Size Effects and Temperature Dependence in Thermal Conductivity of γ-Ga₂O₃ Films

β-Ga₂O₃ is a promising ultra-wide bandgap semiconductor with the potential to provide significant improvements in the performance and the manufacturing cost of power electronic devices. An unexpected γ-phase inclusion as a defect has been reported when β-Ga₂O₃ was doped or alloyed. Because thermal transport in γ-Ga₂O₃ has not been studied, the effect of the γ-phase inclusion on the thermal conductivity of β-Ga₂O₃ is still in question. γ-Ga₂O₃ also has a potential to be used as an efficient catalyst due to its porous structure and high surface area, and thermal conductivity of the catalyst is an important attribute for an effective thermal management. The thermal conductivities of (100) γ-Ga₂O₃ films deposited on (100) MgAl₂O₄ substrates with various thicknesses were measured using frequency-domain thermoreflectance (FDTR). A multi-fit iteration algorithm was implemented to simultaneously extract thermal conductivities and thermal boundary conductances from these measurements. Measured thermal conductivities of γ-Ga₂O₃ are comparable to thermal conductivity of (-201) β-Ga₂O₃, which suggests that the γ-phase inclusion in the doped or alloyed β-phase will not adversely affect its thermal conductivity. The thermal conductivity of γ-Ga₂O₃ increases from 2.3 (+0.9, −0.5) W/mK to 3.5 ± 0.7 W/mK over films from 75 nm to 404 nm in thickness, which demonstrates a prominent size effect on thermal conductivity. The thermal conductivity of γ-Ga₂O₃ also shows a slight increase as temperature increases from 293 K to 400 K. This increasing trend in thermal conductivity can occur due to defect scattering dominating Umklapp scattering in this temperature range. γ-Ga₂O₃ has a cation-defective spinel structure with at least two gallium vacancies in every unit cell, which causes defect scattering to dominate Umklapp scattering at these temperatures.