InGaAs/GaAs 量子井結構中晶格應變造成之缺陷能階電性分析

Abstract

InGaAs/GaAs量子井結構超過臨界厚度，因應力回復造成不相稱差排而產生缺陷。我們以深層能階暫態頻譜、暫態電容的量測與導納頻譜量測研究缺陷能階、載子分佈與不相稱差排。由深層能階暫態頻譜中得知缺陷能階約於0.33eV~0.49eV之間。這個缺陷的分佈大於量子井的範圍，深入至上下兩層GaAs層，造成載子的空乏。 在高應力回復的樣品中，量測頻率會影響缺陷的縱深分佈圖，所以推測樣品於高頻處有一串連電阻。對其做電容對頻率的量測，發現低頻處的缺陷與深層能階暫態頻譜量測得的缺陷相同，而高頻處由串連電阻量測得的活化能亦約於0.33~ 0.49 eV。由電容對頻率量測的電阻值對應至整個樣品結構厚度，其應為樣品的電阻。 由電容對頻率的量測我們得知能帶圖及其等效電路。我們提出一個新的等效電路模型，其中的R、C都有其物理意義存在，不但可以詳細解釋我們高應力回復的樣品電容中對電壓、電容對頻率的關係，且可應用於一般的缺陷能階。

Electric defect states are investigated in relaxed InGaAs/ GaAs quantum well to find its position in the bandgap and its relation to the carrier distribution and misfit dislocations by deep-level transient spectroscopy(DLTS),capacitance transient, and admittance spectroscopy. From DLTS, a dominating 0.33~ 0.49eV trap is observed for relaxed InGaAs/GaAs quantum well. This trap level is related to the defect states created by the misfit dislocations introduced by strain relaxation. The depth profile of the trap is found to extend beyond the quantum well region which is consistent with that of carrier distribution, confirming that this 0.33~0.49eV trap plays a significant role in carrier depletion. The depth profile DC/C is found to depend on the measuring frequency for highly relaxed sample. This effect is explained by an existence of a high-resistive layer as observed in capacitance-frequency(C-w)spectra. For the highly relaxed sample, two step-like C-w spectra is observed, with-frequency step corresponding to the emission effect of the 0.33~0.49eV trap and high-frequency step corresponding to the RC time constant effect of the high-resistive layer. The resistance of the high-resistive layer is derived and found to have an activation energy of 0.33~0.49eV, further confirming that carrier depletion is originated from the 0.33~ 0.49eV trap. The resistance derived from C-w spectra also agrees with that derived from current-voltage relation and theory from bulk-limited resistance. The C-w spectra permit us to derive the band diagram and its equivalent circuit model. A new equivalent circuit model with each parameter corresponding to certain physical meaning and including the effect of applied voltage is derived to explain the voltage- and frequency-dependent capacitance for the highly relaxed sample. This circuit model can also be applied to general cases of trap-containing Schottky diodes. The physical meaning of the low-frequency capacitance is illustrated.