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Title: Structural characterisation of heteroepitaxial zincblende gallium nitride
Author: Lee, Lok Yi
ISNI:       0000 0004 7962 0340
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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To achieve white and colour-tuneable lighting, the mixing of light from red-, green- and blue- wavelength LEDs is desired. At present, the efficiency of green-wavelength LEDs based on InGaN is only about half that of red phosphide and blue nitride LEDs. This is known as the 'green gap' problem. III-nitrides based on the zincblende crystal structure have the potential to bridge the 'green gap' due to the theoretically achievable absence of internal polarisation fields that plague the commonly used c-plane wurtzite crystal structure. In this thesis, metastable zincblende GaN grown by metal-organic vapour phase epitaxy is stabilised on 3C-SiC/Si substrates with a miscut, ranging from 0° to 4°. Crystalline zincblende GaN nucleates as elongated islands which coalesce with increasing GaN deposition. Annealing of nucleation layers reduces the substrate coverage, due to both material desorption and the ripening of islands. Incomplete substrate coverage of the nucleation layer results in a low zincblende phase purity and pits in the subsequent epilayer. For epilayer growth, a temperature of around 885 °C and a V/III-ratio of 38 produce zincblende GaN with a phase purity of more than 98 % and the lowest surface roughness in the range of parameters investigated. The surface morphology of elongated features found in both nucleation layers and epilayers is explained by the anisotropic diffusion of adatoms on the low-symmetry top monolayer of (001)-oriented zincblende GaN lattice. Defects, including stacking faults and dislocations, are found to relax the heteroepitaxial zincblende GaN film in both nucleation layers and epilayers. Miscut of the 3C-SiC/Si substrate, used to avoid the formation of antiphase domains, results in the preferential formation of the {111} stacking fault that is steeper with respect to the GaN/SiC interface. This reduces the efficiency of the annihilation between the oppositely inclined {111} stacking faults and thus limits the reduction in stacking fault density with epilayer thickness. Bunches of {111} stacking faults, with a range of spacings (up to around 10 nm) between stacking faults within a bunch, result in a broad photoluminescence emission peak with energy that extends above band gap of zincblende and wurtzite GaN. The understanding of the physics behind the formation of the surface morphology can aid future decisions on the growth process to produce desired surfaces, while the knowledge of the origin and annihilation of stacking faults is useful for the consideration of defect reduction techniques for LED structures.
Supervisor: Oliver, Rachel A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: cubic ; zincblende ; gallium nitride ; atomic force microscopy ; transmission electron microscopy ; x-ray diffraction ; nucleation ; thin film ; surface morphology ; stacking faults ; dislocations ; semiconductor ; nitride ; III-nitride