Evidence for an electron-hole plasma in the photoluminescence spectra of insulating InSb at very low pump intensities

We have studied the photoluminescence spectra of insulating n-type InSb at 2 K. Spectra are observed at pump intensities as low as 100 mW/${\mathrm{cm}}^2$, more than a factor of 30 lower than any previously reported study. From an analysis of the line shape and intensity dependence of the spectra, the photoexcited carrier density is determined to be significantly larger than the Mott density, indicating that the photoexcited system exists as an electron-hole plasma. Additional analysis shows that nonradiative decay is the dominant recombination process, with a lifetime of the order of 25 nsec. The photoluminescence spectra exhibit intensity-dependent shifts in the spectral features, which we interpret within the context of band-gap renormalization. Quantitative comparison with a standard model of band-gap renormalization yields a 2-K, zero-density band gap of 235.3 meV, in agreement with recent magneto-optical studies. The observation of band-gap renormalization is strong evidence supporting the thesis that even at very low pump intensity, the photoexcited system exists as an electron-hole plasma.