Performance of InGaN Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates with Modified Top-Tip Cone Shapes

InGaN light-emitting diodes (LEDs) were fabricated on cone-shaped patterned sapphire substrates (PSSs) by using low-pressure metalorganic chemical vapor deposition. To enhance the crystal quality of the GaN epilayer and the optoelectronic performance of the LED device, the top-tip cone shapes of the PSSs were further modified using wet etching. Through the wet etching treatment, some dry-etched induced damage on the substrate surface formed in the PSS fabrication process can be removed to achieve a high epilayer quality. In comparison to the LEDs prepared on the conventional sapphire substrate (CSS) and cone-shaped PSS without wet etching, the LED grown on the cone-shaped PSS by performing wet etching for 3 min exhibited 55% and 10% improvements in the light output power (at 350 mA), respectively. This implies that the modification of cone-shaped PSSs possesses high potential for LED applications

Development of nano-patterned sapphire substrates for deposition of AlGaInN semiconductors by molecular beam epitaxy

Thesis (M.Sc.Eng.)This research addressed the design and fabrication of nano-patterned sapphire substrates (NPSS) as well as the growth by molecular-beam epitaxy (MBE) on such substrates of AlGaN and InGaN multiple quantum wells (MQWs). In recent years a number of LED manufacturers are developing nitride LED devices emitting in the visible part of the electromagnetic spectrum on micron-patterned sapphire substrate (MPSS). These devices are reported to have lower threading dislocation densities, resulting in improvement of the LED internal quantum efficiency (IQE). Furthermore, the LED devices fabricated on MPSS were also found to have improved light extraction efficiency (LEE), due to light scattering by the patterned substrate. My research focuses on the development of nano-patterned sapphire substrate aiming to improve the performance of LEDs grown by MBE and emitting at the deep ultraviolet region of the electromagnetic spectrum. In order to optimize the nano-patterning of the sapphire substrates for maximum light-extraction, the Finite-Difference Time-Domain (FDTD) simulation method was employed. The LEE enhancement was calculated as a function of the diameter, height and perion of the pattern. The calculations were performed only at a single wavelength, corresponding to the maximum of the emitted LED spectrum, which was taken to be 280 nm. These calculations have shown that the best sapphire substrate patterning strategy for this wavelength is the cone shape pattern in hexagonal array structure. Based on limited number of calculations I found that the optimum period, diameter and height of this cone shaped pattern are 400nm 375nm and 375nm respectively. Experimentally, nano patterned substrates were fabricated through natural and nano-imprint lithography. In natural lithography the first step for the definition of the nano-pattern consists of coating the sapphire substrate with photoresist (PMMA) followed by depositing a monolayer of polystyrene nanospheres, 400nm in diameter, using the Langmuir–Blodgett method. These spheres assemble on the substrate and form a closed packed hexagonal array pattern. Following this step the size of the spheres was slightly reduced using reactive-ion etching (RIE) in oxygen plasma. This was followed by the deposition a chromium film, lift-off to remove the polystyrene spheres and RIE to remove the PMMA from the footprints of the spheres. The substrate was then coated with a nickel or chromium films followed by another lift-off which defines the final mask for the formation of cone shaped features by RIE in a CHF3 plasma. An alternative method for pattern definition was the nanoimprint lithography; the stamp for this method (2 mm2 in size) was formed on Silicon substrate using e-beam lithography. NPSS with high quality pillar shape was also fabricated by this method, however, this method can produce only small size patterns. AlGaN films and GaN/InGaN MQWs were deposited on the NPSS by MBE, and characterized by Scanning electron microscopy and photoluminescence and cathodoluminescence measurements. The cathodoluminescence and photoluminescence spectra show that films grown on NPSS has much stronger luminescence than the films grown on flat sapphire substrate, consistent with enhanced light extraction efficiency

Quantum Dot–Incorporated Hybrid Light-Emitting Diode

Quantum dots are very promising candidates to enhance the performance of hybrid devices. Their size-dependent wavelength tunability owing to quantum size effect, narrow full width at half maximum, high quantum yield, and several other optoelectronic properties enable their use as potential components in GaN-based light-emitting diodes. This chapter explains methods to fabricate color-converted and white light-emitting diodes with the incorporation of semiconductor quantum dots

