Optical absorption edge of ZnO thin films: The effect of substrate

The optical absorption edge and the near-absorption edge characteristics of undoped ZnO films grown by laser ablation on various substrates have been investigated. The band edge of films on C (0001) and R-plane (1102) sapphire, 3.29 and 3.32 eV, respectively, are found to be very close to the single crystal value of ZnO (3.3 eV) with the differences being accounted for in terms of the thermal mismatch strain using the known deformation potentials of ZnO. In contrast, films grown on fused silica consistently exhibit a band edge ∼0.1 eV lower than that predicted using the known deformation potential and the thermal mismatch strains. This behavior is attributed to the small grain size (50 nm) realized in these films and the effect of electrostatic potentials that exist at the grain boundaries. Additionally, the spread in the tail (E0) of the band edge for the different films is found to be very sensitive to the defect structure in the films. For films grown on sapphire substrates, values of E0 as low as 30 meV can be achieved on annealing in air, whereas films on fused silica always show a value 100 meV. We attribute this difference to the substantially higher density of high-angle grain boundaries in the films on fused silica.