Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe-alloys are all indirect bandgap semiconductors that cannot emit light efficiently. Accordingly, achieving efficient light emission from group-IV materials has been a holy grail 1 in silicon technology for decades and, despite tremendous efforts 2-5 , it has remained elusive 6 . Here, we demonstrate efficient light emission from direct bandgap hexagonal Ge and SiGe alloys. We measure a sub nanosecond, temperature insensitive radiative recombination lifetime and observe a similar emission yield to direct bandgap III-V semiconductors. Moreover, we demonstrate how by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned in a broad range, while preserving a direct bandgap. Our experimental findings are shown to be in excellent quantitative agreement with the ab initio theory. Hexagonal SiGe embodies an ideal material system to fully unite electronic and optoelectronic functionalities on a single chip, opening the way towards novel device concepts and information processing technologies.Silicon has been the workhorse of the semiconductor industry since it has many highly advantageous physical, electronic and technological properties. However, due to its indirect bandgap, silicon cannot emit light efficientlya property that has seriously constrained potential for applications to electronics and passive optical circuitry 7-9 . Silicon technology can only reach its full application potential when heterogeneously supplemented 10 with an efficient, direct bandgap light emitter.The band structure of cubic Si, presented in Fig. 1a is very well known, having the lowest conduction band (CB) minimum close to the X-point and a second lowest * These authors contributed equally to this work. † Correspondence to E.P.A.M.(e.p.a.m.bakkers@tue.nl).minimum at the L-point.As such, it is the archetypal example of an indirect bandgap semiconductor, that, notwithstanding many great efforts 3-6 , cannot be used for efficient light emission.By modifying the crystal structure from cubic to hexagonal, the symmetry along the 111 crystal direction changes fundamentally, with the consequence that the L-point bands are folded back onto the Γ-point. As shown in Fig. 1b, for hexagonal Si (Hex-Si) this results in a local CB minimum at the Γ-point, with an energy close to 1.7 eV 11-13 . Clearly, Hex-Si remains indirect since the lowest energy CB minimum is at the M-point, close to 1.1 eV. Cubic Ge also has an indirect bandgap but, unlike Si, the lowest CB minimum is situated at the L-point, as shown in Fig. 1c. As a consequence, for Hex-Ge the band folding effect results in a direct bandgap at the Γ-point with a magnitude close to 0.3 eV, as shown in the calculated band structure in Fig. 1d 14 .To investigate how the direct bandgap energy can be tuned by alloying Ge with Si, we calculated the band structures of He...

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.

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

Highly oriented Ge 0.81 Sn 0.19 nanowires have been synthesized by a low temperature chemical vapor deposition growth technique. The nanostructures form by a self-seeded vapor-liquid-solid mechanism. In this process, liquid metallic Sn seeds enable the anisotropic crystal growth and act as sole source of Sn for the formation of the metastable Ge 1-x Sn x semiconductor material. The strain relaxation for a lattice mismatch of  = 2.94 % between the Ge (111) substrate and the constant Ge 0.81 Sn 0.19 composition of nanowires is confined to a transition zone of <100 nm. In contrast, Ge 1-x Sn x structures with diameters in the micrometer-range show a fivefold longer compositional gradient very similar to epitaxial thin film growth. Effects of the Sn growth promoters' dimensions on the morphological and compositional evolution of Ge 1-x Sn x are described. The temperature and laser power dependent photoluminescence analyses verify the formation of a direct band gap material with emission in the mid-infrared region and values expected for unstrained Ge 0.81 Sn 0.19 (e.g. band gap of 0.3 eV at room temperature). These materials with band gaps in the mid-IR hold promise in applications such as thermal imaging and photodetection as well as building blocks for group IV-based mid-to near-IR photonics.

No abstract

The growth of Sn-rich group-IV semiconductors at the nanoscale provides new paths for understanding the fundamental properties of metastable GeSn alloys. Here, we demonstrate the effect of the growth conditions on the morphology and composition of Ge/GeSn core/shell nanowires by correlating the experimental observations with a theoretically developed multi-scale approach. We show that the cross-sectional morphology of Ge/GeSn core/shell nanowires changes from hexagonal (bounded by {112} facets) to dodecagonal (bounded by both {112} and {110} facets) upon increasing the supply of the Sn precursor. This transformation strongly influences the Sn distribution as a higher Sn content is measured under {112} facets. Ab-initio DFT calculations provide an atomic-scale explanation by showing that Sn incorporation is favored at the {112} surfaces, where the Ge bonds are tensile-strained. A phase-field continuum model was developed to reproduce the morphological transformation and the Sn distribution within the wire, shedding light on the complex growth mechanism and unveiling the relation between segregation and faceting.pulling during the shell growth. 14 However, at larger core diameters the shell will experience higher strain, eventually inducing plastic deformation with the nucleation of defects and surface roughening. 18,19 In this work, we show how the morphology and composition of the GeSn shell on a Ge core NW are strongly dependent on the growth conditions. At a higher supply of the Tintetrachloride (SnCl4) precursor, the symmetry of the NW cross-section changes from 6-fold to 12-fold by increasing the size of the six {110} (corner) facets in the GeSn shell with respect to the (main) six {112} facets. At the same time, enhanced segregation is observed, with an increasing difference in composition between Sn-poor <110>-oriented stripes and Sn-rich {112}oriented facets. In addition, at the highest supply of the Sn precursor, phase separation occurs and multiple Sn droplets are visible on the NW sidewall. The experimental observations are then rationalized theoretically by a multi-scale approach. First, the shape transition will be interpreted by a continuum kinetic growth model, including surface diffusion. Then, first principle calculations will be exploited to assess the origin of the different composition within the facets and to extend the growth model in order to simultaneously trace the evolution of shape and composition. The agreement between experiments and theory highlights the strong correlation between faceting and segregation dynamics in the Ge/GeSn core/shell NW system. RESULTS AND DISCUSSIONExperimental analysis. The effect of the SnCl4 precursor flow on the morphology of the GeSn shell grown around 100 nm Ge cores is shown in Fig. 1. A fixed growth time of 2 h was used in combination with a Ge/Sn ratio in gas phase ranging from 1285 to 300. An increase in the diameter of the core/shell NWs is visible with increasing (decreasing) supply of SnCl4

Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial planar defect in materials with a type I 3 basal stacking fault and investigate its structural and electronic properties. Electron microscopy and atomistic modeling are used to reconstruct and visualize this stacking fault and its terminating dislocations in the crystal. From band structure calculations coupled to photoluminescence measurements, we conclude that the I 3 defect does not create states within the hex-Ge and hex-Si band gap. Therefore, the defect is not detrimental to the optoelectronic properties of the hex-SiGe materials family. Finally, highlighting the properties of this defect can be of great interest to the community of hex-III-Ns, where this defect is also present.

No abstract