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Abstract The continued downscaling of silicon CMOS technology presents challenges for achieving the required low power consumption. While high mobility channel materials hold promise for improved device performance at low power levels, a material system which enables both high mobility n-FETs and p-FETs, that is compatible with Si technology and can be readily integrated into existing fabrication lines is required. Here, we present high performance, vertical nanowire gate-all-around FETs based on the GeSn-material system grown on Si. While the p-FET transconductance is increased to 850 µS/µm by exploiting the small band gap of GeSn as source yielding high injection velocities, the mobility in n-FETs is increased 2.5-fold compared to a Ge reference device, by using GeSn as channel material. The potential of the material system for a future beyond Si CMOS logic and quantum computing applications is demonstrated via a GeSn inverter and steep switching at cryogenic temperatures, respectively.

Epitaxy of semiconductor-based quantum well structures is a challenging task since it requires precise control of the deposition at the submonolayer scale. In the case of Ge1–xSnx alloys, the growth is particularly demanding since the lattice strain and the process temperature greatly impact the composition of the epitaxial layers. In this paper, the realization of high-quality pseudomorphic Ge1–xSnx layers with Sn content ranging from 6 at. % up to 15 at. % using isothermal processes in an industry-compatible reduced-pressure chemical vapor deposition reactor is presented. The epitaxy of Ge1–xSnx layers has been optimized for a standard process offering a high Sn concentration at a large process window. By varying the N2 carrier gas flow, isothermal heterostructure designs suitable for quantum transport and spintronic devices are obtained.

Energy harvesting for Internet of Things applications, comprising sensing, life sciences, wearables, and communications, requires efficient thermoelectric (TE) materials, ideally semiconductors compatible with Si technology. In this work, we investigate the potential of GeSn/Ge layers, a group IV material system, as TE material for low-grade heat conversion. We extract the lattice thermal conductivity, by developing an analytical model based on Raman thermometry and heat transport model, and use it to predict thermoelectric performances. The lattice thermal conductivity decreases from 56 W/(m·K) for Ge to 4 W/(m·K) by increasing the Sn atomic composition to 14%. The bulk cubic Ge0.86Sn0.14 alloy features a TE figure of merit of ZT ∼ 0.4 at 300 K and an impressive 1.04 at 600 K. These values are extremely promising in view of the use of GeSn/Ge layers operating in the typical on-chip temperature range.

Nanowires are promising platforms for realizing ultra-compact light sources for photonic integrated circuits. In contrast to impressive progress on light confinement and stimulated emission in III-V and II-VI semiconductor nanowires, there has been no experimental demonstration showing the potential to achieve strong cavity effects in a bottom-up grown single group-IV nanowire, which is a prerequisite for realizing silicon-compatible infrared nanolasers. Herein, we address this limitation and present the first experimental observation of cavity-enhanced strong photoluminescence from a single Ge/GeSn core/shell nanowire. A sufficiently large Sn content (~ 10 at%) in the GeSn shell leads to a direct bandgap gain medium, allowing a strong reduction in a material loss upon optical pumping. Efficient optical confinement in a single nanowire enables many round trips of emitted photons between two facets of a nanowire, achieving a narrow width of 3.3 nm. Our demonstration opens new possibilities for ultrasmall on-chip light sources towards realizing photonic-integrated circuits in the underexplored range of extended SWIR.

The continued downscaling of silicon CMOS technology faces big challenges in achieving the required low power consumption. While high mobility channel materials hold promise for improved device performance at low power levels, to date a material system which enables both high mobility n-FETs and p-FETs, that is compatible with Si technology and can be readily integrated into existing fabrication lines is still missing. Here, we present high performance, vertical nanowire gate-all-around FETs based on the GeSn-material system grown on Si. While the p-FET performance is improved by exploiting the small band gap of GeSn as source yielding high injection velocities, the mobility in n-FETs is increased 2.5-fold compared to a Ge reference device, by using GeSn as channel material. The very high potential of the material system for future beyond Si CMOS logic and quantum computing applications is demonstrated via a GeSn inverter and steep switching at cryogenic temperatures, respectively.

