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Pre-treatment on Patterned Sapphire Substrate for Epitaxial Lateral Overgrowth of GaN

  • 김대식 (Kim, Dae Sik, 대학원 신소재공학과 신소재공학전공)

초록/요약

The III-nitride semiconductors such as GaN, InN, AlN and their alloys have been demonstrated for potential applications in the field of light-emitting diodes (LED), high electron mobility transistors, laser diodes and ultra-violet detectors due to their unique physical properties such as wide direct band gaps ranging from ∼0.7 eV to 6.2 eV, high breakdown field and good thermal conductivity. Among these nitrides, the growth of high quality GaN is studied extensively because GaN layers are utilized as templates for the growth of devices and active layers for III-nitrides based structures. Major developments in wide-gap Ⅲ-Ⅴ nitride semiconductors have recently led to the commercial production of very bright LED emitting from green to near UV. However, its full potential has not yet been realized due to a dramatic lack of suitable GaN bulk single crystals. Therefore, the development of nitride-based devices has relied exclusively on hetero-epitaxy. Although other substrates offer some promise as substrate materials, single crystalline sapphire (Al2O3) wafers are usually used as substrates. However, the large lattice and thermal mismatch between GaN and sapphire substrate generates a high dislocation density and even cracks in the epitaxial layer. Therefore, many studies have focused on stress and defect management techniques to reduce the density of cracks and dislocations. Epitaxial lateral overgrowth (ELO) based on selective area growth (SAG) is promising technique whose value has been demonstrated in fabrication of those devices. The structural quality and optical efficiency of GaN-based materials can be improved using both techniques taking advantage of the strain relaxation and reduced piezoelectric field at the sidewalls of selectively grown ridges. Nevertheless, the SAG process has limited use due to the need for regrowth. In addition, mask materials used in the conventional ELOG process, such as SixNy or SiO2, are potential sources of impurities and can induce strain in the epilayer. As the demand for improving the characteristics of the LED device is increased, many researches for improving the light extraction efficiency by forming patterns on the substrate are being carried out. The patterned sapphire substrate (PSS) technique was derived from the ELO technique, which leads to grow the GaN with lower dislocation density on the PSS. Furthermore, the PSS improves the LEE of GaN-LEDs. The substantial advantage of the PSS technique is that the device structure is grown with a single growth process without any mask, which cannot be avoided in the conventional ELO process. Therefore, the PSS technique is contamination-free from the mask material such as SiO2. Additionally, a single growth process gives economical advantage because of shorter growth time. This study comprises of three research scopes. In the first part, preliminary experiments were conducted to investigate the growth mechanism and growth behavior of GaN on PSS. As a result of the preliminary experiments, the growth of non-optimized GaN causes other problems such as poly-grains on patterns. The poly-grains of GaN grown on the patterns not only degrade the crystallinity of GaN films but also adversely affect the surface roughness. The new methods of two pre-treatments on PSS were proposed to solve these problems. In the second part, a novel GaN growth method has been proposed by using the selective N+ ion-implantation on patterns. It was confirmed that nucleation of GaN at the top of the patterns was controlled by the effect of N+ ion-implantation. Additionally the surface roughness were noticeably improved by this new approach. However, despite the improvement of GaN films by controlling the poly-grains, defects were remained due to the lattice mismatch between GaN films and sapphire substrate. In the last part, sputtered aluminum nitride (AlN) buffer layer has been studied to solve problems. The AlN buffer layer grown at various growth conditions by sputter was optimized by identifying the mechanism to mitigate the stress between GaN and sapphire.

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TABLE OF CONTENTS
Chapter 1. Introduction
1.1 Introduction to III-nitrides ...................................................1
1.1.1 Physical Properties ...................................................................................1
1.1.2 History of GaN .........................................................................................8
1.1.3 Growth of GaN MOCVD ..................................................................................9
1.2. Introduction of GaN-based LED .....................................................13
1.2.1 History of GaN Metal-semiconductor Emitters ..................................13
1.2.2 Internal, Extraction, External, and Power Efficiencies of LED .....................15
1.2.3 Improved External Quantum Efficiency in GaN-based LED ..........................17
1.3. GaN on Patterned Sapphire Substrate .........................................21
1.3.1 Advantage of GaN on PSS ..............................................................................21
1.3.2 Growth and Limitations of GaN on PSS .........................................................26
1.3.3 Pre-treatment on PSS .....................................................................................29

Chapter 2. Theoretical Background
2.1 Overview of the Deposition Equipment ...........................................33
2.1.1 Home-made MOCVD System for GaN ...........................................................33
2.1.2 Growth Mode of GaN by MOCVD .................................................................36
2.1.3 Sputter Processes ...........................................................................................38
2.1.4 Sputtering System for AlN Buffer ...................................................................41
2.2 Hetero-epitaxial Growth of GaN ......................................................44
2.2.1 Epitaxial Growth ............................................................................................44
2.2.2 Lattice Mismatches between Substrates and GaN ..........................................45
2.3 Overview of Patterned Sapphire Substrate .....................................47
2.3.1 Crystal Geometry of PSS ...............................................................................47
2.3.2 Surface Energy of PSS ...................................................................................53
2.4 Growth Behavior of GaN on Patterned Sapphire Substrate ..........56
2.4.1 Initial Growth of GaN on PSS ........................................................................56
2.4.2 Optimization of Growth Conditions of GaN by MOCVD ..............................59

Chapter 3. Experimental
3.1 Pre-treatment by Selective Ion-implantation ..................................72
3.1.1 Mask on PSS ..................................................................................................72
3.1.2 Ion-implantation ...........................................................................................72
3.1.3 Epitaxial Lateral Overgrowth of GaN by MOCVD ........................................75
3.2 Pre-treatment by AlN Buffer .............................................................77
3.2.1 AlN on PSS by Sputtering ..............................................................................77
3.2.2 Epitaxial Growth of GaN by MOCVD ...........................................................77
3.2.3 LED Structure Growth on AlN Buffer ............................................................77

Chapter 4. Results and Discussion
4.1 Pre-treatment by Selective Ion-implantation ..................................79
4.1.1 Pre-treatment of PSS by Selective N+ Ion-implantation .................................79
4.1.2 ELO of GaN on N+ Ion-implanted PSS by MOCVD .......................................82
4.2 Pre-treatment by AlN Buffer .............................................................88
4.2.1 Effect of AlN Buffer Crystallinity on Quality of GaN .....................................88
4.2.2 AlN Crystallinity and GaN Growth Behavior on PSS ....................................99
4.2.3 Characteristics of Light Emitting Diode with AlN Buffer ............................114

Chapter 5. Conclusion ..................................................................122

References ...........................................................................................124

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