Direct bandgap group IV materials may thus represent a pathway towards the monolithic integration of Si-photonic circuitry and CMOS technology.Although a group IV direct bandgap material has not been demonstrated yet, silicon photonics using CMOS-compatible processes has made great progress through the development of Si-based waveguides 12 , photodetectors 13 and modulators 14 . The thus emerging technology is rapidly expanding the landscape of photonics applications towards tele-and data communication as well as sensing from the infrared to the mid infrared wavelength range 15-17 . Today's light sources of such systems are lasers made from direct bandgap group III-V materials operated off-or on-chip which requires fibre coupling or heterogeneous integration, for example by wafer bonding 3 , contact printing 4,5 or direct growth 6,7 , respectively. Hence, a laser source made of a direct bandgap group IV material would further boost lab-on-a-chip and trace gas sensing 15 as well as optical interconnects 18 by enabling monolithic integration. In this context, Ge plays a prominent role since the conduction band minimum at the -point of the Brillouin-zone (referred to as -valley) is 3 located only approx. 140 meV above the fourfold degenerate indirect L-valley. To compensate for this energy difference and thus form a laser gain medium, heavy n-type doping of slightly tensile strained Ge has been proposed 19 . Later, laser action has been reported for optically 20 and electrically pumped Ge 21 doped to approx. 1 and 4×10 19 cm -3 , respectively. However, pump-probe measurements of similarly doped and strained material did not show evidence for net gain 22 , and in spite of numerous attempts, researchers failed to substantiate above results up to today. Other investigated concepts concern the engineering of the Ge band structure towards a direct bandgap semiconductor using micromechanicallystressed Ge nanomembranes 9 or silicon nitride (Si 3 N 4 ) stressor layers 23 . Very recently, Süess et al. 10 presented a stressor-free technique which enables the introduction of more than 5.7 % 24 uniaxial tensile strain in Ge µ-bridges via selective wet under-etching of a pre-stressedlayer. An alternative technique in order to achieve direct bandgap material is to incorporate Sn atoms into a Ge lattice, which primarily reduces the gap at the -point. At a sufficiently high fraction of Sn, the energy of the -valley decreases below that of the L-valley. This indirect-to-direct transition for relaxed GeSn binaries has been predicted to occur at about 20 % Sn by Jenkins et al. 25 , but more recent calculations indicate much lower required Sn concentrations in the range of 6.5-11.0 % 26,27 . A major challenge for the realization of such GeSn alloys is the low (< 1 %) equilibrium solubility of Sn in Ge 28 and the large lattice mismatch of about 15 % between Ge and -Sn. For GeSn grown on Ge substrates, this mismatch induces biaxial compressive strain causing a shift of the  and L-valley crossover towards higher Sn concentrations ...

The strong correlation between advancing the performance of Si microelectronics and their demand of low power consumption requires new ways of data communication. Photonic circuits on Si are already highly developed except for an eligible on-chip laser source integrated monolithically. The recent demonstration of an optically pumped waveguide laser made from the Si-congruent GeSn alloy, monolithical laser integration has taken a big step forward on the way to an all-inclusive nanophotonic platform in CMOS. We present group IV microdisk lasers with significant improvements in lasing temperature and lasing threshold compared to the previously reported nonundercut Fabry−Perot type lasers. Lasing is observed up to 130 K with optical excitation density threshold of 220 kW/cm 2 at 50 K. Additionally the influence of strain relaxation on the band structure of undercut resonators is discussed and allows the proof of laser emission for a just direct Ge 0.915 Sn 0.085 alloy where Γ and L valleys have the same energies. Moreover, the observed cavity modes are identified and modeled.

