臺灣博碩士論文加值系統

我們提出於氮化鎵和藍寶石基板介面製作倒六角型金字塔結構，以提高紫外光發光二極體的效率。該倒六角形金字塔結構係由非等向性化學蝕刻而成。該紫外光發光二極體，歷經磊晶中斷、蝕刻後再成長的過程。我們分別從TEM剖面圖及XRD中，觀察及量測到較少的穿透缺陷與較窄的晶體繞射半高寬。我們提出了多階層側向磊晶(ML-ELOG)機制以解釋穿透缺陷減少的原因。該樣品在20毫安培的注入電流下，其電激發光之軸向光發光強度相較於參考片提升了120%，而積分強度則提升了85%。
我們透過光追跡模擬及遠場光學實驗，以佐證紫外光發光二極體具倒六角型金字塔結構下之量測結果。此具倒六角型金字塔結構之發光二極體，可藉由氮化鎵-空氣-藍寶石基板間的折射率差異與幾何結構，使光線得以重新導正至軸向光因而使視角減小。此倒金自塔結構可有效提高發光效率，並可藉由部份的應力釋放及多階層側向磊晶機制以減少氮化鎵中穿透缺陷來提高磊晶的晶體品質。
此外，我們可利用該此倒金自塔結構於介面造成的脆弱特性，近一步使用熱壓法去除藍寶石基板以製作thin-GaN發光二極體。實際上，我們亦已大面積地利用該結構以機械式剝離法剝離藍寶石基板。

We report the fabrication and study of high efficient UV light emitting diodes (UVLED) with inverted pyramid microstructures at GaN-sapphire interface. The HIP structures were created by anisotropic chemical wet etching. The UV LED was fabricated by growth interrupt, chemical etching, and regrowth processes. The as grown sample showed reduction of dislocation density in the cross-section TEM images and narrow XRD rocking curves. The ML-ELOG mechanism might elucidate the reason for threading dislocation reduction. The electroluminescent output power at normal direction was enhanced by 120% at 20 mA operating current and the output power integrated over all directions was improved by 85% compared to a reference sample.
The ray tracing simulation and three-dimension far field pattern experimental results of UVLED with HIP GaN/air structure are reported. The HIP structure could reduce the view angle, which redirects the escape light employed by optical geometric property and refraction index difference within GaN/air/Sapphire. The UVLED with inverted pyramid structures have significantly enhanced the light extraction efficiency and at the same time also improved the crystal quality by partially relieving the strain and reducing the dislocation defects in ML-ELOG regrowth process.
Besides, we might employ the structure weakness near the interface, to remove to sapphire substrate with thermal bonding process for fabricate thin GaN LED. Indeed we have had successfully remove sapphire substrate by mechanical lift-off to develop the large area lift-off by HIP structure.

誌 謝 I
Abstract II
摘要 IV
Catalog of contents V
Chapter 1 Preface
1.1 Introduction 1
1.2 Development status of GaN-based UV LED 2
1.3 Motivation and other techniques comparison 4
1.4 Reference 13

Chapter 2 Properties and Types of defect in crystalline lattice
2.1 Brief introduction about defects 15
2.2 Burgers vector 17
2.3 Point defects 18
2.4 Stacking faults 18
2.5 Screw, Edge and Mixed dislocations 19
2.6 Jogs and Kinks 21
2.7 Climb 22
2.8 Dislocations in crystal growth 22

Chapter 3 Experimental principles and methods
3.1 Etching process in molten KOH 26
3.2 Scanning electron microscope-Cathodoluminescence (SEM-CL) 30
3.3 Atomic force microscope (AFM) 31
3.4 Transmission electron microscope (TEM) 34
3.5 X-ray diffraction (XRD) 36
3.6 Photoluminescence spectroscopy (PL) 40
3.7 Electroluminescence spectroscopy (EL) 41
3.8 Reference 42

Chapter 4 Analysis of UVLED regrown on HIP GaN substrate
4.1 Fabrication of UV LED epitaxial structure 44
4.1.1 Process flow of HIP-structure LED 44
4.2 Material properties analysis
4.2.1 Surface morphology analysis of HIP structure 47
4.2.2 Roughness analysis by AFM after LED regrowth 48
4.2.3 X-ray diffraction analysis 49
4.2.4 Defects formation analysis by TEM 51
4.2.5 Model of multi-layers epitaxial overgrowth (ML-ELOG) 53
4.3 Optical properties analysis of UV-LED regrown on HIP GaN substrate
4.3.1 Photoluminescence spectrum 54
4.3.2 Far-field optical pattern 56
4.3.3 TracePro geometrical optics simulation 57
4.4 Electrical properties analysis of UVLED regrown on HIP GaN substrate
4.4.1 Electroluminescence spectrum 59
4.4.2 Current-dependent EL intensity 60
4.4.3 L-I-V characteristic curve analysis 61
4.4.4 Observation of Leakage current 62
4.5 Reference 63
Chapter 5 Mechanical lift-off by HIP-structure
5.1 Mechanical lift-off process flowchart 65
5.2 Material properties analysis
5.2.1 GaN-sapphire interface morphology analysis after mechanical lift-off by SEM 67
5.2.2 GaN-sapphire interface morphology analysis after mechanical lift-off by AFM 68
Chapter 6 Conclusion 70

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