Harvesting the full potential of single-crystal semiconductor nanowires (NWs) for advanced nanoscale field-effect transistors (FETs) requires a smart combination of charge control architecture and functional semiconductors. In this article, high-performance vertical gate-all-around NW p-type FETs (p-FETs) are presented. The device concept is based on advanced Ge0.92Sn0.08/Ge group IV epitaxial heterostructures, employing quasi–one-dimensional semiconductor NWs fabricated with a top-down approach. The advantage of using a heterostructure is the possibility of electronic band engineering with band offsets tunable by changing the semiconductor stoichiometry and elastic strain. The use of a Ge0.92Sn0.08 layer as the source in GeSn/Ge NW p-FETs results in a small subthreshold slope of 72 mV/dec and a high ION/IOFF ratio of 3 × 106. A ∼32% drive current enhancement is obtained compared to the vertical Ge homojunction NW control devices. More interestingly, the drain-induced barrier lowering is much smaller with GeSn instead of Ge as the source. The general improvement of the transistor’s key figures of merits originates from the valence band offset at the Ge0.92Sn0.08/Ge heterojunction, as well as from a smaller NiGeSn/GeSn contact resistivity.

Despite having achieved drastically improved lasing characteristics by harnessing tensile strain, the current methods of introducing a sizable tensile strain into GeSn lasers require complex fabrication processes, thus reducing the viability of the lasers for practical applications. The geometric strain amplification is a simple technique that can concentrate residual and small tensile strain into localized and large tensile strain. However, the technique is not suitable for GeSn due to the intrinsic compressive strain introduced during the conventional epitaxial growth. In this Letter, we demonstrate the geometrical strain amplification in GeSn by employing a tensile strained GeSn-on-insulator (GeSnOI) substrate. This work offers exciting opportunities in developing practical wavelength-tunable lasers for realizing fully integrated photonic circuits. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) iGrant of Singapore A*STAR IRG (A2083c0053); National Research Foundation Singapore (Competitive Research Program NRF-CRP19-2017-01, NRF-ANR Joint Grant NRF2018-NRF-ANR009 TIGER); Ministry of Education - Singapore (AcRF TIER 1 RG 115/21, AcRF TIER 2 MOE2018-T2-2-011 (S)).

Theoretical investigation of electro-absorption modulators in the mid-infrared range (>∼2 μm) is performed using asymmetric intra-step quantum wells based on Ge1−η1Snη1/Ge1−η2Snη2 heterostructures with SiGeSn outer barriers. After exploring the parameter space of the Sn content difference and width ratio of the intra-layers, a linear and much larger Stark shift is realized, compared to that of a square quantum well, without an increase of the total structure width. A modulator based on an optimized intra-step quantum well structure with a total well width of 12 nm is theoretically predicted to have both a larger peak shift per unit applied field and a larger absorption change than a 12 nm square quantum well device. By analyzing the device performance based on the two figures of merit: (1) absorption change per applied field and (2) absorption change per applied field squared, and taking 10 dB extinction ratio, a 44% higher bandwidth per volt and 46% lower power consumption per bit are achieved in intra-step than in a square well. Although the swing voltage for a square quantum well can be reduced by using a larger on-set applied field and performance could be improved, we found that the intra-step quantum well using zero on-set still retains its advantages when compared to the square quantum well which uses a 0.5 V on-set voltage.

