Semiconductivity in Cd_{1_x}Ca_xF₂:In³⁺ Single Crystals

Abstract

Bridgman-grown Cdi_zCaxF2 single crystals were doped with 0.1 mole % ln203, Ga203, and T1203. Heat treating the crystals in a Cd atmosphere at 500° had no effect on the resistivity (about 107 ohm-cm) of the Ga- and Tl-doped crystals. However, with In-doped crystals Cd firing turned the colorless insulators into dark red, n-type semiconductors. The 300°K resistivity, carrier concentration, and mobility were about 10 ohm-cm, 1017 e/cm3, and 10 cm2/v-sec, respectively. At 77°K the crystals had high resistivity (about 10s ohm-cm). Two activation energies for conduction of 0.15 and 0.20 ev appeared t be present. The dark red crystal color, associated with the conductivity, was caused by the combination of strong infrared and visible absorptions. Yellow-green phosphorescent and thermoluminescent emissions in untreated, In-doped crystals were excited with 2537-A radiation. All of the above properties were strongly dependent on the CaF2 content of the doped Cdi_xCaF2 crystals. Between 0 and 1% CaF2 there was little change in the properties. Between 1 and 10% CaF2 the resistivity increased from 10 to 10s ohm-cm, the carrier concentration and mobility became unmeasurable, the crystal color changed from dark red to colorless, and the phosphorescent lifetimes increased from seconds to minutes. Since the properties exhibited by the In-doped Cdi_xCaxF2 crystals are quite similar to those previously found for RE-doped crystals, it appears that the same conductivity model previously proposed for CdF2:RE3+ crystals is also applicable for In3+ doping. The stability of the lower oxidation states of In can account for the differences between In and RE doping. © 1966, American Chemical Society. All rights reserved.