Observation of exciton-polariton emissions from a ZnO epitaxial film on the a-face of sapphire grown by radical-source molecular-beam-epitaxy

Exciton-polariton emissions were observed at 8 K in the photoluminescence spectrum of a ZnO epitaxial film on the a-face of sapphire grown by radical-source molecular-beam-epitaxy. The resonance energies of corresponding photoreflectance structures agreed with those of longitudinal and transverse excitons, i.e. upper and lower polariton branches, where A, B and C-excitons couple simultaneously to an electromagnetic wave. In contrast to the results obtained for GaN, longitudinal-transverse splitting of the B-excitonic polariton was resolved, which is due to the large oscillator strength.