The thermal conductivity of undoped, Sn-doped, and Fe-doped β-Ga2O3 bulk crystals was measured by the 3ω technique in the temperature range of 295–410 K. A unique approach for extracting the thermal conductivity along the lateral and transverse heat flow directions was used in order to determine the thermal conductivity along different crystallographic directions. The data analysis at room temperature confirmed the expected anisotropy of the thermal conductivity of β-Ga2O3, revealing the highest value of ∼29 W/m K in the [010] direction. The thermal conductivity of the Sn-doped and Fe-doped β-Ga2O3 samples was found to be lower than that of the undoped samples due to the enhanced phonon-impurity scattering contribution, which reduces the thermal conductivity. This tendency was maintained for the thermal conductivity at elevated temperatures. The thermal conductivity in all samples decreased with increasing temperature, but the slope of the temperature dependence was found to depend on both the doping and the crystallographic orientation.

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Dielectric and conducting properties of unintentionally doped bulk and Sn-doped thin film β-Ga2O3 samples were studied using time-domain terahertz spectroscopy. Complex permittivity and optical conductivity spectra from 0.25 to 2.5 THz were obtained experimentally over a broad temperature range. The low-temperature spectra of the unintentionally doped sample were fit using a model involving two oscillators. The parameters of one of them show an unusual temperature dependence, in particular, a pronounced increase in the oscillator strength upon heating above 50 K. This is interpreted as an absorption due to thermally activated charge carriers moving in localized potential minima linked to the unintentional doping. Upon heating, the influence of this optical conductivity mechanism strongly increases, and the sample becomes opaque in the THz range near 100 K. The nanocrystalline Sn-doped Ga2O3 thin film sample exhibits a much higher optical conductivity than the unintentionally doped bulk sample, and its spectra are remarkably stable over a broad temperature range (4–750 K). This first study of β-Ga2O3 based on phase-sensitive THz spectroscopy reveals how the impurities influence the high-frequency conductive properties of the material.