深層能階缺陷對光激發載子於GaAsN/GaAs量子井結構中的影響

Abstract

本篇論文主要研究GaAsN/GaAs結構中的缺陷與量子井之間的關係。經過PL證實，GaAsN中存在一深層能階(deep level)的缺陷，此能階的濃度與摻雜N的濃度並無關係，且對照其能階的缺陷活化能(activation energy)，應與透過低溫成長的GaAs中的VGa為同一缺陷。當GaAsN受光激發時，其內部載子重新分布，使得量子井樣品的電容值上升，此即一般定義的光電容。為了解光電容的來源，我們將缺陷最多的樣品做了快速熱退火(rapid thermal annealing=RTA)的處理，RTA可將樣品內的缺陷消除，藉此區別出量子井及缺陷對光電容的影響。除此之外，為了解釋量子井受光激發的載子分布，我們認為光激發電子有機會與Top n-GaAs中的Nd中和，產生中性區而改變能帶的結構，其光激發效應如同給予樣品一順向偏壓。此模型並非只對受光激發的量子結構適用，對於樣品表面有自生載子中心(generation center)而言，此結構也能提供合理的解釋。在一般情況下，由於量子井中已侷限許多電子，外加激發光源很難再將過量電子填入量子井中。因此我們再針對不同厚度的量子井做光激發的C-V量測，對於結構最厚的250Å而言，由於此厚度的量子效應已經非常微弱，幾乎沒有侷限任何電子，反而使得光激發電子有機會填入量子井中，在C-V量測中也能明顯看到隨激發光源功率的增加，量子井中的電子濃度逐漸達到飽和的現象。透過以上許多電性上的量測，使得我們對於深層能階在光電容上扮演的腳色，以及光激發電子於GaAsN/GaAs量子井結構中的分佈有更完整的認識。

Deep-level traps in the GaAsN/GaAs quantum well structure have been observed in our previous studies[4].This trap, possibly Ga vacancy (VGa), is induced by a low-temperature growth of the GaAsN layer to suppress the composition fluctuation. The properties of these traps and their related emissions in this material system have been reported. However, the optical properties of the deep-level have seldom been reported. This work investigates the photo-capacitance of the GaAsN/GaAs quantum well structure with various well thicknesses. To realize the origin of the photo-capacitance in GaAsN well, we study the 250 Å sample after annealing. In addition, the carriers exited by illumination or thermal emission from generation centers would neutralize Nd in Top n-GaAs. The carrier redistribution would change the energy band around the well, thus the differential capacitance characterizations would be affected on C-V measurement. In general, the quantum well is hardly filled with photo-carriers since many energy states in well are already occupied by electrons. However, for 250Å, this would happen because of the quantum effect is so weak and the quantum levels are empty before illumination. Through these optical and electrical measurements, we know more the role of deep-level on photo-capacitance and the distribution of photo-carriers in the GaAsN/GaAs quantum well structure.