마이크로 발광 다이오드를 위한 사파이어 나노 멤브레인 위 GaN 이종성장 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2021. 2. 장호원.III-nitride based semiconductors have advantages such as high quality, high efficiency, and long lifespan, and thus have attracted considerable interest in optoelectronic device applications such as light-emitting diodes, lasers, and solar cells over the past decades. The GaN epitaxial layer is mainly grown on heterogeneous substrates such as Si, SiC, and sapphire because epitaxial growth using the GaN substrate is not possible for economic and technical reasons. Among them, the sapphire substrate is widely used due to its high quality, transparency, and high temperature stability, but many problems arise due to the difference in lattice constant from the epilayer and the difference in thermal expansion coefficient. Due to the difference in lattice constant, high density of threading dislocation that directly affect the efficiency of the optical device are generated in the epilayer, and due to the difference in thermal expansion coefficient, a large compressive stress on the GaN thin film and wafer bow occurs during cooling at room temperature after the epilayer growth at high temperature. These problems hinder the realization of high-efficiency GaN-based optical devices. Compared to existing display technologies such as liquid crystal displays or organic light emitting diodes, micro light-emitting diodes (micro-LEDs) are in the spotlight as a next-generation display technology because they have excellent characteristics such as high brightness, fast response speed, ultra-high resolution, and low power consumption. In particular, in fields such as virtual reality and augmented reality, which are expected to be highly demanded in the future, displays become closer to the human eye, and ultra-high-definition micro-displays are required. However, low external quantum efficiency (EQE), high level of leakage current, and immature micro-LED transfer technology are obstacles to commercialization. The fabrication of conventional micro-LED uses a plasma etching process to form individual micro-LEDs after growing a LED epilayer on a substrate. Non-radiative recombination is increased by exposure of the multi-quantum wells (MQWs) serving as an active layer, thereby causing high level of leakage current and low external quantum efficiency. In this study, a substrate with sapphire nano-membrane structures was proposed to obtain a high quality GaN epitaxial layer and to solve the problems of micro-LEDs. The growth of GaN on the sapphire nano-membrane was studied using an metalorganic chemical vapor deposition. The GaN epitaxial layer grows in various shapes depending on the growth orientation and growth conditions, and various growth facets appear. In order to grow the desired micro-GaN epilayer on the sapphire nanomembrane, the study on growth behavior of GaN was first conducted to understand the growth aspect of the GaN on the sapphire nano-membrane. The fabrication of sapphire nano-membrane was carried out by photolithography, amorphous alumina deposition using atomic layer deposition (ALD), photoresist (PR) removal, and crystallization through a subsequent heat treatment process. The amorphous alumina layer is crystallized into a single crystal -phase alumina (sapphire) through solid phase epitaxy in the heat treatment process. In order to understand the growth behavior according to the growth direction, the growth of GaN was observed by varying the angle of the stripe pattern. The fastest lateral growth rate was seen in the stripe-shaped sapphire nano-membrane along with the sapphire [112 ̅0] direction, and it was confirmed that the lateral growth rate repeats the maximum and the minimum every 30º rotation. By measuring the growth rate of facets formed differently depending on the direction, it could be understood that the growth shape of GaN was different according to the orientation. In addition, GaN grown in the bottom