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首页> 外文学位 >Thin film epitaxy, defects and interfaces in gallium nitride/sapphire and zinc oxide/sapphire heterostructures (polar and non-polar) for light emitting diodes .
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Thin film epitaxy, defects and interfaces in gallium nitride/sapphire and zinc oxide/sapphire heterostructures (polar and non-polar) for light emitting diodes .

机译：发光二极管的氮化镓/蓝宝石和氧化锌/蓝宝石异质结构（极性和非极性）中的薄膜外延，缺陷和界面。

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摘要

There are three sources of strain in heteroepitaxial growth, lattice misfit; thermal misfit; and growth related defects. The primary aim of the present work was to do a fundamental study of strain and mechanisms for strain relaxation in epitaxial growth of polar-GaN and polar and nonpolar-ZnO thin films grown on sapphire substrates. We have shown that through the paradigm of domain matching epitaxy (DME) these large lattice misfit systems can be grown in a fully relaxed state at the growth temperature. As a result we need to deal with thermal and defect strains only. Growth of GaN and ZnO films on sapphire is characterized by structural inhomogeneities which are caused by impurities, variation in composition or strain. Depending on crystal structure and growth orientation of epitaxial layers, the presence of strain in epilayers can induce various phenomena which can affect device properties. The inhomogeneities due to strain have been favorably used to increase efficiency of solid state light devices based on GaN and ZnO. An understanding of the epitaxial growth mode and strain generation and relaxation processes in these systems is imperative to constructively exploit strain inhomogeneities. Working towards this end, my research work focused on a fundamental study of epitaxial growth and strain relaxation mechanisms in heteroepitaxy of GaN and ZnO and was conducted in the following three parts.;Epitaxial Nucleation Layer (NL) for GaN based LEDs. This work addressed the formation of nanostructured GaN NL which is necessary to obtain smooth surface morphology and reduce defects in h-GaN layers for LEDs and lasers. From detailed X-ray and HR-TEM studies, it was determined that NL consists of nanostructured grains which were found to be faulted cubic GaN (c-GaN) with a small fraction of unfaulted c-GaN. From X-ray scans and modeling, we determined c-GaN fraction to be over 63% and rest h-GaN. From HRXRD and Raman spectroscopy it was determined that the NL contained in-plane tensile strain, presumably arising from defects due to island coalescence during Volmer-Weber growth.;Two-step growth of Polar ZnO to achieve two-dimensional growth for device layers. In this work, the nucleation layer template was grown at a low temperature (230--290 degC) to induce a two-dimensional growth, followed by growth at a moderate temperature ∼430 degC to form high-quality smooth ZnO layers for device structures. The calculation of c and a lattice parameters by HRXRD for the NLs and the epilayers grown on NL showed that with increase in growth temperature c converged towards relaxed values whereas the a values remained strained. The decoupling observed between a and c lattice parameters of the films has been explained by the controlled kinetics of the growth process and the difficulty of dislocation nucleation.;Epitaxial Growth of Nonpolar ZnO Thin Films. Spontaneous and piezoelectric polarization in ZnO grown along c-axis reduces the efficiency of LEDs. To overcome polarization effects, epitaxy, structure-property correlation and strain relaxation mechanism of nonpolar a-plane(11-20) ZnO grown on r-plane(1-102) sapphire by PLD were studied. The lattice misfit in the plane of film for this orientation varies from -1.5% in [0001]ZnO to -18.3% in [-1100]ZnO direction. Based on anisotropic strain relaxation observed along the in-plane [-1100] and [0001]ZnO stress directions and HRTEM investigations of the interface, it is inferred that plastic relaxation occurring in small misfit direction [0001]ZnO by dislocation nucleation is incomplete. This is consistent with DME concept of a complete strain relaxation for large misfits and a difficulty in relaxing film strain for small misfits.
机译：异质外延生长有三种应变源：晶格失配；晶格失配。热失配；和生长相关的缺陷。本工作的主要目的是对在蓝宝石衬底上生长的极性GaN以及极性和非极性ZnO薄膜的外延生长过程中的应变和应变松弛机理进行基础研究。我们已经证明，通过域匹配外延（DME）范式，这些大的晶格失配系统可以在生长温度下以完全松弛的状态生长。结果，我们只需要处理热应变和缺陷应变。蓝宝石上GaN和ZnO薄膜的生长特征是结构不均匀，该不均匀是由杂质，成分变化或应变引起的。取决于晶体结构和外延层的生长取向，外延层中应变的存在会引起各种现象，这些现象会影响器件的性能。应变引起的不均匀性已被有利地用于提高基于GaN和ZnO的固态照明器件的效率。必须了解这些系统中的外延生长模式以及应变的产生和松弛过程，以建设性地利用应变不均匀性。为此，我的研究工作集中在GaN和ZnO异质外延中外延生长和应变松弛机理的基础研究，并在以下三个部分进行了研究； GaN基LED的外延成核层（NL）。这项工作解决了纳米结构的GaN NL的形成，这对于获得光滑的表面形态并减少LED和激光器的h-GaN层中的缺陷是必需的。通过详细的X射线和HR-TEM研究，可以确定NL由纳米结构的晶粒组成，这些晶粒被发现是断层立方GaN（c-GaN）和一小部分未断层的c-GaN。通过X射线扫描和建模，我们确定c-GaN分数超过63％，其余为h-GaN。根据HRXRD和拉曼光谱法，可以确定NL包含面内拉伸应变，这大概是由于Volmer-Weber生长过程中岛聚结引起的缺陷所致。极性ZnO的两步生长以实现器件层的二维生长。在这项工作中，成核层模板在低温（230--290摄氏度）下生长以诱导二维生长，然后在约430摄氏度的中等温度下生长以形成用于器件结构的高质量光滑ZnO层。通过HRXRD对NL和在NL上生长的外延层的c和晶格参数的计算表明，随着生长温度的升高，c收敛到松弛值，而a值保持应变。薄膜的a和c晶格参数之间观察到的解耦已经通过生长过程的受控动力学和位错成核的困难来解释。非极性ZnO薄膜的外延生长。沿c轴生长的ZnO中的自发极化和压电极化会降低LED的效率。为了克服极化效应，研究了通过PLD在r面（1-102）蓝宝石上生长的非极性a面（11-20）ZnO的外延，结构性质相关性和应变松弛机制。对于该取向，膜平面中的晶格失配在[0001] ZnO方向上为-1.5％至在[-1100] ZnO方向上为-18.3％。根据沿面内[-1100]和[0001] ZnO应力方向观察到的各向异性应变弛豫以及界面的HRTEM研究，可以推断出位错形核在小失配方向[0001] ZnO上发生的塑性弛豫是不完全的。这与DME的概念完全一致，即对于较大的失配，将应力完全松弛，而对于较小的失配，则难以减小薄膜应变。

著录项

  • 作者

    Pant, Punam.;

  • 作者单位

    North Carolina State University.;

  • 授予单位 North Carolina State University.;
  • 学科 Physics Condensed Matter.;Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 285 p.
  • 总页数 285
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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