Electron statistics in CdF2 semiconductor crystals with DX centers

Abstract

The degree of compensation and ionization energy of two-electron DX centers in CdF2: In and CdF2: Ga semiconductors are determined by studying the statistical distribution of electrons localized on impurity levels. The sharp temperature dependence of the concentration of neutral donors observed in CdF2: Ga over the temperature range T = 250–400 K is explained by a high compensation degree, K ≥ 0.996. Thus, all Ga ions introduced into a CdF2 crystal lattice during crystal growth form shallow donor levels. However, the concentration of Ga ions that can form bistable DX centers is rather low and is close to the concentration of electrons injected into the crystal during additive coloring. In CdF2: In crystals, the degree of compensation is smaller than that in CdF2: Ga but is sufficiently high and the number of bistable DX centers is not limited. It is concluded that an extremely narrow impurity band forms in the CdF2: Ga semiconductor. For a total charged-impurity concentration of ∼1020 cm−3, the width of the impurity band in CdF2: Ga is not likely to exceed ∼0.02 eV.