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Title: Characterization of (11-20) non-polar and (11-22) semi-polar GaN epitaxial films
Author: Johnston, C. F.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2009
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Abstract:
This thesis describes the microstructural characterization (using transmission electron microscopy [TEM], high resolution X-ray diffraction [HRXRD] and atomic force microscopy [AFM]) of non-polar (11-20) and semi-polar (11-22) GaN epitaxial layers. They were grown under a range of conditions on sapphire by metal-organic vapour phase epitaxy (MOVPE). The majority of the dislocations were partial dislocations bounding BSFs. As a consequence, defects were confined to the c-plane. A 3D-2D growth transition (where 3D islands are grown initially, then coalesced) reduced the defect density of non-polar GaN. Optimised coalescence conditions (i.e. a V/III ratio of 50) produced smooth films with a low RMS roughness. The effect of a 3D-2D growth transition on semi-polar material was less clear, but the microstructure was similar to non-polar GaN, in that the majority of dislocations were partials bounding BSFs. Since the growth conditions for the semi-polar epilayers were not optimised, surfaces were rough. Quantum wells grown on these templates were found to follow the surface roughness resulting in large fluctuations in quantum well width (of a few nanometres) and subsequent broadening of peaks in the photoluminescence spectra. To reduce the defect density, epitaxial laterial overgrowth (ELOG), SiNx interlayers and ScN interlayers were applied to non-polar and semi-polar films. ELOG was effective at reducing both BSF and dislocation density in both examples. Thick SiNx interlayers in non-polar GaN reduced dislocation density by a factor of 500 and BSF density by a factor of 5, compared to the template. An 8.5 nm ScN interlayer in semi-polar GaN reduced the dislocation density by a factor of 200 and BSF density by a factor of 14.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.605657  DOI: Not available
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