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 fabrication of nanohelices by the scrolling of strained bilayers is investigated. It is shown that structure design is dominated by edge effects rather than bulk crystal properties such as the Young's modulus when the dimensions of the structures are reduced below 400 nm. SiGe/Si/Cr, SiGe/Si, and Si/Cr helical nanobelts are used as test structures. Dimensions of the belt width are reduced from 1.30 microm to 300 nm, and parameters controlling helicity angle, chirality, diameter, and pitch of the nanohelices are investigated. An anomalous scrolling direction deviating from the preferred <100> scrolling direction has been found for small structures. Making use of the anomalous scrolling, it is possible to fabricate three-dimensional helices with helicity angles less than 45 degrees , which is advantageous for micro- and nanoelectromechanical systems.

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 report on the observation of photogalvanic effects in epitaxially grown Sb2Te3 and Bi2Te3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the "symmetry filtration" the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature.

Nanowire growth: Catalyst‐free growth of GaN nanowires on Si substrates (see image) is investigated by high‐resolution transmission electron microscopy. Small GaN crystalline clusters are found on top of an interface amorphous layer. High‐crystalline‐quality vertical nanowires are grown on an amorphous oxide layer. These findings open new possibilities for nanowire growth on a variety of nonconventional substrates.

Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet interference lithography (EUV-IL) at a wavelength of lambda = 13.5 nm for fast, large-area exposure of templates with perfect periodicity. Si(001) substrates have been patterned with two-dimensional hole arrays using EUV-IL and reactive ion etching. On these substrates, three-dimensionally ordered SiGe quantum dot crystals with the so far smallest quantum dot sizes and periods both in lateral and vertical directions have been grown by molecular beam epitaxy. X-ray diffractometry from a sample volume corresponding to about 3.6 x 10(7) dots and atomic force microscopy (AFM) reveal an up to now unmatched structural perfection of the quantum dot crystal and a narrow quantum dot size distribution. Intense interband photoluminescence has been observed up to room temperature, indicating a low defect density in the three-dimensional (3D) SiGe quantum dot crystals. Using the Ge concentration and dot shapes determined by X-ray and AFM measurements as input parameters for 3D band structure calculations, an excellent quantitative agreement between measured and calculated PL energies is obtained. The calculations show that the band structure of the 3D ordered quantum dot crystal is significantly modified by the artificial periodicity. A calculation of the variation of the eigenenergies based on the statistical variation in the dot dimensions as determined experimentally (+/-10% in linear dimensions) shows that the calculated electronic coupling between neighboring dots is not destroyed due to the quantum dot size variations. Thus, not only from a structural point of view but also with respect to the band structure, the 3D ordered quantum dots can be regarded as artificial crystal.