鎂摻雜氮化鎵化合物薄膜之X光吸收細微結構研究

Abstract

本論文主要是利用X光吸收細微結構 (XAFS) 研究有機金屬氣相沉積磊晶法 (MOVPE) 成長未摻雜與鎂摻雜氮化鎵薄膜中，鎂的摻雜對結構的變化。包括兩部分：氮原子K-吸收邊緣的近X光吸收細微結構 (NEXAFS) 與鎵原子K-吸收邊緣的延伸X光吸收細微結構 (EXAFS)。氮原子近X光吸收邊緣的譜圖中發現，未摻雜與輕摻雜的氮化鎵樣品譜圖與入射光的角度有很明顯的相依性；重摻雜的氮化鎵樣品中相依性則明顯地減弱。意味著，當鎂的濃度很高時會有立方結構 (cubic)相存在於六角結構 (hexagonal)中。鎵原子的延伸X光吸收細微結構結果中，當入射X光的電場方向平行於氮化鎵六角結構中的C軸方向時，鎵吸收原子的最鄰近兩層原子的配位數，在鎂濃度增加時有很明顯的下降趨勢。模型模擬中指出，單獨的空缺缺陷不可能表現出如此大的配位數減少。間隙 (interstitial)、取代 (substitutional)、反佔據 (antisite)、和其他鎂所造成的缺陷是造成嚴重配位數下降的原因。尤其是鎂間隙的產生，由於相位的偏移導致相抵的作用(cancellation)和明顯的配位數減少。另外，模擬的結果亦發現高鎂摻雜樣品結構上有混合結構出現 (mixing structure).

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.