本論文旨在應用本實驗室研究團隊已建立之垂直結構GaN-基LED(vertical structure light emitting diode, VLED) 相關技術配合微晶粒LED(micro-LED,μ-LED) 設計，進行以微晶粒VLED 為單元之高功率(3~5 W) 類微晶粒VLED(quasi micro-VLEDs, Qμ-VLEDs) 製程技術開發，並研究其晶粒尺寸效應藉以實施大面積LED 製作，並藉此提升大面積LED (20~50 W) 之發光效率。

本論文研究架構主要分為兩部分，第一部分為類微晶粒VLED(quasi micro-VLEDs, Qμ-VLEDs) 之結構設計及製作。包含以COMSOL Multiphysics進行三種不同切割方式之模擬並比較其優劣，並藉以決定Qμ-VLEDs 之實際製作結構、以及實際製作出之兩種Qμ-VLEDs結構，並將之與傳統大面積單顆VLED(regular VLED)進行光電特性比較，後續並利用COMSOLMultiphysics 與CrosslightAPSYS 進行模擬分析。第二部分為以第一部分之結果延伸之分析，包含調變切割道深度與設計一組可以重複組合組成大面積LED 之Qμ-VLEDs 單元。

本研究實作之Qμ-VLED-A 與Qμ-VLED-B 分別為2×2 與4×4之晶粒並聯類微晶粒結構。輸入電流為350 mA 時，相較於regularVLED，Qμ-VLED-A 的光輸出功率提升約18.92%。

後續由Qμ-VLED-A 與Qμ-VLED-B 延伸之分析中，切割道深度調變之模擬結果顯示，當蝕刻n-GaN 至MQW 裸露時，其光輸出功率比不進行切割之regular VLED 提升了9.45%。大面積LED 之Qμ-VLEDs 單元設計之模擬結果則顯示，在3×3 且單元寬度為400μm 時，本研究所提出之設計，比起相等面積之一般VLED，其光萃取效率提升了8.42%；於5×5 時，其光萃取效率提升了15.89%；於7×7 時，其光萃取效率提升了19.31%。

上述結果顯示，本研究所提出之Qμ-VLEDs 結構以及大面積LED之Qμ-VLEDs 單元設計之方法，確實可改善VLED 之發光效率並提升其光輸出功率。

中文關鍵字：類微晶粒、氮化鎵、垂直結構、大面積設計

SUMMARY
This thesis investigates the size effect and the scaling-up design of GaN-based verticalstructured light-emitting diodes (namely, quasi-micro VLEDs or Qμ-VLEDs). Two types of Qμ-VLEDs and a group of scaling-up unit design are proposed. Simulated and experimental results on uniformities of current distribution and light emission are presented. For a 2×2 arrayed Qμ-VLED with a die size of 41×41 mil2, an enhancement in light output power by 18.9% at 350 mA as compared with that of regular VLED is achieved, which could be attributed to improve current spreading and enhanced light emission through the side wall of the intra-cutting way. With etching depth modulation, the type which the whole n-GaN layer was etched has an enhancement in light output power by 9.45% at 300 mA/mm. Scaling-up unit design that has 3×3 units which has 400 μm width in each unit has an enhancement in extraction efficiency by 8.42% at 300 mA/mm as compared with regular VLED of the same area.

Key words: LED, μ-LED, Qμ-VLEDs, scaling-up unit design

INTRODUCTION
GaN-based light-emitting diodes (LEDs) have attracted much attention because of their potential applications in solid-state lighting (SSL). Further enhancing the luminous flux, efficiency, reliability of power LEDs is very crucial to promote SSL. For the past decade, efforts to solve the current crowding issues in high power LEDs by employing various methods such as vertical-conducting structure, current spreading layer (CSL), current blocking layer (CBL), micro-LED (μ-LED) structure, etc., have been demonstrated.

Among those approaches, the μ-LEDs have been shown having a better uniformity in current distribution and enhanced light output power (Lop) as compared to those of conventional lateral LEDs. Most GaN-based μ-LEDs reported in the literatures are with a lateral structure, in this thesis, a vertical-structured GaN-based LED (abbreviated as VLEDs) with intra-cutting ways and arrayed p-electrodes is proposed to release the issues encountered in conventional μ-LEDs. Since the layout of this device is analogous to μ-LED, but each micro LED is not completely separated, it is thus named as quasi micro-VLEDs (Qμ-VLEDs).

