Fermi-edge singularity in degenerate n-type bulk InAs

n-type doped InAs bulk crystals has been studied by Fourier-transform photoluminescence in combination with Fourier-transform photoluminescence excitation (FTPLE) spectroscopy. The FTPLE spectra exhibit a pronounced enhancement of the oscillator strength at the optical band gap even under degenerate doping conditions where a significant Burstein-Moss shift is observed. The spectral position of the observed enhancement changes in the same way as the Fermi level with increasing doping concentration. The data are consistently explained by Mahan's theory of optical interband transitions under degenerate conditions, and thus give direct evidence for the existence of a Fermi-edge singularity in a bulk semiconductor.