Elevated temperature dependence of energy band gap of ZnO thin films grown by e-beam deposition

We report the surface, structural, electronic, and optical properties of the epitaxial ZnO thin films grown on (0001) sapphire substrate at 600 deg. C by an electron-beam deposition technique. ZnO thin films have been deposited in an oxygen environment and post-deposition annealed to improve the stoichiometry and the crystal quality. In order to investigate the free exciton binding energy and the temperature dependence of the energy bandgap, we carried out variable temperature (78-450 K) transmittance measurements on ZnO thin films. The absorption data below the energy bandgap have been modeled with the Urbach tail and a free exciton, while the data above the gap have been modeled with the charge transfer excitations. The exciton binding energy is measured to be E0= 64 ± 7 meV, and the energy band gaps of the ZnO film are measured to be Eg-tilde 3.51 and 3.48 eV at 78 and 300 K, respectively. The temperature dependence of the energy gap has been fitted with the Varshni model to extract the fitting parameters α= 0.00020 ± 0.00002 eV/K, β= 325 ± 20 K, and Eg (T = 0 K) = 3.516 ± 0.0002 eV.