在照光下蕭基二極體受到鑲嵌在內部的InAs量子點影響的機制與等效電路模型

Abstract

本論文主要探討照光後電容明顯抬升的機制。首先，我們將樣品視為產生電流的蕭基二極體(Schottky diode)及具有電容特性的量子點(QDs)兩個部份。而我們發現了，量測到的Schottky diode的電容值將隨不同強度的照射光源而增加，電容有明顯上升的現象，此現象與升溫時Schottky diode的電容值抬升的現象非常類似，且都會在電容-電壓量測(capacitance-voltage, C-V)中產生新的平台並造成原先空乏的平台延後出現。為了解釋此現象，我們建構出等效電路模型，並藉由理論分析，得知當QDs在充電後產生的電位降將有效地使Schottky diode之電容值抬升。而且QDs的此充電過程在照光及高溫的條件下更為顯著。經由電容與電流的關係亦可計算出量子點的電容大小約在4000 pF，且其電容值不隨照光強度而改變。在最後，以此理論模型為基礎也發現了，若在QDs之上成長位障較高的In0.14AlAs層，其厚度越厚則Schottky diode量測到的電容值也上升越多。這是因為較厚的In0.14AlAs層將更有助於增強QDs的載子侷限能力。

In this study, we discuss the mechanism of the apparently increased capacitance under illumination. The InAs/GaAs quantum dots (QDs) sample can be considered as a GaAs Schottky diode with InAs QDs, and the measured capacitance of Schottky diode can be influenced by the carriers stored in QDs. Since the photocurrent significant charges the QDs, the capacitance of Schottky diode can be apparently increased after illumination. An additional plateau is formed after illumination in C-V, and this new plateau is confirmed as the potential drop of charged quantum dots (QDs). Therefore, the QDs is considered as a capacitor, and the equivalent circuit modal can be build. Through the analysis of the modal, the effective capacitance of QDs can be determined as 4000 pF, and the determined capacitance is independent on illumination power. Besides, we found that the QDs can also be significant charged under high temperature. At last, an In0.14AlAs layer capping on the QDs can enhance the carrier confinement of QDs. So a larger capacitance of Schottky diode is measured for the QDs sample with thicker In0.14AlAs layer.