Photoreflectance and photoluminescence of exciton-polaritons in a ZnO epilayer grown on the a-face of sapphire by radical-source molecular-beam epitaxy

Exciton-polariton structures were observed at 8 K in the photoreflectance (PR) and resonantly excited photoluminescence (PL) spectra of a ZnO (0001) epitaxial film on the a-face of sapphire grown by radical-source molecular-beam epitaxy. The resonance energies of near-normal predominant PR structures were close to those of the longitudinal excitons, i.e. upper polariton branches, where A-, B-, and C-excitons couple simultaneously to an electromagnetic wave. In contrast to the results obtained for GaN, the longitudinal-transverse (L-T) splitting of the B-excitonic polariton was observed, which is due to the large oscillator strength. The sample quality was high enough to observe distinct excitonic polariton emissions.