Structural and Optical Characterization of ZnO Thin Films Deposited by Reactive rf Magnetron Sputtering

The aim of this study is to obtain high-quality zinc oxide thin films at room temperature by reactive radiofrequency (rf) magnetron sputtering in an oxygen environment. The films were deposited on glass and silicon substrates at several temperatures, ranging from ambient temperature to 400 degrees C. In order to have the best results at room temperature we adjusted several deposition parameters such as the O(2)/Ar gas pressure, target-substrate distance, rf power, and gas flow rate. The ZnO samples were characterized by several methods. From XRD measurements it was confirmed that ZnO films are c-axis oriented, the line width and intensity are sensitive to the variation of the growth temperature, and the best results were found at room temperature for our growth conditions. From AFM pictures it is seen that the grain size decreases when the temperature is increased to 400 degrees C and the surface roughness of the as-deposited films increases with the deposition temperature. All films exhibited excellent transmission (in excess of 90%) in the visible range with a steep fall off in transmission at 375 nm for the samples grown at room temperature. From the absorbance measurements the optical band-gap energy was extrapolated at about 3.25 eV, which closely agrees with the ZnO single-crystal value. Raman backscattering is used to probe the zone center phonons and evaluate the residual stress in the as-grown ZnO thin films from the position of the E(2)(high) mode.