InGaAs/GaAs量子點與GaAsN/GaAs量子井的電性與光性研究

Abstract

本論文主要探討由晶格不匹配的三五族半導體材料所成長的單一量子井和量子點樣品之電、光特性。首先我們成長三片InAs厚度分別為1.1、2.3及3.4 monolayer (ML)的GaAs/InAs/GaAs結構，在長晶時從反射式高能電子束繞射(RHEED)之圖形，我們可以判斷1.1 ML樣品的InAs是以二維模式成長，但是對於2.3及3.4 ML的樣品而言，InAs是以三維的模式成長，也就是形成了量子點結構，PL與TEM也證實了量子點的成長。由電容-電壓的量測顯示出1.1及2.3 ML的樣品在InAs區域都具有載子堆積的現象，表現出位能井的特性，但是對於3.4 ML的樣品卻出現了載子在InAs區域被空乏的現象，這顯示3.4 ML的樣品已發生了晶格鬆弛，在InAs與GaAs的界面處產生了缺陷，此電性量測的結果與TEM的觀測一致，再由DLTS的量測，我們觀察到3.4 ML的樣品共有五類缺陷存在，其中活化能為0.34及0.54eV的缺陷在其它不同In成份的InGaAs量子井結構中也有觀察到，所以是與In成份無關的缺陷。我們利用導納頻譜對量子井厚度為1000A且已晶格鬆弛的GaAs/In0.2Ga0.8As/GaAs樣品做研究，發現活化能為0.33eV的缺陷會空乏電子而造成高電阻，另外我們提出了新的等效電路模型來解釋本片樣品的頻率響應。 相對於InGaAs/GaAs中的InGaAs在平行方向的壓縮應變，GaAsN/GaAs間的晶格不匹配會造成GaAsN的拉伸應變，從x-ray繞射，我們可看出所有樣品都具有良好的結構品質，從PL的結果來看，與GaAsN相關的放射能量隨GaAsN的厚度增加而降低，顯示有量子侷限效應。在電性上觀察到三個缺陷，其中活化能為0.35及0.45eV的訊號推測是屬於與氮相關或是由strain所造成的缺陷。

The electrical and optical characteristics of GaAs-based strained quantum well and quantum dots have been investigated in this dissertation. In the case of compressed strain, the GaAs/InAs/GaAs quantum dots with the InAs thickness varying from 1.1, 2.3 to 3.4 monolayer (ML) are studied. From photoluminescence (PL), cross-sectional transmission electron microscopy (XTEM) and reflection high-energy electron diffraction patterns (RHEED), we observe the presence of quantum dots for 2.3 and 3.4 ML-thick InAs samples. The capacitance-voltage (C-V) measurement shows a carrier confinement for 1.1 and 2.3 ML samples. For 2.3 ML sample, XTEM images show the presence of defect-free self-assembled quantum dots. With increasing the InAs thickness to 3.4 ML, significant carrier depletion caused by the relaxation is observed. In contrast to 1.1 and 2.3 ML samples in which no traps are detected, two broad traps and three discrete traps at 0.54, 0.40 and 0.34 eV are observed in 3.4 ML sample. The traps at 0.34 and 0.54 eV are found to be similar to the traps observed in relaxed GaAs/In Ga As/GaAs single quantum well structure with 1000A-thick InGaAs. By comparing with the XTEM images, the trap at 0.54 eV is identified to be the relaxation-induced dislocation trap in the GaAs layer. The trap at 0.33 eV is found to cause carrier depletion and produce a resistive region in relaxed In Ga As/GaAs single quantum well structure. In order to analyze the capacitance-frequency spectra in such high-resistive structure, a new equivalent circuit model is developed. In contrast to the compressed strain in InGaAs/GaAs material, GaAsN/GaAs material posses a tensile strain. The effect of tensile strain is investigated by continuously varying the GaAsN thickness in GaAs/GaAsN/GaAs single quantum well structures. The x-ray spectra show the interference patterns for the sample with GaAsN up to 590 A, indicating a high flatness of interfaces. The PL data show the decrease of the GaAsN emission energy with increasing the GaAsN thickness, suggesting the effect of quantum confinement. Carrier depletion is observed for the sample with 250A-thick GaAsN. From such sample, three traps at 0.35, 0.45 and 0.75 eV are observed. Among them, the traps at 0.35 and 0.45 eV are speculated to be N-related or strain-induced defects.