Abstract
Beta-gallium oxide (β-Ga₂O₃) is a promising wide-bandgap semiconductor for high power applications, such as high-voltage DC converters and low frequency power switches. Such applications require devices that exhibit stable output characteristics under bias stress and that can routinely handle high electric fields. The operating temperatures of such devices have to be minimised in order to maximise their lifetimes. Despite reaching a number of key research milestones, several unresolved problems continue to hinder the commercialisation of β-Ga₂O₃. The degradation of devices at high-voltages and a high degree of self-heating both limit the reliability of β-Ga₂O₃ devices, while deep-level states have been observed to increase device on-resistance and induce threshold voltage instabilities. The investigation of these effects is the focus of this thesis.High-bias stressing of trench-MOS Schottky-barrier diodes was used to investigate device breakdown. Analysis of the reverse bias leakage current of these diodes revealed two distinct leakage regimes, a low bias leakage current dominated by leakage over the Schottky-barrier, and a high bias leakage dominated by leakage paths through the Al₂O₃ dielectric. Breakdown of the diodes was attributed to the high-bias degradation of the dielectric.
Self-heating in thin-channel β-Ga₂O₃ MOSFETs was then investigated using Raman nanothermography. These measurements revealed an absence of hot-spots for operating powers up to 0.9 W/mm. This was attributed to distributed heating effects in the MOSFET, with the resistive heating in the channel being comparable to that of heating in the high-field region at the drain side of gate. A comparison of these measurements to a pulsed IV method for extracting averaged channel temperatures, previously reported in the literature, demonstrated a high degree of agreement between the two methods. 3D simulations of the channel temperature profile support this conclusion. Potential anisotropy in the electrical performance of β-Ga₂O₃ MOSFETs was then investigated using pulsed IV characterisation. A large anisotropy in device on-resistance was observed as a function of device orientation relative to the substrate. Raman measurements of the substrate orientation demonstrated that this anisotropy did not correlate with the underlying substrate orientation across two sample sets. The variation in on-resistance was attributed to artifacts in the fabrication of the device.
Electron trapping at a Al₂O₃/Ga₂O₃ interface was investigated using capacitance-voltage characterisation of a MOS-CAP structure. The presence of a distinct ledge in the up-sweep of the capacitance-voltage characteristic, after high bias stressing of the device, was attributed to the presence of an interface trapping state. TCAD simulations demonstrated that the presence of such a state is sufficient to explain the ledge and an observed hysteresis in the capacitance-voltage characteristic. An upper bound of 2.3 eV for the trap state conduction band offset was determined. Finally, bulk trapping in β-Ga₂O₃ MOSFETs was investigated using deep-level transient current spectroscopy. The presence of a trapping state with an activation energy of 120 meV was consistent with a previously reported trapping state in β-Ga₂O₃. Thermal stressing of the device lead to the emergence of a new device behaviour, with activation energy of 510 meV. This new behaviour was suppressed after a 350K anneal of the substrate, with a semi-permanent reduction in the drain-current observed. This behaviour was attributed to the charging of electrically isolated regions in the device. The deep-level transient current spectroscopy method is not well suited to the investigation of such charging effects, with a conductive substrate required to separate the impact of buffer trapping from surface trapping. The fabrication of β-Ga₂O₃ devices on a conductive substrate is necessary to model this behaviour.
| Date of Award | 27 Sept 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Martin H H Kuball (Supervisor) & Michael J Uren (Supervisor) |