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Spontaneous luminescence and optical gain of high density electron-hole phasma (EHP) in GaAs are observed at 4.2 K under nanosecond pulse excitation. Time-resolved luminescence spectra are observed under picosecond pulse excitation. The chemical potential and reduced band-gap energy of the EHP are obtained from the spectral shape analysis. These values and also their time changes are explained qualitatively by the calculation of the EHP ground-state energy. After picosecond pulse excitation, hot carriers cool down rapidly to 50 K within the first 150 ps, but the thermal equilibrium with the lattice is not attained up to 400 ps. The observed gaseous EHP is considered to exist only in a transient state and the electron-hole liquid state is not realized.
Tanaka et al. (Mon,) studied this question.