Picosecond electron-hole droplet formation in indirect-gapAlxGa1−xAs

A phase separation into a liquid and an exciton gas is found for the optically generated electron-hole system in indirect-gap ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As. The formation of electron-hole droplets occurs on a time scale of a few hundred picoseconds, i.e., much more rapidly than in Si or Ge. This rapid droplet formation is found in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As regardless of whether it is an indirect-gap semiconductor due to its alloy composition or due to application of hydrostatic pressure. A subnanosecond phase separation is, however, not observed in GaAs, which is an indirect-gap semiconductor under hydrostatic pressure. This gives strong evidence that the accelerated nucleation dynamics is caused by alloy disorder. We determine the equilibrium density and critical temperature of the liquid phase from a preliminary phase diagram to be 4.5×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and 34 K, respectively. The electron-hole plasma with density and temperature above the critical parameters is still bound with respect to the free exciton and shows self-confinement.