The local arrangement of atoms in an alloy impacts its electronic band structure and, consequently, its fundamental physical properties. This work investigates the short-range order of tin atoms in binary Ge1-xSnx semiconductors epitaxially grown on Ge-buffered Si (001) substrates. Samples featuring Sn content up to 13 at.% were analyzed by x-ray absorption fine structure spectroscopy at the Sn-K edge. The study reveals how the growth method and its process parameters, like temperature, or the epitaxial built-up strain affect the local atomic ordering of tin within the alloys. While the deposition technique seems to have a marginal effect on the Sn short-range ordering, the growth temperature and Sn content systematically influence the bonding angles Sn. Ge-Sn next neighbor coordination shells, whereas the interatomic distances remain largely unaffected. In the first coordination shell Sn-Ge are strongly favored over Sn-Sn, while the opposite happens in the next coordination shell, where Sn-Sn are systematically favored. These insights into the relationship of growth condition and layer properties enable to identify the nature of the large variety of electronic and optical properties measured in Ge1-xSnx layers.

The nascent group IV GeSn alloys are highly attractive for spintronics applications, including quantum computing, due to their ability to enable highly scalable fabrication and all-electrical spin manipulation. In this work, we conduct an in-depth study of a two-dimensional hole gas in a Ge/GeSn quantum well, exhibiting the integer quantum Hall effect and distinct Shubnikov-de Haas oscillations. Emphasis is given to the determination of the Landé g-factor and its pronounced anisotropy in this two-dimensional system, revealing values significantly higher than those in conventional Ge or SiGe/Ge systems. Moreover, by modeling the spin-orbit interaction using the Iordanskii-Lyanda-Geller-Pikus theory, crucial cubic Rashba spin-orbit interaction coefficients, are extracted and their significance is highlighted. This work provides the experimental validation of the theoretically predicted enhancements in spin-orbit interaction and g-factors in GeSn alloys compared to Ge. Additionally, it delivers essential parameters for the design of hole spin devices, such as hole qubits, utilizing GeSn-based structures on the Si platform. GeSn alloys hold promise for spintronics and quantum computing due to their scalable fabrication and spin manipulation capabilities. Here, the authors study a two-dimensional hole gas in a Ge/GeSn quantum well, revealing enhanced spin-orbit interactions and g-factors, providing key insights for designing GeSn-based spintronic devices.

The successful demonstration of (Si)Ge1-xSnx alloys as direct-gap materials for infrared lasers has driven intense research on group IV-based devices for nanoelectronics, energy harvesting, and quantum computing applications. The material palette of direct-gap group-IV alloys can be further extended by introducing carbon to fine-tune their structural and electronic properties, significantly expanding their functionality. This work presents heteroepitaxial growth of C(Si)GeSn alloys using an industry-standard reduced-pressure chemical vapor deposition reactor. The introduction of CBr4 as a precursor enables controlled incorporation of C atoms (<1 at.%) into the epilayer lattice, while simultaneously increasing the Sn content in the CGeSn alloy up to ≈18 at.%. Carbon plays a key role in modulating strain, stabilizing the crystal structure, and influencing material properties. By leveraging alloying and strain engineering, quaternary CSiGeSn bulk layers and CGeSn/GeSn heterostructures are epitaxially grown. The impact of C incorporation on optical emission is investigated in LEDs based on CGeSn/GeSn multiple quantum wells, demonstrating enhanced near-infrared emission at 2.54 µm, which is sustained up to room temperature.

Recent progress in the quest for CMOS-integrable GeSn light sources comprises the optically-pumped laser operating at room temperature and the first demonstrations of electrically pumped lasers. In this work, the performance of electrically-pumped double heterostructure GeSn ring laser diodes are evaluated as a function of their geometry and pumping pulse time. In particular, the trade-off between the band structure, i.e. the directness of the GeSn band gap, and the device heat dissipation is discussed in terms of their impact on the emission intensity and threshold current density.

The integration of thermoelectric devices into mainstream microelectronic technological platforms could be a major breakthrough in various fields within the so-called Green-IT realm. In this article, the thermoelectric properties of heteroepitaxial SiGeSn alloys, an emergent CMOS-compatible material system, are evaluated to assess their possible application in thermoelectric devices. To this purpose, starting from the experimentally low lattice thermal conductivity of SiGeSn/Ge/Si layers of about ∼1 to 2 W/m·K assessed by means of 3-ω measurements, the figure of merits are calculated through the use of Boltzmann transport equation, taking into account the relevant intervalley scattering processes, peculiar of this multivalley material system. Values for the figure of merit ZT exceeding 1 have been obtained for both p- and n-type material at operating temperatures within the 300-400 K range, i.e., at typical on-chip temperatures. In this interval, the predicted power factor also features very competitive values on the order of 20 μW/cm ·K2. Our finding indicates that this emergent class of Si-based materials has extremely good prospects for real-world applications and can further stimulate scientific investigation in this ambit.

