利用穿隧效應以及時間解析量測估算應力鬆弛後InAs量子點之等效電容值

Abstract

本論文對成長厚度為2.7及3.3 ML的兩個樣品進行光性及電性分析，探討應力鬆弛對於InAs/InGaAs量子點光電容特性產生的影響。而在電性量測中發現在3.3 ML樣品中量子點的載子躍遷速率比在2.7 ML樣品中慢了很多，這是因為量子點磊晶的厚度超過臨界厚度約3ML時，會在量子點的後方產生差排缺陷造成很大的空乏區，使得電子的放射速率被缺陷所抑制，也因此形成特殊之光電容現象。我們推論在光激發下，從砷化鎵蕭基接面二極體產生的光電流會受到應力鬆弛後的量子點所影響。當量子點中累積足夠的過量載子後，會在量子點區形成壓降，導致蕭基接面的空乏區改變，造成抑制光電流的現象發生。在本研究中先利用等效RC電路的模型來探討砷化鎵樣品與陶瓷電容器串聯時所造成抑制電流的現象，接著將其結果與應力鬆弛後的量子點(3.3ML)比較後發現，因應力鬆弛會增強量子點的侷限效應(quantum confinement effect)，故形成類似電容器的特性，使得當量子點中累積足夠的載子後會形成壓降來抑制光電流的產生，進而形成特殊之光電容現象。最後我們利用穿隧效應的理論以及實驗來得到量子點之等效電容值約為2000~3000pF。

In this study, we analyze the optical and electrical properties of the strain-relaxed InAs/InGaAs quantum dot (QD) samples with two different InAs deposition thicknesses, 2.7 and 3.3 ML, and discuss the influence of strain relaxation on the Photo-capacitance of InAs QDs with InGaAs capping layer. Relaxation-induced defect reduces the background concentration beneath the QD layer , lowering the carrier emission rate of QDs, and formation of the Photo-capacitance by relaxed QDs Schottky diode. We demonstrate the photocurrent generated by GaAs Schottky diode can be suppressed by the relaxed QDs .When electrons are programmed into the relaxed QDs ,the potential drop is caused by the charged QDs, then reduced the Schottky diode’s depletion width, and suppressed the photocurrent. First, we compared with the effective RC circuit model is proved to interpret the mechanism by using the GaAs bulk sample series connected with a ceramic capacitor and the relaxed QDs have significantly suppress the photocurrent effect , then we can understand the relaxed QDs play a role as a capacitor. Therefore, the photo-capacitance can induced by relaxed QDs sample. Furthermore, we using the time-resolved measurement and tunneling rate theory to estimate the photo-capacitance of relaxed QDs is about 2000~3000 pF.