Strong coupling in bulk GaN microcavities grown on silicon

We have studied GaN microcavities grown on silicon by angle resolved reflectivity at 5 and 300 K. A Bragg mirror made up of λ/4 AlGaN and AlN layers has been grown on a silicon substrate. The GaN active layer whose thickness is equal to λ/2 is covered by a 100 Å thick aluminium layer which acts as the top mirror. Transfer matrix simulations are carried out to interpret the reflectivity spectra from which crucial parameters of the active layer such as the oscillator strengths, the excitonic energies and the associated broadenings of A and B excitons are deduced. At low temperature the strong coupling regime between with A and B excitons and the cavity photonic mode is clearly observed. At room temperature the splitting is evaluated to 45 meV. In order to get further insight in the physics of nitride-based microcavities and especially to improve the exciton-photon coupling, the influence of the Bragg or metallic mirrors on the cavity is analysed. The inhomogeneous broadening of excitons together with the strain gradient experienced by the active layer along the growth axis are also investigated. All of these effects on the Rabi splitting are analysed and discussed.