Density of states study of amorphous CdS

Abstract

The density of states N( E) in evaporated amorphous CdS (a-CdS) and a-CdS:In films has been determined from space-charge-limited current (SCLC) measurements on isotype a-CdSc-Si heterojunctions. The density of states near the Fermi level ( E Fn+2 kT) varies between 1.8×10 15 and 1.3×10 17 cm −3 eV −1 and strongly depends on the substrate temperature, film thickness (in the range 0.1–1.0μm) and indium doping. These results are explained by assuming (i) an increase in the material density and a decrease in the number of micropores in the layer when the substrate temperature and the film thickness increase and the layers are doped with indium, and (ii) an enhancement of the importance of the surface states when the film thickness decreases. On the basis of the SCLC results and those obtained by the constant photocurrent method and dispersive transport measurements a picture of the density of states distribution in the entire mobility gap of the high resistivity (sulphur-rich) a-CdS is proposed.