Investigation of the Reflectivity Spectrum of the a-Plane Oriented ZnO Epilayers Grown by Plasma-Assisted Molecular Beam Epitaxy from the Gaussian Distribution

The Photoluminescence spectra at low temperature of the a-plane oriented ZnO grown on r-plane (011-2) sapphire substrates by plasma-assisted molecular beam epitaxy, showed experimentally three types of excitons A, B and C. In the reflectivity spectra, authors used a program based on the theory of the spatial resonance dispersion Hopfield model to fit the free excitons. The A and B free excitons were fitted together and the C exciton with the band gap. But these fits were not perfect in the transparency zone at low energy. This is mainly due to the fact that the A and B free excitons are closer and the C exciton is closer to the band gap but another reason is the value of the oscillator strength. In the present work, we present a method taking account the Gaussian distribution, to fit perfectly the excitons A, B and C using almost the same physical parameters than the theory of the spatial resonance dispersion Hopfield model.