EL2 缺陷對於InAs/GaAs 量子點的電子放射特性分析:照光的影響

Abstract

本論文主要探討含有EL2缺陷能階之分子束磊晶成長厚度2.2 ML InAs/GaAs單一量子點，藉由有無照射光源下瞭解EL2缺陷對於其電子放射特性的影響。首先，利用電容電壓(C-V)、深層能階暫態頻譜(DLTS)和光激發螢光頻譜(PL)分析探討其基本光電特性，並藉由C-V與暫態電容頻譜(C-t)分析中觀察到缺陷訊號的電容值貢獻。此缺陷訊號受熱激發主導，在高溫時電子躍遷速率變快，使直流偏壓掃過時能貢獻於電容值。進一步以DLTS量測確認此為低溫成長GaAs時易形成的EL2缺陷，接著針對DLTS分析之缺陷訊號，估算出局部侷限能階缺陷濃度，其濃度值相當高接近於長晶時摻雜之背景濃度。再來，藉由照光能量0.8 eV激發EL2缺陷能階，觀察到大量電容值抬升，並引入光強度及光激發之捕捉截面積，說明照光對於缺陷能階及量子點能態的影響，其與電子躍遷速率成正比。另外，也從載子濃度分佈中觀察出量子點能態受照光影響，電子佔據機率改變的現象。除此之外，從照光的DLTS量測中可直接觀察到載子速率變快的情形。最後，利用照光光源能量的不同(0.7 eV ~ 1.56 eV)，藉此了解光電容、光電流和輻射復合三者的關係，並加入rate equation作進一步描述。

The electron emission properties of the EL2 defect state with (without) illumination in 2.2-ML(monolayer) InAs self-assembled quantum dots (QDs) containing an EL2 defect state is presented. Initially, the defect state is observed at the temperature dependence capacitance-voltage (C-V) profiling, leading to the faster electron emission rate with temperature increasing. The source of the EL2 defect state is studied by deep-level transient spectroscopy (DLTS) measurements. Moreover, we calculate the concentration of the EL2 defect state, which is compared with the doping background concentration of top GaAs layer. Under an energy illumination of 0.8 eV, the large capacitance produces, suggesting the electron emission rate of the EL2 defect state increasing. An expression of electron emission rate is dependent with the intensity of the excited light source and optical cross section. Furthermore, the electron occupancy probability is changed upon illumination. The DLTS measurement under illumination also shows the electron emission rate increasing. Finally, the electron-hole pairs produce in the QDs and defect states under illumination energy of 0.7-1.56 eV can explain the relationship between photocapacitance, photocurrent, and carrier radiative recombination. Furthermore, a simple rate equation can describe this phenomenon.