Comparative Study on the Optical Band Edge of ZnO Films with Different Measurement Techniques

ZnO films were prepared by radio frequency (rf) magnetron sputtering. The absorption coefficients as a function of incident photon energy were obtained by means of spectroscopic ellipsometer (SE), as well as by using transmission and reflection measurement (T&R). The optical absorption band gap decided by SE is 3.32 eV, while that decided by T&R is 3.26 eV. The difference is ascribed to the fact that SE probes only the surface of films whereas the T&R measurement probes the bulk of the films. The same fact can also be used to explain that, at optical band edge region, the absorption coefficient decided by SE is lower than that decided by T&R. When the incident photon energy is larger than the optical band gap, exitonic transitions in the bulk of the films are used to discuss the change of the absorption coefficient vs. incident photon energy.