Structure and properties of gallium-oxide films produced by high-frequency magnetron-assisted deposition

The properties of gallium-oxide films produced by the radio-frequency magnetron-assisted sputtering of a $\beta$-Ga$_2$O$_3$ target with deposition onto sapphire substrates are studied. The as-deposited gallium-oxide films are polycrystalline and contain crystallites of the $\alpha$ and $\beta$ phases. Exposure to oxygen plasma does not bring about the appearance of new crystallites but makes crystallites several times larger in average dimensions in the substrate plane. After annealing at 900$^{\circ}$C, the crystallite size becomes twice as large as that in the unannealed film. The films not subjected to thermal annealing exhibit a high resistance at 20$^{\circ}$C. In the range of 50–500$^{\circ}$C, the conductivity of the samples $(G)$ only slightly depends on temperature $(T)$ and, as $T$ is elevated further, exponentially increases with the activation energy 0.7–1.0 eV. After annealing of the films in argon at 900$^{\circ}$C (30 min), the conductivity G starts to sharply increase at $T\approx$ 350$^{\circ}$C. In the dependence of $\ln G$ on $T^{-1}$, a maximum in the range of 470–520$^{\circ}$C and a portion of decreasing conductivity at higher temperatures are observed. The unusual temperature dependence of the conductivity after annealing is attributed to a change in the structure and phase composition of polycrystalline gallium-oxide films and, possibly, to some effects at the surface. The structures produced on insulating substrates are solar blind in the visible wavelength region and sensitive to radiation in the ultraviolet region (222 nm).