預施應力於氮化銦鎵/氮化鎵多重量子井結構與氮化鎵薄膜再成長於奈米柱之穿透式電子顯微術研究

Abstract

在本研究的第一部份中，我們比較三片具有不同結構的氮化銦鎵/氮化鎵多重量子井樣品的奈米結構以及光學特性。其中一片樣品以傳統長晶方法成長高濃度量子井結構，另一片樣品以預施應力長晶法先成長一低濃度量子井結構再成長高濃度量子井結構。由穿透式電子顯微術所得之影像，我們可觀察到量子井間不同程度的銦原子聚集現象和成分不均勻的變化。經由應力分佈分析計算其各個量子井的平均濃度，我們發現以預施應力長晶法成長的樣品擁有較為顯著的銦濃度增加及較強烈的銦原子聚集。並且從電激發螢光特性來看，利用預施應力長晶法的樣品在電流增加時擁有較小的藍位移。我們推論這現象是歸因於在預施應力的樣品中，當電流增加，發光主要來至於底層的高濃度量子井，有較高的銦濃度，並且擁有較強烈的銦原子聚集現象而有較佳的載子侷限能力進而減少量子侷限史塔克效應及其屏蔽效應。第三片樣品是以預施應力成長法來成長黃光量子井結構及堆疊一層藍光量子井結構來混光產生白光光源。從電激發光光譜來看，在50毫安培電流時，色座標非常接近理想白光光源。並且發現此樣品對量子侷限史塔克效應的影響是相當小的。 在本研究的第二部分中，我們比較兩片氮化鎵薄膜成長在氮化鎵奈米柱上的樣品的奈米結構以及光學特性。兩片樣品都是利用分子束磊晶生長方式在矽基板上成長氮化鎵奈米柱，接著利用有機金屬化學氣相沉積生長方式在氮化鎵奈米柱上覆蓋一層氮化鎵薄膜。透過掃描式電子顯微鏡觀察樣品表面形態，我們發現在高溫成長(900 ℃)時，可以得到較為平坦的結晶表面。並且由X光繞射儀結果來看，在高溫成長的樣品之晶體品質是兩片樣品之中較好的。

In this research, we compare the nanostructures and optical characterizations of three InGaN/GaN multiple quantum well light-emitting diode samples with different structures. The first sample is grown with the onventional technique, the second sample is grown with a low-indium QW before the high-indium QWs based on a prestrained technique. From the HRTEM images, different degrees of indium aggregation and composition fluctuation between QWs are observed. From the calibrations of the average indium contents of these QWs based on the strain state analysis images, we can see the higher indium contents and stronger clustering behaviors in the second sample. From the EL spectrum, we can see the smaller blue shift in increasing injection current level in the second sample, which is based on the prestrained growth technique when compared with the first sample, which is based on the conventional growth technique. It is found that the quantum-confined Stark effect (QCSE) under the prestrained growth condition is actually enhanced. The smaller blue shift in the prestrain sample is attributed to the injection-current dependence of hole distribution among QWs and the enhanced carrier localization in the QWs of increased indium contents. Carrier localization can reduce the QCSE and its screening effect. The third sample is a blue/yellow dual-wavelength white-light InGaN/GaN QWs LED epitaxial structure with its EL spectrum close to the ideal condition in the Commission International de l'Eclairage (CIE) chromaticity based on the prestrained growth technique. The screening effect of the QCSE in the implemented white-light LED is quite small. Then, we compare the nanostructures and optical characterizations of two samples of overgrown GaN films on GaN nano-columns at different growth temperatures. GaN nano-columns are grown on silicon substrate with molecular beam epitaxy. GaN films are directly overgrown on nano-columns with metalorganic chemical vapor deposition. From the plane-view scanning electron microscopy images, the overgrown GaN film in the sample of high-temperature growth (900 ℃) has a flatter surface morphology. From the X-ray diffraction results, the crystal quality of the sample of high-temperature growth (900 ℃) is generally better than the one grown at the low temperature (800 ℃).