砷化鎵蕭基二極體中砷化銦量子點在光激發下的電容器特性之暫態測量分析

Abstract

本論文的重點在於具有砷化銦量子點(InAs quantum dots, InAs QDs)結構、或是具有氮摻雜之砷化鎵量子井(GaAsN quantum well, GaAsN QW)結構的砷化鎵蕭基二極體(GaAs Schottky diode)，在光激發下的電容-電壓(C-V)及電流-電壓(I-t)之量測與分析。我們發現了在光激發的條件之下，GaAs Schottky diode的電容值將有效地被內部鑲嵌的量子點(井)所調變。產生巨大光電容的來源，是因為量子點所具有獨特的電容器特性，並且量子點在儲存電荷後可產生高達約2伏特的電位降，進而產生具大光電容。量子點(井)所呈現的電容特性也同時調變了GaAs Schottky diode所產生的光電流值，藉由不同的溫度及不同照射光能量的實驗分析，發現從GaAs Schottky diode產生的光電流來源，與GaAs材料內的EL2 defect與Ga vacancy defect產生的電流特性一致。基於以上的論述可建構出樣品的等效RC電路模型，並推導出量子點的充電時間常數、以及C-V、I-V、C-t、I-t各種量測之數學型式並進行模擬。從模擬與實驗中發現所有量測的結果都是非穩態的，若測量時間遠快於量子點所需的充電時間，則測量到的是純粹Schottky diode的特性；若測量時間與量子點充電時間接近時，則測量到的電容及電容值將被量子點所調變。此外，以此理論模型檢視含有應力誘發差排缺陷的InAs QDs樣品，發現了在量子點周圍存在一定程度的缺陷可增強量子點的電容特性。最後，我們也將GaAs Schottky diode應用在振盪器(oscillator)以及訊號放大器(amplifier)。利用量子點(井)照射光後產生電容效應，將可有效地調變振盪器的輸出訊號週期。以結論來說，我們認為任何半導體材料之元件都有可能存在因未知電容產生的充電效應，了解此效應將有助於更準確地分析半導體元件的電性量測結果。

In this study, the transient measurements of the GaAs Schottky diode with InAs quantum dots (QDs), or with GaAsN quantum well (QW), are investigated. The measured capacitance of GaAs Schottky diode can be modulated by the embedded QDs (QW) under illumination, this modulation for capacitance is attributed to the large potential drop (about 2V) caused by the charged QDs (QW). So we can show a significant capacitive property of QDs (QW) charged by photocurrent, and show that the generation of photocurrent is related to the EL2 defect and Ga vacancy in GaAs Schottky layer. According to the results above, the equivalent RC circuit modal of sample can be established, and the formula of several measurements (C-V, I-V, C-t, and I-t) are derived. The measured capacitance and current are determined by the relation between the charging time constant of QDs and the sweeping rate of applied bias. Besides, by analyzing the experiments of the QDs sample with strain-relaxation- induced misfit-defect, we also demonstrate that the defect around the QDs can improve the capacitive property. At last, we show that the GaAs Schottky diode is applied on the oscillator and on the signal-amplifier, and an optical-controlled IC circuit with Schottky diode is also demonstrated in this study.