, passive device configurations and their practical limitations. It continues with the discussion of emergent classes of active battery-free optoelectronic devices that find applications in behavioral studies involving active, real-time controls of central and peripheral nervous system activities and organ functions. Finally, we include recent trends in hybrid multimodal device constructs which are designed to exploit

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other modes of stimulation and sensing such as drug delivery, photometry and tissue oximetry.

6.2 In-Vivo Optogenetics Applications Optogenetics is a versatile method that has gained tremendous momentum in the last decade due to key advantages over other neuromodulation approaches, especially millisecond time responses [26], commensurate with the dynamics of neuronal cells themselves, and technologies with capabilities for fine spatiotemporal control [27]. The method uses light sensitive 7-transmembrane type-I microbial opsins as single component biological actuators, where light absorption and ion actuation processes occur with the same protein [25, 28]. The use of genetic tools and the abundant presence of retinoids, the natural anchoring compounds for opsins, in the cells of vertebrate species, allow for the effective transfection across wide varieties of neurons, and other cells [29]. Upon light absorption, these proteins participate in ion transactions (ion pumps) or sustained ion fluxes (ion channels) across the cell membrane, thereby disturbing its resting potential (~–70 mV). This process results in depolarization, and increases the potential to positive values, as the result of cations influx into, or anions efflux out of, the cell [30, 31]. Conversely, for hyperpolarization, the potential becomes more negative than –70 mV, due to cations efflux out of, or anions influx into, the cell [15, 32]. These two processes produce stimulation or inhibition of neuronal activity, respectively, each of which is of fundamental importance in neuromodulation. Currently dozens of opsins are available, and in widespread use in optogenetics, categorized into four groups based on their conductance properties [25]. The most studied class is Bacteriorhodopsins (BR)—an haloarchaeal proton pump abundant in different types of marine protobacteria [33]. These opsins, in their natural state and under oxygen deprived conditions, work as alternative energy production mechanisms by pumping protons out of the cytoplasm to the extracellular matrix, see Fig. 6.1, which in turn generates proton-motive forces that mediate adenosine triphosphate (ATP) synthesis. The conductance kinetics of these opsins involves pumping of protons (H+ ) upon light absorption during the opsin photocycle, typically in the wavelength range of 540 nm and with 19 ms deactivation time, which leads to a loss of function associated with cellular hyperpolarization. Proteorhodopsins (PR) follow under the same proton pump category, are optically active at 490 nm (blue absorbing variant) and 525 nm (green absorbing variant), and have a ~10–800 ms activation cycle [34, 35]. The next class of microbial opsins is from halorhodopsins (HR) derived from archaebacterial [36, 37]. HRs are chloride channels (Cl− ) and, in contrast with BRs, produce a constant Cl− influx into the cytoplasm upon light absorption, which leads to hyperpolarization and consequent inhibition, as shown in Fig. 6.1. The peak absorption wavelength is 590 nm and the deactivation time is 4.2 ms. The third, and most commonly class of opsins used in neuroscience and other biological applications, is channelrhodopsin (ChR) derived from Chlamydomonas

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Fig. 6.1 Principles of optogenetics. Optical actuators, such as genetically engineered microbial opsins expressed in the membranes of neurons, enable bidirectional transmembrane signaling in the form of ion fluxes upon light absorption. This process produces gain or loss of neuronal functions observed in measurable behavioral outcomes. Channelrhodopsin (ChR) produces sodium (Na+ ) influx when excited at 470 nm. Halorhodopsin (NpHR) produces chloride (Cl− ) influx when excited at 590 nm. Bacteriorhodopsins/Proteorhodopsins (BR/PR) produces protons (H+ ) outflux when excited at 540 nm or 490–525 nm, respectively. OptoXR, chimeras formed by the covalent binding of rhodopsin and G-protein coupled receptors (GPCR) modulates neurotransmitter fluxes when excited at 500 nm. IP3: Inosital trisphosphate. Gq, Gs, and Gi: G protein alpha subunit. DAG: Diacylglycerol. cAMP: cyclic adenosine monophosphate

reinhardtii, as shown in Fig. 6.1. ChRs are sodium cation (Na+ ) channels that produce a continuous Na+ influx into the cytoplasm to produce depolarization, which results in stimulation [26, 28, 38]. Incorporating mammalian codons into ChR to replace algal codons leads to an improved variant with high expression efficiency in mammals (ChR2). The peak excitation wavelength is 470 nm and the deactivation time is 10 ms. These optically active proteins diversify to a wide range of opsins that vary in ion conductance, excitation wavelength, and response time [25, 32]. Another family of optically active elements modulates intracellular signaling, not in the form of changes in transmembrane ion conductance, but in the form of biochemical signaling based on neurotransmitters, hormones, etc. These compounds employ endogenous G-protein coupled-receptors (GPCR) as natural pathways for transmembrane biochemical signalling [13, 20, 39]. In addition, they use type II opsins, such as those found in the retinas of mammalian eyes, which are also a type of GPCR. The covalent binding of rhodopsin to GPCR, also known as OptoXR (Fig. 6.1), forms an emergent class of light activated GPCR with demonstrated applications in the modulation of transmembrane dopaminergic, serotonergic, adrenergic fluxes, and even faster neurotransmitters such as glutamate and GABA [40]. OptoXRs are typically excited at a wavelength of 500 nm with deactivation times of 0.5–16 s. Examples of OptoXRs are those coupled to Gq, Gs and Gi GPCR subtypes that modulate excitatory or inhibitory pathways, see Fig. 6.1.

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Fig. 6.2 In-vivo optogenetics applications. The genetic tools to express opsins in different types of cells and the implantable devices to deliver light to different locations of the body serve as the basis for the use of optogenetics to modulate different types of cellular activities in-vivo. Recent demonstrations involve modulation of the central nervous system (brain and spinal cord), peripheral nervous system (sciatic nerve) and organs (heart, bladder, and colon), quantitated in the form of behavioral and/or physiological observables in free moving animal models

Optogenetics is of particular interest for its capabilities for modulating cell activity in-vivo and in free behaving animal models [41]. The versatility in exploiting opsin expression in cells other than those in the brain, together with development of advanced technologies for delivery of light to biological systems, expands considerably the scope of optogenetics applications within and beyond neuroscience research [42–44]. Examples (Fig. 6.2) include modulation of spinal cord [45] and peripheral nerve activity [46], cardiac cycles [47], colonic motility [48] and bladder function [49], where responses can be studied through consequent macroscopic physiological observables and change in behavior. Emerging schemes for light delivery allow spatiotemporal control, precision in intensity, timing and wavelength, in miniaturized platforms that can be fully implanted for wireless control and battery free operation [27, 50]. Overall, the synergistic integration of genetics and cell biology with sophisticated, biocompatible optical interfaces suggests a promising future and further expanded scope of optogenetics as a methodology for neuroscience, medicine, and biology research.

6.3 Tools for Light Delivery The use of optogenetics in live animal studies requires device platforms for delivering light at specific wavelengths, with precise control over spatial and temporal characteristics at locations of interest, with minimal tissue trauma and with chronic stability in operation. A simple, yet widely used method relies on standard optical fibers adopted from the telecommunication industry as waveguides to deliver light

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from external sources to target cell populations [51]. Recent technology advances in the form of tapered geometries at the tips of the fibers improve the stimulation quality by increasing the illumination volume and resolution [52–54]. The geometrical taper naturally allows for mode-division demultiplexing (MDD) to control the light emission location. Here, high optical waveguide orders escape at large fiber taper diameters, whereas low waveguide orders escape at small diameters. The waveguide order is determined by the excitation angle at the front-end of the fiber. In addition, milled scattering windows in opaque coatings on these fibers further diversify the illumination profile. Figure 6.3a shows a representative tapered fiber (TF) that allows multi-site optical stimulation through seven emission windows (~20 μm) [52]. Flat silicon shanks (~90 μm wide, ~18 μm thick, and 3 or 5 mm long) integrated with lithographically defined photonic waveguides serve as alternative methods for light delivery shown in Fig. 6.3b [55]. Here, wavelength division demultiplexing (WDD) implemented on a dispersion-engineered arrayed waveguide gratings (AWG) allows selective separation of light with different wavelengths to designated waveguides for a

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Fig. 6.3 Tools for light delivery. a Tapered optical fiber (125 μm base diameter and 3–6º taper angle) allows mode-division demultiplexing (MDD) to control the emission of light through seven milled scattering windows (~20 μm). Subset: zoomed image of scattering window. b Flat silicon shanks (~90 μm wide, ~18 μm thick, and 3 or 5 mm long) with lithographically defined photonic waveguides that implement wavelength division demultiplexing (WDD) selectively deliver light, at different wavelengths, through multiple emitter pixels (E-pixels). AWG: arrayed waveguide gratings. c Silicon shanks (100 μm wide, 40 μm thick, and 3 or 6 mm long) with integrated array of 16 microscale inorganic light emitting diodes (μ-ILEDs, 25 μm diameter) serve as localized illumination sources powered by external electrical sources connected to contact pads located at the back end. d A soft optogenetic neural probe (~20 μm thick) with four μ-ILEDs (50 × 50 μm2 , 6.45 μm thick) mounted on a flexible thin polyester filament with lithographically defined metal traces for external electrical connections allows for chronic operation in the deep brain. Panel a reproduced with permission from ref. [52]; Copyright (2014) Elsevier. Panel b reproduced with permission from ref. [55]; Copyright (2016) Society of Photo-Optical Instrumentation Engineers (SPIE). Panel c reproduced with permission from ref. [58]; Copyright (2016) Springer Nature

