Thin Film Epitaxy, Defects and Interfaces in GaN/Sapphire and ZnO/Sapphire Heterostructures (Polar and Non-polar) for Light Emitting Diodes
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2010-03-15
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Abstract
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 temeperature. 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 inhomogenities 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 inhomogenities 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 inhomogenities. 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 HRTEM 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.  
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Pulsed Laser Deposition, GaN, ZnO, isotropic and anisotropic strain in thin films, Heteroepitaxy
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Degree
PhD
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Materials Science and Engineering
