Bulk Phase Relations, Conductivity, and Transparency in Novel Bixbyite Transparent Conducting Oxide Solution in the Cadmium-Indium-Tin Oxide System

A novel transparent conducting bixbyite solid solution In_2-2x(Cd,Sn)_2xO₃ (0 < x < 0.34 at 1175°C) has been discovered. Four-point dc-conductivity varies widely with x and the [Sn]/[Cd] ratio with a maximum in excess of 1800 S/cm (x = 0.05, [Sn]/[Cd] = 9) and a minimum too low to be measured (x = 0.05, [Sn]/[Cd] = 0). The optical gap measured along the line In_2-2xCd_xSn_xO₃ remains constant near 3 eV while transparency decreases with increasing x possibly because of free carrier absorption. Four-point dc-conductivities measured from biphasic samples prepared by varying the [Sn]/[Cd] ratio suggest that the solution extends between the CdO-In₂O₃ and SnO₂-In₂O₃ binaries for small x. As x increases, the solution width decreases and is found to exist only over a small range of [Sn]/[Cd] ratios slightly greater than unity near x = 0.34. Single-phase bixbyite samples subjected to a reduction anneal showed increased conductivity and slightly higher optical gaps, possibly as a consequence of the Moss-Burstein shift. The ratio of the reduced to as-fired conductivities for specimens prepared along the line In_2-2xCd_xSn_xO₃ decreased with increasing x. This suggests that for small x electrons are generated by oxygen vacancies while at larger x the electron population is fixed by a Sn excess (i.e., [Sn]/[Cd] > 1).