非極性氮化鎵及氧化鋅之結構及光學特性之研究

Abstract

在我的論文裡，將我的研究分成兩個部份:第一個部份是利用熱退火處理來改善a平面的氮化鎵之材料品質，而第二部份是藉由插入a平面的氮化鎵來成長非極性的氧化鋅。改善成長於r平面藍寶石基板上的a平面氮化鎵是在氮氣的環境上利用熱退火於已長成的樣品上。在1000℃的a平面氮化鎵退火下，其表面的粗糙程度只有0.4奈米。X光繞射圖更証實了晶格品質的改善。由x光的Rocking curve實驗，對[0001]方向的氮化鎵而言，半高寬有隨著溫度從850到1100℃逐漸減少的現象。由穿遂式電子顯微鏡(Transmission Electron Microscope)的實驗結果指出在樣口經過1000℃的熱退火後，螺旋差排(Treading Dislocations)沿著[0001]方向的氮化鎵從每平方公分5x1010減少到每平方公分1.5x1010，而疊差的數量從每公分8.7x105減少到每公分4.8x105。室溫的光激發螢光光譜及對應的陰極激發光影像顯示在經過熱退火處理後，比起未經過熱退火處理的a平面氮化鎵而言，a平面氮化鎵的能帶邊緣發光強度有明顯的增加及有較大的發光區域，其中的主要原因是非輻射復合的中心數量的減少。 在實驗的第二部份，藉由插入a平面的氮化鎵當緩衝層，將非極性的氧化鋅層成長於r平面的藍寶石基板。由掃描式電子顯微鏡(Scanning Electron Microscope)的觀察，隨著成長溫度的增加，氧化鋅會從似小草狀的結構轉變成薄膜的結構。X光繞射圖及原子力顯微鏡更証實了成長的樣品具有非極性的方向及平整的表面。由光激發螢光譜的結果，氧化鋅薄膜有最強的近能帶邊緣的發光波長在383奈米並抑制了深層能階的發光。變溫的光激發螢光譜指出，隨著溫度的增加，中性施子束縛激子和中性受子束縛激子會漸漸的轉變成自由激子。藉由活化能的計算，當在高溫的時候，施子束縛的激子和受子束縛的激子的活化能分別對應於激子的束縛能和能量差介於受子束縛激子及施子束縛激子的能量，而在低溫的時候，分別對應於能量差介於施子束縛激子和自由激子及能量差介於受子束縛激子和施子受子對的能量。最後，非極性氧化鋅的薄膜有很大潛力可以運用在新穎的光電材料上。

In this thesis, I divide my experiments into two parts: first is the crystal quality improvement of a-plane GaN by using thermal annealing process and second is growth of non-polar ZnO film via inserting a-plane GaN. The crystal quality improvement of a-plane GaN grown on r-plane sapphire was demonstrated by applying thermal annealing on as-grown samples in nitrogen ambient. The root mean square roughness of the surface was only 0.4 nm in the 1000 °C -annealed a-plane GaN. The crystal quality improvement was confirmed by the X-ray diffraction. Full width at half maximums of X-ray rocking curve for [0001]GaN was gradually decreased when the samples were treated with annealing temperatures from 850 to 1100℃. Transmission electron microscope resulted further indicated threading dislocations were decreased from 5× 1010 cm-2 to 1.5× 1010 cm-2 along [0001]GaN and stacking faults were decreased from 8.7× 105 cm-1 to 4.8× 105 cm-1 after the sample was annealed at 1000 °C. Room-temperature photoluminescence (PL) and corresponding cathodoluminescence image measurements showed band edge emission intensity for a-plane GaN with annealing was enhanced and revealed larger emission area compared to the regular a-plane GaN film, which was attributed to reduction of the non-radiative recombination centers. In the second section, non-polar ZnO film was grown on r-plane sapphire via inserting a-plane GaN layer by using furnace. Scanning electron microscope revealed the morphologies of ZnO were transformed from grass-like structure to thin film with increasing the growth temperature. X-ray diffraction and atomic force microscope measurements confirmed that our sample possessed non-polar crystal orientation and smooth surface. PL results exhibited the film had strongest near band edge emission of 383 nm and quenching of deep level emission. Temperature-dependent PL indicated the neutral donor-bound excitons (DoX) and accepted-bound excitons (AoX) could gradually transit to free excitons (FX) with increasing temperature. The activation energies of DoX and AoX were close to exciton binding energy and the transition variation between AoX and DoX energy for high temperature, while those were close to difference between DoX and FX and between AoX and the acceptor-donor pair for low temperature. Non-polar ZnO films have great potential for applications of novel optoelectronic.