Investigation of subsurface-structural and optical properties of InAs quantum dots grown on GaAs
|摘要:||由於網路資訊之迅速發展，對高速、安全、穩定之資訊傳輸的需求也日益殷切；光纖通訊因具有高頻寬、不受電磁干擾、質量輕、傳輸衰減率低、不易被竊聽等優勢已逐漸成為寬頻通訊之主流。也因此對光通訊中所必須之高效率且低價格的發光光源的需求亦大幅攀升。其中又以量子點（quantum dot）結構為發光原理之LED為重要光源研發方向。而以次表面分析(subsurface-structural analysis)技術探討雙異質結構量子點微結構(nanostructure)的研究相對較少；但如能直接觀測次表面量子點的顯微結構並分析探討其與量子點之電性與光學特性表現間之相關性，必能增進對量子點之了解，並有助於高效率發光元件之研發。故本篇論文主要分析從次表面之氮(Nitrogen，N)的注入(incorporation position)位置與多層堆疊量子點的空間層數(spacer layers)等參數對砷化銦量子點(InAs quantum dots, InAs QDs)的結構與光學特性的影響。從探討量子點的形貌尺寸及其微結構與材料應變之關係，進而討論量子點之光學性質與應變分布之相關性。本研究中之InAs量子點是以(001) n-type砷化鎵(GaAs)做為基板再利用分子束磊晶法系統(molecular beam epitaxy，MBE)所成長。
在次表面分析中以穿透式電子顯微鏡(transmission electron microscopy，TEM)、高角度環形暗場掃描穿透式電子顯微鏡(high-angle annular dark field scanning transmission electron microscopy，HAADF-STEM)量測InAs(N)量子點結構的形貌與尺寸。另外，以光激發螢光量測系統(photoluminescence，PL)量測InAs(N)量子點的光學性質，傅利葉轉換軟體影像(fast Fourier filtered image)及X光倒易空間圖譜技術(reciprocal space maps，RSM)推測量子點應變狀態，闡明氮注入(incorporated)InAs量子點、砷鎵化銦層(InGaAs)或多層堆疊InAs量子點的空間層數時，對於InAs量子點的光學特性、形貌結構和物理特性變化的影響。
由研究結果顯示，InAsN量子點尺寸比InAs量子點高度小9 %、寬度小36 %。InGaAsN/InAs量子點尺寸比InGaAs/InAs 量子點高度小9 %、寬度集中在20~22 nm；多層堆疊InAs量子點尺寸比單層InAs量子點高度小27 %、寬度大19 %。而在光學特性分析中，InAsN量子點之主要發光波長(峰值位於1250 nm與1350 nm)比InAs量子點分別紅移50 nm與40 nm；InGaAsN/InAs量子點之主要發光波長(峰值位於1252 nm與1335 nm)比InGaAs/InAs量子點分別紅移52 nm與25 nm。多層堆疊InAs量子點之主要發光波長(峰值位於1145 nm與1154 nm)比單層InAs量子點分別藍移70 nm與140 nm。從應變觀點方面分析，可知InGaAsN層與多層堆疊InAs量子點中的空間層數，會使原本InAs量子點的壓應變釋放，而影響發光波長。本研究對InAs量子點結構對其光學性質之影響有進一步之了解，雖未能使量子點波長達到1.5μm，但以朝向目標前進，對開發更接近理想波長之光通訊用量子點應可提供一研發之參考與依據。|
The rapid expansion in the amount of data being sent across the Internet has resulted in the need for faster, safer and more stable ways and media of data transmission. Having many advanced properties such as high speed broadband transmission, free of electromagnetic interference, low loss, no crosstalk, much lower weight and compact in size, fiber-optic communication is becoming the mainstream in the Broadband Global Network. As a result, light sources with high efficiency and low cost are in great demand. Quantum dots (QDs) based LED is one of the most promising light sources and is attracting a lot of attentions of researchers. However, very limited researches are related to the subsurface structural analysis of the quantum dot hetero-structures. In order to further the understanding of QDs so as to improve its efficiency, it is critical to be able to correlate the subsurface nanostructure of QDs to its electronic/optical properties. This thesis aimed to analyze the effects of nitrogen incorporation position and parameters such as thickness/number of spacer layers on the subsurface-structure and optical properties of InAs(N) QDs. The shape/size and subsurface-structure of the QDs were firstly correlated to the resulted material strain. Efforts were then made to study the relationship between optical properties and strain conditions of QDs. InAsN QDs used in this study were grown by molecular beam epitaxy (MBE) on (001) n-type GaAs substrates. The transmission electron microscopy (TEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) were used to observe the morphology of InAs(N) QDs and to analyze their size/shape. Photoluminescence measurements and fast Fourier filtered images together with reciprocal space maps were conducted to measure the emission wavelength and the strain condition of the InAs(N) QDs respectively. The results showed that nitrogen incorporated into InAs QDs, InGaAs layer or spacer layers of multi-layer InAs QDs could have profound effects on the obtained morphology, optical and physical properties of InAs(N) QDs. It is also found in this study that the height and width of InAsN QDs’s are smaller than InAs QDs by 9% and 36% respectively. The height of InGaAsN/InAs QDs is smaller than that of InGaAs/InAs QDs by 9% and the width falls in the range of 20~22 nm. The height of multi-layer InAs QDs are taller than that of single-layer InAs QDs by 27% and their width is wider by 19%. In comparison to InAs and InGaAs/InAs QDs, the main emission peaks of InAsN and InGaAsN/InAs QDs have red shifted 50 nm/40 nm and 52 nm/25 nm to 1250 nm/1350 nm and 1252 nm/1335 nm respectively. In the case of multi-layer InAs QDs, the main emission peaks have blue shifted 70 nm/140 nm from that of single-layer InAs QDs to 1145 nm/1154 nm. From the viewpoint of strain, the presence of InAsGaN and spacer layer(s) in multi-layer InAs QDs tends to relax the existing strain and leads to InAs QDs becoming larger and flatter. As a result, the emission wavelength of QDs is changed correspondingly. Though the wavelength has not quite reached 1.55μm yet, this study has proved to be moving at the right direction.
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