TEMPERATURE AND POLARISATION DEPENDENT REFLECTIVITY OF THE C FREE EXCITON OF A-PLANE ORIENTED ZNO EPILAYERS GROWN BY PLASMA-ASSISTED MOLECULAR BEAM EPITAXY.

The photoluminescence (PL) and reflectivity (Ref) properties of a-plane oriented ZnO epilayers, grown by molecular beam epitaxy (MBE) on r-plane oriented sapphire substrate, have been investigated. At low temperature donor-bound exciton (DX) emission (3.355-3.383eV) and the C free excitonic transitions (3.427-3.436eV) were identified through polarization dependent photoluminescence and reflectivity measurements. The DX plays a major role in PL spectra at low temperature, while the free exciton transition (FX) gradually dominates the spectrum with increasing temperatures. In the ? polarisation, first the temperature-dependent variations of the PL spectra and of the integrated intensity of the C-free exciton were studied. The activation energy of the C-free exciton was calculated to be 59 meV from the temperature dependent quenching of the integral intensities. Then, the temperature dependence of the reflectivity peak energy position of the C-free exciton, was plotted and fitted. Our results substantiated the excitonic nature of the PL emission of the a-plane ZnO thin film, in the ? polarisation (E?c) axis.