This work investigates the electronic band structures of bulk Ge1-xSnx alloys using the empirical pseudopotential method (EPM) for Sn composition x varying from 0 to 0.2. The adjustable form factors of EPM were tuned in order to reproduce the band features that agree well with the reported experimental data. Based on the adjusted pseudopotential form factors, the band structures of Ge1-xSnx alloys were calculated along high symmetry lines in the Brillouin zone. The effective masses at the band edges were extracted by using a parabolic line fit. The bowing parameters of hole and electron effective masses were then derived by fitting the effective mass at different Sn compositions by a quadratic polynomial. The hole and electron effective mass were examined for bulk Ge1-xSnx alloys along specific directions or orientations on various crystal planes. In addition, employing the effective-mass Hamiltonian for diamond semiconductor, band edge dispersion at the Γ-point calculated by 8-band k.p. method was fitted...

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Electronic band structure and effective mass parameters of Ge1-xSnx alloys

Si–Ge–Sn alloys: From growth to applications

A surface-illuminated photoconductive detector based on Ge0.91Sn0.09 quantum wells with Ge barriers grown on a silicon substrate is demonstrated. Photodetection up to 2.2µm is achieved with a responsivity of 0.1 A/W for 5V bias. The spectral absorption characteristics are analyzed as a function of the GeSn/Ge heterostructure parameters. This work demonstrates that GeSn/Ge heterostructures can be used to developed SOI waveguide integrated photodetectors for short-wave infrared applications.

https://biblio.ugent.be/publication/3159522/file/4398481.pdf

GeSn/Ge heterostructure short-wave infrared photodetectors on silicon

Using empirical pseudopotential theory, the direct (Γ) and indirect bandgaps (L and X) of unstrained crystalline SixGe1−x−ySny have been calculated over the entire xy composition range. The results are presented as energy-contour maps on ternary diagrams along with a ternary plot of the predicted lattice parameters. A group of 0.2 to 0.6 eV direct-gap SiGeSn materials is found for a variety of mid-infrared photonic applications. A set of “slightly indirect” SiGeSn alloys having a direct gap at 0.8 eV (but with a smaller L-Γ separation than in Ge) have been identified. These materials will function like Ge in various telecom photonic devices. Hetero-layered SiGeSn structures are described for infrared light emitters, amplifiers, photodetectors, and modulators (free carrier or Franz-Keldysh). We have examined in detail the optimized design space for mid-infrared SiGeSn-based multiple-quantum-well laser diodes, amplifiers, photodetectors, and quantum-confined Stark effect modulators.

The direct and indirect bandgaps of unstrained SixGe1−x−ySny and their photonic device applications

Recent demonstrations of optically pumped lasers based on GeSn alloys put forward the prospect of efficient laser sources monolithically integrated on a Si photonic platform. For instance, GeSn layers with 12.5% of Sn were reported to lase at 2.5 um wavelength up to 130 K. In this work, we report a longer emitted wavelength and a significant improvement in lasing temperature. The improvements resulted from the use of higher Sn content GeSn layers of optimized crystalline quality, grown on graded Sn content buffers using Reduced Pressure CVD. The fabricated GeSn micro-disks with 13% and 16% of Sn showed lasing operation at 2.6 um and 3.1 um wavelengths, respectively. For the longest wavelength (i.e 3.1 um), lasing was demonstrated up to 180 K, with a threshold of 377 kW/cm2 at 25 K.

Optically pumped GeSn micro-disks with 16 % Sn lasing at 3.1 um up to 180K

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

SixGe1−x−ySny alloys were grown by molecular beam epitaxy at low temperature, followed by ex-situ annealing. The crystal quality of SixGe1−x−ySny layers was characterized by atomic force microscopy and transmission electron microscopy. The compositions and lattice constants of the alloys were studied by x-ray photoelectron spectroscopy and x-ray diffraction. The results show that Vegard’s law is a good approximation for SixGe1−x−ySny alloys. Photoreflectance spectroscopy at room temperature was used to determine the direct bandgap energy of SixGe1−x−ySny layers. Analyzing the relationship between composition and direct bandgap energy reveals a negative energy bowing parameter for SiSn.

Structural and optical characterization of SixGe1−x−ySny alloys grown by molecular beam epitaxy

In recent years, GeSn and SiGeSn alloys have been widely studied, because of their potential optical properties as well as their compatibility with Si technology. GeSn alloys become direct bandgap materials with about 6–7% Sn1−2, thus a promising candidate as a laser gain material. SiGeSn alloys provide independent control of lattice constant and band structure3−4, which makes them ideal candidates as buffer layers and quantum well barriers. Therefore, SiGeSn/GeSn/SiGeSn heterostructures, with or without strain, can be designed to achieve pure group IV lasers, in which Type-I band offset alignment might be realized.5−6 Simulations of optical gain show very promising results, but direct photoluminescence (PL) of this heterostructure has not been previously demonstrated. In our group, we have obtained high-quality single GeSn and SiGeSn layers by low-temperature MBE growth and characterized their structural and optical properties1, 3, 7−9. We will describe the experimental realization and optical characterization of the GeSn/SiGeSn heterostructures.

Weisheng Lu

Papers

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

Structural and optical characterization of SixGe1−x−ySny alloys grown by molecular beam epitaxy

Optical properties of Ge 1−z Sn z / Si x Ge 1−x−y Sn y heterostructures