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+ 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 λ2-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(-1) for n=4×10(19) cm(-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 λ2-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.

Large inherent optical gain from the direct gap transition of Ge thin films

“…Our result confirms that reducing C!L intervalley scattering may significantly increase the optical gain in Ge and decrease the lasing threshold. Together with our recent observation of enhanced L!C intervalley scattering in tensilestrained n þ Ge, 14 these results indicate that the performance of Ge lasers can be further improved by engineering the intervalley scattering for enhanced electronic occupation of the C valley. Additionally, the result on injection dependence of fs transmittance spectra is directly relevant to the saturation absorption behavior of Ge, which can be applied to saturable absorbers and other important nonlinear photonic devices.…”

Analysis of Threshold Current Behavior for Bulk and Quantum-Well Germanium Laser Structures

“…A first-principle model [10] has been employed to describe FCA in GaAs in the near-infrared regime, which takes into account the band structure as well as optical phonon, acoustic phonon, and charged impurity scattering mechanisms. Our recent studies on the infrared absorption of tensile-strained n + Ge also show that the classic λ 2 -dependent Drude model of intravalley free electron absorption only holds true at λ ≥ 15 μm [11]. At λ < 15 μm, the free electron absorption decreases much faster with wavelength than the prediction of the Drude model.…”

Predictions of Free-Carrier Electroabsorption and Electrorefraction in Germanium

“…This model has previously been applied by Wang et al [27] to the case of n-type Ge. In intravalley FCA the absorption is an indirect process, where absorption of a photon raises a carrier to a higher energy level within the same band, assisted by a phonon for momentum conservation.…”

“…This conclusion is supported by the shape of the n-type Ge absorption spectra, which corresponds well to that expected from intravalley FCA. It is worth noting that this may only be true for unstrained Ge; in [27], the absorption spectrum of n-type tensile strained Ge was measured, and displayed a response in one part of the spectrum that was ascribed to intervalley absorption.…”