The current density as a function of applied voltage, optical excitation, and temperature in photoconducting CdS crystals with different non-Ohmic contacts (Au, Ag, Cu, Pt, Ni, and Sn) and the transient behavior of this current after switching-on of the applied voltage is investigated. High-field domains adjacent to these contacts are used as tools to measure the fields near the contacts. These domains are analyzed for different light intensities and temperatures. It is concluded from the experimental results that the observed high current density is caused by tunneling through the barrier and that in a slab of about 600-Å thickness at the cathode, the field must increase by about one order of magnitude from the field in the high-field domain. The space charge necessary for this must be stored in levels inaccessible for detection with conventional methods. It is proposed that in CdS about ${10}^{18}$-${\mathrm{cm}}^{-3\mathrm{}}$ levels caused by native defects at about 0.8 eV below the conduction band are responsible for this space charge. These levels are probably also responsible for the fact that CdS is never observed to be $p$ type under equilibrium doping conditions.

• Received 9 June 1971

DOI:https://doi.org/10.1103/PhysRevB.7.1433