GeSn alloys have emerged as a promising material for group IV light sources because alloying Ge with Sn increases the directness of the bandstructure, thus improving the efficiency of light emission. Despite several years of progress in GeSn lasers, however, the integration of such lasers into practical applications still faces challenges such as high threshold, low operating temperature, and large device footprint. In this report, we address these challenges via each of the studies containing thermal management, defect reduction, and nanowire growth approach. First, we demonstrate improved lasing characteristics including reduced threshold and increased operating temperature in GeSn microdisks directly sitting on Si, which is allowed by the enhanced thermal management over the conventional suspended microdisks. Although there is a concern about poor optical confinement of the sitting approach, we confirm the simultaneous achievement of excellent heat dissipation and superior optical confinement from the microdisk released on Si through experiments and theoretical simulations. We also demonstrate a decreased threshold in microdisk lasers fabricated using a high-quality GeSn-on-insulator (GeSnOI) substrate. Photoluminescence measurements show that the reduction of defects in GeSnOI leads to enhancement of spontaneous emission and reduction of the lasing threshold. Lastly, we present the potential for GeSn nanowire lasers having smaller footprints by observing clear cavity resonances in a single nanowire grown by a bottom-up growth approach. Our demonstrations provide guiding principles to push the performance of GeSn lasers to the limit towards a realization of practical group IV light sources.

GeSn alloys have been regarded as a promising material for creating a complementary metal-oxide-semiconductor (CMOS)-compatible light source. Despite the remarkable progress in demonstrating GeSn lasers, an unavoidable intrinsic compressive strain introduced during epitaxial growth has prevented researchers from pushing the directness of GeSn gain media to the limit and realizing practical GeSn lasers. In this paper, we demonstrate a GeSn-based 1D photonic crystal nanobeam laser on a high-quality GeSn-on-insulator (GeSnOI) substrate which allows releasing the limiting compressive strain, thus improving the threshold and operating temperature. Pump-power-dependent photoluminescence measurements show a lasing threshold density of 18.2 kW cm−2 at 4 K for the released strain-free GeSn nanobeam, which is ~2 times lower than that of the unreleased GeSn nanobeam with compressive strain. The improved bandgap directness in the released GeSn nanobeam also allows achieving lasing action at higher operating temperatures up to 90 K compared to the unreleased laser device (

We present comprehensive modeling of a SiGeSn multi-quantum well laser that has been previously experimentally shown to feature an order of magnitude reduction in the optical pump threshold compared to bulk lasers. We combine experimental material data obtained over the last few years with k · p theory to adapt transport, optical gain, and optical loss models to this material system (drift-diffusion, thermionic emission, gain calculations, free carrier absorption, and intervalence band absorption). Good consistency is obtained with experimental data, and the main mechanisms limiting the laser performance are discussed. In particular, modeling results indicate a low non-radiative lifetime, in the 100 ps range for the investigated material stack, and lower than expected Γ-L energy separation and/or carrier confinement to play a dominant role in the device properties. Moreover, they further indicate that this laser emits in transverse magnetic polarization at higher temperatures due to lower intervalence band absorption losses. To the best of our knowledge, this is the first comprehensive modeling of experimentally realized SiGeSn lasers, taking the wealth of experimental material data accumulated over the past years into account. The methods described in this paper pave the way to predictive modeling of new (Si)GeSn laser device concepts.

ZnO is a widely studied material for several applications, such as a photocatalyst, a working electrode for dye-sensitized solar cells, and for thermoelectric devices. This work studies the effects of an increase in the number of carriers by doping ZnO with Al and Ga. The 6.25 mol% Al-doped ZnO, 6.25 mol% Ga-doped ZnO, and 12.5 mol% (Al, Ga)-co-doped ZnO nanoparticles were prepared using the combustion method. The prepared samples were then characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and UV–visible spectroscopy techniques. Moreover, the density functional theory (DFT) was also employed for computational study of Al and Ga doped ZnO. Optimized crystal structures, density of states (DOS) and band structure of these systems were calculated using Vienna Ab initio Simulation Package code. From this study, Al and Ga are found to play an important role in both the morphology and optical properties of the ZnO: Al and Ga doping can change the band gap and the Fermi level position in the ZnO. The prepared samples were characterized for their thermoelectric properties, and these were also modelled, using BolzTraP code, for ZnO, Al-doped ZnO, Ga-doped ZnO and (Al, Ga)-co-doped ZnO. The Seebeck coefficient, electrical conductivity, relaxation time, electronic thermal conductivity and power factor were all analysed. The experimental and computational results all point in the same direction, indicating that the thermoelectric properties of ZnO change because the semiconductor ZnO transforms into metallic ZnO when doped with Al and Ga. This leads to ZnO showing different thermoelectric properties, particularly Ga-doped ZnO and (Al, Ga)-co doped ZnO: they provide a high electrical conductivity and power factor. Therefore, it is expected that these favorable properties might promote the ZnO to be a potential candidate for improved efficiency thermoelectric devices.

