Analysis of ultraviolet photoconductivity in ZnO films prepared by unbalanced magnetron sputtering

Photoresponse characteristics of polycrystalline ZnO films prepared by the unbalanced magnetron sputtering technique have been analyzed for ultraviolet photodetector applications. Changes in the crystallographic orientation and the microstructure of the films due to in situ bombardment effects during film growth have been studied. Variations in photoresponse are correlated with the observed changes in the optical properties and the defect concentration in the films. ZnO films with (100) and (101) orientation possessing a small grain size exhibited a slow response with a rise time=1.99 s, whereas porous ZnO films with a mixed orientation (100), (002), and (101) and a larger grain size exhibited a fast response speed with a rise time=792 ms. The influence of trap levels on the slow and fast rising components of the photoresponse characteristics and the origin for a fast and a stable response have been identified. A slow rise in the photocurrent directly relates to the adsorption and desorption of oxygen on the film surface, and the fast rise is due to a bulk-related phenomena involving embedded oxygen. The magnitude of the photocurrent and the rise time are found to decrease considerably with increasing number of trap levels.