region between the membrane patterns was also observed. The Ga diffusion to the bottom region was inhibited by the GaN layer grown laterally on the membrane, showing the possibility that the GaN layer could be removed from the substrate by breaking the sapphire nano-membrane. Secondly, a discrete micro-sized GaN layers were grown by designing a pattern, based on an understanding of the growth behavior on the sapphire nano-membrane. Since the sapphire nano-membrane was a closed structure, ashing method using an oxygen plasma was proposed to remove the PR. By controlling the thickness and density of the membrane, the PR removal rate was observed, and as a result, sapphire nano-membrane of various sizes could be successfully manufactured using suitable conditions. The direction and size of the pattern were appropriately designed using the direction with the fastest and slowest lateral growth rates, and the GaN layers were merged only in the desired area, so that the micro-GaN separated from each other was grown. The micro-GaN layer had a 40% decrease in threading dislocation density (TDD) and a 36.5% increase in photoluminescence (PL) intensity compared to GaN grown on a planar sapphire substrate. Finally, a discrete core-shell-like micro-LED array was grown on the 100 nm-thick sapphire nano-membrane without a harmful plasma etching process. It was confirmed that the sidewalls of MQWs were protected by p-GaN, and self-passivation by p-GaN is expected to prevent the decrease in EQE caused by the plasma etching process. TDD in the micro-LED formed on sapphire nano-membrane was reduced by 59.6% due to the sapphire nano-membranes, which serve as compliant substrates, compared to GaN formed on a planar substrate. In addition, Enhancements in internal quantum efficiency by 44% and 3.3 times higher PL intensity were also observed from it. Cathodic emission of 435 nm was measured in the c-plane multi-quantum well, while negligible levels of emission were observed in the lateral semipolar plane. Cathodoluminescence emission at 435 nm was measured from c-plane multiple quantum wells (MQWs), whereas negligible emissions were detected from semi-polar sidewall facets. A core-shell-like MQWs were formed on all facets, hopefully lowering concentration of non-radiative surface recombination centers and reducing leakage current paths. This study provides an groundbreaking platform for micro-LEDs by using sapphire nano-membrane.3족 질화물 기반 반도체는 고품질, 높은 효율, 긴 수명 등의 장점을 가지고 있어 지난 수십 년 동안 발광 다이오, 레이저, 태양전지 등 광전자 소자 응용 분야에서 상당한 관심을 받고 있다. 질화갈륨 기반 에피층은 경제적, 기술적인 이유로 동종 기판을 사용한 에피 성장이 불가능하여 주로 Si, SiC 및 Sapphire와 같은 이종 기판에서 성장한다. 그 중에서 사파이어 기판은 고품질, 투명성 및 고온 안정성으로 인해 광범위하게 사용하는데 에피층과의 격자 상수 차이와 열팽창 계수 차이로 인해 많은 문제점들이 발생하게 된다. 격자상수 차이로 인하여 광소자 효율에 직접적으로 영향을 미치는 고밀도의 전위 결함이 에피층 내에 생성되고 열팽창 계수 차이로 인하여 고온에서의 에피층 성장 후 상온 냉각 시에 기판 휨 현상과 함께 박막에 큰 압축 응력이 작용하게 된다. 이는 질화갈륨 기반 광소자의 구현을 방해한다. 마이크로 발광 다이오드는 기존 디스플레이 기술인 액정 표시 장치나 유기 발광 다이오드에 비해 고휘도, 빠른 응답 속도, 초고해상도 구현 가능, 낮은 전력 소모 등 우수한 특성을 가지고 있어 차세대 디스플레이 기술로 각광받고 있다. 특히, 앞으로 많은 수요가 기대되는 가상 현실과 증강 현실과 같은 분야에서는 디스플레이가 사람의 눈에 가까워지며, 초고화질의 마이크로 디스플레이를 요구하고 있다. 하지만, 낮은 외부양자효율, 높은 수준의 누설 전류 및 미숙한 마이크로 발광 다이오드 전사 기술 등이 상용화에 걸림돌이 되고 있다. 기존 마이크로 발광 다이오드 제작은 기판 위에 발광 다이오드 에피층을 성장한 후, 개별 마이크로 발광 다이오드를 형성하기 위하여 플라즈마 식각 공정을 이용한다. 이는 활성층 역할을 하는 다중양자우물 구조를 외부에 드러나게 하여 비발광 재결합을 증가시키고 따라서 낮은 외부양자효율과 높은 수준의 누설 전류를 발생하게 한다. 본 연구에서는 사파이어 나노멤브레인 구조가 형성된 기판을 제안하여 고품질의 질화갈륨 에피층을 얻고 마이크로 발광 다이오드의 문제점들을 해결하고자 하였다. 사파이어 나노멤브레인 위의 질화갈륨 성장은 유기금속화학증착법을 이용하여 연구하였다. 질화갈륨 에피층의 성장은 성장 방향과 성장 조건에 따라 다양한 모습으로 성장하며, 여러 결정면들이 나타난다. 사파이어 나노멤브레인 위에 원하는 형태의 마이크로 질화갈륨 에피층을 성장하기 위해, 먼저 사파이어 나노멤브레인 위 질화갈륨의 성장 양상을 이해하는 연구를 진행하였다. 사파이어 나노멤브레인 제작은 포토리소그래피, 원자층 증착 장비를 활용한 비정질 알루미나 증착, 포토리지스트 제거, 후속 열처리 공정을 통한 결정화로 진행된다. 비정질 알루미나층은 열처리 과정에서 고상에피택시를 통해 사파이어 기판과 같은 단결정 알파상 알루미나층으로 결정화된다. 성장 방향에 따른 성장 거동을 파악하기 위해, 스트라이프 패턴의 각도를 다양하게 바꾸며 질화갈륨의 성장을 관찰하였다. 사파이어 [112 ̅0] 방향의 스트라이프 형태 사파이어 나노멤브레인에서 가장 빠른 측면 성장 속도를 볼 수 있었고, 30도 회전할 때마다 측면 성장 속도가 최고와 최소를 반복하는 것을 확인하였다. 