GeSn alloys are the most promising semiconductors for light emitters entirely based on group IV elements. Alloys containing more than 8 at. % Sn have fundamental direct band-gaps, similar to conventional III-V semiconductors and thus can be employed for light emitting devices. Here, we report on GeSn microdisk lasers encapsulated with a SiN x stressor layer to produce tensile strain. A 300 nm GeSn layer with 5.4 at. % Sn, which is an indirect band-gap semiconductor as-grown with a compressive strain of −0.32 %, is transformed via tensile strain engineering into a truly direct band-gap semiconductor. 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. Continuous wave (cw) as well as pulsed lasing are observed at very low optical pump powers. Lasers with emission wavelength of 2.5 µm have thresholds as low as 0.8 kW cm −2 for ns-pulsed excitation, and 1.1 kW cm −2 under cw excitation. These thresholds are more than two orders of magnitude lower than those previously reported for bulk GeSn lasers, approaching these values obtained for III-V lasers on Si. The present results demonstrate the feasabiliy and are the guideline for monolithically integrated Si-based laser sources on Si photonics platform. arXiv:2001.04927v1 [physics.app-ph] 14 Jan 2020 at an Sn concentration of 8 at. % 3 . The lattice mismatch between Sn-containing alloys and the Ge buffer layer, the typical virtual substrate for their epitaxial growth, generates compressive strain in the grown layer, which counteracts the effect of Sn incorporation, decreasing the directness ∆E L−Γ = E L − E Γ . On the contrary, applying tensile strain will increase the directness. Finding a proper balance between a moderate Sn content to minimize crystal defects and to maintain thermal stability of the GeSn alloy on one hand and making use of tensile strain on the other hand are the keys to bring lasing threshold and operation temperature close to application's requirements. The mainstream research to increase ∆E L−Γ focuses on high Sn content alloys 5, 12 , obtained by epitaxy of thick strain-relaxed GeSn layers. A large directnesss is obtained, leading to higher temperature operation, although at the expense of steadily increasing laser threshold 13 . We have recently theoretically proposed an alternative approach, which is based on two key ingredients: employing moderate Sn content GeSn alloys, and inducing tensile strain in them 14 . This study indicated that, if a given directness is reached via tensile strain rather than by increasing Sn content, the material can provide a higher net gain. The underlying physics originates in the valence band splitting and lifting up of the light hole, LH, band above the heavy hole, HH, band. Its lower density of states (DOS) reduces the carrier density required for transparency, hence reduces the lasing threshold, as will be shown below. GeSn alloys with a moderate Sn content offer a couple of ...

The recent observation of a fundamental direct bandgap for GeSn group IV alloys and the demonstration of low temperature lasing provide new perspectives to the fabrication of Si photonic circuits. This work addresses the progress in GeSn alloy epitaxy aiming at room temperature GeSn lasing. Chemical vapor deposition of direct bandgap GeSn alloys with a high -to L-valley energy separation and large thicknesses for efficient optical mode confinement is presented and discussed. Up to 1 µm thick GeSn layers with Sn contents up to 14 at.% were grown on thick relaxed Ge buffers, using Ge 2 H 6 and SnCl 4 precursors. Strong strain relaxation (up to 81 %) at 12.5 at.% Sn concentration, translating into an increased separation between -and L-valleys of about 60 meV, have been obtained without crystalline structure degradation, as revealed by Rutherford backscattering/ion channeling spectroscopy and Transmission Electron Microscopy. Room temperature transmission/reflection and photoluminescence measurements were performed to probe the optical properties of these alloys. The emission/absorption limit of GeSn alloys can be extended up to 3.5 µm (0.35 eV), making those alloys ideal candidates for optoelectronics in the mid-infrared region. Theoretical net gain calculations indicate that large room temperature laser gains should be reachable even without additional doping.

We investigate spin and charge transport in both single and bilayer graphene non-local spin-valve devices. An inverse dependence of the spin lifetime τs on the carrier mobility µ is observed in devices with large contact resistance area products (RcA > 1 kΩµm 2 ). Furthermore, we observe an increase of τs with increasing RcA values demonstrating that spin transport is limited by spin dephasing underneath the electrodes. In charge transport, we measure a second contact-induced Dirac peak at negative gate voltages in devices with larger RcA values demonstrating different transport properties in contact covered and bare graphene parts. We argue that the existence of the second Dirac peak complicates the analysis of the carrier mobilities and the spin scattering mechanisms.Graphene has drawn strong attention because of measured spin diffusion length of some µm at room temperature. While most spin transport devices only exhibit spin lifetimes up to several hundred picoseconds at room temperature [1][2][3][4][5][6][7][8][9] there are only few reports with spin lifetimes above one nanosecond.10-12 Nevertheless, all experimental values of the spin lifetimes are some orders of magnitude shorter than theoretically predicted 13,14 indicating that in present devices spin transport is limited by extrinsic sources of spin scattering. These include spin-orbit coupling by adatoms, edge effects and ripples.6,10,13,15-18 Additionally, spin scattering may result from the underlying substrate or the spin injection and detection contacts. 14,19,20 The importance of the latter might be indicated by recent electron spin resonance (ESR) experiments on graphene nanoribbons and small flakes that were only weakly coupled to the substrate and had no electrodes.21,22 Interestingly, the measured spin lifetimes of localized spin states are at least 200 ns while the estimated spin lifetimes of conduction electrons are 30 ns, which is larger than any reported values from electrical Hanle spin precession measurements.In this Rapid Communication, we investigate the influence of MgO barriers on spin and charge transport properties by fabricating both single layer (SLG) and bilayer graphene (BLG) non-local spin-valve devices with variable contact resistance area products R c A of the MgO/Co electrodes. We explore the relationship between spin lifetime τ s and charge carrier mobility µ in SLG and find a similar 1/µ dependence as seen in previous spin transport studies on exfoliated bilayer graphene (BLG) devices.12 This dependence is only seen in samples with R c A > 1 kΩµm 2 . In fact, we observe that devices with long τ s additionally exhibit a second Dirac peak in charge transport, which stems from the electrodes. This contact-induced Dirac peak overlaps with the Dirac peak of the bare graphene which complicates the analysis of the carrier mobility and thus complicates a clear assignment of the dominant spin scattering mechanism in graphene. For devices with low R c A we find an overall strong decrease of τ s showing that transparent contacts yie...