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delivery to emitter pixels (E-pixels) as localized optical stimulators. Further extensions of this device combine probes into three-dimensional arrays to enable optical stimulation of neuronal activity across broad brain circuits [56]. The emergence of micro-scale inorganic light emitting diodes (μ-ILEDs) revolutionizes light delivery techniques and relevant applications. The dimensions of these μ-LEDs can approach the sizes of individual neurons or even smaller, to allow strategies for localized illumination at the sites of interest without external tethered connections for waveguides [57]. Figure 6.3c shows a representative scheme of a silicon shank (100 μm wide, 40 μm thick, and 3 or 6 mm long) integrated with an array of 16 μ-ILEDs that are individually powered using external electrical sources [58]. Each μ-ILED (25 μm diameter) controls the activation of corresponding neurons. One main disadvantage of this method, as with the waveguides, is the large modulus mismatch at the interface between the rigid probes (~150 GPa for silicon) and soft neural tissues (~0.1–10 kPa) [43]. Such differences cause tissue injury that leads to inflammation and foreign body responses resulting from micromotions, especially in applications that require chronic implantation. Substituting rigid glass waveguides and silicon shanks with compliant substrate materials can reduce this tissue disruption [59]. Figure 6.3d shows an example of this type of soft neural probe [60]. This device uses a thin flexible polyester filament (~6 μm thick) as a substrate and four μ-ILEDs (50 × 50 μm2 , 6.45 μm thick) mounted against lithographically defined thin metal traces with reduced bending stiffness.

6.4 Representative Optogenetic Studies in-Vivo Behavioral studies consist in the observation of induced or suppressed behaviors invivo that result from causal optogenetic excitation or inhibition of the target neuron cell population. Waveguides and μ-ILED probes are common light delivery tools that offer controlled optical stimulation at selected brain regions. Figure 6.4a–c provide a neuroscience research example that employs optical stimulation over different brain regions [53]. In this context, a TF implanted across the dorsal and ventral medial striatum of adult mice expressing ChR2 in indirect pathway striatal projection neurons (iSPNs) provide multi-site illumination (Fig. 6.4a). Selective optogenetic stimulation over iSPNs in the ventral and dorsal striatum can be achieved by adjusting the coupling angles of waveguides (θ1 = 80 ventral, θ2 = 22.50 dorsal). These experiments explore the effects of iSPN activity in these regions on locomotion by sequentially and repetitively delivering 3-min optogenetic stimulation after a 3-min non-stimulation period (Fig. 6.4b). Stimulation in both brain regions results in contraversive spinning and reduced locomotion, while stimulation in ventral striatum has an enhanced effect as shown by the average travelling distance in 3-min period (Fig. 6.4c). Another neuroscience research example that uses soft neural probes with integrated μ-ILEDs as localized illumination sources is summarized in Fig. 6.4d–f [60] In this context, the neural probe, powered by a head-mounted module, implants

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Fig. 6.4 Representative optogenetic studies in-vivo. a–c A representative optogenetic neuroscience study that uses tapered optical fiber (TF) for stimulation over different brain regions. a Schematic illustration of an implanted TF that offers multi-site optogenetic stimulations of indirect pathway striatal projection neurons (iSPNs) across ventral and dorsal striatum through waveguides coupled at 80 (θ1 ) and 22.50 (θ2 ) respectively. b Illumination protocol used for this experiment. c Average traveling distances among 3-min period with respect to optogenetic stimulations at ventral (θ1 ) and dorsal (θ2 ) striatum compared to non-stimulated (NS) scenario. d–f A representative optogenetic neuroscience study that uses soft neural probe with integrated micro-inorganic light emitting diode (μ-ILED) for localized brain stimulation of ventral tegmental area (VTA)— dopaminergic (DA) neurons. d Photograph of implanted mouse. e Heat map of mice’s movement inside a Y-maze where only one side of the maze activates VTA-DA neurons. Left: mice expressing channelrhodopsin (ChR2). Right: mice expressing fluorescence protein (eYFP) for control experiments. f Time difference that represents the mice’s place preference towards the Y-maze branch with optical stimulation. Panels a–c reproduced with permission from ref. [53]; Copyright (2017) Springer Nature. Panels d–f reproduced with permission from ref. [60]; Copyright (2013) American Association for the Advancement of Science

into the ventral tegmental area (VTA) to activate VTA-dopaminergic (DA) neurons (Fig. 6.4d). The behavioral paradigm focuses on stimulation of these neuron populations at levels sufficient to elicit and sustain DA release for behavior conditioning. Here, a mouse moves inside a Y-maze where only one side of the maze activates optogenetic stimulation. The results confirm the hypothesis that mice expressing ChR2 in VTA develop a conditioned preference at the side of stimulation (Fig. 6.4e left), while mice expressing eYFP, a fluorescent protein for control experiments, spend similar time across all sides of the maze (Fig. 6.4e right and Fig. 6.4f).

6.5 Power Delivery Mechanisms The use of μ-ILEDs as local light sources enables designs that operate in a purely wireless fashion with fully implantable device configurations. The power source is

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as important as the optoelectronic components themselves and plays a critical role in shaping the capabilities and limitations of the devices. Recent developments demonstrate different powering mechanisms used in biomedical devices, in the form of electrochemical energy storage (batteries) [61, 62] as well as wireless power transfer via pressure [63, 64] and electromagnetic waves [27, 60]. This last option represents the most powerful basis for wireless, battery-free operation in optogenetic devices and other biomedical systems with small form factors for subdermal implantations [65]. The semiconductor material that defines the active regions of the μ-ILEDs sets the electrical requirements, e.g. electrical potential and current density. For example, InGaN alloys commonly used in μ-ILEDs emit in the blue region of the visible spectrum, 460 nm, due to a bandgap that is ~2.5 eV. In contrast, μ-ILEDs that emit light in the red region of the visible spectrum, 630 nm, use AlGaInP alloys with a bandgap around ~1.8 eV. The inherent reduced efficiency of this material results in a demand for comparatively large current densities. These sets of fundamental material properties define the minimum electrical power conditions to drive the electroluminescence process, of particular importance in designs that use wireless power transfer schemes. Electrochemical energy storage devices, such as batteries, are the most widely employed forms of power sources in consumer electronics, wearables and medical devices [66]. Batteries produce electrical energy during the chemical reduction/oxidation (redox) processes that take place between the electrode materials and the electrolyte [67]. Parameters such as electrolyte and electrode materials determine the operating voltages whereas the energy storage capacity follows primarily from the concentration of the chemical compounds that participate in the redox reactions. Therefore, the energy capacities of batteries are directly related to their volume. For example, a typical lithium ion battery with a 15 mAh capacity has a 360 mm3 volume, compared to 2100 mm3 for a 150 mAh capacity based on the same technology. Such large sizes prevent the realization of fully implantable devices for small animal studies, in particular for optogenetics, as well as other biomedical devices that require minimally invasive form factors. The use of batteries most commonly exploit externalized hardware, where risk of injury is low compared to implanted versions and where battery replacement is straightforward. The disadvantages are in heavy headsets that impose mobility constraints on small animal models such as mice, with associated alterations in behavioral outcomes. Wireless power transfer represents a non-contact battery-free mode of operation to power optoelectronic subdermal devices. These mechanisms use the energy density carried by propagating waves, such as pressure waves in the 0.1–10 MHz frequency range (ultrasound) where absorption in soft biological tissue is low (~1 dB/cm at 1 MHz) [68]. Piezoelectric materials serve as the basis for energy transducers that harvest power in such cases [69, 70]. For example, piezoelectric transducers employed in peripheral nerve stimulations (2 × 3 × 6.5 mm3 , 78 mg) provide up to 3 mW of ultrasonic wireless electrical power transfer to drive electronic circuits [71]. This technology has been demonstrated in implantable biomedical devices such as sensing and wireless ultrasonic transmission of electromyogram and electroneurogram signals [64], nerve stimulators [72], and brain optogenetic stimulators [73]. One

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of the main fundamental limitations, besides ultrasound absorption in tissue, is the reflection losses at the interfaces between the ultrasound transducer and the biological tissue. As a result, all practical uses of ultrasound require that the source is placed directly in physical contact with the tissue, typically the skin. Furthermore, the need for ultrasound focusing imposes technological limitations on the transducer and the orientation of the receiver with respect to the ultrasound phase front which prevents the deployment of these technologies in experimental conditions that involve free moving animals. Electromagnetic (EM) waves support a different wireless power delivery mechanism that is now employed in a wide range of implantable biomedical devices. The fundamental energy conversion phenomena depend on the operation frequency. For example, photovoltaic devices such as solar cells convert absorbed electromagnetic radiation in the visible and near-infrared spectrum (300–750 THz) into electrical power [74–76]. Photovoltaic power cells appropriate for subdermal implantation are those that use NIR frequencies to avoid the high levels of absorption that occur in the visible frequency range. Recent demonstrations in 4 mm porcine skin/fat model suggest power supply in the order of 73.5 mW/cm2 can be delivered to subdermal implanted devices [76]. At much smaller frequencies (0.1–100 MHz) the energy conversion does not involve the absorption of EM radiation but instead it uses magnetic induction between two coils, in resonance with the operating frequency, to transfer power from a transmitter coil to a coil integrated in the device [27, 65]. For a given transmitted power, the induced electrical power depends on the area of the receiver coil as well as the orientation of the receiver plane with respect to the magnetic field. This mode of wireless energy transfer is highly versatile and is common in realistic applications, especially, but not limited to, those in optogenetics where small animal models such as mice can mingle inside an experimental enclosure while a transmitting coil antenna provides wireless power transfer anywhere within the experimental arena.

