氮化銦鎵/氮化鎵藍光發光二極體效率衰退現象之改善;Improving the efficiency droop of InGaN/GaN blue light-emitting diodes

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題名:	氮化銦鎵/氮化鎵藍光發光二極體效率衰退現象之改善;Improving the efficiency droop of InGaN/GaN blue light-emitting diodes
作者:	劉學興;Liu,Hsueh-hsing
貢獻者:	電機工程學系
關鍵詞:	有機金屬化學氣相沉積;發光二極體;效率衰退;MOCVD;LED;efficiency droop
日期:	2014-08-28
上傳時間:	2014-10-15 17:11:40 (UTC+8)
出版者:	國立中央大學
摘要:	本篇論文描述以低壓有機金屬化學氣相沉積法成長不同的發光二極體結構去改善氮化銦鎵量子井發光二極體中效率衰退現象，內容分為兩個主要部分: 第一部分是研究在一般c面發光二極體中不同磊晶結構對效率衰退現象的影響，第二部分則是開發半極化發光二極體的磊晶與製程技術，以實現效率衰退現象不明顯的發光二極體。利用氮化鋁鎵/氮化鎵/氮化銦鎵層插入在量子井下方增加電流散佈，並經由實驗的結果討論電流散佈和效率衰退的關係。由模擬的結果顯示，在氮化銦鎵發光二極體結構中加入一層n型氮化鋁鎵/氮化鎵/氮化銦鎵的電流散佈層於量子井的下方，會比使用n型氮化鋁鎵/氮化鎵和n型氮化鎵/氮化銦鎵兩種結構產生較高的電子濃度，因此此n型電流擴散層可以減緩電流聚集並提升外部量子效率，實驗結果也證實整個LED的發光均勻度更加改善，在350 mA的情況下，外部量子效率以及功率轉換效率可以分別提升18.2 % 和22.2%；而效率衰退在46 A/cm2下，則可以從20.9%改善至12.5%。本研究亦設計一個氮化銦鎵的預先插入層經由三甲基銦處理後，降低量子井中銦含量的自聚的實驗，用以探討量子井中銦含量波動對效率衰退的影響。這實驗是以氮化銦鎵(In0.03Ga0.97N)/氮化銦鎵(In0.13Ga0.87N)/氮化鎵的複合式量子井為主體，利用三甲基銦處理氮化銦鎵(In0.03Ga0.97N)/氮化銦鎵(In0.13Ga0.87N)的介面，預期可以減緩發光二極體中的效率衰退。經由表面點缺陷密度、X光繞射、倒晶格空間圖及光激螢光等量測分析結果顯示，此方法降低了銦的自聚使得效率衰退效應減少，並且不會犧牲外部量子效率。與標準結構比較，在大面積的發光二極體中外部量子效率衰退點可以從44 A/cm2移至86 A/cm2，而效率衰退在176 A/cm2下則可以從13.7%改善至5.5%。為了降低電子溢流情形，本論文提出將一p型氮化鎵插入層加在最後一個位障層和p型氮化鋁鎵之間，經由模擬顯示，此結構除了可以增加輸出功率還可以改善效率衰退。實驗結果顯示，發光二極體外部量子效率在25 A/cm2下可以提升12.8 %，其效率衰退的趨勢與模擬結果相當符合。本研究開發出一種利用現場沉積SiNx遮罩形成自組式雙層島狀氮化鎵緩衝層的技術，可以降低磊晶層差排密度至少至原來的三分之一，因此亦可用以探討差排密度對效率衰退的影響。此實驗是在成長氮化鎵於矽基板上進行的，成長過程中插入適當的氮化矽薄層，即可形成雙層島狀結構，此結構可以有效地降低缺陷密度從9.6×109 cm-2到2.6×109 cm-2，利用穿透式電子顯微鏡的觀察結果，本論文亦提出此雙層島狀結構形成的機制及減少差排的原因。實驗結果顯示，將氮化銦鎵發光二極體成長在雙層島狀結構上，可以改善27%的外部量子效率，而效率衰退在100 A/cm2下則可以從52.1%改善至10.8%。根據變溫光激螢光量測結果推測，差排降低會減少量子井中的銦簇集現象，故可改善效率衰退效應。本論文的第二部分敘述開發矽基板上成長半極化（1-101）發光二極體技術的過程，並探討此類發光二極體之效率衰退情形。所開發的技術可在有V行凹槽的7°傾斜的（100）矽基板上獲得高品質的半極化（1-101）氮化鎵，實驗結果顯示，1.2 m厚的半極化氮化鎵之表面粗糙度為 0.3 nm，X光繞射量測的(101) and (002)搖擺曲線半高寬分別為523和581 arcsec。在多條紋半極化發光二極體結構上所測得的光譜半高寬為35.9 nm，在350 mA操作時，此半極化發光二極體幾乎沒有效率衰退現象。 ;This dissertation describes InGaN light-emitting diodes (LEDs) of various structures prepared by low-pressure metal-organic chemical vapor deposition (MOCVD) for improving efficiency droop. The content is composed of two parts: One is investigation of the efficiency droop in c-plane LEDs and the other is developing the technologies of realizing semi-polar (1-101) LEDs on Si for low efficiency droop. An n-type AlGaN/GaN/InGaN heterostructure layer is inserted below the multiple quantum wells (MQWs) to improve current spreading and efficiency droop. As indicated by one-dimensional simulation, an n-type Al0.1Ga0.9N/GaN/In0.06Ga0.93N heterostructure induces a higher electron concentration than either the n-AlGaN/GaN cladding layer or n-GaN/InGaN current spreading layer used in conventional LEDs. Experimental results show that the light output uniformity across a chip is indeed greatly improved. Consequently, the external quantum efficiency (EQE) and wall-plug efficiency is improved by about 18.2% and 22.2%, respectively, at an injection current of 350 mA. The efficiency droop at 46 A/cm2 is also improved from 20.9% to 12.5%. As In content variation in the InGaN quantum well might worsen efficiency droop, inserting