Infrared absorption of n-type tensile-strained Ge-on-Si.

We analyze the IR absorption of tensile-strained, n-type Ge for Si-compatible laser applications. A strong intervalley scattering from the indirect L valleys to the direct Gamma valley in n(+) Ge-on-Si is reported for the first time to our knowledge. The intervalley absorption edge is in good agreement with the theoretical value. On the other hand, we found that the classical lambda(2)-dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at lambda < 15 mu m. A first-principle model has to be employed to reach a good agreement with the experimental data. The intravalley FCA loss is determined to be < 20 cm(-1) for n = 4 x 10(19) cm(-3) at lambda = 1.5-1.7 mu m, an order lower than the results from Drude model. The strong L -> Gamma intervalley scattering favors electronic occupation of the direct Gamma valley, thereby enhancing optical gain from the direct gap transition of Ge, while the low intravalley free-electron absorption at lasing wavelengths leads to low optical losses. These two factors explain why the first electrically pumped Ge-on-Si laser achieved a higher net gain than the theoretical prediction using lambda(2)-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers. (C) 2013 Optical Society of America