非極性"a"平面氧化鋅多重量子井結構之光學特性

Abstract

在本篇論文中，我們利用脈衝雷射蒸鍍製程技術成長不同的非極性”a”平面氧化鋅/氧化鋅鎂多重量子井作為研究對象。在特性分析中，包含了結構與光學特性上的相關研究。在發光特性方面，所利用的是光激發螢光光譜(PL)，以及吸收光譜(absorption spectra)技術進行樣品的光學特性分析,結構及相關成分的研究包含了穿透式電子顯微鏡圖像、能量散佈光譜、選區繞射…。 在低溫PL可以觀察到比較薄的量子井有較高的量子侷限效應,同時在變功率的PL實驗中,發現樣品的發光波長並不隨者雷射功率增加而改變,意味者非極性面成長的氧化鋅多重量子井確實有效的抑制了QCSE的效應。從變溫PL的光譜變化顯示激子有效的侷限在量子井中以及藉由變溫PL的曲線擬合的結果可以得到激子的束縛能有效的提高。成長非極性結構的另一優勢在於極化發光(polarized light emission)的價值，我們所測得的發光極化率大約在93%左右(@20K)。偏振的吸收光譜解析出了價電帶的能帶位置,而第一個價電帶和第二個價電帶的能量差正是使得氧化鋅的多重量子井有高極化率的主因。 將雷射聚焦到微米等級,並且使用YVO4脈衝雷射去光激發我們的樣品可以觀察到氧化鋅非線性光學的發光現象。在極高的雷射能量密度下光激發,觀察到了電子電洞所形成離子團的發光特性,並且出現了random lasing的特殊現象,此種不需要反射鏡即可產生半高寬小於0.37nm的光源,值得未來更進一步的開發成電激發的光電雷射元件。

In this thesis, a-plane ZnO/ZnMgO multiple quantum wells were grown on r-plane sapphire by pulsed laser deposition for investigation. We utilized several methods including photoluminescence (PL), polarized absorption spectra, and micro photoluminescence (μ-PL) to investigate the optical characteristics of our samples. Energy dispersive spectrometer revealed the magnesium concentration of barrier layer is 20%. Transmission electronic microscopy was further carried out to determine the growth direction of MQWs along [11-20] and the barrier and well thicknesses. The low temperature PL experiments revealed that the larger quantum confinement in thinner quantum well. Meanwhile, the power dependent PL measurement indicated no apparent emission peak shift for all samples due to no built-in electric field in a-plane MQWs. Moreover, the temperature dependent PL reveals that excitons were confined well in MQWs. From the fitting result of activation energy, the exciton binding energy was further enhanced by quantum well. High degree polarization ratio (92%) was observed in ZnO/Zn0.8Mg0.2O MQWs (@20K). From the polarized absorption spectra experiment results, electronic band structure was observed. Larger △E (E2-E1) was the main reason which induced high polarization ratio. Further, we demonstrated the random lasing behavior of a-plane ZnO/ZnMgO MQWs. At high excitation intensity (~55.7MWcm-2), many sharp peaks emerge on the spectra. The FWHM of each individual peaks was about 0.37nm and the mode spacing was about 0.65nm. From the equation we had calculated the exciton binding energy is around 66meV which is closed to the activation energy. We attributed this lasing behavior from the forming of electron-hole plasma and the threshold was about 47.33MWcm-2. For constant pump intensity of 113 MWcm-2, larger excitaition area produces EHP lasing behavior easier. This is also a characteristic behavior in random lasing system.