Optical investigations on the existence of phase transition in ZnO:Li thin films prepared by DC sputtering method

We investigated the effect of temperature on the absorption spectra of Zn0.8Li0.2O thin films (ZnO:Li), deposited at 573 K, in the wavelength range 190-800 nm. The films were deposited on sapphire, MgO or quartz substrates by DC sputtering method. The results show a shift of the optical energy gap (E-g), with direct allowed transition type near the fundamental edge, to lower wavelengths as the temperature increases. The temperature rate of E-g changes considerably showing an anomaly around 320 K depending on type of substrate. The founded results indicated that replacement of Zn ions with Li ions induces a ferroelectric phase in the ZnO wurtzite-type semiconductor. The exponential dependence of the absorption coefficient on the incident photon energy suggests the validity of the Urbach rule. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.