6.6 Passive Wireless Battery-Free Devices Traditional light delivery methods for optogenetic studies require physical tethered connections to serve as waveguides from external light sources and/or electrical power delivery to implanted LEDs. These encumbrances can strongly affect animals’ mobility and constrain their natural social interactions. Moreover, group studies (n > 2) are nearly impossible with these light delivery setups as connecting wires will entangle during animal’s routine activities. Fully implantable, wireless battery free devices that use resonant magnetic inductive power delivery schemes described previously are rapidly emerging as practical solutions to these challenges. The mostly commonly used approach exploits electromagnetic coupling between a transmission antenna, driven at a selected radio frequency (RF), that encompasses the experimental enclosure and a receiver antenna integrated in the device.

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Figure 6.5a shows a representative wireless, battery-free device for brain optogenetic stimulation that employs a flexible printed circuit board (fPCB, Copper 18 μm/polyimide 75 μm/Copper 18 μm) as a substrate for a receiving antenna (~10 mm diameter) [27]. This antenna, in the form of a spiral planar inductor, is tuned with a matching capacitor to resonate with the incoming radio frequency (RF) for maximized efficiency of electrical power harvesting from the transmission antenna. The area and power of the transmission antenna as well as the area of the receiving antenna determine the harvested power. For example, a ~10 mm diameter receiving antenna can generate ~10–15 mW electrical power across a 30 cm × 30 cm cage fitted with a transmission antenna powered at 8 W [77]. A narrow, flexible probe, defined in the same fPCB, supports one or more commercial μ-ILEDs (270 × 220 × 50 μm3 , 470 nm), typically near the tip end. A red μ-ILED (1 × 0.5 × 0.2 mm3 , 630 nm) in a parallel electrical connection with the stimulating μ-ILED and located near the receiving antenna serves as a visual indicator for operation after subdermal implantation (Fig. 6.5b). A bilayer encapsulation of parylene (14 μm, 2.8 GPa) and polydimethylsiloxane (PDMS, ~30 μm, ~400 kPa) isolates the device from the biological environment and provides soft interfacial contact with surrounding tissues to reduce foreign body responses. During implantation, the probe penetrates the brain through drilled holes on the skull to deliver the stimulating μ-ILED to targeted regions of the brain, whereas the rest of the device lies over the skull underneath the skin (Fig. 6.5b). Passing RF power through the transmission antenna activates the μ-ILEDs, and modulation of this RF produces on–off patterns of operation that are relevant for optogenetic stimulation of neuronal activities. These devices offer attractive applications for optogenetic studies in freely behaving animals, especially those that involve complex behaviors, intricate environments, social engagements and/or chronic operation. For example, the standard light–dark box (LDB), a model that uses the natural behavior of mice to avoid open spaces and seek shelter in dark confined zones to characterize anxiety behaviors, requires a sealed enclosure to fully block ambient light. This experimental implementation can be cumbersome to use with wired systems but is highly convenient with wireless platforms. Figure 6.5c shows a typical neuroscience behavior study that uses this behavior paradigm with wireless technology [39]. In this context, the hypothesis that “chimeric rhodopsin/ β2 adrenergic receptor (opto-β2 AR) modulates neuronal activity through cAMP generation” can be verified by photoactivating opto-β2 AR in the basolateral amygdala (BLA) of mice to increase their anxiety states. The experimental results show that optogenetic stimulation in the BLA decreases the latency to enter the dark zone and increases their time for remaining in this location, consistent with a significant increase in anxiety. The wireless power transfer and light delivery approaches employed in these devices can also translate to other optogenetic studies, including those that involve spinal cord and branching peripheral nerves, by adopting slight modifications in form factors. Figure 6.5d shows a representative device (10 (l) × 5 (w) × 0.2 (thickness) mm3 ) for optogenetic stimulation of the spinal cord [45]. Fabrication follows similar protocols to those for brain devices with slight differences in encapsulation methods. In this case, the device is encapsulated with polyisobutylene (PIB, ~30 μm) followed

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by dip coating with PDMS (~10 μm). The indicator LED is also not required. Here, the probe is plastically deformed into a bent architecture to facilitate implantation. In practice, the probe inserts into the targeted spinal cord segment through a midline incision at the dorsal surface over the vertebral bone. The rest of the device lies on the vertebrae and is secured with cyanoacrylate glue (Fig. 6.5e). One study uses this device to explore the optogenetic regulation of nocifensive behaviors by stimulating (470 nm) transient receptor potential vanilloid 1 (TRPV1) afferents at L4-L6 spinal cord segments in mice [45]. Optically triggered reversible pain behaviors such as flinching, hind paw licking, jumping, and vocalization (Fig. 6.5f), indicate successful modulation of sensible pain through optical activation of spinal afferents. In addition, the use of these devices, in similar experimental configurations, address other peripheral nerve functions, such those associated with the sciatic nerve [78], is straightforward. These devices also satisfy requirements for optogenetic studies in regulating organ function, such as cardiac pacing [47], colonic motility [48], and sensory pain across the bladder [79]. Figure 6.5g shows a representative device (1 cm (l) × 1 cm (w) × 1 mm (thickness)) for optogenetic stimulation of the bladder [79]. This device follows similar fabrication protocols as those for brain and peripheral nerve devices. Different from applications in the central nervous system (brain) and peripheral nervous system, this device adopts a soft patch form factor obtained using bilayer coatings of PIB (5 μm) and PDMS (500 μm) to provide conformal interfacial contact with tissues during optogenetic stimulation in free moving animals. In practice, the device, comprised of the power harvesting module and stimulating μ-ILED (530 nm), implants subcutaneously between the skin and muscle through a small incision on the skin of the abdomen to illuminate the bladder (Fig. 6.5h). One example exploits this device to study whether inhibition of nociceptive bladder sensory afferents reduces sensible pain in awake, free moving mice [79]. In this context, mice injected with cyclophosphamide (CYP) experience bladder inflammation and visceral pain, and gain free access to both sides of a V maze. One side of the V maze activates the device to optogenetically inhibit the nociceptive bladder afferents of mice expressed with inhibitory archaerhodopsin acting on the sensory neuron-specific sodium channels (SNS-Arch). Results show that SNS-Arch mice exhibit a strong place preference for the inhibition side (LED on), while wildtype mice do not show any preferences, indicating an ongoing attenuation in sensory pain from inhibition of nociceptive bladder sensory afferents (Fig. 6.5i).

6.7 Limitations of Passive Wireless Battery-Free Devices Wireless battery-free devices offer promising opportunities for the study of many behavioral paradigms in free moving animals. The illumination patterns for controlling neuronal activities, however, remain passive as all stimulation parameters (frequency, duty cycle) rely on simple on–off modulation of the RF field generated by the transmission antenna. Although this mode of operation does not limit the

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simultaneous operation of several devices within the same experimental enclosure, it restricts individual device addressability and prevents control of independent multichannel modes of illumination. This limitation prevents application to sophisticated scenarios at the frontier of neuroscience research. For example, certain social interaction studies require the use of different stimulation protocols, dynamically and individually selected among groups of animals (n > 2) within the same experimental enclosure. Passive devices cannot satisfy this inclusive mode of operation because all of them share the same modulation dynamics as imposed by the transmission antenna. In addition, the nonuniform magnetic field density distribution within the volume of the experimental enclosure leads to spatially-dependent harvested power in the implanted devices, along with associated fluctuations in optical intensity. The interest in active independent control over optical stimulation, preferably in realtime, with multichannel configurations and with the capability for individual device addressability within a large group enable important neuroscience studies where neither passive wireless devices nor tethered systems are feasible.

