Lasing is experimentally demonstrated in a direct bandgap GeSn alloy, grown directly onto Si(001). The authors observe a clear lasing threshold as well as linewidth narrowing at low temperatures.

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Lasing in direct-bandgap GeSn alloy grown on Si

This review paper explores considerations for ultimate CMOS transistor scaling Transistor architectures such as extremely thin silicon-on-insulator and FinFET (and related architectures such as TriGate, Omega-FET, Pi-Gate), as well as nanowire device architectures, are compared and contrasted Key technology challenges (such as advanced gate stacks, mobility, resistance, and capacitance) shared by all of the architectures will be discussed in relation to recent research results

Considerations for Ultimate CMOS Scaling

Hybrid silicon lasers based on bonded III–V layers on silicon are currently the best contenders for on-chip lasers for silicon photonics. On-chip silicon light sources are highly desired for use as electrical-to-optical converters in silicon-based photonics. Zhiping Zhou and Bing Yin of Peking University in China and Jurgen Michel of Massachusetts Institute of Technology assess the three main contenders for such light sources: erbium-based light sources, germanium-on-silicon lasers and III-V-based silicon lasers. They consider operating wavelength, pumping conditions, power consumption, thermal stability and fabrication process. The scientists regard the power efficiencies of electrically pumped erbium-based lasers as being too low and the threshold currents of germanium lasers as being too high. They conclude that III–V quantum dot lasers monolithically grown on silicon show the most promise for realizing on-chip lasers.

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On-chip light sources for silicon photonics

GeSn is predicted to exhibit an indirect to direct band gap transition at alloy Sn composition of 6.5% and biaxial strain effects are investigated in order to further optimize GeSn band structure for optoelectronics and high speed electronic devices. A theoretical model has been developed based on the nonlocal empirical pseudopotential method to determine the electronic band structure of germanium tin (GeSn) alloys. Modifications to the virtual crystal potential accounting for disorder induced potential fluctuations are incorporated to reproduce the large direct band gap bowing observed in GeSn alloys.

Achieving direct band gap in germanium through integration of Sn alloying and external strain

We synthesized up to Ge0.914Sn0.086 alloys on (100) GaAs/InyGa1−yAs buffer layers using molecular beam epitaxy. The buffer layers enable engineered control of strain in the Ge1−xSnx layers to reduce strain-related defects and precipitation. Samples grown under similar conditions show a monotonic increase in the integrated photoluminescence (PL) intensity as the Sn composition is increased, indicating changes in the bandstructure favorable for optoelectronics. We account for bandgap changes from strain and composition to determine a direct bandgap bowing parameter of b = 2.1 ± 0.1. According to our models, these are the first Ge1−xSnx samples that are both direct-bandgap and exhibit PL.

Increased photoluminescence of strain-reduced, high-Sn composition Ge1−xSnx alloys grown by molecular beam epitaxy

Highly tensile-strained layers of Ge were grown via molecular beam epitaxy using step-graded InxGa1−xAs buffer layers on (100) GaAs. These layers have biaxial tensile-strain of up to 2.33%, have surface roughness of <1.1 nm, and are of high quality as seen with transmission electron microscopy. Low-temperature photoluminescence (PL) suggests the existence of direct-bandgap Ge when the strain is greater than 1.7%, and we see a greater than 100× increase in the PL intensity of the direct transition with 2.33% tensile-strain over the unstrained case. These results show promise for the use of tensile-strained Ge in optoelectronics monolithically integrated on Si.

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Strong enhancement of direct transition photoluminescence with highly tensile-strained Ge grown by molecular beam epitaxy

Unstrained and compressive-strained Ge1−xSnx alloys were grown on InGaAs buffer layers by molecular beam epitaxy. Photoreflectance at room temperature determines the direct bandgap energies of Ge1−xSnx alloys from the maxima of the light- and heavy-hole bands to the bottom of Γ valley. The lowest transition energies from photoreflectance are consistent with the energies derived from photoluminescence. The calculated bowing parameter is 2.42 ± 0.04 eV for the direct band gap of Ge1−xSnx alloys. The dilational and shear deformation potentials of the direct band gap are −11.04 ± 1.41 eV and −4.07 ± 0.91 eV, respectively. These basic material parameters are important in designing optoelectronic devices based on Ge1−xSnx alloys.

Investigation of the direct band gaps in Ge1−xSnx alloys with strain control by photoreflectance spectroscopy

First principles study showed indicated band gap of Ge can be tuned by alloying with Sn and metastable GeSn alloys can be synthesized at or above room temperature. Subsequently, high quality GeSn layers were grown using low temperature MBE. PL indicated good crystal quality of GeSn material with a reduced direct bandgap. Challenges involved in CMOS processing on GeSn were addressed through effective surface cleaning and a low thermal budget process flow. To the best of our knowledge this work is the first demonstration of a high-κ pMOSFET using 3% GeSn as channel material showing 20% improvement in hole mobility compared to Ge. Alloying Ge with Sn has thus been shown as a performance booster for Ge based devices. Further improvements in material quality and incorporation of higher substitutional Sn, coupled with strain and bandgap engineering, significant performance gains can be achieved from this alloy system.

GeSn technology: Extending the Ge electronics roadmap

The Ge-Ge longitudinal optical Raman peak has been measured in strained Ge1−xSnx alloy layers grown on top of relaxed InyGa1−yAs buffer layers on GaAs substrates by molecular beam epitaxy. The experimental result shows that the peak frequency shift increases linearly from the value for bulk Ge with the Sn fraction x and the strain ɛ, Δω = ω − ωGe = ax + bɛ. In these experiments alloy and strain contributions are decoupled and measured separately, and a and b are determined to be a = − 82 ± 4 cm−1 and b = − 563 ± 34 cm−1, over the entire composition and strain range investigated.

Hai Lin

Papers

Increased photoluminescence of strain-reduced, high-Sn composition Ge1−xSnx alloys grown by molecular beam epitaxy

Strong enhancement of direct transition photoluminescence with highly tensile-strained Ge grown by molecular beam epitaxy

Investigation of the direct band gaps in Ge1−xSnx alloys with strain control by photoreflectance spectroscopy

GeSn technology: Extending the Ge electronics roadmap

Raman study of strained Ge1−xSnx alloys