Group IV semiconductor optoelectronic devices are now possible by using strain-free direct band gap GeSn alloys grown on a Ge/Si virtual substrate with Sn contents above 9%. Here, we demonstrate the growth of Ge/GeSn core/shell nanowire arrays with Sn incorporation up to 13% and without the formation of Sn clusters. The nanowire geometry promotes strain relaxation in the GeSn shell and limits the formation of structural defects. This results in room-temperature photoluminescence centered at 0.465 eV and enhanced absorption above 98%. Therefore, direct band gap GeSn grown in a nanowire geometry holds promise as a low-cost and high-efficiency material for photodetectors operating in the short-wave infrared and thermal imaging devices.

A comprehensive study of optical transitions in direct bandgap Ge 0.875 Sn 0.125 group IV alloys via photoluminescence measurements as a function of temperature, compressive strain and excitation power is performed. The analysis of the integrated emission intensities reveals a strain-dependent indirect-to-direct bandgap transition, in good agreement with band structure calculations based on 8 band k•p and deformation potential method. We have observed and quantified  valley -heavy hole and  valley -light hole transitions at low pumping power and low temperatures in order to verify the splitting of the valence band due to strain. We will demonstrate that the intensity evolution of these transitions supports the conclusion about the fundamental direct bandgap in compressively strained GeSn alloys. The presented investigation, thus, demonstrates that direct bandgap group IV alloys can be directly grown on Ge-buffered Si(001) substrates despite their residual compressive strain.2 KEYWORDS: direct bandgap, photoluminescence, germanium tin, group IV, compressive strain TOC GRAPHIC 3 Group IV semiconductors are known for their excellent electronic transport properties but limited optical applicability due to their indirect bandgap nature, turning them into inefficient light emitters. However, the pioneering work of R.Soref and C.H. Perry 1 and, later He and Atwater 2 as well as subsequent theoretical studies [3][4][5] indicated that alloying two group IV elements, i.e. semiconducting Ge and semimetallic -Sn, should result in a group IV semiconductor which could be tuned from a fundamental indirect to a direct bandgap material by increasing the substitutional Sn concentration in the Ge lattice. Although it was unanimously accepted that the -valley of the conduction band can be decreased below the Lvalley, theoretical estimates of the required Sn content for this transition as well as the impact of strain on the transition are widely spread. 6,7 This prospect has driven large efforts to grow device-grade GeSn epilayers, [8][9][10][11] prove their fundamental direct bandgap and finally to demonstrate photonic functionality. Recently, advances in Chemical Vapor Deposition (CVD)of GeSn binaries with high Sn contents of up to 14% has been reported 9,10,12-15 which enabled not only the proof of the direct bandgap nature but also the unambiguous demonstration of laser action at 2.3 µm under optical pumping. 16 Hence, direct bandgap GeSn alloys are CMOS-compatible IV-IV semiconductors with novel optical and electrical properties that are similar to those of III-V and II-VI compounds, used today in optoelectronic applications, i.e. The growth temperature (350°C), the total pressure and the partial pressures of the source gases were kept constant, resulting in the growth of GeSn alloys with a Sn content of 12.5 ± 0.5%. 11,15 Due to the lattice mismatch between the GeSn film and Ge virtual substrate, the GeSn layers were biaxially compressively strained. The films are fully strained for thicknesses below the critical t...

GeSn and SiGeSn are promising materials for the fabrication of a group IV laser source offering a number of design options from bulk to heterostructures and quantum wells. Here, we investigate GeSn/SiGeSn multi quantum wells using the optically pumped laser effect. Three complex heterostructures were grown on top of 200 nm thick strain relaxed Ge0.9Sn0.1 buffers. The lasing is investigated in terms of threshold and maximal lasing operation temperature by comparing multiple quantum well to double heterostructure samples. Pumping under two different wavelengths of 1064 nm and 1550 nm yield comparable lasing thresholds. The design with multi quantum wells reduces the lasing threshold to (40 ± 5) kW/cm 2 at 20 K, almost 10 times lower than for bulk structures. Moreover, 20 K higher maximal lasing temperatures were found for lower energy pumping of 1550 nm.