Abstract
This dissertation focuses on investigating the thermal response of various properties in ultra-wide bandgap-phase nanocrystalline gallium oxide, β-phase Ga2O3 semi- conductor thin films. Optical and phonon interactions were examined, revealing a redshift of the optical bandgap by ∼ 200 meV within a temperature range of 77 − 622K. The optical gap at room temperature measured approximately ∼ 4.85 eV, which is typical for nanocrystalline thin films. Analysis of the electron phonon interaction showed the involvement of a low-energy phonon at around 30 meV. Defects, including disordered graphite form, Sp2 network incorporation, and granular morphology, significantly influenced the band-edge properties at elevated temperatures. The emission at 3.56 eV and 4.85 eV in the deep-UV region was attributed to bandgap recombination, confirming the theoretical prediction that trapped holes inhibit bandgap emission. The ratio of large self-trapped holes (STH) to bandgap intensity was determined to be 6 : 1, making beta-gallium oxide a viable emitter in the 3.56 eV STH spectral range. The distinction between STH and self-trapped excitons (STE) was investigated, indicating consistency between STH photoluminescence (PL) at around 3.5 eV and STE emission. Furthermore, the thermal response of STH photoluminescence and the role of phonon interaction were examined in the β-Ga2O3 nanocrystalline films across a temperature range of 77 − 622K. The PL intensity decreased with increasing temperature, exhibiting an activation energy of ∼ 72 meV. Raman mode analysis revealed a decrease in the intensity of high-frequency modes associated with the GaIO4 site, suggesting a phonon annihilation process. These modes, having energy comparable to the STH activation energy, can couple to the STH and transition it from a radiative to a non-radiative regime according to the configurational coordinate model. The broad linewidth of the PL indicated a strong STH-phonon coupling. The low-frequency Raman modes followed the thermal Bose-Einstein phonon population. The peak position of the STH exhibited negligible temperature response compared to the redshift of ∼ 220 meV observed in the film’s band edge, attributed to its deep-level energy position. The temperature-dependent linewidth (FWHM) of the STH intensity showed weak temperature dependence, confirming STH as an intrinsic defect property in Ga2O3 crystals. Hysteresis experiments demonstrated the reversibility and consistency of the temperature-dependent measurements, highlighting the resilience of the β-Ga2O3 semiconductor films for high-temperature, high-power, and optoelectronic device applications such as for UV emitters.