以X光吸收細微結構研究熱退火對鎂摻雜氮化鎵化合物的影響

Abstract

本文我們描述利用X-光吸收細微結構光譜（XAFS）技術研究二次熱退火（two-step thermal annealing）效應對於有機金屬氣相沈積磊晶法（MOVPE）所成長的鎂摻雜氮化鎵（GaN）薄膜上的晶格結構和晶相轉變之影響。我們利用此半導體化合物中氮和鎵兩元素的K-吸收邊緣能量量測了氮原子的近邊緣X-光吸收細微結構（NEXAFS）與鎵原子的延伸X-光吸收細微結構（EXAFS）。 在K-吸收邊緣的近邊緣X-光吸收細微結構方面，當入射X-光被引導在切向入射的方向時（即電場平行c-軸方向 E // c-axis），我們發現許多實驗數據有較顯著的變化。因此，所有的分析和討論皆著重在這個特別的偏振方向。研究的結果顯示經過退火後，在沿著六方結構c-軸方向上傳導帶的p軌域的部分能階密度（p-DOS）增加了。與配位數（CN）相關的傅立葉轉換譜圖的強度（｜FT｜）也相對增強。經過第二次30分鐘的退火之後，擬合（fitting）結果顯示，第一層配位層（氮原子）的配位數幾乎不變，而第二配位層（鎵原子）的配位數已經回升且接近飽和。而且擬合結果也顯示德拜-瓦拉因數（Debye-Waller factor）值也隨著退火而下降了。這兩者的擬合結果指出在晶體中較有序的結構增多了。我們也以模型模擬（simulation）的方法，配合最合適德拜溫度（Debye temperature）和晶相的混合比例，去找出和證實在氮化鎵樣品中的常見缺陷結構，以及立方到六方晶相的轉變。綜合以上所有的X光吸收光譜（XAS）的觀測，我們可以推論，經過二次熱退火步驟之後，結構的缺陷和薄膜的品質都獲得了相當程度的修復和改善。

Here we report the x-ray absorption fine structure spectroscopy (XAFS) study of a two-step thermal annealing effects on crystal structure and phase transformation of Mg-doped GaN films grown by MOVPE. Measurements were made on the K-edge energies of both elements of the compound semiconductor. Nitrogen was with the near-edge absorption fine spectroscopy (NEXAFS), whilst gallium was with the extended one (EXAFS). In the NEXAFS study, the cubic phase component of Mg-doped GaN films was found to reduce, through the increasing angular dependency of the spectra response after thermal annealing. For the highest doped sample, the mixing ratio x of the cubic to the hexagonal polytypes was found to decrease from ~ 50% to ~ 40% in favor of the more stable hexagonal polytype. In the Ga K-edge XAFS investigation, more appreciable variation of the data were found when the incident x-ray was introduced in the grazing angle (i.e. E // c-axis). Accordingly, all analyses and discussions were made for this particular polarization. It was found, after thermal annealing, p-partial density of states (p-DOS) in the conduction band along the hexagonal c-axis was increased. Fourier transform magnitude, | FT |, related to coordination number (CN) also increased. The fitting results showed that the CN of the first-shell (N) remained approximately the same, whereas that of the second-shell (Ga) increased and tended to saturate after the second 30 min thermal treatment. Analysis further showed a decreasing of Debye-Waller (DW) factor through annealing. All fitting results indicated a more ordered structure of the crystal was prevailed. Simulations were performed to fit and to prove the common defects and the cubic to hexagonal transformation of the GaN samples with optimized Debye temperature and the mixing ratio x. In summarizing all these XAS observations, one can conclude that the structure defects were repaired and the film quality was improved to a limited extent after performing the two-step thermal annealing.