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Atomic short-range order (SRO) in direct-bandgap GeSn for infrared photonics has recently attracted attention due to its notable impact on band structures. However, the SRO in GeSn thin films grown by different methods have hardly been compared. This paper compares SRO in GeSn thin films of similar compositions grown by molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) using atom probe tomography. An $\sim$15% stronger preference for Sn-Sn 1$^{st}$ nearest neighbor (1NN) is observed in MBE GeSn than CVD GeSn for both thin film and quantum well samples with Sn composition ranging from 7 to 20%. Interestingly, samples grown by different deposition tools under the same method (either MBE or CVD) showed remarkable consistency in Sn-Sn 1NN SRO, while MBE vs. CVD showed clear differences. Supported by theoretical modeling, we consider that this difference in SRO originates from the impact of surface termination, where MBE surfaces are exposed to ultrahigh vacuum while CVD surfaces are terminated by H to a good extent. This finding not only suggests engineering surface termination or surfactants during the growth as a potential approach to control SRO in GeSn, but also provides insight into the underlying reasons for very different growth temperature between MBE and CVD that directly impact the strain relaxation behavior.

Temperature dependence of vibrational modes in semiconductors depends on lattice thermal expansion and anharmonic phonon-phonon scattering. Evaluating the two contributions from experimental data is not straightforward, especially for epitaxial layers that present mechanical deformation and anisotropic lattice expansion. In this work, a temperature-dependent Raman study in epitaxial Ge and Ge$_{1-x}$Sn$_{x}$ layers is presented. A model is introduced for the Raman mode energy shift as a function of temperature, comprising thermal expansion of the strained lattice and anharmonic corrections. With support of x-ray diffraction, the model is calibrated on experimental data of epitaxial Ge grown on Si and Ge$_{1-x}$Sn$_{x}$ grown on Ge/Si, finding that the main difference between bulk and epitaxial layers is related to the anisotropic lattice expansion. The phonon anharmonicity and other parameters do not depend on dislocation defect density (in the range $7\cdot 10^6$ - $4\cdot 10^8$ cm$^{-2}$) nor on alloy composition in the range 5-14 at.%. The strain-shift coefficient for the main model of Ge and for the Ge-Ge vibrational mode of Ge$_{1-x}$Sn$_{x}$ is weakly dependent on temperature and is around -500 cm$^{-1}$. In Ge$_{1-x}$Sn$_{x}$, the composition-shift coefficient amounts to -100 cm$^{-1}$, independent of temperature and strain., Comment: 8 pages, 5 figures

The local ordering of atoms in alloys directly has a strong impact on their electronic and optical properties. This is particularly relevant in nonrandom alloys, especially if they are deposited using far from the equilibrium processes, as is the case of epitaxial Ge1-xSnx layers. In this work, we investigate the arrangement of Ge and Sn atoms in optoelectronic grade Ge1-xSnx epitaxial layers featuring a Sn content in the 5-14% range by using polarization-dependent Raman spectroscopy and density-functional-theory calculations. The thorough analysis of the polarization-dependent spectra in parallel and perpendicular configuration allowed us to properly tag all the observed vibrational modes, and to shed light on that associated to disorder-assisted Raman transitions. Indeed, with the help of large-scale atomistic simulations, we were able to highlight how the presence of Sn atoms, that modify the local environments of Ge atoms, gives rise to two spectral features at different Raman shifts, corresponding to distortions of the atomic bonds. This analysis provides a valuable framework for advancing the understanding of the vibrational properties in Ge1-xSnx alloys, particularly with regard to the impact of local ordering of the different atomic species., Comment: 20 pages, 10 figures, Accepted Manuscript of a Published Paper in Physical Review

Here, we present a micro-electromechanical system (MEMS) for the investigation of the electromechanical coupling in graphene and potentially related 2D materials. Key innovations of our technique include: (1) the integration of graphene into silicon-MEMS technology; (2) full control over induced strain fields and doping levels within the graphene membrane and their characterization via spatially resolved confocal Raman spectroscopy; and (3) the ability to detect the mechanical coupling of the graphene sheet to the MEMS device with via their mechanical resonator eigenfrequencies., Comment: 5 pages, 6 figures, conference proceeding