The success of GeSn alloys as active material for infrared lasers could pave the way toward a monolithic technology that can be manufactured within mainstream silicon photonics. Nonetheless, for operation on chip, lasing should occur at room temperature or beyond. Unfortunately, despite the intense research in recent years, many hurdles have yet to be overcome. An approach exploiting strain engineering to induce large tensile strain in micro-disk made of GeSn alloy with Sn content of 14 at% is presented here. This method enables robust multimode laser emission at room temperature. Furthermore, tensile strain enables proper valence band engineering; as a result, over a large range of operating temperatures, lower lasing thresholds are observed compared to high Sn content GeSn lasers operating at similar wavelength.

GeSn alloys are promising candidates for complementary metal-oxide-semiconductor (CMOS)-compatible, tunable lasers. Relaxation of residual compressive strain in epitaxial GeSn has recently shown promise in improving the lasing performance. However, the suspended device configuration that has thus far been introduced to relax the strain is destined to limit heat dissipation, thus hindering the device performance. Herein, we demonstrate that strain-free GeSn microdisk laser devices fully released on Si outperform the canonical suspended devices. This approach allows to simultaneously relax the limiting compressive strain while offering excellent thermal conduction. Optical simulations confirm that, despite a relatively small refractive index contrast between GeSn and Si, optical confinement in strain-free GeSn optical cavities on Si is superior to that in conventional strain-free GeSn cavities suspended in the air. Moreover, thermal simulations indicate a negligible temperature increase in our device. Conversely, the temperature in the suspended devices increases substantially reaching, for instance, 120 K at a base temperature of 75 K under the employed optical pumping conditions. Such improvements enable increasing the operation temperature by ~40 K and reducing the lasing threshold by 30%. This approach lays the groundwork to implement new designs in the quest for room temperature GeSn lasers on Si., 16 pages, 7 figures

A direct bandgap can be engineered in Ge-rich group-IV alloys by increasing Sn content and by introducing tensile strain in GeSn. Here, we combine these two routes in quantum well (QW) structures and systematically analyze the properties of SiGeSn/GeSn quantum wells for a range of Sn content, strain, and well width values, within realistic boundaries. Using the k ⋅ p method, and including L-valley within the effective mass method, we find that 13–16 nm is a preferred range of well widths to achieve high gain for tensile-strained SiGeSn/GeSn quantum wells. Within the range of the well widths, a loss ridge caused by inter-valence band absorption and free carrier absorption is found in the region of parameter space where Sn content and strain in the well are related as Sn ( % ) ≈ − 7.71 e x x ( % ) + 17.13. Limited by a practical strain boundary of 1.7%, for a 14 nm quantum well, we find that 7.5 ± 1 % Sn and 1 ± 0.2 % strain is a promising combination to get a good net gain for photon transition energy higher than ∼0.42 eV. A maximum utilization of strain is preferred to obtain the best gain with lower energies (

