應力鬆弛對InAs/InGaAs量子點特性之影響

Abstract

本論文對成長厚度為3.06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量子點的影響。雖然在光激螢光頻譜(PL)中，可以看到兩個樣品的量子點都發生了應力鬆弛的現象。但在電容電壓量測(C-V)和導納頻譜量測(C-F)中卻發現，在3.3 ML樣品中量子點的載子躍遷速率比在3.06 ML樣品中慢了很多。從深層能階暫態頻譜(DLTS)上可以發現這是兩個樣品中缺陷濃度的不同，並且由C-V模擬可以印證缺陷造成了底層GaAs背景濃度的下降。 從3.06 ML樣品的PL頻譜及TEM (Transmission Electron Microscopy)圖，可以看到存在著兩群量子點，分別來自於In向外擴散(高能量量子點)及產生差排缺陷(低能量量子點)的應力鬆弛量子點。在3.06 ML樣品的變溫PL頻譜，發現當溫度升到140 K時，有高能量量子點半高寬下降以及低能量量子點積分強度的上升的現象。這是由於兩群量子點間產生載子轉移效應所造成。量子點間載子轉移的效應使我們在3.3 ML樣品的C-F低溫量測中看到，電子躍遷速度在溫度升高到140 K時，有不尋常的下降現象。 此外，我們觀察在光激發下的量子躍遷行為。在光激發下，有更多的電子注入量子點中。而由於導帶中載子濃度的增加，會造成量子點和缺陷的電子躍遷速率變快。

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, 3.06 and 3.3 ML, and discuss the influence of strain relaxation on the InAs QDs with InGaAs capping layer. While the strain relaxation is observed from the photoluminescence (PL) spectra of both QD samples, the carrier emission rate from the QDs, which can be obtained from the capacitance-voltage (C-V) and admittance (C-F) measurements, is much slower in the 3.3 ML sample than that in the 3.06 ML sample. It is attributed to the different defect concentration of two samples, estimated from deep-level transient spectroscopy (DLTS) measurements. The C-V simulation demonstrates the background concentration in the bottom GaAs is lowered by the defect. From the PL spectra and transmission electron microscopy (TEM) data of the 3.06 ML sample, we discover there are bimodal QDs in the samples. One of the QD families is strain-relived by indium out-diffusion, and the low-energy QD family is strain relaxed by the generation of misfit defect. PL properties of the 3.06 ML sample are studied as a function of temperature from 10 to 300 K. At about 140 K, an enhancement of the integrated PL intensity of the low-energy QDs and a decrease of the FWHM of the high-energy QDs are observed. These abnormal temperature behaviors are attributed to the carrier transfer from the high-energy QD family to the low-energy one. From the C-F measurements of the 3.3 ML sample, the interdot carrier transfer can be seen as the electron emission rate from the QDs exhibiting an unusual reduction at about 140K. Furthermore, the electron emission of the QD samples under illumination is investigated. During illumination, more electrons are injected into the QDs. The electron emission rate becomes faster due to the amount of electron on the conduction band is increased.