Impurity Conductivity in Single-Crystal CdS

The transport properties of electrons in CdS grown with Group I impurities which show the phenomenon of "storage" have been studied to determine the mechanism of conduction. The mobility parallel to the $c$ axis at low temperature is of the order of 1 to 10 ${\mathrm{cm}}^2$/ volt sec and is very anisotropic, the mobility perpendicular to the $c$ axis being 10 to 30 times that parallel to the $c$ axis. The resistivity of the crystals shows an activation energy of about 0.001 eV at low temperature. The resistivity is very sensitive to either donor or acceptor concentration. These characteristics indicate that the conduction mechanism is not a result of electrons in the normal conduction band but is a result of an impurity conductivity. The activation energy of 0.001 eV is thought to correspond to ${\epsilon }_3$ in silicon and germanium impurity-conduction theory.