EFFECTS OF ANNEALING ZNO FILMS PREPARED BY ION-BEAM-ASSISTED REACTIVE DEPOSITION

Annealing ZnO films prepared by ion-beam-assisted deposition (IBAD) in air above 773 K can change the resistivity from almost-equal-to 10(-4) OMEGA cm to more than 10(6) OMEGA cm, changing the films from a highly degenerate transparent material to a thermally activated semiconductor. No change in the transmission through the visible region of the spectrum (more than 90%) is observed but the optical band gap can change from almost-equal-to 3.41 to almost-equal-to 3.24 eV, as a result of a Burstein-Moss shift. However, for films that are not initially very transparent, the average transmission can be increased from almost-equal-to 28% to more than 85%. High temperature annealing increases the average size of crystallites by almost-equal-to 15%. After annealing, a large photoresponse is observed, with the conductivity increasing by more than a factor of 10(5) when the film is illuminated with UV light (almost-equal-to 4 x 10(16) photons cm-2 s-1). The rise and decay time constants are very large, indicating that previously reported surface structural changes occur. These constants strongly depend on the intensity of the light and the ambient gases present. Any true photoconductiity present appears to be very small. Encapsulating the free surface of a ZnO film with a layer of amorphous SiN(x) can help to stabilize the film, and it decreases the photoresponse time as well as the magnitude of the photoresponse.