6.8 Active Wireless Battery-Free Devices for Studies of the Central Nervous System Active wireless devices typically use a microcontroller to expand the device capabilities. In addition, unidirectional or bidirectional communication enables real-time control and, in some cases, to broadcast physiological data collected by integrated sensors [46, 47, 50, 80]. These upgrades in electronic functionality demand additional power and space for extra electronic components, as compared to passive configurations. The original form factors and surgical procedures are no longer appropriate because these devices spatially organize stimulation components and electronic circuitry together for the convenience of fabrication and implantation. This configuration restricts the implantation location to areas near the target nerves or organs. One strategy to remove this constraint separates the electronic control module from the stimulation or sensing components (Fig. 6.6a) and connects them with soft, mechanically compliant interconnections. Figure 6.6b shows a representative device with these characteristics, as recently developed for optogenetics applications in mice and rats [77]. The electronic module resides at the back of the animal, where sufficient space is available for a large receiver coil and for collections of electronic components to offer intense optical stimulation with active multichannel control (Fig. 6.6a). Compared to passive brain devices (Fig. 6.5a) confined to the area of skull (~1.2 × 1.2 cm2 ), this architecture offers extended space for further expanded functional options. The electronic control module and the injection probes are encapsulated in bilayer films of parylene (14 μm) and PDMS (~30 μm for probes, ~200–800 μm for electronics). These two components connect with soft, stretchable metal serpentine traces (~400 nm) that are symmetrically embedded in parylene (5 μm) and silicone elastomer (Ecoflex, ~200 μm). These traces do not limit any routine activities of the

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mice, because they allow free physical deformations associated with body contortions around the neck and back regions [77]. Video analysis demonstrates that these traces experience up to 28% compression to 41% elongation as well as bending to radii of curvature from 1.7 to 22 cm 95% of the time. Photolithographically defined metal serpentine patterns allow stretching of up to 50% elongation without causing plastic deformation (0.3%) as demonstrated by finite element analysis (FEA) shown in Fig. 6.6c. Meanwhile, the symmetrical bilayer encapsulation creates a mechanical neutral plane for these metal traces to minimize their strain during bending (Fig. 6.6d) [77]. The operation of NFC-enabled active devices of these types appears in the block diagram in Fig. 6.6e. Here the device harvests power using the receiver coil and provides a regulated output voltage to power the electronic components. In parallel connection to the power harvesting module, an NFC memory (M24LR04E, ST Microelectronics) connects to the receiving antenna. This device supports the standard ISO15693 NFC communication protocol for bidirectional communication. The main core of the device is an 8-bit low power microcontroller (Attiny84, Microchip) that contains firmware designed to operate four independent illumination channels. Each channel supports voltage regulation, implemented with a low pass filter, for selecting the illumination intensity. The illumination protocol, including temporal parameters such as frequency and duty cycle, and the intensity levels are programmed in real time using a graphical user interface on a computer. This computer interfaces with the RF power control that supports the ISO15693 communication protocol to write commands on the NFC chip on an event basis. The real-time control over stimulation parameters achieved with this configuration significantly expands the illumination protocols relevant for optogenetic experiments. For example, as shown by Fig. 6.6f, bidirectional modulation over interconnected brain regions with dynamic control of four independent μ-ILEDs with different wavelengths allows study of cooperative effects between distal brain regions [77]. Comprehensive analysis for optogenetic behavior studies also requires quantitative characterization of the volumes of optically activated neurons. Numerical simulations based on FEA methods (i.e. COMSOL) quantify light propagation through brain tissues or other biological media. Light transmission within these media is determined by wavelength dependent scattering and absorption coefficients, spatial and angular emission profiles, and input optical intensity. Figure 6.6g shows an optical irradiance distribution calculated using this simulation method for the case of a single μ-ILED (270 × 220 × 50 μm3 , 460 nm, ~16 mW/mm2 surface optical irradiance) in brain tissue [77]. Combined with the activation threshold for opsins, 0.1mW/mm2 for ChR2 [81] and the average neuron cell density in the brain, the illumination volume and number of activated neurons can be calculated. In addition, temperature distributions at the tissue-probe interface can be numerically simulated similarly to ensure that designs avoid excessive heating of tissues during operation. The temperature increment depends on the thermal power dissipation from the μ-ILED, the optical absorption, thermal properties of the device and surrounding tissues, and the biological environment (such as blood perfusion). Figure 6.6h shows an example of the simulated temperature increment above a μ-ILED (460 nm) operated at 5 Hz in

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brain tissue [77]. The computed dependence on optical irradiance and duty cycle, indicates that the duty cycle should be carefully selected during operation at high power levels to avoid excessive heating.

6.9 Active Wireless Battery-Free Devices for Peripheral Nerves Active wireless, battery-free devices used for brain optogenetics can also be applied in studies for the regulation, or modulation, of peripheral organ function. The example in Fig. 6.7a shows a hybrid optogenetic device designed for controlling bladder dysfunction such as irregular voiding [49]. The system includes a soft, stretchable resistive strain gauge (SG), a pair of green μ-ILEDs (530 nm), and a wireless control and power module (WCP). The SG and stimulation μ-ILEDs are integrated on a flexible copper (9 μm)/polyimide (18 μm) substrate that includes a thin stainless steel filament (25 μm) as a heat sink to prevent overheating during optical stimulation, and supported on a soft patch that mounts as a ring around the bladder. These a

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Fig. 6.7 Active wireless battery-free device for studies of the peripheral nervous system. a Photograph of an active hybrid wireless battery-free optogenetic device designed for real-time regulation of bladder dysfunction. This device contains an optoelectronic stimulation and sensing module (OESS) comprising a soft resistive strain gauge (SG) and a pair of micro-inorganic light emitting diodes (μ-ILEDs, 530 nm), and a wireless control and power (WCP) module. b Schematic illustration of this device implanted in a rat model. c Photograph of a rat implanted with the device. d Block diagram of closed-loop low energy Bluetooth (BLE) operation implemented in the WCP module. A low energy BLE microprocessor activates μ-ILEDs upon irregular void identification sensed from the SG. e Optogenetic regulation on sensed irregular bladder voiding in-vivo. Closed loop operation successfully detects frequent (over 3 times per hour) and smaller voiding events and activates μ-ILEDs for two hours to optogenetically activate inhibitory opsin (HSV-Arch) at the bladder wall. Substantial decrease in voiding frequency in HSV-Arch rats compared to eYFP rats (control) indicates successful treatment of bladder dysfunction. Panels a–e reproduced with permission from ref. [49]; Copyright (2019) Springer Nature

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optoelectronic components are encapsulated in PIB and with PDMS on the side that faces the bladder. The SG consists of a narrow, carbon-doped (15 wt%) silicone film (15 μm thickness) as a resistive strain sensor embedded in a symmetric bilayer of PDMS (~40 μm). The organ unit connects to the WCP, also encapsulated in PIB and PDMS, with flexible copper wires embedded in a polyurethane tube. In practice, the soft patch bonds onto the bladder with biocompatible adhesives and sutures to the muscle tissue to prevent physical drift. Meanwhile the WCP implants ventrally between the skin and muscle (Fig. 6.7b, c) [49]. The active peripheral nerve system follows a similar architecture to that used in brain optogenetics devices. The receiving antenna on the device harvests power from the RF field and passes regulated power to the electronic components. In addition, a supercapacitor bank (two of 80 mF each) provides temporary charge storage to compensate when the device, once implanted in the animal, suffers from position or angular rotation that affects the power harvesting efficiency and compromises the function of the device. In this configuration the device gains wireless control with a Bluetooth low energy (BLE) microprocessor, instead of NFC, and transmits data recorded from the SG sensor to a user interface, hosted on a tablet that processes the data. Upon void identification, the tablet communicates to the BLE chip with a command to activate the μ-ILEDs and optogenetically stimulate the bladder (Fig. 6.7d) [49]. The bidirectional capability of this active device can be used in a closed loop feedback to detect abnormal voiding events in the bladder of a rat based on voiding frequency and volume measured by SG, such that optogenetic stimulation can correct such dysfunction. Frequent (over 3 times per hour) and smaller (below threshold volume) voids define an abnormal scenario, which triggers the activation of the μILEDs (530 nm). In this study the inhibitory opsin Arch is expressed at the wall of the bladder. Studies involve rats virally transduced with Arch-eYFP (positive control group) and eYFP (negative control group). Both groups of rats are then injected with CYP to induce bladder dysfunction. Closed loop operation successfully detects abnormal voiding in both groups of rats and activates the μ-ILEDs for two hours after an average of 265 min following injection of CYP. Arch rats exhibit substantial decreases in voiding frequency compared to eYFP rats, indicating successful detection and treatment for CYP induced bladder dysfunction (Fig. 6.7e) [49].

6.10 Hybrid Devices for Other Applications Battery-free active devices with wireless communication capabilities open up broad exploratory opportunities, not only in applications that employ light activation of transgenetic cells but in other techniques where light stimulation is essential or complementary [22, 82, 83]. Some examples that exploit this type of device configuration are in optofluidics [46, 61, 84–86], photometry [80, 87] and oximetry [88], all of great relevance for neuroscience and other biomedical science.

