Structural, electrical, and optical characterization of bulk and thin-film transparent conducting oxides in the cadmium-indium-tin-oxide system

The system CdO-In₂O₃-SnO₂ was investigated for novel transparent conducting oxides (TCOs), materials that conduct electricity and transmit light. The bulk subsolidus phase relations were determined in the system CdO-In₂O₃-SnO₂ at 1175°C. A distorted orthorhombic perovksite Cd_1−xSn_1−x In_2xO₃ solution (0 < x < 0.045), a bixbyite solution In_2−2x(Cd,Sn)_2xO₃ (0 < x < 0.34), and a spinel solution (1−x)CdIn₂O₄—(x)Cd ₂SnO₄ (0 < x < 0.75) were discovered. These phases appear to exist over a range of compositions such that there is a slight excess of In+3 and/or Sn+4 relative to their stoichiometric compositions. The cation distribution in the spinel solution evolves from primarily normal CdIn₂O₄ to inverse Cd₂SnO ₄ as two octahedral In+3 cations are replaced by one Cd+2 and one Sn+4 cation. The solution terminates near x = 0.75, perhaps because there is no longer sufficient octahedral In +3 present to reduce short-range order effects between Cd+2 and Sn+4. The electrical and optical properties of the bixbyite and spinel solutions were investigated. The bixbyite phase has conductivities near 300 S/cm for x = 0.34 and optical gaps of 3.0 eV. Bulk spinel specimens with x = 0.70 were produced with conductivities of 2000 S/cm and optical gaps of 3.0 eV. Conductivities near 4000 S/cm, optical gaps near 3.7 eV, and absorption less than 10% in the visible were measured on spinel films 0.8 to 0.9 μm thick with x = 0.45 and 0.70.; The optical gaps in thin films increase from 3.5 eV for 0 < x < 0.2 to 3.7 eV for 0.2 < x < 0.70 while bulk specimens show a decrease from 3.0 eV for 0 < x < 0.2 to 2.8 eV for 0.2 < x < 0.70. The optical gap decrease with increasing x in bulk specimens of the spinel solution stems from the decrease in the fundamental band gap related to a change in the cation distribution between normal CdIn₂O₄ and inverse Cd₂SnO₄. The optical gap increase with increasing x observed in thin-film spinel specimens stems from an increase in Burstein-Moss shift with increasing x that offsets the drop in fundamental band gap with increasing x. Because the fundamental band gap can be tuned in the spinel by changing composition or synthesis temperature, an additional degree of freedom useful in tuning the optical window is gained.