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Title: Development of multi-microscopy techniques for the characterisation of nitride semiconductors
Author: O'Hanlon, Thomas James
ISNI:       0000 0004 7968 4459
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Heteroepitaxially-grown nitride semiconductors typically contain a high density of extended defects, particularly threading dislocations (TDs). Despite the relative defect tolerance of these materials, it is important to understand how particular defects impact the electronic and optical properties. The direct correlation of measurements on the same nanoscale feature between different microscopy techniques can allow a deeper understanding of the interplay between the structural, optical and electronic properties and is termed 'multi-microscopy'. In this work, after reviewing the basic material properties and the characterisation techniques used, the development of novel multi-microscopy techniques for cross-sectional samples is presented. In particular, different marker strategies and methods for preparing transmission electron microscopy (TEM) lamellae from previously characterised features in the focused ion beam (FIB) microscope are discussed. These were then applied to investigate a number of nanoscale features. Firstly, this approach was applied to trench defects in InGaN/GaN quantum well (QW) structures, which display unusual emission behaviour. After imaging a trench defect population by directly correlated atomic force microscopy (AFM) and scanning electron microscopy cathodoluminescence (SEM-CL), representative trench defects were lifted out, using the FIB methods developed previously, for examination by scanning TEM and energy dispersive X-ray spectroscopy. Trench defects with greater cathodoluminescence redshift initiated earlier in QW growth, had higher indium fractions in the enclosed QWs and did not appear to contain the gross well width fluctuations of the surrounding material. Secondly, the possible role of late-forming GaN island coalescence boundaries (CBs) in TD generation was investigated. Silicon-doped marker layers, scanning capacitance microscopy and SEM-CL were used together to identify the latest coalescing boundaries on cleaved cross- sections of fully-grown epilayers. Directly correlated TEM on the CB regions revealed TDs at each late-forming boundary (with a screw component seen for each TD examined in detail) suggesting these boundaries may contribute to TD populations, though their density is low. AFM and electron channelling contrast imaging (ECCI) are fast, non-destructive alternatives to TEM for dislocation characterisation. AFM and ECCI were used to assign TD type on the exact same region of an epitaxial lateral overgrowth GaN sample. Direct agreement was seen for vertical TDs, and inclined TDs (which could not be assigned in ECCI) were mostly a-type in AFM. Finally, the preparation of exposed metallic droplets and nano-rings on an InGaN surface for in situ TEM annealing experiments was explored.
Supervisor: Oliver, Rachel Sponsor: ERC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: gallium nitride ; semiconductor ; microscopy ; multi-microscopy ; fib ; scm ; light emitting diodes ; led ; nitride ; materials science ; trench defects ; coalescence boundaries ; directly correlated microscopy