以X-光繞射技術探討氮化銦鎵/氮化鎵奈米結構

Abstract

在本篇論文中，我們利用了X光繞射技術來探討氮化銦鎵/氮化鎵奈米結構。首先，我們比較預施應力量子井結構與一般長法之量子井結構的X光繞射結果。所謂預施應力之量子井結構即在生長高銦量子井前，先生長一層低銦量子井。由倒置晶格圖譜的結果，我們發現一般樣品的量子井結構是完全受到完全應變，然而在預施應力樣品中，則發現量子井有應變鬆弛的現象。假設預施應力樣品中各高銦量子井的應變鬆弛程度不同，並藉由X光繞射模擬軟體來估算兩個樣品各量子井層之銦濃度及量子井厚度。發現預施應力量子井中，最靠近低銦量子井之高銦量子井之應變鬆弛最大且銦濃度最高，導致最長的發光波長。在增加電子穿透深度的陰極射線螢光頻譜中所觀察到的紅移現象，與我們所計算出來的應變鬆弛及銦濃度分佈相符合。著，我們發展一套可以探測樣品不同深度之X光繞射技術。藉由這套技術，我們可以得到不同深度中之螺旋式差排、邊緣差排之密度及平均橫向區域之範圍。我們應用這項技術以探討利用金屬有機化學氣相沈積法在週期性氮化鎵奈米柱上再接合生長之差排演變。我們固定了奈米柱直徑與間隙之比例，且在不同奈米柱直徑上再生長氮化鎵。由量測結果發現，在直徑與間隙最小之奈米柱上再生長之氮化鎵，其差排密度最小，橫向區域範圍最大，同時也有最高的發光效率。後，藉由X光繞射以及能量散佈X光頻譜，我們探討了氮化銦鎵薄膜生長在氮化鎵上，在臨界厚度附近銦濃度與應變鬆弛程度之相依性。我們發現生長氮化銦鎵至臨界厚度時，異質結構所導致之應變開始鬆弛。然而，在應變鬆弛程度到達大約40% 之前，銦濃度都維持固定。在這點之後，隨著長晶厚度增加，銦濃度與應變鬆弛程度也增加直至應變完全鬆弛。最後，銦濃度也維持在應變完全鬆弛這個階段。

In this dissertation, we utilize X-ray diffraction technique to characterize InGaN/GaN nanostructures. First, we compare the X-ray diffraction (XRD) results of two InGaN/GaN quantum-well (QW) structures to observe the effects of prestrained growth by depositing a low-indium QW before the growth of five high-indium QWs. From the results of reciprocal space mapping, we observe the fully strained condition in the QWs of the control sample. However, in the sample of prestrained growth, the average strain is partially relaxed. By using an XRD fitting algorithm for calibrating QW parameters, we obtain reasonable values for the compositions and thicknesses of the QWs in both samples. In particular, by assuming a non-uniform strain relaxation distribution among the five high-indium QWs in the prestrained sample, we obtain reasonable composition variations among the QWs. The high-indium QW closest to the low-indium one is most strain-relaxed and has the highest indium incorporation, leading to the longest-wavelength emission. The observed red shift with increasing electron penetration depth in the cathodo-luminescence spectra of the prestrained sample is consistent with the distributions of calibrated strain relaxation and indium composition. hen, depth-dependent X-ray diffraction techniques are demonstrated. Screw/edge dislocation density and lateral domain size in each depth can be obtained through depth-dependent X-ray diffraction technique. We apply this technique to study the threading dislocation (TD) evolution during coalescence overgrowth on patterned GaN nanocolumn with metalorganic chemical vapor deposition. From the measurement results, it is found that among the overgrowth samples of different nanocolumn diameters and spacing sizes with fixed nanolumn diameter/spacing ration, the one with the smallest size and spacing leads to lowest TD density, the largest lateral domain size, and the highest photoluminescence efficiency. inally, the dependencies of indium composition and degree of strain relaxation on the growth thickness of an InGaN thin film on GaN near the critical thickness are calibrated based on the measurements of reciprocal space mapping of X-ray diffraction and energy-dispersive X-ray spectroscopy. It is observed that when the growth of InGaN reaches the critical thickness, the hetero-structure-induced strain starts to relax. However, before the degree of strain relaxation reaches ~40 %, the indium composition is fixed at the fully-strained level. Beyond this point, the indium composition increases with growth thickness until the strain becomes fully relaxed. After this point, the indium composition is fixed at this high level.