방향에 따라 다르게 형성되는 질화갈륨 결정면들의 성장 속도를 측정하여 질화갈륨의 성장 형태가 달라지는 것을 이해할 수 있었다. 또한, 멤브레인 패턴 사이의 바닥 영역에서 성장한 질화갈륨도 관찰하였다. 측면 성장이 원활하게 진행된 멤브레인 위 질화갈륨층에 의하여 바닥 영역으로의 갈륨 확산이 저해되었고, 이는 사파이어 나노멤브레인을 부러뜨려 질화갈륨층을 기판에서 떼어낼 수 있다는 가능성을 보여주었다. 다음으로, 사파이어 나노멤브레인 위 성장 양상에 대한 이해를 토대로 패턴을 설계하여 서로 분리된 마이크로 크기의 질화갈륨층을 성장하였다. 사파이어 나노멤브레인이 막힌 구조이기 때문에 포토리지스트를 제거하기 위한 산소 플라즈마 방법을 제안하였다. 멤브레인의 두께와 밀도를 조절하며 포토리지스트의 제거 속도를 관찰하였고 결과적으로 적합한 조건을 이용하여 다양한 크기의 사파이어 나노멤브레인을 성공적으로 제작할 수 있었다. 측면 성장 속도가 가장 빠른 방향과 가장 느린 방향을 이용하여 패턴의 방향과 크기를 적절히 설계하였고 원하는 영역에서만 질화갈륨층이 합쳐지게 하여 서로 분리된 마이크로 크기의 질화갈륨이 성장하게 하였다. 마이크로 크기의 질화갈륨층은 평면 사파이어 기판에서 성장한 질화갈륨에 비해 관통전위 밀도가 40% 감소하였고, 광 발광 세기는 36.5% 증가하였다. 마지막으로, 100 nm 두께의 사파이어 나노멤브레인 위에 마이크로 발광 다이오드 어레이를 해로운 플라즈마 식각 공정 없이 성장하였다. 서로 분리되어 성장한 마이크로 발광 다이오드의 측면 다중양자우물 층은 p형 질화갈륨에 의해 보호되는 것을 확인하였고 이는 플라즈마 식각 공정으로 인한 광 효율의 저하를 막을 수 있을 것으로 기대된다. 사파이어 나노멤브레인 위에 성장한 마이크로 발광 다이오드의 관통전위 밀도는 평면 사파이어 기판 위에 성장한 발광 다이오드에 비해 59.6% 감소하였다. 또한, 내부양자효율이 44% 향상되었고 광 발광 세기가 3.3배 증가한 것을 관찰하였다. 435 nm의 음극 발광 방출이 c면 다중양자우물에서 측정된 반면, 측면의 반극성면에서 무시할 수 있는 수준의 방출이 관측되었다. 코어쉘 형태의 다중양자우물이 모든 면에 형성되어 있어 비복사 표면 재결합을 감소시키고 누설 전류 경로를 줄이게 된다. 본 연구를 통해, 사파이어 나노멤브레인 기술이 마이크로 발광 다이오드를 위한 획기적인 플랫폼 기술로 발전될 가능성을 기대한다.Chapter 1. Introduction 1 1.1 III-nitride semiconductors 1 1.1.1 General properties of III-nitride materials 1 1.1.2 GaN based LEDs and micro-LEDs 2 1.2 Technical issues with GaN based LEDs and micro-LEDs 8 1.2.1 Lattice mismatch and high dislocation density in GaN epilayers 8 1.2.2 Film stress and wafer bow 9 1.2.3 Low light extraction efficiency 10 1.2.4 Current issues with micro-LEDs 11 1.3 Compliant substrate and pendeo epitaxy 18 1.4 Epitaxial growth of GaN on sapphire nano-membrane 24 1.4.1 Solid-phase epitaxy in amorphous Al2O3 nano-membrane 24 1.4.2 The growth of GaN on ultra-thin sapphire nano-membrane 24 1.5 Thesis contents and organization 31 1.6 Bibliography 34 Chapter 2. Experiments and analysis 41 2.1 Growth equipment 41 2.1.1 Metalorganic chemical vapor deposition (MOCVD) 41 2.1.2 Atomic layer deposition (ALD) 41 2.2 Analysis tools 44 2.2.1 Field emission scanning electron microscopy (FE-SEM) 44 2.2.2 Transmission electron microscopy (TEM) 44 2.2.3 Micro-Raman spectroscopy 44 2.2.4 Micro-photoluminescence (Micro-PL) 45 2.2.5 Cathodoluminescence (CL) 45 Chapter 3. The study on growth behavior of GaN on sapphire nano-membrane 47 3.1 Introduction 47 3.2 Experimental procedure 53 3.3 Results and discussion 57 3.3.1 Fabrication of sapphire nano-membrane 57 3.3.2 Growth behavior of GaN on sapphire nano-membrane with various orientations 60 3.3.3 Growth behavior of GaN on spacing region with various orientations 67 3.4 Summary 70 3.5 Bibliography 71 Chapter 4. The growth of a discrete micro-GaN array with various sizes on sapphire nano-membrane 77 4.1 Introduction 77 4.1.1 Micro-LEDs as an emerging display technology and current issues 77 4.1.2 Sapphire nano-membrane for micro-LEDs 81 4.1.3 Pattern design for discrete GaN dies by using growth behaviors of GaN on sapphire nano-membrane 81 4.2 Experimental procedure 86 4.3 Results and discussion 89 4.3.1 PR removal by asher for fabrication of the sapphire nano-membrane array 89 4.3.2 Growth of a discrete micro-GaN array 101 4.3.3 Threading dislocation density of micro-GaN layer 105 4.3.4 Optical property of micro-GaN layer 108 4.4 Summary 110 4.5 Bibliography 111 Chapter 5. A core-shell-like micro-LED array grown on sapphire nano-membrane 116 5.1 Introduction 116 5.2 Experimental procedure 123 5.2.1 Fabrication of a sapphire nano-membrane array 123 5.2.2 Epitaxial growth of a micro-LED array 124 5.2.3 Characterization 124 5.3 Results and discussion 127 5.3.1 Fabrication of a sapphire nano-membrane array 127 5.3.2 Epitaxial growth of a discrete micro-LED array 129 5.3.3 Characterization of a core-shell-like micro-LED array 132 5.3.4 Transfer of a micro-LED array for device fabrication 145 5.4 Summary 152 5.6 Bibliography 153 Chapter 6. Conclusions 161 국 문 초 록 165 List of publications 167Docto

