Characterization of the Nucleation Layer in GaN/Sapphire Heterostructures

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Title: Characterization of the Nucleation Layer in GaN/Sapphire Heterostructures
Author: Pant, Punam
Advisors: Prof. J. Narayan, Committee Chair
Abstract: The objective of this work was to investigate the formation of the GaN nucleation layer during low temperature and high temperature growth and to study domain matching epitaxy in GaN and AlGaN nucleation layers deposited on sapphire by MOCVD. GaN nucleation layer was observed to be crystalline in nature as opposed to the reported amorphous nature by several groups. This observation is supported by the SAD pattern and high-resolution TEM images. The GaN nucleation layer shows island growth and has a cubic (zinc blende) structure during the initial growth step at low temperature (500-600°C). The islands are characterized by defects such as stacking faults, twins and grain boundaries. During the second stage of the growth process at high temperature the nucleation layer transforms to hexagonal (wurtzitic) structure and retains a high a density of stacking faults. The growth mechanism of GaN and AlGaN nucleation layer grown on sapphire (0001) substrate was investigated based on the the concepts of domain matching epitaxy. It is observed that the initial misfit strain which is calculated by considering the planar spacing between the nucleation layer and the substrate is very large ~ 15-16%, but the strain reduces to very low values by matching of integral multiples of lattice planes across the film substrate interface. Calculations revealed that the misfit strain in the nucleation layer remains constant with temperature. However we find a spatial variation in strain within the interface which seems to result from lowering of total system energy. We discuss the role of nucleation layer in the formation of 'defect-free' device quality GaN-layers on basal plane sapphire substrates.
Date: 2005-10-26
Degree: MS
Discipline: Materials Science and Engineering

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