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Enhancing Light Extraction Efficiency of the AlGaN Deep Ultraviolet Light-emitting Diodes by Utilizing Strong TM Polarized Emission and p-AlGaN Transparent Region

Fayisa, Gabisa Bekele (포항공과대학교)

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초록 moremore
AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) have been attracting increasing attentions for the wide range of applications due to their compact in size, energy-efficient and environmentally friendly. However, their external quantum efficiency (EQE) is still too low to replace widel...
AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) have been attracting increasing attentions for the wide range of applications due to their compact in size, energy-efficient and environmentally friendly. However, their external quantum efficiency (EQE) is still too low to replace widely used highly toxic low-pressure mercury lamps because of their extremely poor light extraction efficiency (LEE). Thus, increasing the LEE of the deep-UV LEDs is very important to improve the EQE, and thereby widely utilize them as deep-UV light sources in the different areas. In this dissertation, the effective approaches to overcome inherently poor LEE of the AlGaN-based deep-UV LED are proposed. A different number of arrays of micro-ring deep-UV LED having an inclined sidewall at the outer perimeter and a p-GaN-removed inner circle of the micro-ring, together with MgF2/Al omnidirectional reflectors have been fabricated. The micro-ring deep-UV LED shows remarkably higher light output power by 70% than the reference, consistent with the calculated result, as well as a comparable turn-on and operational voltages, which are attributed to the effective extraction of strong TM polarized anisotropic emission and the reduction of the absorption loss by the p-GaN contact layer, simultaneously. Moreover, we showed the light output power from the micro-ring LEDs is increased and the operational voltage is reduced while the size of micro-ring is getting smaller owing to the effective extraction of the TM polarized light and better ohmic contact formation by increased n-contact areas. Furthermore, micro-ring with additional circular p-contact at the center of the ring and reduced p-contact size were fabricated. These LEDs show higher light output power as compared with that of the micro-ring LEDs due to the combining effects of increasing the current spreading throughout the active region and further reduction in the deep-UV photon loss at the top layer. Finally, the three terminal semiconductor device known as light emitting triodes (LETs) were fabricated to investigate the holes injection efficiency, together with LEE.
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Contents
Abstract……………………………………………………………………………i
Contents……………………………………………………………………………..v
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Contents
Abstract……………………………………………………………………………i
Contents……………………………………………………………………………..v
List of Figures…………………………………………………………………….vi
List of Tables………………………………………………………………………xi

Chapter 1. Introduction ………………………….……………………………1
1.1 Deep ultraviolet light……………………………………. .…………………2
1.2 Deep-UV light sources and their applications ……………………………3
1.3 Working principle and materials for deep-UV LEDs...………………………5
1.3.1 Basic properties of the AlGaN alloys.…………………………8
1.4 References……..…………………………………………………………….11

Chapter 2. Low light extraction efficiency in AlGaN-based deep-UV light-emitting diodes…………………...................................................................16
2.1 AlGaN-based deep ultraviolet light emitting diodes……………………….17
2.2 Fundamental challenges in AlGaN deep-UV LEDs……………………..19
2.3 Low light extraction efficiency in deep-UV LEDs…………………………25
2.4 Major limiting factors for light extraction efficiency of deep-UV LEDs….28
2.4.1 Strong absorption of deep-UV light in p-GaN layer……….………28
2.4.2 Predominant TM polarized emission…………………………34
2.4.3 Current crowding effects on the light extraction efficiency……......37
2.5 Conclusion…………………………………………………………………40
2.6 References…………………………………………………………………42
Chapter 3. Enhanced light extraction efficiency of the micro-ring array AlGaN deep ultraviolet light emitting diodes....………………………50
3.1 Micro-ring array deep ultraviolet light emitting diodes…………………………51
3.2 Numerical simulation of micro-ring deep-UV LEDs……...……………………53
3.3 Experimental…………………...………………………………………………56
3.4 Characterizations of the micro-ring deep-UV LEDs………………………….59
3.4.1 Optical and electrical property of micro-ring………………………..61
3.4.2 Effect of the micro-ring sizes on the light extraction efficiency……….66
3.4.3 Investigating the effect of the current crowding effects on the light extraction efficiency ………………………………………….............73
3.4.4 Effect of ring-shaped p-contact size on the light output power………..82
3.5 Conclusion …………………………………………………………………86
3.6 References……………………………………………………………………88

Chapter 4. Investigating the holes injection efficiency together with light extraction efficiency by utilizing light emitting triode………...............................89
4.1 Mesh type deep-UV light emitting triodes………...………………..……92
4.2 Deep-UV light emitting triode design and fabrication.…………………98
4.3 Optical and electrical properties of deep-UV light emitting triodes …101
4.3.1 External quantum efficiency and efficiency droop………..………103
4.3.2 Light extraction efficiency of the light emitting triodes……………110
4.4. Conclusion……………………………………………………….……….113
4.5. Reference………………………………………………………..………..115

Chapter 5. Summary and Conclusion………………………………………117
요약문 (Summary in Korean) ………………………………………………120
Acknowledgements……………………………………………………………122
Curriculum Vitae………………………………………………………………