Electron-hole plasma in photoexcited indirect-gapAlxGa1−xAs

We report luminescence studies of pulse photoexcited, indirect-gap ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ in the temperature range of 2-150 K. Two spectral features are observed: excitons scattered by random potential fluctuations, and the electron-hole plasma (EHP). For $0.44\le x\le 0.53$ we find a plasma density between 0.5×${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The plasma binding energy, relative to the free exciton, is very small and may be negative. A strong no-phonon band is observed, which we attribute to scattering by potential fluctuations, as well as momentum-conserving phonon-assisted transitions. The lifetime of the EHP below 20 K is about five times shorter than that of the "intrinsic" exciton and is predominantly radiative. There is good, if not definitive, evidence that plasma-exciton phase separation occurs. Stimulated emission from the EHP is observed in nominally undoped, indirect-gap material, for $x\le 0.46$. Further spectral features are present at moderate to high excitation, which may result from multiexciton complexes.