Optical Properties and Donor States in Semiconducting CdF2

The infrared absorption spectra associated with Y, Gd, Tm, and In donors in semiconducting Cd${\mathrm{F}}_2$ have been measured. Except for the In-doped sample, in which the infrared absorption has to be induced at low temperature, the absorption spectrum is practically independent of the particular dopant. The results are discussed in terms of both a tight-binding approximation and a hydrogenlike effective-mass approximation. The hydrogenic model, in conjunction with an assumption that the conduction band in Cd${\mathrm{F}}_2$ is multivalleyed, is reasonably consistent with all the experimental results, and leads to a value for the polaron mass in Cd${\mathrm{F}}_2$ of $0.23m_0$, where $m_0$ is the electron mass in vacuum.