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Pharmacological stimulation uses endogenous receptors in neurons, and other cells, to induce loss or gain of cellular activity. This technique uses drugs to control biochemical signaling processes that enhance function (agonist) by activating the target receptor, or suppress function (antagonist) by blocking it [20]. The strength of this technique is that it provides cell specificity, although it lacks high spatiotemporal resolution. Drugs administered to biological systems diffuse in the surrounding tissue, such that only a portion reaches the target region. Furthermore, the activation and deactivation timescales are not commensurate with neuronal activity. This limitation, along the poor localization, prevents real time operation. Nevertheless, pharmacology stimulation represents a complementary technique when combined with optogenetics [22]. In this case pharmacology provides precise activation or deactivation of neuron cells for long periods of time and optogenetics serves as a real-time interrogation technique. This combination allows for the study of complex neuronal circuitry and neuronal projections across different brain regions. Recent trends in engineering research are in the development of systems for rapid, actively programmed and spatially precise delivery of drugs to targeted regions. This approach includes soft microfluidic networks, miniaturized pumping systems and wireless control strategies as that shown in Fig. 6.8a. For example, this technology can be used in a behavioral paradigm associated with the role of the dorsal hippocampus (DH) in the control of locomotion [89]. Here, mice transfected with the ChR2 in the DH, are implanted three weeks afterwards with devices shown in Fig. 6.8b. Mice are first optogenetically stimulated, in the absence of pharmacology stimulation. Then the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5phosphonovalerate acid (D-APV) is delivered before optogenetics stimulation. In both cases the location of the mice is recorded to determine the locomotion activity. As seen in Fig. 6.8c such activity increases when optogenetically stimulated, which confirms that photostimulation of DH neurons increases glutamate-dependent locomotion [89, 90]. In contrast, the blockage of these glutamate receptors using the NMDA antagonist D-APV inhibits this behavior, as observed in negligible locomotion activity with respect to the baseline, see Fig. 6.8c. Other similar paradigms using optogenetics and pharmacology in combination are possible such as in the context of anxiety inhibition [91], pain modulation [46]. Furthermore, the emergence of bioengineered or genetic approaches for pharmacological stimulation such as photoactivatable drugs or DREADDs, in the context of the capabilities of these optofluidic devices, represent an exciting additional avenue for enhanced precision in neuromodulation and for expanded options in spatiotemporal control in free moving animal models. Another interesting example is the use of optoelectronic devices, here a μILED and a microscale inorganic photodetector (μ-IPD), as a fluorescence recording device, for measurements techniques referred to as photometry [80, 87]. This concept follows a similar level of device integration as other active battery-free devices, but in this case the μ-ILED excites genetically encoded calcium indicators in the surrounding tissue and a μ-IPD records the photocurrent generated by the emitted light. This device allows for non-contact, all-optical recording of cell-activity. Wireless battery free platforms in form factors that allow fully subdermal implantation

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offer superior photometric applications in freely behaving animals compared to traditional methods, such as those that employ optical fibers or large head stages that restrict or alter an animal’s natural behaviors. Figure 6.8d–e shows a representative device configured with a μ-ILED that emits at the calcium indicator excitation wavelength (468 nm) and a μ-IPD, coated with a thin film optical filter (~7 μm thick), adjacent to the μ-ILED at the end of the probe to monitor calcium dynamics via fluorescence [87]. The system uses RF inductive coupling for wireless power delivery, as previously described, and a microcontroller that synchronizes optical stimulation and fluorescence recording. The data captured by the devices transmits wirelessly using a standard on–off keying modulation of an infrared LED, subsequently detected and demodulated by a photodetector placed in the vicinity of the experimental enclosure. This wireless communication approach transmits data at a rate of 27 samples per second with 12 bit precision. The fabrication, encapsulation, and implantation procedures follow similar protocols as described above (Fig. 6.8a). A behavioral experiment demonstrates the use of this device to study neuronal activity in the BLA under induced anxiety in free moving mice [80]. Transduced mice that express the encoded calcium indicator (AAV5-CaMKII-GCaMP6f) in BLA neurons are electrically shocked to induce anxiety states. The BLA plays an important role in anxiety, such that monitoring BLA neurons provides indirect measures of the anxiety state of the subject. The implanted devices record neuronal activity via fluorescence of calcium indicators in the extracellular matrix. The fluorescence intensity increases after electrical stimulus (Fig. 6.8f), indicating increased neuronal activities in BLA during anxiety states. In addition to transgenetic approaches widely used in neuroscience studies, other sensing techniques exploit μ-ILEDs to transduce physiological variables. Oximetry is an example where the differential optical absorption between oxyhemoglobin (HbO2 ) and hemoglobin (Hb) in blood provides a strategy to infer the oxygenation level in the surrounding tissue [88]. In this application, wireless, battery-free devices, such as those described previously, incorporate a pair of μ-ILEDs (540 and 625 nm) and a μ-IPD affixed at the probe tip in between the μ-ILEDs to record regional tissue oxygenation in free moving animals, as shown in Fig. 6.8g. In this platform, the microcontroller multiplexes the μ-ILEDs illumination while recording the photocurrent. The data is then broadcast using an infrared LED to an external photodiode that demodulates the incoming stream of data. In a practical application, mice implanted with this device in the brain are placed in a hypoxia chamber to change the fraction of inspired oxygen (FiO2 ) between 21 and 8% (Fig. 6.8h). The measured regional tissue oxygen saturation of hemoglobin (rStO2 ) damps over time as the FiO2 decreases, indicating a vasoconstriction of micro vessels due to severe hypoxia induced by interchanging the oxygen flow (Fig. 6.8i). This platform supports the use of oximetry as a measurement modality that is relevant for the monitoring of organ transplants, tissue oxygenation across the body or pulse oximetry.

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6.11 Conclusion Optogenetics has evolved into an essential method in biological research, capable of providing fine spatiotemporal control of cell-specific responses of neurons across all parts of the nervous system. Optical fibers and implantable μ-ILEDs represent engineering systems that can support programmed light delivery to optogenetically activated tissues, for spatially localized optical modulation and, in other contexts, for sensing of individual cells or populations, all applicable to freely moving animals. Wireless, battery-free platforms in various form factors are particularly valuable in these latter contexts, where chronically stable operation as fully implantable devices are possible, without any significant mass-loading or tethering constraints that could limit natural behaviors of the animals. The latest in active devices of these types provide real-time control over their operation, for advanced stimulation protocols with individual addressability among groups of animals. Closed loop feedback control systems that built on these wireless technologies further allow real-time treatment of organ dysfunction based on sensor responses. In addition, hybrid devices that combine optical and pharmacological approaches to neuromodulation and recording of neuronal activity or other physiological signals in designs with wireless, battery-free features represent powerful directions for future research. Additional work involves multimodal stimulation (optical, electrical, pharmacological, thermal) and sensing (electrophysiological, mechanical, chemical) methodologies to further expand the range of possibilities for engaging across cells, tissue regions and complete organs. The results will create many new opportunities in the field of neuroscience, to decipher neural circuits in-vivo and in complex interaction scenarios that involve social groups and naturalistic environments.

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Chapter 7

Flexible and Stretchable Micro-LED Display Luhing Hu and Jong-Hyun Ahn

7.1 Introduction Displays, as a representative technology that visually presents the output from electronic devices, have become omnipresent in our daily lives. Display technology has evolved from bulky cathode ray tubes (CRTs) to liquid crystal displays (LCDs) [1, 2] and organic light-emitting diode (OLED) [3, 4] flat-panel displays. LCDs and OLEDs dominate the global display market because they exhibit excellent properties, such as thin thickness, low power consumption, quick response time, and high resolution [5]. However, LCDs have a limited contrast ratio (1000–5000:1) and shortcomings when applied to flexible applications [6]. In case of OLED displays, though they are available for use as flexible displays, the short lifespan of the panel remains a challenging issue [7, 8]. Moreover, the OLED display requires an additional complicated encapsulation process to protect the panel, as it is sensitive to temperature and humidity [9–11]. Recently, inorganic material-based LEDs have attracted considerable attention owing to their excellent properties, such as high dynamic range (HDR), long lifespan, low power consumption, and high luminance, which is higher than 50,000,000 cd/m2 [12–16]. Furthermore, LEDs on the microscale are promising candidates for nextgeneration displays because they exhibit higher efficiency for converting electrical power to optical power and better current density distribution compared to their larger counterparts [17–20], as well as high tolerance to temperature and humidity, which enables them to be outstanding indoor and outdoor displays [21, 22]. In addition, smaller pixels provide higher resolution in the same area, which is the key to realizing displays for augmented reality (AR) and virtual reality (VR) applications [23]. Unlike L. Hu · J.-H. Ahn (B) School of Electrical and Electronic Engineering, Yonsei University, Seoul 03722, Republic of Korea e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2021 J.-H. Ahn and J.-H. Kim (eds.), Micro Light Emitting Diode: Fabrication and Devices, Series in Display Science and Technology, https://doi.org/10.1007/978-981-16-5505-0_7

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LCDs that require extra layers such as polarizers, liquid crystals, color filters, and backlight units, self-emissive micro-LEDs can have higher luminance and efficiency without optical loss due to the light modulation layers. Since micro-LEDs are based on thin-film III-V semiconductors, they have limitations in bending or stretching in their natural forms. For over a decade, considerable effort has been made to realize such deformable displays, for example, device designs [24, 25], materials synthesis [26, 27], and advanced fabrication processes [28]. After removing the growing substrate, the micro-LED device thickness is approximately several tens of microns, and it can be assembled and integrated on foreign substrates to form flexible and stretchable displays. The reduction in the total thickness of the display leads to a decrease in mechanical stiffness, and hence, in achieving high mechanical reliability while maintaining excellent luminous properties. Furthermore, stretchability has become an important criterion in futuristic displays such as skin-attachable health care monitoring devices and smart clothes [29]. Display technology can be categorized into passive-matrix and active-matrix configurations. The passive matrix type is relatively easier to be fabricated, but has some drawbacks. For instance, each pixel is sequentially scanned at one time, which leads to a slower response time compared to that in the active-matrix configuration [30]. Moreover, power consumption is higher in the passive-matrix type. In contrast, each pixel in the active-matrix display is individually controlled by a thin-film transistor (TFT). With this configuration, the display can be refreshed at a higher rate, and the crosstalk effect can be reduced [31]. In this chapter, various types of flexible and stretchable micro-LEDs integrated with active-matrix backplanes are discussed.

