Growth, microstructure, charge transport, and transparency of random polycrystalline and heteroepitaxial metalorganic chemical vapor deposition-derived gallium-indium-oxide thin films

Gallium-indium-oxide films (Ga_xIn_2-xO₃), where x = 0.0-1.1, were grown by low-pressure metalorganic chemical vapor deposition using the volatile metalorganic precursors In(dpm)₃ and Ga(dpm)₃ (dpm = 2,2,6,6-tetramethyl-3,5-heptanedionato). The films were smooth (root mean square roughness = 50-65 Å) with a homogeneously Ga-substituted, cubic In₂O₃ microstructure, randomly oriented on quartz or heteroepitaxial on (100) yttria-stabilized zirconia single-crystal substrates. The highest conductivity of the as-grown films was found at x = 0.12, with σ = 700 S/cm [n-type; carrier density = 8.1 × 10¹⁹ cm^-3; mobility = 55.2 cm²/(V s); dσ/dT < 0]. The optical transmission window of such films is considerably broader than that of Sn-doped In₂O₃, and the absolute transparency rival or exceeds that of the most transparent conductive oxides known. Reductive annealing, carried out at 400-425°C in a flowing gas mixture of H₂ (4%) and N₂, resulted in increased conductivity (σ = 1400 S/cm; n-type), carrier density (1.4 × 10²⁰ cm^-3), and mobility as high as 64.6 cm²/(V s), with little loss in optical transparency. No significant difference in carrier mobility or conductivity is observed between randomly oriented and heteroepitaxial films, arguing in combination with other data that carrier scattering effects at high-angle grain/domain boundaries play a minor role in the conductivity mechanism.