| Summary: | 碩士 === 國立交通大學 === 電子物理系 === 88 === Crystal structures of pure and Mg-doped GaN films grown by MOVPE were studied using X-ray absorption fine structure spectroscopy (XAFS) to examine the doping effects of Mg. Whereas the lighter nitrogen was investigated with the near-edge spectroscopy (NEXAFS) on its K-edge, the heavier Ga was performed with the extended one (EXAFS) also on the K-edge. The spectral response of nitrogen NEXAFS was found to dependence eased or became less obvious, however, for the heavily doped samples, revealing the formation of cubic structure in the film when the Mg concentration is high. In the EXAFS results, the coordination numbers of the first two shells of the absorber Ga drastically reduced in increasing Mg concentration when the incident X-ray electric field vector was set to parallel to the GaN hexagonal c-axis. Model simulation results indicated that vacancy defects alone cannot be responsible for the huge depletion in coordination numbers. Interstitial, substitutional, antisite, cluster and other Mg-induced defects in one hand, and mixing of the cubic phase in the hexagonal one on the other, together with the vacancy defects, large Debye-Waller factor and inelastic losses all contributed to the drastic attenuation of the polarized EXAFS envelope that resulted in the huge reduction of the coordination numbers.
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