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 $\mathrm{\Gamma }$ 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 ${\mathrm{\lambda }}^2$-dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at $\lambda <15\mathrm{\mu m}$. 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 $<20{\mathrm{cm}}^{-1}$ for $n=4\times {10}^{19}{\mathrm{cm}}^{-3}$ at $\lambda =1.5–1.7\mathrm{\mu m}$, an order lower than the results from Drude model. The strong $L\to \mathrm{\Gamma }$ intervalley scattering favors electronic occupation of the direct $\mathrm{\Gamma }$ 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 ${\mathrm{\lambda }}^2$-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.

© 2013 Optical Society of America

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