고효율 GaN 기반 발광다이오드 제작을 위한 중공 구조가 제어된 사파이어 기판에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 윤의준.The GaN-based white light-emitting diodes (LEDs) have attracted much attention as a substitute for conventional illumination such as incandescent light bulbs and fluorescence lamps because of its high efficiency and long life. However, rapid penetration of the LEDs into lighting market has been limited due to its high cost. A major drawback of epitaxial growth of the GaN layer is that native substrates are not yet available in large scale, so heteroepitaxy using sapphire substrate have been typical method for the epitaxial growth. The large differences in the lattice constant and thermal expansion coefficient between GaN and sapphire substrate cause high density of threading dislocations and severe wafer bow. In addition, total internal reflection of the emitted light due to the large difference in the refractive index between the GaN epitaxial layer and outside (air) is one of the factors that reduce light extraction efficiency. To achieve the high efficiency and productivity for cost reduction of the GaN-based LEDs, these technical issues need to be resolved. In this research, in order to overcome the problems, we proposed a growth scheme using cavity engineered sapphire substrate (CES) in which a two-dimensionally patterned cavities are arrayed on a sapphire substrate. Amorphous alumina film was deposited by atomic layer deposition on a photoresist patterned sapphire substrate, and subsequent high temperature annealing resulted in the formation of a cavity array surrounded by a crystallized sapphire shell by solid-phase epitaxy (SPE). It was confirmed that well-defined air-cavity array was successfully incorporated into the sapphire substrate. Also, the amorphous alumina layer was fully crystallized into single crystalline α-phase from the sapphire substrate, indicating that the CES can act as a substrate for the epitaxial growth of GaN. In the growth scheme using the CES, the GaN layer is grown on the SPE α-Al2O3 layer and resultantly crystalline quality of the GaN layer could be dependent on the characteristics of the SPE α-Al2O3, which arouses the importance of the fundamental understanding on the SPE mechanism. Accordingly, we investigated the SPE of stripe-shaped cavity amorphous Al2O3 membrane structure on a sapphire substrate. TEM analysis revealed that the SPE process occurred through 2 stages of the phase transformation from amorphous to γ-Al2O3 and subsequently to α-Al2O3. During the phase transformation to γ-Al2O3, beside SPE at the interface between the amorphous alumina layer and sapphire substrate, nanocrystalline γ-Al2O3 was formed in the upper part of the membrane. However, during the SPE from γ- to α-phase, random nucleation was not observed in our investigation condition, resulting that the whole alumina membrane was transformed into α-Al2O3 by SPE. During the phase transformations, volume of the alumina membrane was contracted by the density increase, which induces stresses and deflections in the Al2O3 membrane structure. Furthermore, the activation energies of the SPE procedure from amorphous to γ-phase and that from γ- to α-phase were obtained as 3.1 eV and 3.9 eV, respectively, by precise measurement of the SPE rate using TEM analysis. In addition, SPE mechanism of the amorphous alumina into the intermediate γ-phase was investigated in detail by phase/orientation mapping using a scanning nanobeam diffraction technique of TEM. This evidently revealed presence of the two stacking-mismatched domains in the epitaxial γ-Al2O3 layer, which can be distinguishable only at the specific projecting direction. More importantly, distribution of the stacking-mismatched domains in the SPE γ-Al2O3 layer gives significant information for understanding the formation mechanism of the γ-Al2O3 domains. The growth behavior of GaN on the CES was investigated. The GaN film was observed to fill the spaces between the cavities at the initial stage of growth and then grow laterally over the cavities, leading to a completely coalesced pit-free smooth surface. CL dark spot density was reduced from 1.9 × 108 cm-2 to 1.4 × 108 cm-2, demonstrating that the threading dislocation density was reduced using the CES. Also, the incorporation of cavities was observed to significantly reduce the stress in the GaN film by ~30%. The output power of LED on CES at an input current of 20 mA was measured to be 2.2 times higher than that on the planar sapphire substrate, indicating that the cavity pattern at the interface significantly enhanced the light extraction. To suppress the undesired growth of GaN on the cavity pattern, we suggested growth of GaN layer on a partially crystallized CES (PCCES) in which only the planar region between the patterns was crystallized into single crystalline (0001) α-Al2O3 while the alumina shell surrounding the cavities consisted of nanocrystalline γ-Al2O3. Due to limited growth rate of nanocrystalline GaN islands on the nanocrystalline alumina shell, c-plane GaN from the planar region laterally overgrows the nanocrystalline GaN islands on cavity patterns without interrupting by them. By using the PCCES, threading dislocations in the GaN region above the cavity patterns was significantly reduced compared to that on the existing CES. As a result, reverse leakage current for the GaN Schottky diode on PCCES was reduced by one order of magnitude compared to that on the existing CES.Chapter 1. Introduction 1 1.1. GaN-based LEDs 1 1.2. Technical issues in GaN-based LEDs 5 1.2.1 High density of threading dislocations 5 1.2.2 Low light extraction efficiency 6 1.2.3 Wafer bow 7 1.3. Patterned sapphire substrate 15 1.4. Cavity engineered sapphire substrate 18 1.5. Solid-phase crystallization 22 1.5.1 Thermodynamics and kinetics of solid-phase crystallization 22 1.5.2 Random nucleation vs. solid-phase epitaxy 23 1.6. Solid-phase epitaxy in amorphous Al2O3 thin film 29 1.6.1 Crystal structure of γ- and α-Al2O3 29 1.6.2 SPE procedure in amorphous Al2O3 thin film 30 1.6.3 Kinetics of SPE in amorphous Al2O3 thin film 31 1.7. Thesis contents and organiation 40 1.8. Bibliography 42 Chapter 2. Fabrication of cavity engineered sapphire substrate 48 2.1. Introduction 48 2.2. Experimental details 49 2.3. PR patterning and thermal reflow 52 2.4. Atomic layer deposition of amorphous Al2O3 layer 56 2.4.1 Optimization of the ALD process for the fabrication of CES 56 2.4.2 Properties of the ALD Al2O3 layer 57 2.5. Thermal treatment for fabrication of CES 66 2.5.1 Annealing condition for fabrication of CES 66 2.5.2 Microstructure and crystalline quality of the annealed Al2O3 layer 68 2.6. CES with various cavity shape 78 2.7. Summary 81 2.8. Bibliography 82 Chapter 3. Investigation on SPE of 3-dimensional amorphous alumina nanomembrane structure on c-plane sapphire substrate 85 3.1. Introduction 85 3.2. Experimental details 87 3.3. Crystallizaton procedure of the 3-D alumina nanomembrane structure 89 3.3.1 Phase transformation from amorphous to γ-phase 89 3.3.2 Phase transformation from γ- to α-phase 92 3.3.3 Fully crystallized α-Al2O3 nanomembrane structure by SPE 96 3.4. Finite elecment simulation for calculation of stress induced in 3-D alumina nanomembrane structure 108 3.5. Kinetics in SPE of amorphous Al2O3 layer 111 3.6. Summary 117 3.7. Bibliography 119 Chapter 4. Investigation on stacking-mismatched domain structure of γ-Al2O3 layer formed on c-plane sapphire substrate by solid-phase epitaxy 124 4.1. Introduction 124 4.2. Experimental details 127 4.3. TEM analysis on the SPE γ-Al2O3 layer 128 4.3.1 Phase/orientation mapping of SPE γ-Al2O3 layer 128 4.3.2 Selected area diffraction pattern and dark field image analysis 131 4.4. Discussion on SPE mechanism of γ-Al2O3 domain structure 137 4.5. Summary 146 4.6. Bibliography 147 Chapter 5. Characteristics of GaN layer and performances of GaN-based LEDs on CES 150 5.1. Introduction 150 5.2. Experimental details 152 5.3. Growth of GaN epitaxial layer on CES 155 5.4. Characteristics of GaN epitaxial layer on CES 159 5.5. Fabrication and performances of LED chips on CES 164 5.6. Summary 170 5.7. Bibliography 171 Chapter 6. Growth of GaN epitaxial layer on partially crystallized cavity engineered sapphire substrate for suppression of parasitic GaN growth on pattern surface 176 6.1. Introduction 176 6.2. Experimental details 181 6.3. Microstructure of the partially and fully crystallized CES 184 6.4. Growth of GaN epitaxial layer on partially and fully crystallized CES 187 6.5. Characteristics of GaN layers on partially and fully crystallized CES 197 6.6. Summary 204 6.7. Bibliography 205 Chapter 7. Conclusion 209Docto

