Thesis
Optical and electrical characterisation of alpha-Ga2O3 thin films
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2025
- Thesis identifier
- T17322
- Person Identifier (Local)
- 202077787
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Ga2O3 is an emerging ultra-wide bandgap semiconductor for next-generation power electronics, deep ultraviolet photodetectors, and high-temperature optoelectronic devices. However, the performance of Ga2O3 devices is often limited by the presence defects. Ga2O3 is a polymorphic compound, with the α phase offering several advantages over the other phases such as it exhibits the widest bandgap and demonstrates the ability for bandgap engineering through isomorphic compounds. To improve the efficiency and reliability, it is crucial to understand the influence of defects on the electrical and optical properties of the material. This thesis aims to generate new knowledge on the metastable α Ga2O3 that will aid the future development of α Ga2O3 devices. Through temperature-dependent photoluminescence spectroscopy, a previously unreported emission line at 3.8 eV was observed at temperatures below 90 K and was attributed to donor-acceptor pair recombination between H decorated VGa, and HO or HI serving as the acceptor and donor, respectively . Low temperature cathodoluminescence mapping further revealed a spatial dependence, indicating a nonuniform distribution of Hydrogen decorated gallium vacancies. To investigate the photoelectric properties of α-Ga2O3, a custom-built photoelectrical characterisation setup was developed with capabilities for time-resolved and power-dependent measurements. Using this setup, we evaluated the performance of α-Ga2O3 photodetectors and found that commonly reported metrics such as responsivity and response time are strongly influenced by experimental conditions and device architecture. These findings highlight the need for careful consideration when comparing detector performance across different studies. We propose that photoconductance serves as a more reliable parameter for such comparisons and emphasise the importance of conducting optical power-dependent measurements when benchmarking photodetectors in the literature. Further work on the photoelectrical properties presents the first application of the constant photocurrent method to α-Ga2O3 , enabling the determination of sub-bandgap absorption coefficients of 1x105cm−1 at the band edge (5.3 eV) to 0.8 cm−1 close to the midgap (2.7 eV) . Variations in the absorption edge slope indicated the presence of an exponentially distributed defectrelated states below the Fermi level. Complementary thermally stimulated current measurements allowed for the identification of traps above the Fermi level, identifying three traps at approximately 138 K, 220 K, and 235 K, which correspond to trap activation energies of 0.28 eV, 0.47 eV, and 0.51 eV, respectively. Tentavively, we assign trap E1 (0.28 eV) to be related to related to O related divancies such as OGa-VGa, where the Sn related complexes also demand attention. Trap E2(0.47 eV) and E3 (0.51 eV) were tentatively assigned to SiGa -H. together providing a near full bandgap mapping of defect states. Finally, we make use the broad absorption spectrum of our samples, introduced by native defects identifed in previous chapters, as proof-of-concept study to demonstrate the capability of α-Ga2O3 -based photodetectors to monitor water quality contamination. Photocurrent measurements reveal 3 distinct regions that match key water contaminant absorption characteristics. Region (i) (200-250 nm) corresponds to band-to-band transitions and is ideal for nitrate detection, region (ii) (250-350 nm) is related to band tail transitions and fits the absorption peak of DOC, and finally, region (iii) (350-470 nm) addresses SSC detection using defect-mediated transitions. For the wavelengths selected for the contamination test, strong correlations (R2 > 90) were observed between the concentration of the contaminant and the photocurrent. The R2 correlations strongly depended on the selected illumination wavelength, which demonstrates good selectivity of the photodetectors. This work opens the door to a more sensitive, compact, and energy-efficient system to monitor water quality.
- Advisor / supervisor
- Reynolds, Stephen
- Massabuau, Fabien
- Resource Type
- DOI
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