以時域解析螢光光譜方法研究氧化鋅內之超快激子動態

Abstract

在本研究中我們有系統的探討在氧化鋅薄膜材料中的光學性質。首先我們比較不同型態氧化鋅樣品其自由激子及束縛激子溫變衰減時間，由其螢光頻譜可以看出以有機金屬化學氣相磊晶法成長之氧化鋅在低壓的成長條件下，受體較容易產生，導致較短的激子生命。在高壓下成長的氧化鋅薄膜，和雜質相關的低能量發光頻帶較弱，代表較佳的樣品品質，此類樣品有較長的激子生命期。 為了進一步暸解在高功率激發下激子的多體效應，我們進行一系列變溫及變激發功率的發光量測實驗。我們觀察到雙激子以及施體束縛雙激子的發光特性。由兩階段的衰變期時間常數，我們成功的建立一個四能階的模型來解釋在自由激子、施體束縛激子、雙激子以及施體束縛雙激子間的載子流動。 另外，我們將自由及施體束縛激子視為一混合系統，利用熱衰減理論來探討其激子發光生命期。這些自由及束縛激子的發光生命期成功的結合震盪強度在頻寬改變關係中的預測，顯示自由激子的發光模型在熱能小於施體束縛能的情況下，可成功的用於束縛激子上。

In this research, we systematically investigate the optical properties in a ZnO thin film. First, we compare the photoluminescence spectrum and temperature-dependent PL decay time of FX and D0X in ZnO samples of different morphologies. PL spectra indicate that growth at lower pressures in MOCVD may enhance the incorporation of acceptors in ZnO and hence lead to a shorter lifetime in TRPL calibration. The lack of lower-energy impurity-related emission in high-pressure growth implies the high quality characteristic of the ZnO thin film. This leads to the long lifetime of FX in thin film due to the exciton radiative lifetime nature over the thermal quenching effect. Second, for further understanding the optical properties of excitonic manybody interaction under high excitation, we observe not only the emission line (the M line) of XX, but also that (the D0M line) of donor-bound biexciton (D0XX) in the excitation-power dependent measurement. The calibrated two-stage decay times are used to build the model of ultrafast biexciton dynamics in such a ZnO sample. The interplay between free exciton (FX), donor-bound exciton (D0X), biexciton (XX) and donor-bound biexciton (D0XX) in ZnO will be discussed. We explain the trends of the calibrated decay times successfully with a four-level model. III Finally, the radiative lifetime of the mixed system of FX and D0X is calibrated based on the thermal quenching rate of the integrated PL intensity of the system. With the radiative lifetime data, the FX radiative lifetimes are estimated by using a theoretical relation between the lifetime and the spectral width. From the results of FX radiative lifetime, we also calibrate the D0X radiative lifetimes. The results support our model that the D0X radiative behavior is similar to that of FX when the thermal energy is smaller than the donor binding energy.