7.2 Flexible Micro-LED 7.2.1 Technical Processes to Transfer Micro-LEDs The main fabrication processes of micro-LED displays include epitaxy growth of III-V semiconductors [27, 32–35], lift-off technology of LED chips from substrates [35–39], and assembly of micro-LEDs on backplane circuitry [40, 41]. Since the first high-efficiency gallium nitride (GaN) blue LED was presented in 1994 [42], numerous studies have been carried out to enhance the performance of LEDs and have led to a revolution in lighting and display applications. From the perspective of micro-LEDs, the large-area synthesis of GaN is one of the main challenges in commercializing micro-LED displays. This is because the fabrication of LED chips on a large scale will eventually reduce the manufacturing cost. Even though the technology of GaN epitaxial growth on sapphire substrates is mature and high-quality thin films can be obtained, it is still not preferable to use large-area sapphire substrates because of the high cost of the material. Therefore, the epitaxial growth of GaN on large-area silicon wafers is in high demand.

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After micro-LED chip fabrication on an epitaxial wafer, the devices must be released from the rigid wafer and transferred onto flexible substrates. The representative methods are as follows: (1) chemical lift-off, (2) laser lift-off (LLO), and (3) mechanical lift-off, which is followed by the integration of micro-LED on the target substrate using the fluidic self-assembly method. High-quality GaN thin films can be grown on Si (111) substrates with an internal quantum efficiency of 55% [43]. Arrays of micro-LED devices can be produced on wafers using conventional techniques [44]. Micro-LEDs can be released from the substrate by adopting anisotropic etching of silicon. The etching rate of the Si (110) plane is higher than that of the Si (111) plane in warm potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) solution [45, 46]. Because of this phenomenon, micro-LED arrays were fabricated on the wafer perpendicular to the direction of < 110 > [36]. In addition, a pair of supporting anchors is formed at the edges of the device to hold the freely suspended micro-LED after the silicon etching process. Scanning electron microscopy (SEM) images show the micro-LED device before and after etching (Fig. 7.1) [42]. Importantly, both the electrical and optical properties remained unchanged after the process. This chemical lift-off method is not only limited to GaN-on-Si wafers but can also be applied to GaAs-based red microLEDs [47–49]. The semiconductor layer with AlInGaP quantum wells is epitaxially grown on a GaAs wafer with an AlAs sacrificing layer that can be etched in a hydrofluoric (HF) acid solution. Similarly, anchors that serve as a support for the micro-LED can be created using UV-curable epoxy to hold the free-standing devices in the HF solution. Choi et al. applied a similar technique to lift off red GaAs microLED arrays with bridge-structured anchors for accurate alignment with backplane circuitry (Fig. 7.1) [50]. Another well-known method to release the micro-LED arrays from the wafer is the LLO, which is a technique to manipulate the laser beam to selectively scan and release the devices in a defect-free manner [51–53]. This technique requires a transparent substrate, such as sapphire, to enable the laser beam to irradiate through and weaken the adhesion force between the devices and the substrate. Therefore, this technique is a specific method for GaN-on-sapphire devices. LLO is an advanced technology that provides high efficiency and high process yield over a large area coverage with minimum damage to the devices. The common laser used for GaN-based devices is the KrF excimer laser because of the band-gap difference of the GaN (3.4 eV), the sapphire substrate (~10 eV), and the 248 nm excimer laser (5 eV). Hence, the photon energy from the laser is absorbed by the GaN, which causes a rapid increase in temperature at the interface, leading to the formation of Ga metal and nitrogen gas. By applying pressure simultaneously to this process, the devices detach harmlessly from the sapphire substrate. The schematic in Fig. 7.1 illustrates the selective LLO process and bonding process of the micro-LED on the carrier substrate [54]. Once the device is released from the substrate, a temporary carrier substrate, such as a silicon wafer with a bonding metal layer [55], thermal release tape (TRT) [54], and PDMS elastomer [56], is used to lift the devices and transfer them onto a flexible substrate. Kim et al. successfully demonstrated flexible micro-LED arrays on a polyethylene

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Fig. 7.1 Technical process to transfer the micro-LED for flexible applications. The techniques to release the thin film from growth substrates include chemical lift-off, laser lift-off, and mechanical lift-off. Each method has its pros and cons, and the details can be found in the main text. One of the representative methods to integrate the micro-LED on the target substrate is the fluidic self-assembly method that uses the capillary force of the liquid

terephthalate (PET) substrate, as well as high flexibility and deformability of the device by tying the array into a knot (Fig. 7.1) [37]. The mechanical lift-off method of releasing the micro-LED arrays from the wafer is a chemical-free process that does not require additional complicated equipment compared with chemical and laser-assisted approaches. However, this method has a disadvantage as it may cause damage to the thin film, and the transferable area is also limited. Therefore, numerous studies have been conducted to overcome the limited

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throughput of this method [33, 35]. To take advantage of the atomic layered structure of two-dimensional (2D) materials, which enable easy mechanical exfoliation, researchers have attempted to synthesize GaN layers on graphene using a buffer layer, such as a zinc oxide (ZnO) nano-wall [27]. After thin film synthesis, the device can be easily released from the growing substrate and transferred onto a flexible substrate for various purposes. In addition, researchers have found that the graphene layers exhibit lattice transparency to the underlying substrate, which enables the epitaxial orientation of the substrate to penetrate the atomic-thin graphene layers. Therefore, a high-quality thin film can be remotely grown on a graphene-coated substrate. The graphene layer acts as an ideal releasing layer to mechanically lift the thin film from the host substrate (Fig. 7.1) [35]. After depositing a stressor metal layer (e.g., nickel) on the thin film surface, a flexible handling substrate was used to exfoliate the III-V layers from the graphene-coated substrate. When the applied external stress is higher than the van der Waals force of the III-V thin film/graphene or graphene/substrate interface, the thin film can be peeled off from the substrate harmlessly. Similarly, single-crystal hexagonal boron nitride (h-BN) is used as an intermediate layer for III-nitride synthesis, as well as a releasing layer for the mechanical lift-off process [57]. GaN thin films grown on h-BN exhibit similar optical and electrical properties before and after the transfer process. After the separation of the micro-LED arrays from the growing wafer, the devices are ready to be transferred to foreign substrates for flexible and stretchable applications. The state-of-the-art transfer process can be divided into four categories: elastomer-based pick-and-place [49], electrostatic transfer [58], electromagnetic transfer [59], and fluidic self-assembly methods [60]. The pick-and-place transfer technology using the elastomer stamp is a method that picks up micro-LED arrays to a target substrate using the van der Waals adhesion of the elastomer materials [61]. Researchers at KIMM developed an advanced technology to precisely transfer the micro-LEDs using a roll with a PDMS stamp [50]. The roll transfer method enables a high-speed, large-area micro-LED transfer system with significantly improved transfer yield. Both electrostatic transfer and electromagnetic transfer methods require a unique pick-up head that uses either electrostatic or magnetic forces to lift the micro-LED arrays. In the case of the electromagnetic transfer method, a coil that generates electromagnetic force is needed, and the equipment becomes complicated when the size of the micro-LED chip decreases. Considering the transfer yield and transfer speed of these methods, small-scale micro-LED displays are the target applications. Another approach to assemble micro-LEDs on the backplane circuitry is to adopt the fluidic self-assembly transfer method (Fig. 7.1) [60, 62]. This transfer technology is based on capillary interactions between the device and the liquid solder. Using this method, high-speed assembly and high production yield of micro-LEDs assembled on foreign substrates can be realized at a relatively low cost. Recently, over 19,000 GaN micro-LED chips have been assembled accurately on a target substrate with a throughput yield of 99% within 1 min [63].

