Surface photovoltage measurements in vapour-deposited CdS

Abstract

Surface photovoltage has been measured in vapour deposited CdS using metal-insulator-semiconductor structures. The photovoltage was measured using a.c. techniques. The surface potential at thermal equilibrium was studied as a function of the types of insulators used and the various surface treatments given to the CdS films. A depletion region with a surface barrier as high as 200 mV was measured with either vapour deposited CaF 2 or MgF 2 as the overlying insulator. For the CdS-SiO interface, a depletion region was found with a surface potential which varied between 20 and 100 mV, depending on the deposition procedure for the SiO films. The dependence of the photovoltage on light intensity is in good agreement with calculated values neglecting charge changes in the fast surface states, and is therefore at least consistent with previously reported results on fast surface states in vapour deposited CdS. The values of surface potential obtained have been compared to values obtained from field-effect capacitance measurements and good agreement was obtained. The significance of these results as pertains to the thin-film field-effect transistor is discussed.