METHOD AND EXPERIMENTS
For the Qμ-VLEDs fabrication, the epi-layer structure of samples using in this work were grown by metalorganic chemical vapor deposition (MOC.V.D). For chip processing, first, an 8-period quarter-wave SiO2/TiO2 double-layer distributed Bragg reflector (DBR) was formed on p-GaN surface over the intra-cutting way region, which could provide excellent light reflection and electrical isolation after inter-cutting. It was followed by deposition and patterning of a reflective/ohmic Ni/Ag/Ni/Au metal system onto the bare p-GaN surface (i.e., regions without DBR) to form 2×2 and 4×4 arrays for Qμ-VLEDs. This metal system was then annealed in O2 ambient at 400 °C for 10 min to maximize its reflection and ohmic contact properties. After that, a blanket deposition of Ni/Au metal systems to serve as an ohmic and adhesive layer was conducted. An 80-μm-thick nickel substrate was formed on the adhesive layer by electroplating, followed by a patterned laser lift-off (LLO) process using a 248 nm excimer laser at a reactive energy of 850 mJ/cm2. After the removal of sapphire substrate, the exposed buffer layer (undoped GaN layer) was removed by an inductive coupled plasma (ICP) etching. The samples were then dipped into a 6 mol/L KOH solution at 60 °C for 5 min for surface roughening to enhance light extraction. For device isolation and formation of intra-cutting ways, the epitaxial structure was etched by ICP all the way down to the DBR layer using patterned photoresist mask. Note that the 20-μm-wide intra-cutting ways are designed to have no intersections with the n-electrodes to avoid step-coverage issue. Finally, a metal pad consisting of Ti/Al/Ti/Au was deposited on the exposed n-GaN layer. The light emission intensity, voltage-current (V-I) and light output power–current (Lop-I) characteristics of the prepared VLEDs were measured using a beam profile, an LED tester and a calibrated integrating sphere, respectively.

Crosslight APSYS and COMSOL Multiphysics were used for the simulation of Qμ-VLEDs' fabrication, effect of the etching depth of cutting way and scaling-up unit design case that has 3×3 units which has 400 μm width each unit. With COMSOL Multiphysics, we can calculate the current distribution of Qμ-VLEDs, and for a quantitative evaluation of the uniformity of current distribution, the coefficient of variation (C.V.) of simulation results were calculated.

RESULTS AND DISCUSSION
Two types of Qμ-VLED devices with 2×2 (each unit with a size of 20×20 mil2) and 4×4 arrays (each unit with a size of 10×10 mil2) for the p-electrodes, denoted as Qμ-VLED-A and Qμ-VLED-B, were prepared. For comparisons, conventional VLEDs (i.e., with no inter cutting), denoted as regular VLED. It is noted that the available light emission area of Qμ-VLED-A and Qμ-VLED-B are about 88.1% and 66.6% of that of the regular one.

Tracepro and Crosslight APSYS were employed to investigate the effects of the proposed scheme on light extraction and current distribution, the calculated optical raytracing simulation results obtained from Tracepro showns that the improvements in extraction efficiency are 8.0% and 12.5% for Qμ-VLED-A and Qμ-VLED-B, respectively, as compared with the regular VLED. With suitable arrangement of the sampling points (n=100), the values of C.V. for current distributions within the active region of regular VLED, Qμ-VLED-A and Qμ-VLED-B at 350 mA/mm are 31.86%, 27.14% and 28.55%, respectively. These results indicate that the Qμ-VLEDs have a better uniformity of current distribution than regular VLED to compensate for less in available emission area. With varying in the etching depth of cutting way, the type which the whole n-GaN has been etched has an enhancement in light output power by 9.45% at 300 mA/mm. Scaling-up unit design case that has 3×3 (5×5, 7×7) units which has 400 μm width each unit has an enhancement in extraction efficiency by 8.42% (15.89%, 19.31%) at 300 mA/mm as compared with same area regular VLED. Design case that has 3×3 units has an enhancement in extraction efficiency by 1% at 300 mA/mm as compared with same area regular VLED.

CONCLUSION
In conclusion, the use of intra-cutting ways and arrayed p-electrodes to improve Lop of VLEDs through a better uniformity in current distribution and an enhanced side-wall light emission has been demonstrated. It is expected that the proposed Qμ-VLEDs could be a potential candidate for the fabrication of high power GaN-based LEDs for SSL in the near future.

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