- 學位論文
利用時間解析量測技術進行掺氮化鋰氧化鋅薄膜的瞬態反射光譜研究
Time-resolved Photoreflectance of LiN-ZnO thin films by femtosecond mode-locked laser
摘要
這篇研究中，我利用反射式泵-探量測技術進行掺雜氮化鋰之氧化鋅薄膜的超快載子量測，並使用可調飛秒鈦藍寶石雷射倍頻後所產生的紫外光短脈衝作為光源，其中心波長可以由362nm調整到381nm，此波段對應量測樣品的導電帶至淺層能帶。在反射式泵-探量測中，當激發光子的能量大於材料能隙時，且激發的載子密度比較低的時候，所量測的折射率的變化為正，這是由於能隙填滿的效應所造成。在接近激子共振的位置，會量測到接近1ps快速的載子鬆弛時間，這是由於激子與聲子的散射所造成的。由所測得的瞬態反射率峰值對激發光波長作圖，發現當光子能量小於激子能隙時，所量測到的瞬態反射率訊號會由正轉負，同樣也是由於「能隙填滿」的效應所造成；且在光脈衝中心波長為376nm的時候，測得瞬態反射率變化接近零，這是由於此波長對應材料的激子共振使得材料的吸收變得非常強烈，使得對應的折射率變化為零。由此我們可以推論樣品的激子束縛能為62.5meV，與氧化鋅薄膜的束縛能60meV非常接近。
並列摘要
In this thesis, we used the reflection pump-probe technique with the UV pulses as light source, that is produced by the frequency doubled tunable femtosecond Ti-sapphire laser, to investigate the carrier dynamics of LiN-ZnO thin film. The center wavelength of the UV pulse is ranging from 362 nm to 381 nm that corresponds to the conduction band to the shallow bandtail of this sample. As the excited photon energy is larger than the bandgap, the transient differential change of the reflection is positive with lower excited carrier density due to the bandfilling effect. By the fitting, the fast carrier lifetime about 1ps can be obtained at near exciton resonance due to the exciton and phonon scattering. The peak of transient reflectance change is plotted as the function of the excited wavelength. When the excited photon energy is lower than the exciton resonance, the transient differential reflection change is switched from positive to negative through the band-filling effect according to the theory. Due to the serious absorption at exciton resonance using the excited wavelength about 376nm, the transient reflection change is around zero due to KK relation. Thus, we recognize the binding energy of the sample is about 62.5meV close to the 60meV binding energy of ZnO thin film