Spectroscopic ellipsometry of ZnO thin films: choice of optimum parameterization of dielectric function

The aim of the study is to test various parameterizations of ZnO dielectric function. Epitaxial thin films grown by reactive magnetron sputtering at different oxygen partial pressures were chosen for analysis by spectroscopic ellipsometry. Three different model dielectric functions (MDF) were applied: Lorent oscillator, critical point parabolic band (CPPB) and parametric semiconductor (PSEMI) model. All three approaches provide reasonable fit of the model to the experimental data below 3 eV. However, Lorentz oscillator provides bad agreement with experimental data at the energies of photons higher than 3 eV. The CPPB model provides unphysically high values of dielectric function imaginary part in the transparency region. The PSEMI model is more adequate compared to Lorentz oscillator and CPPB. PSEMI model yields the energy gap of the film EG=3.30±0.01 eV which does not depend on the O2 partial pressure during growth. The broadening of the film MDF increases with oxygen pressure while the film strain decreases. The variation of the defect structure inside ZnO grains with oxygen pressure can be discussed as a reason.