Silicon photonics in the near-IR, up to 1.6 µm, is already one of key technologies in optical data communications, particularly short range. It also is being prospected for applications in quantum computing, artificial intelligence, optical signal processing, where complex photonic integration is to be combined with a large-volume fabrication. However, silicon photonics does not yet cover a large portion of applications in the mid-IR. In the wavelength range of 2–5 µm, environmental sensing, life sensing and security, all rely on optical signatures of molecular vibrations to identify complex individual chemical species. The markets for such analysis are huge and constantly growing, with a push for sensitivity, specificity, compactness, low-power operation and low cost. An all-group-IV, CMOS-compatible mid-IR integrated photonic platform would be a key enabler in this wavelength range. As for other wavelengths, such a platform should be complete with low-loss guided interconnects, detectors, eventually modulators, and most important an efficient and integrated light sources. This chapter reviews the recent developments of mid-IR silicon-compatible optically and electrically pumped lasers, light emitting diodes and photodetectors based on Ge, GeSn and SiGeSn alloys. It contains insights into the fundamentals of these developments, including bandstructure modelling, material growth and processing techniques.

GeSn alloys have been regarded as a potential lasing material for a complementary metal-oxide-semiconductor (CMOS)-compatible light source. Despite their remarkable progress, all GeSn lasers reported to date have large device footprints and active areas, which prevent the realization of densely integrated on-chip lasers operating at low power consumption. Here, we present a 1D photonic crystal (PC) nanobeam with a very small device footprint of 7 ${\mu}m^2$ and a compact active area of ~1.2 ${\mu}m^2$ on a high-quality GeSn-on-insulator (GeSnOI) substrate. We also report that the improved directness in our strain-free nanobeam lasers leads to a lower threshold density and a higher operating temperature compared to the compressive strained counterparts. The threshold density of the strain-free nanobeam laser is ~18.2 kW cm$^{ -2}$ at 4 K, which is significantly lower than that of the unreleased nanobeam laser (~38.4 kW cm$^{ -2}$ at 4 K). Lasing in the strain-free nanobeam device persists up to 90 K, whereas the unreleased nanobeam shows a quenching of the lasing at a temperature of 70 K. Our demonstration offers a new avenue towards developing practical group-IV light sources with high-density integration and low power consumption., Comment: 14 pages, 6 figures

GeSn alloys are a promising emerging complementary metal-oxide-semiconductor compatible technology for applications in photonics and electronics. However, the unavoidable intrinsic compressive strain introduced during epitaxial growth has prevented researchers from pushing the performance of GeSn devices to the limit and realizing real-world applications. In this paper, we present a straightforward geometric strain-inversion technique that harnesses the harmful compressive strain to achieve beneficial tensile strain in GeSn nanowires, drastically increasing the directness of the band structure. We achieve ∼2.67% uniaxial tensile strain in ∼120 nm wide nanowires, surpassing other values reported thus far. Unique pseudo-superlattices comprising of indirect and direct bandgap GeSn are demonstrated in a single material only by applying a periodic tensile strain. Improved directness in tensile-strained GeSn significantly enhances the photoluminescence by a factor of ∼2.5. This work represents a way to develop scalable band-engineered GeSn nanowire devices with lithographic design flexibility. This technique can be potentially applied to any layer with an intrinsic compressive strain, creating opportunities for unique tensile strained materials with diverse electronic and photonic applications. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Published version The research of the project was in part supported by Ministry of Education, Singapore, under Grant No. AcRF TIER 1 (RG 115/ 21). The research of the project was also supported by Ministry of Education, Singapore, under Grant No. AcRF TIER 2 [MOE2018- T2-2-011 (S)]. This work was also supported by the National Research Foundation of Singapore through the Competitive Research Program (No. NRF-CRP19-2017-01). This work was also supported by the National Research Foundation of Singapore through the NRF-ANR Joint Grant (No. NRF2018-NRF-ANR009 TIGER). This work was also supported by the iGrant of Singapore A-STAR AME IRG (No. A2083c0053).

We present an experimental setup capable of time-resolved photoluminescence spectroscopy for photon energies in the range of 0.51 to 0.56 eV with an instrument time response of 75 ps. The detection system is based on optical parametric three-wave mixing, operates at room temperature, has spectral resolving power, and is shown to be well suited for investigating dynamical processes in germanium-tin alloys. In particular, the carrier lifetime of a direct-bandgap Ge 1 − x Sn x film with concentration x = 12.5 % and biaxial strain − 0.55 % is determined to be 217 ± 15 ps at a temperature of 20 K. A room-temperature investigation indicates that the variation in this lifetime with temperature is very modest. The characteristics of the photoluminescence as a function of pump fluence are discussed.

