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...

One of the current challenges in nanoscience is tailoring the phononic properties of a material. This has long been a rather elusive task because several phonons have wavelengths in the nanometer range. Thus, high quality nanostructuring at that length-scale, unavailable until recently, is necessary for engineering the phonon spectrum. Here we report on the continuous tuning of the phononic properties of a twinning superlattice GaP nanowire by controlling its periodicity. Our experimental results, based on Raman spectroscopy and rationalized by means of ab initio theoretical calculations, give insight into the relation between local crystal structure, overall lattice symmetry, and vibrational properties, demonstrating how material engineering at the nanoscale can be successfully employed in the rational design of the phonon spectrum of a material.

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.

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