Implantable Neural Probes for Electrical Recording and Optical Stimulation of Cellular Level Neural Circuitry in Behaving Animals.

In order to advance the understanding of brain function, it is critical to monitor how neural circuits work together and perform computational processing. For the past few decades, a wide variety of neural probes have been developed to study the electrophysiology of the brain. This work is focused on two important objectives to improve the brain-computer interface: 1) to enhance the reliability of recording electrodes by optimizing the shank structure; 2) to incorporate optical stimulation capability in addition to electrical recording for applications involving optogenetics. For the first objective, a flexible 64-channel parylene probe was designed with unique geometries for reduced tissue reactions. In order to provide the mechanical stiffness necessary to penetrate the brain, the miniaturized, flexible probes were coated with a lithographically patterned silk fibroin, which served as a biodegradable insertion shuttle. Because the penetration strength is independent from the properties of the probe itself, the material and geometry of the probe structure can be optimally designed without constraints. These probes were successfully implanted into the layer-V of motor cortex in 6 rats and recorded neural activities in vivo for 6 weeks. For the second objective, either optical waveguides or μLEDs were monolithically integrated on the probe shanks for optogenetic applications. Compared to existing methods, this work can offer high spatial-temporal resolution to record and stimulate from even subcellular neural structures. In the experiments using wild type animals, despite optimized recording of spontaneous neural activities, the cells never responded to illumination. In contrast, for the ChR2 expressed animals, light activation of neural activities was extremely robust and local, which phase-locked to the light waveform whenever the cell was close to the light source. In particular, the probes integrated with μLEDs were capable of driving different neural circuit behaviors using two adjacent μLEDs separated only by a 60-μm-pitch. With 3 μLEDs integrated at the tip of each of the 4 probe shanks, this novel optogenetic probe can provide more than 480 million (12!) different spiking sequences at the sub-cellular resolution, which is ideal to manipulate high density neural network with versatility and precision.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111604/1/wufan_1.pd

