氮化銦鎵奈米結構之結構與光學特性

Abstract

本論文旨在探討氮化銦鎵奈米結構之結構與光學特性，其主要研究內容涵蓋兩個主題。首先，在論文的第一部份，我們針對氮化銦鎵奈米結構，進行應變釋放及成核過程的研究。其中，我們利用拉曼光譜，針對不同大小且未覆蓋的氮化銦島狀物進行應變釋放的研究。實驗結果指出不管透過何種方式所成長的氮化銦島狀物，其拉曼譜線的峰值將隨著高寬比的增加而紅移。另外，我們也探討存在於氮化銦與氮化鎵介面中的初始壓應變量，發現此應變量在島狀物剛形成時便透過塑性鬆弛的方式進行釋放，且其釋放程度高達 96 % 以上。而殘餘約 的應變量將隨著島狀物的成長再以彈性鬆弛的方式進行釋放。最後根據簡化之 2-D 模型分析及 3D 模擬結果，建立氮化銦島狀物應變釋放的過程與大小及形貌之間的關係。此外，我們也利用有機金屬氣相磊晶系統，透過成長溫度的改變 (550-750 °C) 成長高銦組成的氮化銦鎵奈米點，並探討其表面形貌、合晶的組成及螢光光譜特性。我們觀察到氮化銦鎵奈米點的成核過程主要是由銦原子所主導。其中，鎵原子溶入氮化銦的過程主要是由原子的遷移能力所決定。因此當氮化銦鎵成長於較高的溫度時，傾向於分別形成高銦組成的島狀結構與高鎵組成的薄膜。其中高銦組成的島狀結構其螢光訊號位於近紅外區域，而高鎵組成的薄膜則被認為是可見光訊號的主要來源。在討論完氮化銦島狀物在成長初期時應變釋放的過程後，我們也研究氮化銦鎵薄膜，在不同銦組成下 (0.13<x<0.38) 其應變釋放的情況。並透過高解析度x光繞射儀及倒置空間圖，進而探討薄膜中，應變釋放所導致銦含量分佈不均勻的特性。根據結構上的分析，我們發現在螢光光譜上所觀察到的雙螢光譜峰與氮化銦鎵的應變釋放情況有關。此外，我們更利用變溫及時間解析螢光光譜進一步分析此雙螢光譜峰，並透過螢光強度隨溫度的變化及載子動態的討論，進而了解其光學特性。最後，我們歸納出氮化銦鎵薄膜中，發光效率與侷限態效應相關的特性。 在論文的第二部分，我們利用顯微螢光光譜技術，量測奈米線寬度介於 9-27 奈米的單根氮化銦鎵/氮化鎵奈米線。在低溫環境下，我們可觀察到氮化銦鎵中來自於侷域化激子複合所產生的單一譜線。透過能量隨激發功率的變化，譜線隨時間飄移的情況，及時間解析螢光光譜，我們發現極化場在氮化銦鎵/氮化鎵異質結構中並不顯著。最後經由組成的分析及有限元素法的模擬，我們猜測銦-鎵原子在介面的擴散情況將對氮化銦鎵/氮化鎵異質結構的應力釋放有所影響。此外，我們也透過壓電材料，討論氮化銦鎵/氮化鎵奈米線中，能隙隨外加應力調變的情況。實驗結果指出，螢光譜峰的能量將隨外加壓應力而藍移。根據數值模擬的結果，發現到所外加的雙軸應力將產生一沿著奈米線之非等向性的應變。由於在我們研究的奈米線中，並無內建壓電場及量子侷限效應，因此實驗所觀察的螢光譜峰位移可歸納於應變所導致的能隙變化量。

The dissertation is devoted to the structural and optical properties of InGaN nanostructures. The main focus of this dissertation can be divided into two parts. First, strain relaxation and nucleation process of InxGa1-xN nanostructures grown by MOCVD are comprehensively studied. The Raman spectroscopy has been utilized to study the strain relaxation in uncapped InN/GaN islands of different sizes. A redshift in the Raman peak with the island’s aspect ratio was observed, regardless of how the InN islands were grown. Most of the initial compressive strain at the InN-GaN interface was found to be released plastically, with a relaxation degree up to > 96 %, during the initial stage of island formations. After that, the residual strain of only , was further relaxed elastically via the surface islanding. Based on a simplified 2D model analysis and full 3D simulations, we established the relationship of the strain relaxation in InN/GaN islands with their size and shape. Besides, the surface morphologies, alloy compositions and PL properties of In-rich InGaN nanodots grown by MOCVD at 550-750 °C have also been investigated. The nucleation of InGaN dots was found to be dominated by the surface migration of In adatoms. In particular, we found that the incorporation of Ga into InN during the growth of InGaN dots is governed by adatom migration capability, which tends to decompose into In-rich islands and a thin Ga-rich layer at higher growth temperatures. In-rich islands exhibit PL emission in the NIR range, while the formation of a thin Ga-rich layer is likely to be responsible for the observed visible emission band. After discussing the strain relaxation of InN islands during the initial stage, the strain relaxation in thick InxGa1-xN films with x ranging from 0.13 to 0.38 are also discussed. The In composition inhomogeneity accompanied by strain relaxations in the InGaN films are studied by high-resolution x-ray diffractions (XRD) and reciprocal space mapping (RSM) along an asymmetric axis. According to the structural analysis, the observed double PL peaks are associated with the strain relaxation in the InGaN films. Besides, further studies on double peaks are also performed by temperature dependent and time-resolved PL. The optical characterizations such as integrated PL intensities versus temperatures and carrier dynamics are demonstrated clearly. Finally, the correlation between emission efficiency and localization effect in InGaN film is also concluded. In the second part of the dissertation, we present microphotoluminescence (μPL) measurements on single InGaN/GaN NWs with diameters in the range of 9-27 nm. Sharp emission lines originated from the recombination of localized excitons in the InGaN layers can be resolved at low temperatures. Excitation dependent energy shifts, spectral diffusions, time-resolved PL measurements of these emission lines indicate that the polarization field is insignificant in the InGaN/GaN heterostructures. Composition analyses together with FEM simulations suggest that In-Ga intermixing also play a role in the strain relaxations in InGaN/GaN NW heterostructures. Besides, we also demonstrate the band-gap tuning of InGaN/GaN NW heterostructures in the part by applying an external stress using a piezoelectric actuator. The luminescence peak energy of InGaN/GaN NWs shows blueshift with the applied compressive stress. According to numerical simulations, applying a biaxial stress will induce an anisotropic strain along the NW. Due to the lack of internal electric field and quantum confinement effect in NW heterostructures, the shift in the PL peak energy can be attributed to the strain induced change in the energy gap.