Direct-gap gain and optical absorption in germanium correlated to the density of photoexcited carriers, doping, and strain.

Direct-gap gain up to 850 cm(-1) at 0.74 eV is measured and modeled in optically pumped Ge-on-Si layers for photoexcited carrier densities of 2.0 × 10(20) cm(-3). The gain spectra are correlated to carrier density via plasma-frequency determinations from reflection spectra. Despite significant gain, optical amplification cannot take place, because the carriers also generate pump-induced absorption of ≈7000 cm(-1). Parallel studies of III-V direct-gap InGaAs layers validate our spectroscopy and modeling. Our self-consistent results contradict current explanations of lasing in Ge-on-Si cavities.