Epitaxy and Device Design for High Efficiency Blue LEDs and Laser Diodes

The (Al,Ga,In)N materials system has impacted energy efficiency on the world-wide scale through its application to blue light-emitting diodes (LEDs), which were invented and developed in the 1990s. Since then, cost reductions and performance improvements have brought GaN-based LEDs into the mainstream, supplanting outdated lighting technology and improving energy efficiency.One of the main challenges that still limits commercial LEDs, however, is “efficiency droop,” which refers to the reduction in efficiency as the input current density (and with it, the carrier density) increases. This phenomenon especially plagues high power LEDs, which operate in the current density range of 100-1000 A/cm2.Few practical options exist to directly eliminate efficiency droop, however we investigated two complementary approaches to circumvent the phenomenon. The first “high power solution” would employ blue laser diodes as the engine of solid state white lighting in lieu of LEDs. When laser diodes reach the threshold current density for stimulated emission, the carrier density in the active region clamps, simultaneously clamping droop. The wall plug efficiency of the laser diodes can then continue to rise as input current density increases until another effect (usually thermal) overrides it. The second “low power solution” maintains the blue LED as the solid state lighting engine, but shifts the operation point to low current density (and low carrier density) where efficiency droop effects are negligible and other thermal and electrical constraints in the device design are alleviated, enabling designs for high wall-plug efficiency. Both approaches to circumventing efficiency droop are likely to find a home in diverse future technologies and applications for lighting and displays.The challenge to produce high performance blue laser diodes was approached from an m-plane epitaxy platform. m-Plane is a non-polar orientation of the wurtzite (Al,Ga,In)N, which is free from deleterious polarization-related electric fields in the growth direction. m Plane is a naturally occurring crystal plane with high material gain due to its non-degenerate valence band structure, and thus should be well-suited for laser diode applications. However, m plane blue emission suffers from low indium uptake and broad spontaneous emission linewidth. The use of surface “double miscut” was investigated to improve the local step structure and morphology, resulting in higher indium uptake, narrower linewidth and higher peak power in the blue spectrum.The complementary challenge to improve the wall-plug efficiency for LEDs at low power operation focuses primarily on improved light extraction efficiency and low voltage operation. The main sources of extraction efficiency losses in typical c-plane blue LEDs on patterned sapphire substrates are absorption on the metal contacts, in the current spreading layer and on the metallic reflector, which also doubles as the heat sink. With the relaxed constraints at low power operation, new designs become possible. High light extraction designs were vetted with ray tracing software prior to experimental implementation. The highest demonstrated wall-plug efficiency resulting from these designs was 78.2%, and was accompanied by a greater than unity electrical efficiency (1.03) resulting from thermoelectric pumping, suggesting a pathway for 100% or greater wall-plug efficiency

GaN Micro-LED Integration with Thin-Film Transistors for Flexible Displays

The research presented provides a systematic attempt to address the major challenges for the development of flexible micro-light-emitting diode (LED) displays. The feasibility of driving GaN-based micro-LEDs with a-Si:H-based thin-film transistors by using a thin-film bonding and transfer process was initially proposed. This approach was implemented to create an inverted pixel structure where the cathode of the LED is connected directly to the drain contact of the drive TFT resulting in a pixel circuit having more than 2× higher brightness compared to a standard pixel design. This “paste-and-cut” technique was further demonstrated for the development of flexible displays, enabling the study of the effect of mechanical strain and self-heating of the devices on plastic. Through a finite-element analysis, it was determined that the applied stress-induced strain near the quantum wells of the micro-LEDs are negligible for devices with diameters smaller than 20 microns. Thermal simulation of the LEDs on plastic revealed that a copper bond layer thicker than 600 nm can be used to alleviate self-heating effects of the micro-LEDs. Using these design parameters, micro-LED arrays with 20 micron diameter were integrated onto flexible substrates to validate the theoretical predictions. Further scaling of the LED size revealed substrate bending also tilts the direction of the LED structure, allowing further extraction of light. This effect was demonstrated using nanowire LEDs with a 250 nm diameter transferred onto plastic, where the light output could be enhanced by 2× through substrate bending. Finally, through the removal of bulk defect and surface states, fabrication of highly efficient micro-LEDs having > 400% increase in light output (compared to conventional diodes) was achieved. This outcome was accomplished through the removal of the defective buffer region adjacent to the active layers of the LED and minimization of the non-radiative recombination at the sidewalls. The former was accomplished through the removal of the buffer layer after separation of the LED from the process wafer while the latter is accomplished using a surround cathode gate electrode to deplete free carriers from the sidewall of the forward-biased LED. The resulting performance enhancements provided a basis for high-brightness flexible micro-LED displays developed in this dissertation

Design, growth and fabrication and characterisation of InGaN Micro Light Emitting Diodes using a Direct Epitaxial Approach

