Cooling of a hot electron-hole plasma inAlxGa1−xAs

Time-resolved photoluminescence experiments in the picosecond regime reveal the density dependence of the cooling of a photogenerated electron-hole plasma in ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As with x=0–0.44. A strong reduction of the cooling process is observed for samples with a separation between Γ and X valleys larger than the LO-phonon energy. This reduction is nearly independent of the initial excess energy of the carriers. No reduction, however, is observed for samples with a direct band gap if this separation is smaller than the LO-phonon energy and for samples with indirect band gap. These results show that screening is negligible up to densities of 7×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$. Nonthermal optical phonons created during the cooling process by intravalley scattering of electrons with small effective masses can explain the observed reduction for the electron system.