We analyze the IR absorption of tensile-strained, -type Ge for Si-compatible laser applications. A strong intervalley scattering from the indirect valleys to the direct valley in 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 -dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at . 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 for at , an order lower than the results from Drude model. The strong intervalley scattering favors electronic occupation of the direct 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 -dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.
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