Self‐Activated Semiconductivity in CdS Single Crystals

The results of this paper show that self‐activated conductivity is present in moderately doped CdS crystals above 220 °C. Measurements are made of the dark conductivity s̀ in vacuum between 220 and 400 °C, at different heating and cooling rates. For the stationary conductivity, two straight lines are obtained in a log s̀ vs T−1 plot, with activation energies of 0.67 and 0.98 eV below and above 300 °C, respectively, which are independent of doping. It is shown that Frenkel disorder is dominant and determines conductivity above 300 °C, whereas at lower temperatures its influence is exceeded by a surface layer of high conductivity. The formation energy ϵF of Frenkel defects is estimated to be between 1.2 and 3.2 eV. The influence of sulfur vapor on conductivity is investigated between 450 and 650 °C. The observed decrease in conductivity is attributed to the diffusion of singly ionized Cd interstitials from the interior to the surface of the crystal. The diffusion coefficient for this process is deduced to be 2.1 × 10−5 exp cm2 s−1. {article}{empty}{document}( - 0.6 eVkT ){document} Furthermore, a relation is obtained for the ionization energies of Cd interstitials and Cd vacancies, and an estimate given of the band gap for thermal excitation.