Strained GeSn alloys are promising for realizing light emitters based entirely on group IV elements. Here, we report GeSn microdisk lasers encapsulated with a SiNx stressor layer to produce tensile strain. A 300 nm-thick GeSn layer with 5.4 at% Sn, which is an indirect-bandgap semiconductor as-grown, is transformed via tensile strain engineering into a direct-bandgap semiconductor that supports lasing. In this approach, the low Sn concentration enables improved defect engineering and the tensile strain delivers a low density of states at the valence band edge, which is the light hole band. We observe ultra-low-threshold continuous-wave and pulsed lasing at temperatures up to 70 K and 100 K, respectively. Lasers operating at a wavelength of 2.5 μm have thresholds of 0.8 kW cm−2 for nanosecond pulsed optical excitation and 1.1 kW cm−2 under continuous-wave optical excitation. The results offer a path towards monolithically integrated group IV laser sources on a Si photonics platform. Continuous-wave lasing in strained GeSn alloys is reported at temperatures of up to 100 K. The approach offers a route towards a group-IV-on-silicon laser.

Terahertz quantum cascade-lasers (QCLs) are able to produce higher optical output power at higher temperatures when operated in pulsed mode. Predicting a laser's behavior under pulsed operation in order to achieve performance requirements is, however, a nontrivial exercise: the complex and nonlinear interplay between current, electric field, and thermal transients gives rise to complex responses in both optical output power and emission frequency. In applications where it is important to predict and control these behaviors, establishing the link between current drive, emission frequency, and optical output power is necessary. In this paper, we demonstrate, via a realistic laser-specific model, that by appropriate manipulation of the drive pulse we can not only obtain a higher optical output at increased operating temperature but also both extend and linearize a QCL's frequency sweep. We suggest that consideration of laser behavior through realistic and comprehensive modeling is not only useful but is also required in any pulsed application in which emission frequency change is likely to affect performance.

In this work, a novel CMOS compatible process for Si-based materials has been presented to form SiGe nanowires (NWs) on SiGe On Insulator (SGOI) wafers with unprecedented thermoelectric (TE) power ...

International audience; In recent years much effort has been made to increase the Sn content in GeSn alloys in order to increase direct bandgap charge carrier recombination and, therefore, to reach room temperature lasing. While being successful for the former, the increase of Sn content is detrimental, leading to increased defect concentrations and a lower thermal budget regarding processing. In this work we demonstrate strong photoluminescence enhancement in low Sn content Ge 0.94 Sn 0.06 layers by implementing tensile strain. Fitting of the calculated photoluminescence spectra to reproduce our experimental results indicates a strain of ~1.45%, induced via an SiN x stressor layer, which is strong enough to transform the investigated layer into a direct bandgap semiconductor. Moreover, theoretical calculations, using the 8-band k·p model, show the advantages of using low Sn content tensile strained GeSn layers in respect to gain and lasing temperature. We show that low Sn content GeSn alloys have a strong potential to enable efficient room temperature lasers on electronic-photonic integrated circuits. Si photonics is entering the market for short haul interconnects to construct energy efficient data centers and high performance computers. It offers high bandwidths and thus high speed data communication. In this regard, the conventional chip design is replaced or extended by optical devices like waveguides, modulators, detectors and lasers, in which information is processed via photons. For the latter, GeSn lasers have emerged as a viable solution for Si-compatible integrated group-IV laser sources 1,2

We characterise the thin-film structural properties and photoluminescence of femtosecond (40 fs, 800 nm) pulsed laser deposited Er3+-doped zinc-sodium tellurite glass on Si as a function of laser fluence. The laser fluence regime required for the formation of films composed of nanoparticles without droplets is found, the composition and crystallinity of the deposited material is reported and the photoluminescence of the films is characterised in dependence of film thickness.

