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

Abstract

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 Γ valley in n[superscript +] 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 λ[superscript 2]-dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at λ < 15 μm. 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[superscript −1] for n = 4 × 10[superscript 19] cm[superscript −3] at λ = 1.5–1.7 μm, an order lower than the results from Drude model. The strong L → Γ intervalley scattering favors electronic occupation of the direct Γ 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 λ[superscript 2]-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.