We have investigated the influence of post-deposition annealing on the optical and electrical properties of c-axis oriented zinc oxide films prepared on sapphire substrates by electron beam evaporation. The ZnO films as-deposited and annealed in air were colourless and transparent in visible range and had sharp ultraviolet absorption edges. It is found that the optical bandgap energy of the films lies in the range of ∼3.27 to ∼3.30 eV depending on the annealing regime. From the analysis of the Urbach tail at the absorption edge, the width of the tail of localized states extending into the bandgap was obtained and a value of 44 meV can be achieved by annealing in air.

Lithium-doped zinc oxide (ZnO) films were prepared on C-plane sapphire substrates by the e-beam deposition technique in vacuum. The dependences of the optical absorption edge characteristics, structural, and photoelectrical properties on Li content (from 0 to 10 at.%) were investigated. It is found that the variations of the energy bandgap E g , the spread in the tail of the band edge (parameter E 0 ), the electrical resistivity ρ, and the lattice constant c with impurity concentration do not have monotonic character. Abrupt jumps of the optical and electrical characteristics were observed as the impurity concentration changed near 0.8 at.% Li. The significant enhancement of photoconductivity response for 0.8 at.% Li-doped ZnO films also correlates with the enhanced electrical resistivity up to 2 × 10 6 Ω cm and the appearance of a photoinjection current.

Lithium doped (0–10 at. % Li) ZnO films were grown in the wurtzite structure on sapphire (001) substrates and investigated in the 200–1200 cm−1 frequency range at 300 K by far-infrared reflectivity spectroscopy using polarized oblique (45°) incidence. This technique has enabled us to determine the longitudinal optical phonon frequency E1(LO) at 576 cm−1 of the fundamental lattice vibration at the center of the Brillouin zone, as well as to investigate the LO phonon-plasmon coupling in the low carrier density (N⩽1018 cm−3) ZnO films. The energy shift and halfwidth broadening of the LO phonon band in comparison with the uncoupled mode in high-ohmic ZnO:0.8 at. % Li (ρdc=0.6×106 Ω cm) film have been analyzed to get the concentration and “optical” mobility of charge carriers in the Li doped ZnO films. The results of optical, x-ray diffraction and dc resistivity measurements are discussed.