Abstract
摘 要我們運用時間解析激發探測的技術，在735~775nm 的波長同時做光穿透與反射量測 ，以瞭解低溫成長砷化鎵薄膜中光激載子的動態特性。由實驗結果推論，光激載子乃藉 由能帶填充、熱載子鬆弛、載子再結合與光致吸收等效應，使砷化鎵薄膜的吸收係數與 折射係數發生變化，以至於所量測到的光反射波形有正、負的兩種可能，但光穿透波形 則都是正的，這是由於光致吸收隨載子濃度減少而消失之故。由於樣品的光反射行為所蘊含的物理機制較為複雜，因此無法直接求出其載子生命期，我們遂運用三分量曲線擬 合的技巧萃取載子捕捉時間，其結果與經由光穿透及電脈衝量測所得者是一致的。另外 ，我們也藉著改變激發光的功率去觀察光穿透與反射波形的變化，由實驗結果發現，當 光功率降低時，穿透與反射波形的各個分量所對應之時間常數並未改變，但其中負的分 量的震輻大小在某些波長則明顯的降低，我們認為這是由於光功率下降時，光致吸收效 應受到抑制的結果所致，也因此，在我們的樣品中所觀測到的光穿透波形在各個波長都 是正的。abstractPump-probe photoreflectance (PR) and transmission (PT) weremeasured simultaneously from in situ annealed low-temperature grown (LT-) GaAs at wavelengths ranging from735 to 775nm to investigate the dynamics of photo- carriers.From the experimental results, we conclude that the absorptionand refraction coefficients were changed by the effects ofbandfilling, hot carrier relaxation, carrierrecombination and induced-absorption. While strongwavelength-dependent PR traces were observed, the PT traceswere found to be insensitive to the laser wavelength. Thisinsensitivity was due to the elimination of induced-absorptionwith decreasing carrier density. Since the physicalmechanisms of PR trace were more complicated, the carrierlifetime cannot be extracted directly by a simple curvefitting. A three- component curve fitting technique was usedto extract carrier trapping time, and the results from thefitting are consistent with that from PT and fromphotoconductive response measurement. Besides, we alsoinvestigate the change of PT and PR with decreasing pump beampower. We found that the relaxation time of carriers leavingtheir initial states is still the same, but theamplitude of negative component in the PR traces is greatlyreduced; that is, the induced-absorption effect is greatlysuppressed when at decreased pump beam power. This resultscould just explain the fact why the peaks in the PT traces werealways observed to be positive.