鎂摻雜氮化鎵薄膜之冷激光及拉曼光譜的研究

Abstract

在本論文中，我們利用拉曼光譜(Raman)、冷激光光譜(Photoluminescence,PL)、X射線繞射、諾馬爾斯基顯微鏡(Nomarski)及X射線吸收光譜(XAFS)等方法研究成長在氧化鋁(Al203,Sapphire)基板上之鎂摻雜氮化鎵薄膜的結構及光學特性。關於鎂摻雜之氮化鎵薄膜的實驗結果，隨著鎂融入量的提高，拉曼光譜及X射線吸收光譜顯現其結構由六方最密堆積漸漸轉為偏向立方結構，且 X射線繞射及拉曼光譜結果觀測到往高能量位移之藍位移現象。而由冷激光光譜結果，提高鎂融入量時會降低發光效率，顯示增加鎂融入量其薄膜品質變差，且在較高的鎂摻雜氮化鎵薄膜(約為1019 cm-3)之冷激光光譜結果觀察到兩個現象，第一、冷激光訊號強度會有隨時間衰減的現象，且發光波長會因激發強度而改變。第二、固定激發強度時，不同發光波長的衰減時間常數也會改變。由上述現象，我們認為受子的分佈是一個寬的能帶，可能和淺階受子能階重疊，此雜質或缺陷造成的深階受子能階之位置是比鎂取代鎵原子所形成的受子能階還深，且受子能階的分佈並不均勻。另外，增加激發強度時，施子及受子對數目增加，降低了施子及受子對的平均距離，使發光波長往短波長位移。而經過熱處理之後的樣品，會降低雜質或缺陷造成的深階受子能階的分佈，且使受子能階的分佈變窄，達到較穩定的狀態。

The structural and optical properties of Mg-doped GaN films grown on sapphire were characterized by using Raman, Photoluminescence, X-ray, Nomarski microscopy and X-ray absorption fine structure (XAFS). The experimental results show that high Cp2Mg flow rate is favorable to form cubic structure. Too high Mg concentration would deteriorate the structural and optical quality. We also observe the intensity-bleaching phenomenon at high Cp2Mg flow rate. The photon energy is blue-shifted with the increasing excitation power density due to Coulomb interaction. The characteristic time constants are dependent on the photon energy and the sample temperature. The deep Mg-related subband may extend to overlap with the shallow acceptor levels. The analyses of our results suggest that the acceptor states are not distributed evenly and the recombination rate for the high energy DAPs is greater than that for the low energy pairs. However, thermal annealing treatment would diminish the defects and imperfections and thus sharpen the acceptor band.