Carrier cooling and exciton formation in GaSe

The initial cooling of hot carriers and the subsequent exciton formation in GaSe are studied by time-resolved photoluminescence (PL) using femtosecond up-conversion techniques. From the time-resolved PL spectra of this layered III-VI semiconductor two different energy relaxation channels are derived. After an initial subpicosecond cooling due to Frohlich-type interaction of carriers with longitudinal optical E'(2(2)) phonons a slower regime follows. which is dominated by deformation potential interaction with the nonpolar optical A(1)'(1(2)) phonons. The coupling constant for nonpolar optical phonon scattering is derived. The subsequent formation of excitons is studied at different carrier densities and detection energies. A cross section for the free-exciton formation is determined based on a rate equation model.