(Si)GeSn semiconductors are finally coming of age after a long gestation period. The demonstration of device quality epi-layers and quantum-engineered heterostructures has meant that tunable all-group IV Si-integrated infrared photonics is now a real possibility. Notwithstanding the recent exciting developments in (Si)GeSn materials and devices, this family of semiconductors is still facing serious limitations that need to be addressed to enable reliable and scalable applications. The main outstanding challenges include the difficulty to grow high crystalline quality layers and heterostructures at the desired Sn content and lattice strain, preserve the material integrity during growth and throughout device processing steps, and control doping and defect density. Other challenges are related to the lack of optimized device designs and predictive theoretical models to evaluate and simulate the fundamental properties and performance of (Si)GeSn layers and heterostructures. This Perspective highlights key strategies to circumvent these hurdles and bring this material system to maturity to create far-reaching new opportunities for Si-compatible infrared photodetectors, sensors, and emitters for applications in free-space communication, infrared harvesting, biological and chemical sensing, and thermal imaging.

We report on the design of a waveguide coupled GeSn microdisk-laser cavity in which the germanium virtual substrate serving as a template for GeSn growth is repurposed for the definition of passive on-chip interconnection waveguides. A main challenge resides in transferring the optical power from the upper (Si)GeSn gain stack to the underlying virtual substrate layer and is solved with laser mode engineering. Designs are based on experimentally realized layer stacks and waveguide outcoupling efficiencies as high as 27% are shown in compact resonator geometries with a small, 7 $\mu$m radius, with 42% of the power being recycled in the laser cavity.

Germanium-Tin (GeSn) alloys have emerged as a promising material for future optoelectronics, energy harvesting and nanoelectronics owing to their direct bandgap and compatibility with existing Si-based electronics. Yet, their metastability poses significant challenges calling for in-depth investigations of their thermal behavior. With this perspective, this work addresses the interdiffusion processes throughout thermal annealing of pseudomorphic GeSn binary and SiGeSn ternary alloys. In both systems, the initially pseudomorphic layers are relaxed upon annealing exclusively via thermally induced diffusional mass transfer of Sn. Systematic post-growth annealing experiments reveal enhanced Sn and Si diffusion regimes that manifest at temperatures below 600{\deg}C. The amplified low-temperature diffusion and the observation of only subtle differences between binary and ternary hint at the unique metastability of the Si-Ge-Sn material system as the most important driving force for phase separation., Comment: 15 pages, 3 figures

Sn-containing Si and Ge alloys belong to an emerging family of semiconductors with the potential to impact group IV semiconductor devices. Indeed, the ability to independently engineer both lattice parameter and band gap holds the premise to develop enhanced or novel photonic, optoelectronic, and electronic devices. With this perspective, we present detailed investigations of the influence of Ge1-y-xSixSny layers on the optical properties of Si- and Ge-based heterostructures and nanowires. We found that adding a thin Ge1-x-ySixSny capping layer on Si or Ge greatly enhances light absorption especially in the near IR range leading to an increase in short-circuit current density. For the Ge1-y-xSixSny structure at thicknesses below 30 nm, a 14-fold increase in the short-circuit current is predicted with respect to bare Si. This enhancement decreases by reducing the capping layer thickness. Conversely, decreasing the shell thickness was found to improve the short-circuit current in Si/Ge1-y-xSixSny and Ge/Ge1-y-xSixSny core/shell nanowires. The optical absorption becomes very important when increasing the Sn content. Moreover, by exploiting optical antenna effect, these nanowires show an extreme light absorption reaching an enhancement factor, with respect to Si or Ge nanowires, on the order of ~104 in Si/Ge0.84Si0.04Sn0.12 and ~12 in Ge/Ge0.84Si0.04Sn0.12 core/shell nanowires. Furthermore, we analyzed the optical response of the addition of a dielectric capping layer consisting of Si3N4 to the Si/Ge1-y-xSixSny core-shell nanowire and found about 50% increase in short-circuit current density for a dielectric layer thickness of 45 nm and a core radius and shell thickness superior to 40 nm. The core/shell optical antenna benefits from a multiplication of enhancements contributed by leaky mode resonances in the semiconductor part and antireflection effects in the dielectric part., Comment: 47 pages, 11 figures