We propose and compare two back-side illuminated GeSn avalanche photodiode (APD) mesa structures with 15% tin content operating at photon wavelengths up to 3.3 mu m, suitable for applications like methane gas sensing and analysis of tampered olive oil. The two structures have different multiplication materials: a) silicon, which requires an additional Ge Strain-Relaxed Buffer (SRB) layer for high-quality GeSn growth; b) germanium, which is acting also as SRB layer. The latter design is innovative compared to the state-of-the-art and it proposed to: i) reduce the space charge region (SCR) width by avoiding a too thick Ge SRB, which is required for growing high-tin-content GeSn; ii) avoid one supplementary non-lattice matched heterojunction in the SCR. Physical models for GeSn are discussed for the most relevant parameters of APD design. Simulations are performed in the electrical and optical domains, for evaluating the main figures of merit of APDs and comparing the expected performances between the two designs. Finally, we present the modeling and design of a focalizing all-dielectric metalens, integrated on the detector back-side, for improving the photon collection efficiency at the same active volume size, thus improving the signal-to-noise ratio.

AbstractOver the last 30 years, group-IV semiconductors have been intensely investigated in the quest for a fundamental direct bandgap semiconductor that could yield the last missing piece of the Si Photonics toolbox: a continuous-wave Si-based laser. Along this path, it has been demonstrated that the electronic band structure of the GeSn/SiGeSn heterostructures can be tuned into a direct bandgap quantum structure providing optical gain for lasing. In this paper, we present a versatile electrically pumped, continuous-wave laser emitting at a near-infrared wavelength of 2.32 µm with a low threshold current of 4 mA. It is based on a 6-periods SiGeSn/GeSn multiple quantum-well heterostructure. Operation of the micro-disk laser at liquid nitrogen temperature is possible by changing to pulsed operation and reducing the heat load. The demonstration of a continuous-wave, electrically pumped, all-group-IV laser is a major breakthrough towards a complete group-IV photonics technology platform.

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 paper, a temperature-dependent Raman study in epitaxial Ge and Ge1-xSnx 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 Ge1-xSnx 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 7x106 - 4x108 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 Ge1-xSnx is weakly dependent on temperature and is around -500 cm-1. In Ge1-xSnx, the composition-shift coefficient amounts to -100 cm-1, independent of temperature and strain.

The local lattice distortion in a crystal, the lattice strain, greatly influences the physical mechanisms underlying the operating principles of semiconductor devices. For example, strain is widely used in the bandgap engineering of group IV-based optoelectronic devices to improve their performance. In the case of GeSn-based light emitters, optimization of the lattice strain allowed the demonstration of optically pumped lasing at room temperature [1] and cw operation in an electrically pumped µ-disk laser [2], thanks to its beneficial effect on the "directness" of the bandgap, leading to increased material gain. It is therefore of paramount importance to characterize the strain with high sensitivity and sub-micron spatial resolution. Here we show how scanning X-ray diffraction microscopy, a recently developed model-free method based on synchrotron radiation [3,4], can be used to fully determine the landscape of mechanical deformation and stoichiometry fluctuation in a lithographically fabricated Ge1-xSnx/Ge suspended µ-disk, a structure of the same type as that used to demonstrate the first GeSn laser [5]. The full strain tensor of the entire microstructure, including all normal and shear components, is tomographically reconstructed with a lateral resolution of less than 100 nm and a sensitivity to strain variations of the order of 10-4. By comparing the lattice deformation in different sections of the microdisk, we observe a marked difference between the central pillar region in contact with the virtual Ge substrate and the free-standing outer rim where the Ge layer has been removed. Interestingly, although the misfit dislocation network at the GeSn/Ge relaxed heterointerface in the rim region has been removed during the fabrication process, we observe that a "fossilized" footprint of the dislocations is still present in the strain landscape of the layer. We attribute this to stoichiometric fluctuations that we measure in the Ge1-xSnx alloy generated by dislocation-driven strain fields during epitaxial growth, which are unaffected by etching and in turn generate a local strain field. We then exploit the symmetries of both the microdisk and the Ge1-xSnx material system itself. This allows us, for the first time, to calculate maps of the surface normal stress in an alloyed epitaxial thin film, which is traditionally difficult to impossible to disentangle from stoichiometric fluctuations in diffraction-based data, and is a key piece of information to evaluate in epitaxial layer growth. We complement the synchrotron experiments with electron microscopy and dedicated finite element method (FEM) simulations of both elastic and plastic relaxation processes in the model system, finding excellent agreement between experiment and theory. Furthermore, the effects of strain on the band structure are predicted in the light of the measured local variations in strain and composition. [1] A Elbaz, et al. Nature Photon. 14, 375 (2020) [2] L. Siedel et al. Nature, submitted (2024) [3] C. Corley-Wiciak et al. ACS Appl. Mater. & Interfaces 15, 3119 (2023) [4] C. Richter et al. Phys. Rev. Applied 18, 064015 (2022) [5] S Wirths, et al. Nature Photonics 9, 88 (2015)

