有機金屬氣相沈積生長氮化銦鎵/氮化鎵量子井之應變操控及其應用

Abstract

本論文中，我們介紹預施應力之化學氣相沈積技術並描述其應用。預施應力生長法可有效地提高氮化鎵/氮化銦鎵量子井主動發光層中的銦濃度，而延伸主動層的發光波長。其生長方式為預先生長一低濃度量子井層於底部，接著在相同的生長條件下，可把原先於綠光波段的主動發光層紅移至橘光波段。從陰極射線螢光頻譜顯示，靠近低濃度量子井的主動量子井層，由於受到低濃度量子井較強的應力效應，致使產生較高的銦濃度而發出橘光，反之，於其上遠離低濃度層的量子井則受到較小的作用。從X光繞射量測、穿遂式電子顯微鏡和應力分析軟體等實驗結果也驗證了利用預施應力技術生長的樣品，其不同深度的量子井有不同的銦含量，越靠近低濃度量子井層的量子井有較高的銦濃度。我們利用此生長技術把原本發綠光的磊晶樣品拉長了80 nm的波長，而製作出橘光發光二極體。再者，我們也利用此技術在不需使用螢光粉下，製作白光發光二極體，其電激螢光頻譜可接近理想白光的色座標位置 (1/3, 1/3)。 為了進一步探討預施應力樣品的頻譜藍移現象，我們比較了傳統長晶法的短波長發光二極體和預施應力長法的長波長發光二極體。在注入相同電流的情況下，預施應力元件有較小的頻譜藍移現象。其原因為當注入電流漸增時，所發出的螢光來自深層較高濃度的量子井，而減少頻譜藍移。同時，我們也比較不同厚度的預施應力位障層所產生的效應，較薄的位障層產生較大的應力效果，使輸出頻譜有較明顯的紅移，也減小電流密度引起的頻譜藍移現象。 此外，我們利用預施應力生長法提高綠光樣品的長晶溫度，使得綠光發光二極體有較好發光效率。相較於傳統長晶法的綠光樣品，預施應力樣品可針對內部量子效率、室溫光激螢光強度、電流20 mA下的電激螢光強度，分別提高至167 %、140 %和182 %，從不同激發強度的光激螢光實驗中，發現預施應力樣品有較小的量子侷限史塔克效應。另外從阿瑞尼茲作圖分析和穿遂式電子顯微鏡實驗中也發現預施應力生長法可降低載子侷限效應，因此預施應力樣品發光效率的提高，主要歸功於樣品缺陷密度的降低。

In this dissertation, we introduce the prestrained growth technique of metal organic chemical vapor deposition and its applications. Indium incorporation in InGaN/GaN multiple quantum wells (QWs) can be effectively enhanced based on the prestrained growth technique. The growth technique means the spectral red-shift of the QWs designated for green emission into the orange range in a light-emitting diode (LED) by adding a low-indium QW at the bottom. The cathodo-luminescence spectra indicate that the long-wavelength QWs close to the low-indium one are strongly influenced by this added QW and mainly emit orange photons. Those near the top are less affected. The techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and strain state analysis (SSA) are used to calibrate indium average contents among the high-indium InGaN/GaN QWs. The results confirm that the high-indium QW closest to the low-indium one has the highest indium content. With the pre-strained growth, orange LEDs are fabricated for elongating the emission wavelength by more than 80 nm. Also, we grow a phosphor-free white-light InGaN/GaN QW LED epitaxial structure with its electroluminescence (EL) spectrum close to the ideal condition in Commission International de l'Eclairage chromaticity based on the presrained growth technique. Furthermore, we demonstrate the smaller blue shift in increasing injection current level of an InGaN/GaN QW LED of a longer EL peak wavelength based on the prestrained growth technique when compared with the result of an LED of a shorter EL peak wavelength based on the conventional growth technique. The smaller blue shift can be attributed to the more contribution to light emission from the deeper QWs of higher indium contents when the injection current level is increased in the prestrain sample. Also, the dependencies of output spectral overall red shift and current-density-induced spectral blue shift on the prestrained barrier thickness in InGaN/GaN QW LEDs of prestrained growth are demonstrated. Due to the stronger prestrain effect in a sample of a thinner prestrained barrier, the overall spectral red-shift range increases and the current-density-induced blue-shift range decreases with decreasing prestrained barrier thickness. Besides, the enhanced emission efficiency and reduced spectral shifts of a green InGaN/GaN QW LED epitaxial structure by using the prestrained growth technique, when compared with a control sample of the similar emission spectrum with conventional growth, are demonstrated. The internal quantum efficiency, room-temperature PL intensity, EL intensity at the injection current of 20 mA are increased by ~167, ~140, and ~182 %, respectively. Based on the pump-power dependent PL measurement, it is found that the quantum-confined Stark effect becomes weaker in the prestrained growth sample. Also, from the calibration of the Arrhenius plots and the transmission electron microscopy study, the carrier localization effect is observed to become weaker under prestrained growth. Therefore, the enhanced emission efficiency is attributed to the decreased defect density in the prestrained sample.