an In0.03Ga0.97N pre-layer in the quantum wells and using trimethylindium (TMIn) treatment is proposed to reduce In-rich clusters and improve droop effect. Experimental results indicate that the efficiency droop behavior of InGaN LEDs can be alleviated as expected by using an In0.03Ga0.97N/In0.13Ga0.87N/ GaN composite MQW active layer and TMIn treatment at the In0.03Ga0.97N/ In0.13Ga0.87N interface. Growth pit density, x-ray diffraction, reciprocal space mapping and photoluminescence (PL) measurements indicate that this approach reduces the occurrence of In-rich clusters in the MQWs and leads to reduction in the efficiency droop without sacrificing the EQE of the LEDs. The droop point of the EQE shifts from 44 A/cm2 to 86 A/cm2 compared to the reference sample. There is also an improvement in the efficiency droop at 176 A/cm2 from 13.7% to 5.5%. A In order to decrease the electron overflow current, this study also propose a new InGaN-based LED structure, which has an extra p-GaN spacer layer between the last p-side GaN QW barrier layer and the AlGaN electron blocking layer (EBL). Based on the simulation, this structure is expected to exhibit reduced efficiency droop and enhanced internal quantum efficiency. Experimental results indicate that the EQE is improved by about 12.8 % at an injection current of 25 A/cm2. The experimental results agree very well with the simulation. This dissertation also reports a novel method to produce a self-assembled double-island buffer layer for reducing threading dislocation density in GaN epilayers grown on (111) Si substrates by using in situ SiNx mask during MOCVD. The effect of dislocation density on efficiency droop can thus be analyzed based on these samples. Experimentally, this method effectively reduces the threading dislocation density from 9.6×109 cm-2 to 2.6×109 cm-2. The mechanisms of double-island formation as well as dislocation reduction are described based on transmission electron microscopy investigations. It is also shown that the InGaN LEDs fabricated on the double-island buffer layer exhibit a 27% improvement in their EQE. The efficiency droop at 100 A/cm2 is improved from 52.1% to 10.8%. Based on the temperature dependent PL results, threading dislocations seem to be the driving force of phase segregation in InGaN and cause In-rich regions in MQWs. Reducing threading dislocations will decrease the appearance of indium clusters in QWs and hence the efficiency droop effect. The second part of this work aims at the fabrication of (1-101) semi-polar LEDs on 7°-off (001) Si substrates and their efficiency droop behavior. High quality semi-polar (1-101) GaN epilayers on V-grooved 7°-off (001) Si substrates have been realized as evidenced by the root-mean-square roughness of 0.3 nm as well as the (101) and (002) x-ray rocking curves of 523 and 581 arcsec, measured on a 1.2 m (1-101) GaN. Novel multi-stripe (1-101) blue LEDs show electroluminescence full- width at half-maximum of 35.9 nm. At 350 mA, the LEDs are nearly droop-free.
顯示於類別:	[電機工程研究所] 博碩士論文

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