以穿透式電子顯微術探討氮化物半導體奈米結構

Abstract

本論文中的第一部份，是以高解析穿透式電子顯微鏡術，來研究單一和雙重氮化銦鎵/氮化鎵異質結構樣品的奈米結構和光學特性。此兩系列的樣品皆是以有機金屬化學氣相沉積法的方式來成長四片不同厚度的氮化銦鎵層的樣品，分別是25,50,100,和200奈米。由於異質結構所導致的應變和相分離現象，我們利用測量倒置空間圖和光激螢光的結果來了解不同深度下的晶體品質和能隙。在掃描式電子顯微鏡的結果圖中，我們發現當應變開始釋放時，樣品表面的粗糙度會上升。在雙重異質結構的樣品中，由高解析穿透式電子顯微術所得到的影像，我們可以觀察到銦滴的結構，而且發現當氮化銦鎵層的厚度下降時，銦滴的密度也會下降。 在本論文的第二部份，我們來比較兩片不同結構的氮化銦鎵/氮化鎵多層結構的晶體品質。我們利用測量光激螢光的結果發現氮化銦鎵層較薄氮化鎵層較厚的樣品(8奈米-6對)有比較好的光學特性。從高解析穿透式電子式顯微鏡的影像中也發現，(8奈米-6對)樣品的銦分佈較均勻，而且氮化銦鎵和氮化鎵的界面較清晰。透過應力分布分佈軟體去計算氮化銦鎵井內的平均濃度及銦原子濃度的變化範圍，再加上能量散佈光譜儀的測量結果，我們發現在(8奈米-6對)樣品中，氮化銦鎵井內的銦濃度的變化範圍較小。

The objective of the proposed research uses high resolution transmission electron microscope (HRTEM) to compare the nanostructures and optical properties of single and double MOCVD-grown InGaN/GaN heterostructures as well as the nanostructures of two InGaN/GaN multi-layers of different structures. The single and double heterostructures comprised of varying InGaN thicknesses of 25, 50, 100, and 200 nm. RSM and PL measurements demonstrated the depth-dependent crystal quality and band gap due to the effects of heterostructure-induced strain and phase separation in these samples. SEM results showed that as the strain started to relax the surface roughness rose. HRTEM results of the DH samples showed formations of indium droplets depicting a trend where the density of indium droplets decreased as the InGaN thickness decreased. The second part of this research compared the crystal quality of two InGaN/GaN multi-layers of different structures. PL result implied a better optical quality in the 8nm-6P sample (thinner InGaN thickness and thicker GaN barrier) than in the 10nm-5P sample. HRTEM results showed that the indium distribution was more uniform and the interfaces between the InGaN wells and GaN barriers were clearer in the 8nm-6P sample than in the 10nm-5P sample. The SSA calibrated average indium contents and indium composition fluctuation, which gave us local information about the two samples. We find that the 8nm-6P sample has a weaker indium composition fluctuation than the 10nm-5P sample based on SSA results. EDX demonstrated the same results as SSA.