Radiative recombination in heavily dopedp-type germanium

Low-temperature photoluminescence studies on heavily doped $p$-type germanium are reported. In addition to the indirect-band-gap (0.75 eV) luminescence, direct recombination of nonthermalized minority carriers located at the conduction-band minimum at point $\Gamma$ is observed. The direct recombination occurs across the gaps $E_0$ and $E_0+{\Delta }_0$ (0.89 and 1.19 eV, respectively) and resonates strongly for exciting photon energies close to the energy gap $E_0$. The emission across the gap $E_0$ can be described as a mixture of band-to-band transitions with and without momentum conservation. From the luminescence spectra the shift of the indirect band gap as well as the direct band gap $E_0$ is determined as a function of carrier concentration and compared to full pseudopotential band-structure calculations.