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

• Main Authors: Wang, Xiaoxin (Author), Li, Haofeng (Author), Cai, Yan (Contributor), Kimerling, Lionel C. (Contributor), Michel, Jurgen (Contributor), Liu, Jifeng (Author), Camacho-Aguilera, Rodolfo Ernesto (Contributor)
• Other Authors: MIT Materials Research Laboratory (Contributor), Massachusetts Institute of Technology. Department of Materials Science and Engineering (Contributor), Massachusetts Institute of Technology. Microphotonics Center (Contributor)
• Format: Article
• Language: English
• Published: Optical Society of America, 2013-07-30T19:00:06Z.
• Subjects: Article
• Online Access: Get fulltext

 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.