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7.2.2 Micro-LEDs on a Flexible Substrate After transferring the micro-LEDs onto flexible substrates, such as PET, polyimide (PI), polyurethane (PU), and rubber substrate, the optical, electrical, and mechanical properties were investigated. The GaAs-based red micro-LED was separated from the wafer through chemical etching in HF solution and then transferred to a PU substrate [49]. The electrical properties and photoluminescence (PL) exhibited similar characteristics before and after the transfer process. The optical image in Fig. 7.2a indicates the uniform emission of the micro-LED device, which can be attributed to the excellent current spreading property of smaller devices. In the case of GaN-on-Si micro-LEDs after the chemical lift-off process in a KOH bath, the devices worked well on the plastic substrate stably without degradation [36]. These

Fig. 7.2 Micro LEDs on flexible substrate. a Integration of GaAs-based micro-LED on the flexible polyurethane substrate. The thin micro-LED device is transferred from the mother substrate after the chemical lift-off process and attached to the flexible substrate. The electrical and optical properties remain similar before and after the process. b Laser lift-off process is performed on the GaN micro-LED on sapphire substrate and transfer-printed on the PET substrate. EL spectra of the micro-LEDs show slight differences at the peak owing to the stress released after the lift-off process. The inset indicates the light emission of the LED on PET substrate. c The GaN micro-LEDs are integrated on thin plastic substrate and bent along a metal rod. The I-V characteristics show the stable operation of micro-LEDs on different bending radius

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results prove that the chemical solutions used in the lift-off process do not harm the multi quantum-wells (MQWs) of the micro-LEDs. Next, GaN-based micro-LEDs on sapphire substrates were mostly picked up using the LLO technique. The LLO process requires complicated equipment, and its performance is determined by the laser type, laser power, and pulse width. After the GaN micro-LED is exposed to a laser, thermochemical decomposition occurs, and the residues remaining on the surface have to be removed by dilute hydrochloric (HCl) solution. Subsequently, on the flexible substrate, metal interconnected lines were patterned and deposited for electrical characteristic measurements. Both the device and plastic substrate did not exhibit degradation during the measurements. Furthermore, the measured electroluminescence (EL) spectra of the micro-LEDs exhibited peaks at 457 and 455 nm before and after the LLO process because of the relaxed compressive stress of the GaN layers (Fig. 7.2b) [56]. Mechanical stability is another important parameter for flexible display applications. To achieve high-performance flexible displays, suitable substrates with essential requirements are important, including substrate compatibility with conventional fabrication processes, excellent chemical stability, and most importantly, appropriate mechanical stiffness of the materials. Therefore, thin polymers and ultra-thin glass substrates are the most common substrates used in flexible applications. The stiffness of the substrates depends mainly on the thickness and modulus of the materials. When the total thickness of the device is low, the mechanical stiffness is reduced, and hence, high flexibility can be achieved. The mechanical stability of the microLEDs transferred onto the plastic substrate after the LLO process was analyzed. The devices operated stably at bending radii of 8.2, 3.5, and 2.1 mm, and degradation did not occur as compared with that in the flat condition (Fig. 7.2c) [64]. The electrical properties of the flexible device also exhibited minimum variations after the 2000 cyclic test. Furthermore, researchers operated successfully the micro-LEDs under folding conditions (bending radius = 0.7 mm) [47].

7.2.3 Flexible Micro-LED Display Fundamental studies of micro-LEDs on flexible substrates allow us to understand the behaviors of the devices and enable the integration of passive-matrix type flexible micro-LED displays after metal electrode interconnections. The application of GaN-based blue micro-LEDs has been widely explored by researchers. For example, micro-LED arrays were transferred and laminated onto PET and glass substrates (Fig. 7.3a) [36]. The micro-LED display exhibited stable operation on both types of substrates, and importantly, the PET sample exhibited excellent flexibility and mechanical reliability. In addition, by integrating phosphor particles onto the microLEDs, the display can yield a white color output, as the chromaticity can be tuned by controlling the phosphor concentration. Similarly, in the case of GaAs-based red micro-LEDs, the device arrays are printed on a plastic substrate and interconnected in a passive-matrix layout to realize a flexible micro-LED display (Fig. 7.3b) [49].

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Fig. 7.3 Flexible micro-LED displays. a Optical images of flexible micro-LED displays based on blue GaN. The micro-LED arrays are released from growth substrates and printed on target substrate to produce flexible displays. By introducing phosphor into the platform, yellowish emission can be achieved, and laminated diffuser film provides large area emission. b The integration of GaAs micro-LED on the flexible substrate. The micro-LED array is interconnected in the matrix form to demonstrate display application. Optical images show the operation of the flexible displays while attached on different surfaces. c GaAs-based micro-LED in a vertical structure fabricated on the flexible PI substrate. The schematic figures illustrate the fabrication process of the vertical micro-LED chips

The 16 × 16 array was controlled by external driving circuits and displayed letters B and N. This flexible display can work perfectly with a bending radius of ~7 mm without significant degradation. The calculated maximum strain of the micro-LED in the display was 0.21%. Another notable advantage of such micro-LED displays is the high transparency offered by the small pixels, where only the metal electrodes are opaque. LED chips can be categorized into two types: lateral and vertical. The

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displays discussed above are based on lateral LED chips. Researchers have also tried to implement flexible micro-LED displays using vertical micro-LEDs [25, 48]. As illustrated in Fig. 7.3c, the vertical micro-LED is chemically lifted off using the PI substrate by anisotropic conductive film (ACF) bonding, followed by deposition of the epoxy passivation layer and top metal electrodes [65]. The flexible vertical microLED display on the PI substrate exhibited robust mechanical stability under bending conditions with a bending radius of 5 mm. Although the vertical micro-LED exhibits excellent light extraction efficiency, this method limits the design of the display as the final device has an identical layout to that of the source wafer. The practical applications of these flexible micro-LED displays were investigated. For example, micro-LEDs are integrated into the fabric for wearable applications (Fig. 7.4a) [66]. The mechanical properties of the display play an important role in ensuring that the devices work stably even under extreme deformations. The research team also evaluated the thermal, humid, and chemical stability of the display on fabric (100% cotton). The display worked well under an accelerated stress test of 85 °C/85% humidity test, which indicated that the long lifespan of the display can be assured owing to the natural properties of the materials. Moreover, the display worked properly in detergent solution, exhibiting high chemical resistance. Next, a

Fig. 7.4 Application of flexible micro-LED displays. a Micro-LED display in the passive-matrix configuration is stitched on the fabric for wearable application. b GaAs-based micro-LED display is attached on the fingertip of a glove and shows excellent reliability even in the extreme environment, such as soapy water (Left). Optical image of the micro-LED display after crumbled on aluminum foil (Right). c Flexible micro-LED display produced by X-celeprint. d Flexible and stretchable micro-LED display on textile integrated with driving transistors. Photo courtesy of IMEC

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waterproof display that can be mounted on unusual substrates such as vinyl gloves and metal foil, which can be applied in robotics, was developed using micro-LED devices (Fig. 7.4b) [67]. By integrating functional layers with flexible micro-LEDs, such as photodiodes capable of detecting changes in bio-environments, advanced equipment for bioengineering such as robotic surgery arms can be realized. In the display market, considerable effort has been put into realizing affordable large-area high-performance displays. The manufacturing cost can be controlled and reduced if the assembly of the micro-LED array and repair action can be effectively performed. Recently, a prototype micro-LED display with high resolution was developed by Xceleprint using the elastomer stamp mass transfer method [40]. The photographs in Fig. 7.4c show the passive-matrix type micro-LED display with a resolution of 254 PPI on a plastic substrate. The total thickness of the display is ~130 µm, which enables the robust operation of the display when flexed. Researchers from IMEC demonstrated flexible and stretchable micro-LEDs on textiles with integrated driving transistors (Fig. 7.4d). The fabrication processes of advanced micro-LEDs are more likely to be foundry-compatible for rapid commercialization.

7.2.4 Flexible Micro-LED for Optogenetics Optogenetics is a technology that investigates the genetics of a specific neuron or cell using optical methods to stimulate or provide some functions, such as tissue investigation and analysis of neurons [68, 69]. Since Karl Deisseroth’s research group from Stanford University introduced photosensitive proteins into neurons to allow light to control cell activities [70], numerous studies have been carried out to investigate whether different wavelengths of light are capable of tuning and manipulating neuronal activity, including neuronal excitation and inhibition [71]. Micro-LEDs have accelerated the research of optogenetics because the thin-film device enables direct biological integration, whereas other types of optical sources such as fiber optics [72–74] and waveguides [75–78] are too bulky and exhibit several engineering limitations. Furthermore, small-sized micro-LED devices enable the minimization of cell damage and inflammation during deep brain stimulation (DBS) [79–82]. Researchers have integrated micro-LEDs on a probe that can control the pixels individually [81]. The micro-LED probe was fabricated on a sapphire substrate with a thickness of 100 µm. The goal is to reduce tissue damage during the insertion of micro-LEDs into brain tissue. In addition, the micro-LEDs fabricated on sapphire substrates exhibit excellent optical power and the heat dissipation is under control, which does not harm the surrounding tissue during stimulation and recording. This design, however, lacks flexibility, and the sapphire substrate causes scattering of light, consequently reducing the spatial resolution. By integrating micro-LEDs on flexible substrates, such as PI and biocompatible epoxy, flexible injectable microLEDs can be produced, and the flexibility of the polymer substrate can minimize tissue damage. Kim et al. proposed one of the first flexible optoelectronic devices that can be injected into the animal brain for DBS studies (Fig. 7.5a) [80]. The integrated

