Mechanism of the Conversion of CdF2 from an Insulator to a Semiconductor

Abstract

In a previous publication, the authors proposed a mechanism for the conversion of CdF₂ containing small amounts of trivalent ion impurities from an insulator to an n‐type semiconductor when heated in Cd vapor at 500°C. Additional experimental evidence is presented in support of this mechanism in which the interstitial F^— ions, which normally compensate the extra positive charge of the trivalent ion impurity, diffuse to the surface of the crystal and combine with Cd atoms from the gas phase to form a surface layer of CdF₂. Simultaneously electrons from the Cd atoms diffuse into the crystal and give rise to observed conductivity and optical absorption. It is demonstrated using Cd¹⁰⁹ that no appreciable diffusion of Cd into the crystals takes place during heating in Cd vapor. The dependence of the amount of added Cd upon the Cd pressure is in good agreement with the proposed mechanism. Finally the observed changes in the fluorescence spectrum of Sm³⁺ after heating in Cd vapor are used to show that the symmetry of the crystal field at the Sm site is changed from noncubic symmetry to a higher symmetry, most probably cubic.