Photoreflectance spectroscopy of ZnO for ordinary and extraordinary rays

Photoreflectance (PR) measurements have been carried out to determine the fundamental-absorption-edge (E-0) structure of ZnO for light polarization E perpendicular (E perpendicular to c) and parallel to the c-axis (E parallel to c) at temperatures T between 15 and 300 K. The measured PR spectra revealed distinct structures at similar to3.3-3.5 eV. These structures could be successfully interpreted using the three-dimensional (3D) excitonic plus one-electron line shapes over the entire temperature range. The temperature dependence of the 3D critical-point and excitonic parameters (energy, amplitude and broadening parameter) has also been determined and analyzed using the Varshni equation and an analytical four-parameter expression recently developed for the explanation of the band-gap shrinkage effect in semiconductors. The 3D-exciton binding energies were determined to be 65 (A), 68 (B) and 63 meV (C), respectively.