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Fig. 7.5 Flexible micro-LED for optogenetics. a Flexible micro-LED integrated with a photodiode, temperature sensor, and actuator that can be injected into the deep brain for brain activity analysis. b Optofluidic neural probe based on micro-LED for optogenetics and in vivo pharmacology purposes

thin-film micro-LED had a size of 50 µm × 50 µm and a thickness of 6.45 µm. This small device allows better spatial precision, excellent heat management, and causes less harm to the tissue, prolonging in vivo stimulation and recording. In addition, this flexible optoelectronic device, which consists of micro-LEDs and other electronic components, such as photodiodes, temperature sensors, and actuators, can be injected into the deep brain to record, stimulate, and analyze brain activity simultaneously. Each layer was fabricated and laminated with epoxy, and the microneedle shape was defined using standard photolithography methods. The system was tested on a healthy, freely moving mouse model using a wireless power system. Furthermore, an advanced optofluidic neural probe was demonstrated using micro-LED arrays and integrated with a soft microfluidic drug delivery system (Fig. 7.5b) [83]. In vivo pharmacology enables researchers to investigate complex physiological effects of a specific drug on neurons. By combining in vivo pharmacology and optogenetics, the integrated optofluidic probe was programmed to deliver the drugs and light to the targeted area in the deep brain of an awake animal model. A flexible system with low bending stiffness was adapted to the micromotion caused by movement and blood flow, thereby reducing the possibility of damage and irritation to the brain tissues during stimulation [84].

7.3 Stretchable Micro-LED Display Stretchable optoelectronic is an emerging field that has attracted considerable attention, and technologies to overcome the mismatch of the mechanics in different materials have been developed to realize applications that cannot be achieved by traditional methods [85]. Applications such as skin-attachable displays [31, 86], soft robotics [87], and biomedicine [88] can be integrated into soft materials to provide various functions even under mechanical deformation. III-V compound semiconductors are rigid and brittle in their natural forms and therefore cannot be bent or stretched easily.

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Therefore, several approaches based on (1) wavy geometry of semiconductors with ultrathin thickness and (2) serpentine interconnects with island design have been investigated to overcome the fundamental limitations of the materials. When the thickness of any bulky material is sufficiently reduced to a thin structure, the bending strain of the material is reduced as the strain decreases linearly with the thickness. The relationship between thickness and strain is as follows: ε=

t 2r

where ε is the bending strain, t is the thickness, and r is the bending radius. For example, as the thickness of a rigid, bulk Si semiconductor with a fracture tensile strain of less than 1% is reduced to the nanoscale regime, the Si membrane can form stable crumpled or wavy structures without mechanical failure [89]. Similarly, a wavy structure of III-V compound semiconductor, which is rigid and brittle in its natural form, can be formed. The GaAs thin film was transferred in a ribbon shape and bonded on a pre-strained elastomeric substrate (Fig. 7.6a, left) [49].

Fig. 7.6 Strategies to achieve high stretchability. a Wavy structure that provides stretchability. Thin GaAs ribbon is transferred and attached to the pre-strained rubber substrate to yield a wavy structure when the strain on the rubber substrate is released (left). Wavy interconnects based on graphene electrodes (right). b Serpentine electrode that connects the device bonded on the island to minimize the strain. The FEA simulations prove that the strain applied on the device is negligible even after stretching at 106% (left). The serpentine electrode is moved out of the plane when the device is 360° twisted (right)

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When the pre-strained rubber substrate was released, a wavy structure was formed, and the strain applied to the quantum well region was 0.053%. This wavy structure can yield a stretchable LED device, but its application is limited because of the limited fabrication process on a large area. Therefore, instead of having a wavy structure at the active layer, the electrode that connects the device is designed similarly to yield high stretchability. Recently, atomically thin two-dimensional (2D) materials have become promising candidates for advanced electronics owing to their excellent electrical, optical, and mechanical properties. Graphene, which is well known for its mechanical properties, is suitable for application in stretchable electronics [90, 91]. By applying the concept of the wavy profile to the graphene electrode on a rubber substrate, while the devices are bonded on the rubber surface, a high-performance electronic system can be realized (Fig. 7.6a, right) [92]. The resulting device exhibited stable mechanical properties even under severe deformation because the applied strain was distributed at the interconnect part with low stiffness instead at the electronic device itself. Interconnects in the serpentine layout are another strategy for producing high stretchability electronic systems [93]. One of the most common methods to yield stretchable electronics is to introduce island-bridge structures into the systems, where the active components are bonded firmly on the island, whereas the interconnects are designed in an optimized shape that can withstand high strain when stretched [94– 96]. The thin interconnects are twisted out of the plane when stretched to minimize the applied strain. The mechanical investigation of the strain distribution on the device was conducted using finite element analysis (FEA) and showed low strain applied to the devices even after stretching at 106% (Fig. 7.6b, left) [97]. The microLED array, which is attached to the island, is electrically connected through the metal interconnects in the serpentine layout and assembled on a rubber substrate. The metal interconnects are elongated and create out-of-plane motion to reduce the strain when twisted by 360° (Fig. 7.6b, right) (ref) [67]. These device engineering approaches enable the production of optoelectronic systems with high stretchability. For example, researchers integrated micro-LED arrays on rubber substrates using wave-shaped metal interconnects to connect the LEDs in a passive-matrix configuration (Fig. 7.7a) [49]. The device can be stretched up to 22% while inducing no damage or degradation to the light emission of the microLED. Furthermore, it also exhibited high mechanical reliability after 500 cyclic tests. Serpentine-shaped metal interconnects are utilized in the micro-LED display to serve as a bridge connecting each pixel in the system. The device was fabricated and bonded to a pre-strained rubber substrate. The serpentine interconnects experience a controlled and nonlinear buckling effect when the strain is released. This effect allows the micro-LED display to operate in a stable and robust manner, while a uniaxial strain of 48% is applied during stretching. Another example shows that the micro-LED display operates without failures even under severe deformation when stretched onto a sharp pencil tip. The peak strain was estimated to be approximately 100%, according to the distances between adjacent micro-LEDs (Fig. 7.7b) [67]. Such stretchable micro-LED displays are suitable for applications in biomedicine where high stretchability of the system is required when interacting with soft skin or bio-environment.

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Fig. 7.7 Application of stretchable micro-LED. a Stretchable micro-LED connected using wavy interconnects. b Stretchable micro-LED with the bridge-island structure. The optical images show that the device is stretched, twisted, and deformed at the tip of a pencil. c Serpentine interconnects based on graphene electrodes. Optical images of the micro-LED before and after stretching in a uniaxial direction

Furthermore, researchers also exploited the applications of graphene electrodes in micro-LED displays. Because graphene electrodes are optically transparent, they can replace the opaque metal pads in micro-LEDs to yield higher output power [97]. By applying the serpentine graphene electrode to the micro-LED display, a highperformance stretchable transparent display can be produced. As shown in Fig. 7.7c, the micro-LED display works stably, and uniform emission of the micro-LED can be observed when stretched at 86% uniaxially. Recent advanced display technology requires an active-matrix configuration to overcome the multiplexing limitation and crosstalk effect in passive-matrix type displays. Active-matrix micro-LEDs have been reported using low-temperature polycrystalline silicon (LTPS) [98], oxide TFTs [99], CMOS [40], and high-electronmobility transistors (HEMTs) [100] as the switching component of the display. However, none of them have the ability to bend or stretch. Choi et al. reported the first stretchable active-matrix micro-LED display using a Si TFT [50]. The activematrix micro-LED display was assembled using the roll transfer technique, and the serpentine electrodes connected the pixel in a matrix form. The Si TFTs and microLED arrays were fabricated separately and transferred onto an intermediate substrate for the integration process. Finally, the active-matrix display was transferred onto a soft elastomeric substrate to produce a stretchable display (Fig. 7.8a). Different

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Fig. 7.8 Active-matrix stretchable micro-LED. a Integration of Si TFT and GaAs micro-LED on the rubber substrate using serpentine interconnects. b Dynamic operation of the active-matrix micro-LED displaying characters “Y,” “O,” “N,” “S,” “E,” and “I”. c Optical image of the microLED showing uniform light emission under 40% of strain. The electrical properties of the integrated pixel when strain is applied at 10% step

characters were displayed on the integrated device by controlling the data lines and gate lines (Fig. 7.8b). Because the pixels are integrated using metal electrodes in a serpentine layout, the display can withstand a strain of up to 40% while maintaining the emission properties of the micro-LED. The devices, micro-LED and Si TFT, are bonded on the island; therefore, the applied strain is negligible when stretched at 40%. However, the serpentine electrodes experienced a well-distributed strain to prevent device failure as a result of the applied strain. The electrical properties of the integrated pixel also exhibited small variations when strain is applied at a 10% step.

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7.4 Conclusion As outlined in this chapter, LED chips in the micro-scale regime (