Hot excitons in CuCl and CuBr crystalline thin films grown by vacuum deposition

We report on the luminescence properties of CuCl and CuBr thin films with layer thickness of 3.5--100 nm from the viewpoint of hot excitons. Crystalline thin films preferentially oriented along the 〈111〉 crystal axis were grown on (0001) Al₂{O}₃$ substrates by vacuum deposition. The relative intensity of the free-exciton luminescence to the bound-exciton one in the CuCl and CuBr thin films remarkably increases with a decrease in the film thickness. The excitation spectra of the free-exciton luminescence clearly exhibits oscillatory structures with the period of the LO-phonon energy in all of the thin films. The profiles of the excitation spectra, which reflect the relaxation processes of the hot excitons, hardly depend on the film thickness. It is concluded that the enhancement of the radiative efficiency of the free excitons results from nonequilibrium at the final stage of the relaxation in the polariton bottleneck region. Furthermore, it has been found that the energy of the free-exciton luminescence in the CuBr thin films has an oscillatory dependence on the excitation energy with the period and amplitude of the LO-phonon energy. This indicates the intense nonequilibrium of the free excitons.