Free standing micro disks have been the focus of a significant amount of research in recent years. This is due to the ease in creating low threshold micro lasers and micro array devices. Unfortunately, there are issues with the fabrication process that limit the overall efficiency and quality of such devices. The top-down approach used with these micro disk leads to severe sidewall damage from the etching process. Therefore, we present a novel approach using a direct epitaxial method to selectively grow micro disks in a patterned SiO2 template. This thesis presents the design process in which we made these devices and the use of characterisation to optimise the method to create highly efficient micro-LEDs. We also take these devices further and created micro laser cavities using a hybrid epitaxial/ dielectric cavity. Using lattice matched nanoporous GaN/undoped GaN Distributed Bragg Reflectors (DBR) and a dielectric SiO2/SiN based DBR, we can create optically pumped micro disks arrays with stimulated emission with a wavelength of 510nm. Finally, we investigate a new limiting factor in the growth of ultra-small micro disks (<3.5μm) in the form the circularity of the micro disks themselves rather than the roughness of the sidewall

Device Engineering for Internal Quantum Efficiency Enhancement and Efficiency Droop Issue in III-Nitride Light-Emitting Diodes

Over the past few decades, III-Nitride semiconductors have found the tremendous impacts in solid state lighting, power electronics, photovoltaics and thermoelectrics. In particular, III-nitride based light-emitting diodes (LEDs) with long lifetime and eco-friendliness are fundamentally redefining the concepts of light generation due to the superior material properties of direct bandgap, efficient light emission and robustness. The industry of LED based solid state lighting is fulfilling the potential of reducing the 20% of the total US energy consumed by lighting to half of this usage. However, several major obstacles are still hindering the further development of LEDs for general illuminations. They include efficiency droop phenomenon at high operating current, low efficiency in green spectrum, and low extraction efficiency due to the large difference in refractive index. The report will present both experimental and theoretical works on III-nitride semiconductor materials and devices for solid state lighting, including 1) novel barrier design for efficiency-droop suppression, 2) novel active region design for radiative efficiency enhancement, and 3) fabrication of ultrahigh density and highly uniform III-nitride based quantum dots (QDs) for high efficiency optoelectronics and photovoltaic cells. In addition to the three main topics, a new topic on the p-type III-nitrides doping sensitivity will be investigated in the latter part of this report.Firstly, the use of large bandgap thin barrier layers surrounding the InGaN QWs in LEDs will be proposed for efficiency droop suppression. The efficiency of LED devices suffers from reduction at high current injection, which is referred as efficiency droop phenomenon. Although the origin is still inconclusive up till now, the carrier leakage issue is widely considered as one of the major reasons. The increased effective barrier heights from the use of a thin (d \u3c 2 nm) lattice-matched AlGaInN barriers are shown to improve current injection efficiency and internal quantum efficiency. The optimization of epitaxial conditions of lattice-matched AlInN material has been carried out by metal-organic chemical vapor deposition (MOCVD) for the fabrication of InGaN QW LEDs with the insertion of AlInN thin barrier. The device characterizations of cathodoluminescence and electroluminescence show the great potential of the InGaN-AlInN design in addressing the efficiency droop issue at high current density. Secondly, the staggered InGaN QW and InGaN-delta-InN QW are investigated for the high efficiency LEDs emitting at green or longer emission spectrum region to provide solutions for greengap challenge. The introduction of energy local minima in QW region by the novel structures of staggered InGaN QWs enables the spatial shift of electron and hole wavefunction towards the center of active region. Therefore, the approach leads to the enhancement of electron-hole wavefunction overlap and thus the radiative recombination rate and optical gain. The analysis of InGaN-delta-InN QW LED with the potential of effectively extending the emission wavelength without sacrificing the radiative recombination rates will also be presented. Thirdly, the sensitivity study of the doping levels of p-type layers in InGaN/GaN MQW LEDs will be discussed for industrial application. Due to the difficulty in activating the acceptor magnesium in III-nitrides, thermal annealing process is employed to increase the hole concentration in p-type semiconductors. The uniform temperature distributions in the annealing chambers will lead to non-uniformity in p-type doping levels. The effect of doping levels on LED device performance will be examined, and the doping sensitivity of light output power and internal quantum efficiency will be investigated in this report. The results will provide guidance for the parameter optimization of the fabrication process for commercial product line to increase the yield.Fourthly, the growths of ultra-high density and highly uniform InGaN QDs on GaN/ sapphire template as an important alternative active region for high-efficiency optoelectronic devices will be discussed. The growths of ultra-high density and highly uniform InGaN QDs by employing selective area epitaxy were realized on nanopatterned GaN template fabricated by diblock copolymer lithography. It results in well-defined QD density in the range of 8x1010 cm-2, which represents the highest QD density reported for nitride-based QDs. In comparison, the InGaN QD density by the prevailing Stranski-Krastanow (S-K) growth mode is around mid 109 cm-2 with non-uniformity in dot sizes and distributions. The availability of highly-uniform and ultra-high density InGaN QDs formed by this approach has significant and transformational impacts on developing high-efficiency light-emitting diodes for solid state lighting, ultra-low threshold current density visible diode lasers, and intermediate-band nitride-based solar cells