Influence of n-type doping in germanium crystalline films grown on silicon substrates on room-temperature photoluminescence and optical absorption

We investigate the influence of n-type doping in germanium crystalline films grown on silicon substrates on room-temperature light emission and absorption. The photoluminescence and optical absorption of germanium films on silicon with various carrier concentrations were measured at room temperature. The photoluminescence intensity increased with increasing carrier concentration under low-power excitation, but it quenched at high carrier concentration under high-power excitation. This carrier concentration quenching works against the heavy n-type doping proposed for achieving germanium lasing action. For the undoped germanium film, the absorption at energies near the band edge was enhanced by excitonic effects. There was no enhancement in the absorption spectra of the doped germanium films. The direct band-gap narrowing observed in these absorption measurements was estimated to be about 14 meV at a carrier concentration of 4 × 10¹⁹ cm^-3. This value was considerably smaller than the value (40 meV) reported in a study employing photoluminescence measurements.