\This paper reports on SiGeSn/GeSn multi-quantum-well microdisk lasers. The fabrication of the devices includes a selective under-etching step, which enhances the guiding of the whispering gallery modes inside the cavity. Lasing occurs under different electrical pumping conditions with a very low threshold current and for long, quasi-continuous wave pulses compared to previously reported GeSn-based microdisk lasers. Furthermore, the lasing threshold current is reduced by a factor of ten compared to similar double-heterostructure devices.

The reconstruction of thick GeSn crystalline layers implanted with phosphorus is explored. Our study demonstrates (i) the potential for recrystallizing amorphized GeSn crystal under specific conditions and (ii) functional LEDs incorporating an ex-situ doped GeSn injection layer. Mid-infrared direct band-gap light-emitting diodes are compared with either ex-situ GeSn:P or in-situ Ge:P top contact layers.

Silicon photonics is widely used for near InfraRed (IR) applications up to 1.6 mu m. It plays a key role in short-range optical data communications. However, silicon photonics does not really address mid-IR applications, particularly in the 1.6-5 mu m wavelength range. This spectral region is essential for environmental/life sensing and safety applications relying on the optical features of molecular vibrations, the aim being to discern and categorize complex chemical entities. Growing markets for such analysis prioritise sensitivity, specificity, compactness, energy-efficient operation and cost effectiveness. The need for a CMOS-compatible integrated photonic platform for the mid-IR is obvious. Such fully-group-IV semiconductor platform should include low-loss guided interconnects, detectors, modulators and, critically, efficient integrated light sources. This paper provides a comprehensive review of recent advances in GeSn-based mid-IR silicon-compatible devices, including optically and electrically pumped lasers, light-emitting diodes and photodetectors. It also discusses the principles underlying these developments, with focuses on material growth techniques and processing methods.

Neighborhood at atomic scale is important for the properties of advanced alloys. The preference or avoidance between neighboring atomic species is known as chemical short-range order (SRO). While SRO in metallic medium/high entropy alloys has garnered substantial attention recently, understanding SRO in semiconductor alloys remains underdeveloped. Motivated by theoretically predicted SRO and its dramatic impact on band structure, here we perform statistical analyses of atom probe tomography data to quantify SRO in SixGe1-x-ySny alloys (x<12 at.%, y<16 at.%). Leveraging a side-by-side experiment-theory comparison at the same spatial scale enabled by machine-learning neuroevolution potentials of first-principles accuracy, we reveal, for the first time, a notable SRO favoring Si-Si 1st nearest neighbors even at dilute Si and Sn compositions. The SRO can be tuned by varying precursors, offering a new degree of freedom for band engineering beyond composition and strain, and enabling new phase-change materials based on SRO transitions for Si electronics/photonics.

CMOS-compatible materials for efficient energy harvesters at temperatures characteristic for on-chip operation and body temperature are the key ingredients for sustainable green computing and ultralow power Internet of Things applications. In this context, the lattice thermal conductivity (kappa) of new group IV semiconductors, namely Ge1-xSnx alloys, are investigated. Layers featuring Sn contents up to 14 at.% are epitaxially grown by state-of-the-art chemical-vapor deposition on Ge buffered Si wafers. An abrupt decrease of the lattice thermal conductivity (kappa) from 55 W/(mK) for Ge to 4 W/(mK) for Ge0.88Sn0.12 alloys is measured electrically by the differential 3 omega-method. The thermal conductivity was verified to be independent of the layer thickness for strained relaxed alloys and confirms the Sn dependence observed by optical methods previously. The experimental kappa values in conjunction with numerical estimations of the charge transport properties, able to capture the complex physics of this quasi-direct bandgap material system, are used to evaluate the thermoelectric figure of merit ZT for n- and p-type GeSn epitaxial layers. The results highlight the high potential of single-crystal GeSn alloys to achieve similar energy harvest capability as already present in SiGe alloys but in the 20 degrees C-100 degrees C temperature range where Si-compatible semiconductors are not available. This opens the possibility of monolithically integrated thermoelectric on the CMOS platform.