Photoluminescence of germanium near the screening ionization limit of excitons

Photoluminescence spectra from highly excited germanium at 25–30 K are presented. The density of free electrons and holes is determined from induced free carrier absorption and achieved values up to 10¹⁷ cm^-3. At such densities the electron-hole system is pumped to well above the screening ionization limit of excitons. The band-to-band luminescence. however, is strongly Coulomb enhanced. The spectral shape of the measured luminescence fits well with calculations based on a Yukawa interaction potential.