Silicon, arguably the most important technological semiconductor, is predicted to exhibit a range of new and interesting properties when grown in the hexagonal crystal structure. To obtain pure hexagonal silicon is a great challenge because it naturally crystallizes in the cubic structure. Here, we demonstrate the fabrication of pure and stable hexagonal silicon evidenced by structural characterization. In our approach, we transfer the hexagonal crystal structure from a template hexagonal gallium phosphide nanowire to an epitaxially grown silicon shell, such that hexagonal silicon is formed. The typical ABABAB... stacking of the hexagonal structure is shown by aberration-corrected imaging in transmission electron microscopy. In addition, X-ray diffraction measurements show the high crystalline purity of the material. We show that this material is stable up to 9 GPa pressure. With this development, we open the way for exploring its optical, electrical, superconducting, and mechanical properties.

Group IV materials with the hexagonal diamond crystal structure have been predicted to exhibit promising optical and electronic properties. In particular, hexagonal silicon-germanium (SiGe) should be characterized by a tunable direct band gap with implications ranging from Si-based light-emitting diodes to lasers and quantum dots for single photon emitters. Here we demonstrate the feasibility of high-quality defect-free and wafer-scale hexagonal SiGe growth with precise control of the alloy composition and layer thickness. This is achieved by transferring the hexagonal phase from a GaP/Si core/shell nanowire template, the same method successfully employed by us to realize hexagonal Si. We determine the optimal growth conditions in order to achieve single-crystalline layer-by-layer SiGe growth in the preferred stoichiometry region. Our results pave the way for exploiting the novel properties of hexagonal SiGe alloys in technological applications.

Recent advances in the synthetic growth of nanowires have given access to crystal phases that in bulk are only observed under extreme pressure conditions. Here, we use first-principles methods based on density functional theory and many-body perturbation theory to show that a suitable mixing of hexagonal Si and hexagonal Ge yields a direct bandgap with an optically permitted transition. Comparison of the calculated radiative lifetimes with typical values of nonradiative recombination mechanisms indicates that optical emission will be the dominant recombination mechanism. These findings pave the way to the development of silicon-based optoelectronic devices, thus far hindered by the poor light emission efficiency of cubic Si.

Semiconductor nanowires have increased the palette of possible heterostructures thanks to their more effective strain relaxation. Among these, core-shell heterostructures are much more sensitive to strain than axial ones. It is now accepted that the formation of misfit dislocations depends both on the lattice mismatch and relative dimensions of the core and the shell. Here, we show for the first time the existence of a new kind of defect in core-shell nanowires: cracks. These defects do not originate from a lattice mismatch (we demonstrate their appearance in an essentially zero-mismatch system) but from the thermal history during the growth of the nanowires. Crack defects lead to the development of secondary defects, such as type-I1 stacking faults and Frank-type dislocations. These results provide crucial information with important implications for the optimized synthesis of nanowire-based core-shell heterostructures.

Silicon-Germanium in a hexagonal crystal-structure is a candidate material for a direct band-gap group IV semiconductor that can be integrated into the CMOS process. It has recently been synthesized as a crystalline shell grown epitaxial around a nanowire core of hexagonal Gallium-Phosphide. In order to study the optical properties of this newly generated material and evaluate its potential for building optical devices it is necessary to grow defect and impurity free hexagonal Silicon-Germanium. Impurity detection and mapping in nano-structures is however challenging as most bulk and thin film characterization methods cannot be used. Here we show that Atom Probe Tomography can be used to map the impurities in hexagonal shells of Silicon-Germanium and Silicon. This will allow to optimize growth of hexagonal Silicon-Germanium nanocrystals towards impurity free, optically active crystals.

Segregation of Ge-rich nanoprisms in hexagonal GaP–Si–SiGe core–shell nanowires explained by STEM-EDX analysis and phase-field simulations.

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New opportunities with nanowiresBakkers, Erik; Hauge, H. I.T.; Li, Ang; Assali, S.; Dijkstra, A.; Tucker, R.; Ren, Y.; Conesa Boj, Sonia; Verheijen, M. A.