  Thin films of laser molecular-beam epitaxy grown ZnO films were studied with
respect to their optical properties. 4-K reflectivity was used to analyze
various samples grown at different biaxial in-plane strain. The spectra show
two structures at 3.37 eV corresponding to the A-free exciton transition and at
3.38 eV corresponding to the B-free exciton transition. Theoretical
reflectivity spectra were calculated using the spatial dispersion model. Thus,
the transverse energies, the longitudinal transversal splitting (ELT,), the
oscillator strengths, and the damping parameters were determined for both the
A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the
energy E_LT for the A-excitons is characterized by a negatively-peaking
behavior with a minimum around the zero strain, while ELT for the B-excitons is
an increasing function of the strain field values.
