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Title: Strain relaxation study of Si1-xGex & Ge buffer layers on Si(001) and InSb on Ge/Si(001) virtual substrates
Author: Sivadasan, Vineet
ISNI:       0000 0004 6348 5877
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2016
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Due to their direct and tuneable bandgap, III-V semiconductors offer variations in electrical properties, compared to silicon. However wafers of III-V materials are more expensive to manufacture and have higher defect densities than Si(001). Epitaxially depositing high quality thin films of III-V materials onto Si(001) substrates offers a more cost effective route to manufacturing state of the art III-V electronic devices, whilst mitigating defect generation through lattice and thermal expansion coefficient mismatches. In this study, pure Ge and Si1−xGex layers are deposited as thin film heterostructures on on-axis and 6° off-axis Si(001) substrates using reduced pressure chemical vapour deposition to act as strain tuned “buffer layers” to integrate a particular III-V compound onto the substrate. The films are characterised using transmission electron microscopy (TEM), atomic force microscopy (AFM), high resolution X-ray diffraction (HR-XRD) and defect etching & differential interference contrast (DIC) optical microscopy. The first key finding in this study relates to the development of an 78nm Ge buffer which is comprised of a LT seed layer followed by controlled annealing only and at a fraction of the tensile strain of state of the art thick LT/HT Ge buffer layers. The second key finding comes from the comparisons between the established linear Si1−xGex grading (LG) process with the recently developed reverse linear Si1−xGex/Ge grading (RLG) process. Continuous tensile strain relaxation in RLG even up to x=0.45 yields high quality buffer layers with ≤ 3.7nm roughness, ×107cm−2 TDD and consistently delivering 0.2% tensile strain albeit with a rise in stacking faults past 70% Ge. The third major discovery comes when omitting the reverse graded layer entirely in the RLG structure and depositing a constant composition Si1−xGex step on the Ge buffer layer. High misfit dislocation densities and surface roughening is observed leading to the formation of Kirkendall voids in the Ge underlayer as a strain relieving mechanism. 5 The final chapter of this study investigates solid source molecular beam epitaxy growth of high quality indium antimonide which has the highest lattice mismatch of any III-V compound to Si(001) at 19.3% using the highest quality pure Ge buffer layers on 6° off-axis Si(001). HR-XRD reciprocal space maps shows identical levels of strain in Ge buffer layers grown on on-axis and 6° off-axis substrates, however an increased degree of tilt is measured in the off-axis Ge buffer layer, with reduced degree of tilt in the InSb layer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics