Stimulated emission in indirect gap AlGaAs

Stimulated emissions from both direct and indirect valleys are observed simultaneously in the picosecond time-resolved photoluminescence spectra of the indirect gap semiconductor AlGaAs, for samples with Al concentrations near (but slightly above) the direct-to-indirect band-gap crossover value. Indirect stimulated emission is possible in this material because the long-wave-vector scattering is enhanced by the near degeneracy of the direct and indirect conduction-band valleys and by alloy scattering. Two distinct bands are observed in the luminescence spectra. One of these we associate with the stimulated indirect recombination of electrons from states in the X valley that are below the Γ band gap and are, therefore, not resonantly coupled to states in the direct valley. The other we associate with the recombination of electrons from the central Γ valley or of electrons from the indirect X valley that are energetically resonant with states in the direct valley. The dramatically different temporal dynamics of these two stimulated processes are determined by the generation, relaxation, accumulation, and recombination of the electron-hole plasma.