Exciton-exciton and exciton-carrier scattering in GaSe

An investigation is presented of the photoluminescence at high excitation intensities, in monocrystals of GaSe which contain only low impurity concentrations. Four lines appear at high excitation, one of which corresponds to stimulated emission. The properties of these lines are described as functions of excitation intensity, temperature between 4.2 and 300\ifmmode^\circ\else\textdegree\fi{}K, and of excitation energy. Two lines, which at 4.2\ifmmode^\circ\else\textdegree\fi{}K lie 4 and 35 meV below the free direct exciton, are attributed to exciton-carrier scattering giving rise to spontaneous and stimulated emission, respectively. The two other lines are located 15 and 20 meV below the exciton. They correspond to two different exciton-exciton scattering processes. In the first, an exciton is scattered from the ground state $n=1$ to the excited state $n=2$, whereas the other recombines radiatively. In the second process, one of the excitons dissociates giving rise to an electron-hole pair in their respective bands, the other one emits a photon. The shifts of these lines with temperature and excitation intensity are interpreted by phenomenological theory.