Ionized donor pairs and microwave and far-infrared absorption in semiconductingCdF2

Abstract

The dielectric permittivity $\ensuremath{\varepsilon}={\ensuremath{\varepsilon}}_{1}\ensuremath{-}i{\ensuremath{\varepsilon}}_{2}$ of semiconducting ${\mathrm{CdF}}_{2}:\mathrm{In},$ ${\mathrm{CdF}}_{2}:\mathrm{Ga},$ and ${\mathrm{CdF}}_{2}:\mathrm{Y}$ crystals was studied over the frequency range from 34.0 to 37.5 GHz at temperatures from 1.8 to 100 K. The photoinduced transition from a semi-insulating to a conducting state in photochromic ${\mathrm{CdF}}_{2}:\mathrm{In}$ and ${\mathrm{CdF}}_{2}:\mathrm{Ga}$ crystals results in a significant increase of both the dielectric constant (by $\ensuremath{\Delta}{\ensuremath{\varepsilon}}_{1}=0.5$ to 1.4) and the dielectric-loss factor (by about an order of magnitude). The low-field dielectric-loss factor in these photodecolored crystals and in ${\mathrm{CdF}}_{2}:\mathrm{Y}$ $({\ensuremath{\varepsilon}}_{2}=0.1$ to 0.3) may be decreased by approximately an order of magnitude with an increase in the microwave-field power at 1.8 K. However, ${\ensuremath{\varepsilon}}_{2}$ ceases to depend on the field at $T>4\mathrm{K}.$ These features are explained by the theory of Tanaka et al. for resonant-saturated absorption of ionized donor pairs. We have modified this theory to cover the far-IR range of the absorption spectrum of semiconductors with various degree of compensation. Results following from the modified theory were compared with those obtained by three other groups, namely, Blinowski and Mycielski, Efros and Shklovskii, and Baranovskii and Uzakov. We have shown that the ionized donor pairs are responsible for the far-IR absorption in semiconducting ${\mathrm{CdF}}_{2}$ studied experimentally by Eisenberger, Pershan, and Bosomworth. The role of impurity clusters in storage of an ``excess'' impurity and as a source of interstitial ${\mathrm{F}}^{\ensuremath{-}}$ ions during thermochemical treatment of as-grown crystals (which determines the ultimate semiconductive properties of ${\mathrm{CdF}}_{2})$ is also discussed.