In this work we present a generalization of the Liouvillian superoperator for periodic quantum systems that can be formulated through partitioned Hamiltonians. We formulate a compact algebraic form of the superoperator that allows efficient numerical implementation along with the possibility of further generalization and the inclusion of the system’s boundary effects (i.e. device contacts). We apply this formalism to Quantum Cascade Laser structure where we compare the second nearest and the nearest on approximation, and present the laser dynamics that is independent from the number of states considered.

GeSn is proven as a good candidate to achieve CMOS-compatible laser sources on silicon. Lasing demonstrations in this alloy were based on directness of the band structure, this directness being increased with increasing the Sn content above 8 at.%. These past few years the research were consequently focused on incorporating the highest Sn content as possible to achieve high directness and high temperature laser operation. This unfortunately results is increased threshold. In this contribution we discuss the advantages in combining tensile strain engineering with lower Sn content alloys. This approach is motivated by the higher material quality in lower Sn content. The case with Sn content as small as 5.4 at.% Sn will be discussed. The alloy is initially compressively strained, and exhibits an indirect band gap that is turned to direct by applying tensile strain. A specific technology based on transfer On Insulator stressor layer on metal was developed to address strain engineering, thermal cooling and defective interface with the Ge-VS. This led to lasing in Ge0.95Sn0.05 microdisk cavities with dramatically reduced thresholds, by two order of magnitude, as compared to the case with high Sn alloys and as consequence enables cw operation.

GeSn is discussed as solution to realize the dream of a group IV light source integrated on a Si chip. Sn added into a Ge lattice decreases the conduction band energies leading to a direct bandgap semiconductor band structure. However, the compressive strain increases the direct band energy imposing a large Sn content in the GeSn bulk. In spite of many difficulties regarding the growth of epitaxial GeSn alloys on Si, several hundred nm thick GeSn layers with various Sn concentrations up to 15% could be realized and used as gain material for lasers. Nowadays research concentrates on increasing the Sn content towards 20 at% as well as structural layout. The challenge here is the decreasing quality at high Sn contents and the isolation of the active layer from the mists formed at the interface with Ge/Si which increase the laser threshold. In this direction we discuss the influence on lasing and threshold of MQW SiGeSn/GeSn heterostructures with different quantum well thicknesses. Other solution proposed is the change of intrinsic strain type from compressive into tensile by introducing Si3N4 stressors and also GeSn on Insulator technology. These methods are well known in CMOS technology and can be applied to very low Sn content GeSn alloys. The discussion on the best way to reach room temperature laser is addressed both theoretical and experimental.

We consider the influence of additional carrier confinement, achieved by application of strong perpendicular magnetic field, on inter Landau levels electron relaxation rates and the optical gain, of two different GaAs quantum cascade laser structures operating in the terahertz spectral range. Breaking of the in-plane energy dispersion and the formation of discrete energy levels is an efficient mechanism for eventual quenching of optical phonon emission and obtaining very long electronic lifetime in the relevant laser state. We employ our detailed model for calculating the electron relaxation rates (due to interface roughness and electron-longitudinal optical phonon scattering), and solve a full set of rate equations to evaluate the carrier distribution over Landau levels. The numerical simulations are performed for three- and four-well (per period) based structures that operate at 3.9 THz and 1.9 THz, respectively, both implemented in GaAs/Al0.15Ga0.85As. Numerical results are presented for magnetic field values from 1.5 T up to 20 T, while the band nonparabolicity is accounted for. (C) 2016 Elsevier B.V. All rights reserved.

The optimal design of asymmetric quantum well structures for generation of entangled photons in the mid-infrared range by spontaneous parametric downconversion is considered, and the efficiency of this process is estimated. Calculations show that a reasonably good degree of entanglement can be obtained, and that the optical interaction length required for optimal conversion is very short, in the few μm range.