Infrared Optical Absorption in Semiconducting CdF2:Y Crystals

The infrared optical absorption band associated with yttrium (Y) donors in semiconducting Cd${\mathrm{F}}_2$ has been measured over a broad range of donor concentration and temperature. The results are presented and discussed in terms of continuum theories for the electron energy states of isolated hydrogenlike centers in polar materials. The general features of the optical absorption are approximately described in terms of the nonadiabatic approximation, in which the lattice is polarized to a considerable extent by the instantaneous position of the electron. By assuming that the donors tend to form clusters, apparent discrepancies between the optical and transport properties can be resolved. Impurity-banding effects, which are extremely important in determining the transport properties, are relatively unimportant in interpreting the observed optical absorption.