Investigation of new semiconductor materials for wide band-gap devices

This thesis investigates the compositional and optical properties of wide bandgap III-nitride and gallium oxide semiconductor materials using scanning electron microscopy techniques. The primarily used techniques used here were wavelength dispersive X-ray spectroscopy (WDX) and cathodoluminescence (CL) with other electron microscopy and spectroscopic techniques used to assist in the characterisation of the semiconductor materials. Compositional measurements were performed to determine bulk alloying levels within the semiconductor and trace element analyses e.g doping concentrations. Investigations benefited from the correlative nature of the WDX and CL measurements, allowing simultaneous compositional and optical mapping of the semiconductor, attributing the sample luminescence to compositional spatial regions or features of the material. Semi- and non-polar AlxGa1−xN alloys showed similar Al incorporation as polar material, consistent across the entire AlN range. Si incorporation within semi-polar AlxGa1−xN was found to be independent of bulk composition however the dopant concentration resulting in onset of Si self-compensation increased with AlN composition unlike polar AlxGa1−xN. CL measurements show good agreement with the near-band edge (NBE) emission energy and the material bandgap expected from the alloy concentration for all AlxGa1−xN crystal orientations. Compared to polar AlxGa1−xN NBE emission showed a high degree of broadening with increasing AlN content due to compositional variation in the samples. NBE broadening wasalso accompanied with intense defect luminescence attributed to oxygenated VIII complexes.Indium-gallium oxide (IGO) alloys were grown with low to high In contents with notable crystallographic phase changes from monoclinic to mixed to cubic as the In content was increased. CL measurements show UV, blue and green luminescence with the spectral intensities shifting from dominant UV in near pure Ga2O3 to enhanced blue and green in high In containing materials. Spectral band energies also decreased as the In composition increased due to the reduction of the material bandgap. The incorporation of Sn into Ga2O3 alloys was investigated for thin films grown on various material substrates. Sn alloying levels was found to be dependent on multiple growth factors: Sn availability, substrate choice and growth temperature. Optical properties exhibited similar to the IGO samples for both intensity and energy shifts of the three spectral bands. Electronic investigations into the photodetectorproperties of the TGO films showed superior responsivity and high gain compared to Ga2O3 devices while operating in lower energy UV due to the bandgap reduction. An investigation into the impact of X-ray secondary fluorescence on WDX measurements on semiconductor thin films was completed as a separate computation study. Results show there was a negligible effect of secondary fluorescence on WDX measurements on multiple semiconductor families with various material substrates, unlike for geological samples. The reduced beam energy required for semiconductor thin film analyses minimises the impact of secondary fluorescence on bulk quantitative measurements. However trace element analyses may be impacted by secondary fluorescence when using the typical beam conditions for thin film specimens, particularly when the substrate also contains the trace element, the impact of secondary fluorescence may be reduced however by operating with the lowest suitable beamenergy possible, minimising the size of the secondary excitation volume.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2023

Author dc:creator

• Hunter, Daniel Alasdair

Advisors dc:contributor.advisor

• Martin, Robert W.
• Edwards, Paul Robert

DOI

10.48730/ztp9-av57

Identifier

T16757

Author Identifier

201969403

OAI identifier oai:identifier

oai:strathclyde:bv73c104d