Electrical Properties of Semiconducting CdF2:Y

Measurements of resistivity and the Hall coefficient in the temperature range 425-4.2°K have been made on single crystals of semiconducting Cd${\mathrm{F}}_2$ doped with yttrium (Y). The Y concentrations ranged from 0.002-1 mole%. The characteristic behavior of impurity-band conduction is observed, with the activation energies ${\epsilon }_1$, ${\epsilon }_2$, and ${\epsilon }_3$. The onset of impurity-band conduction occurs at higher temperatures than in other semiconductors. No degeneracy effects are seen for the highest concentrations observed. The data are analyzed on the basis of simple semiconductor theory, assuming a hydrogenic nature of the Y donor. Using the value of effective polaron mass $m_p\approx 0.9m_e$ and the static dielectric constant ${\epsilon }_0=8.1\mathrm{}$, we find that the calculated ionization energy is larger than the experimental value. The impurity-banding behavior cannot be characterized in a fashion similar to what has been done in other semiconductors. Alternative analyses are suggested. The Hall coefficient is analyzed on the basis of the two-band model: the conduction band and the impurity band. At temperatures above 250°K, the Hall-mobility data can be fitted by assuming polar optical mode and an acoustic mode as the dominant scattering mechanisms. The deformation potential was found to be 8.2±1 eV. Below 250°K, ionized impurity-scattering is the additional mechanism. At temperatures below ∼80°K, the mobility falls off sharply owing to impurity banding.