Comparison of Thin Film and Bulk Forms of the Transparent Conducting Oxide Solution Cd1+xIn2-2xSnxO4

Physical and structural properties of thin films prepared via rf magnetron sputtering of the transparent conducting oxide spinel Cd_1+xIn_2-2xSn_xO₄ are compared to those reported for bulk specimens (prepared via high-temperature solid state reaction at 1175 °C). Optical band gaps measured on thin films of Cd_1+xIn_2-2xSn_xO₄ were 3.5, 3.70, and 3.65 eV for x=0.15, 0.45, and 0.70, which where 0.57, 0.94, and 0.95 eV higher than their bulk counterparts. Thin film Seebeck coefficients were -18.0, -15.5, and -15.5 μV/K for x=0.15, 0.45, and 0.70, respectively, which were 27, 24, and 19 μV/K smaller in magnitude than their bulk counterparts. Sn-Mössbauer spectroscopy revealed isomer shifts that averaged 0.2 mm/s for both bulk and thin films specimens. The presence of quadrupole splitting, which averaged near 0.48 mm/s for film specimens and 0.39 mm/s for bulk specimens, suggests that Sn⁺⁴ in all specimens is in octahedral coordination. The difference in quadrupole splitting suggests that thin films have a different cation distribution than their bulk counterparts. The effective mass at the base of the conduction band, measured via the method-of-four-coefficients, was found to be 0.25, 0.18, 0.21, and 0.22 m_e for x equal to 0.15, 0.45, 0.70, and 1.0, respectively. A model that explains the changes in optical gap and thermopower as a result of differences in the fundamental band gap (resulting from a changing cation distribution), conduction band curvature, and carrier density is presented.