Photoluminescence of bulk germanium

We have performed photoluminescence (PL) measurements on intrinsic and doped bulk Ge substrates as a function of temperature and excitation power. A weak luminescence feature at 738 meV is observed at 7 K for Ge substrates with Al and Sb dopants, but not for intrinsic and Ga-doped Ge. This feature can be explained by no-phonon (NP)-assisted recombination of free excitons. Besides the NP feature, we observe intense luminescence peaks at 730, 710, and 702 meV, which can be attributed to phonon-assisted exciton recombination of transverse acoustic (TA), longitudinal acoustic (LA), and transverse optical (TO) phonons in the (111) direction, respectively. Besides these features, we observe luminescence at energy below 700 meV (1.77 mu m). This luminescence shows much lower intensity (about 500 times) than the LA replica. The observed luminescence can be explained as a two-phonon-assisted recombination with the previously mentioned TA, LA, and TO phonons with momentum in the (111) direction (at the L point) and a TO phonon at the Gamma point. The power dependence of the integrated PL shows a log-log dependence with power factor of 2.02 at 9 K. This confirms that the PL originates from recombination of excitons, which are indirectly generated by creation of electrons and holes. The low-energy PL edge follows the temperature dependence of the band gap as a function of temperature, whereas the high-energy PL edge shows an exponential broadening as a function of temperature.