Paramagnetic defects and electrical conductivity mechanisms in thin films of amorphous material which had been deposited using radio-frequency sputtering were studied. Electron spin resonance measurements showed that the defects were mainly S dangling bonds. The electron resonance spin density was independent of temperature, and this suggested that the effective electron-electron correlation energies were positive for these defects. Additional electron spin resonance centers could be created by optical excitation at low temperatures. These optically induced centers annealed at temperatures above about 100K. The electrical conductivity scaled with the electron resonance spin density. Transient optically-induced changes in the photoconductivity and the dark conductivity were accompanied by similar transient optically-induced changes in the electron spin resonance. The results constituted strong experimental evidence that the transport mechanism was controlled by S dangling-bond defects that supplied holes to the valence band for transport.

B.Yan, S.Girlani, P.C.Taylor: Physical Review B, 1997, 56[16], 10249-54