Carrier Mobility and Shallow Impurity States in ZnSe and ZnTe

Electrical transport measurements have been made on $p$-type ZnTe and $n$-type ZnSe. In ZnTe crystals, doping with Cu, Ag, and Au produces acceptor levels at 0.15, 0.11, and 0.22 eV, respectively. An acceptor with an ionization energy of 0.048 eV was found in the undoped crystals and is identified as the first charge state of the Zn vacancy. A shallow donor state, at approximately 0.01 eV below the conduction band, was found in $n$-type ZnSe. It also proved possible to prepare degenerate ZnSe. The scattering mechanisms limiting the lattice mobilities of both materials were considered. It was found that the polar interaction with the longitudinal optical phonons dominates the scattering of electrons in ZnSe. This mechanism probably also predominates in the scattering of the holes in ZnTe. However, the nonpolar interaction with the optical modes could also contribute significantly if the appropriate coupling parameter is larger than we presently believe.