以X-光繞射技術探討氮化銦鎵/氮化鎵奈米結構

Abstract

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. Then, 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. Finally, 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.