Single-crystalline hexagonal silicon-germanium

Group IV materials with the hexagonaldiamond crystal structure have been predicted to exhibit promising optical and electronic properties. In particular,\u3cbr/\u3ehexagonal silicon−germanium (Si1−xGex) 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 Si1−xGex 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 Si1−xGex growth in the preferred stoichiometry region. Our results pave the way for exploiting the novel properties of hexagonal Si1−xGex alloys in technological applications

Cracking the Si shell growth in hexagonal GaP-Si core-shell nanowires

\u3cp\u3eSemiconductor 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-I\u3csub\u3e1\u3c/sub\u3e stacking faults and Frank-type dislocations. These results provide crucial information with important implications for the optimized synthesis of nanowire-based core-shell heterostructures.\u3c/p\u3

InP nanowire solar cell with high open circuit voltage and high fill factor

\u3cp\u3eWe demonstrate an InP axial pn-junction nanowire solar cell array with 107 diode rectification factor, 0.66V open-circuit voltage, 72% fill factor, and 5.08% power efficiency by using post-growth nanowire sidewall etching.\u3c/p\u3

New opportunities with nanowires

Summary form only given. Light emission from Si, would allow integration of electronic and optical functionality in the main electronics platform technology, but this has been impossible due to the indirect band gap of Si. This talk will discuss 2 different approaches, using unique properties of nanowires, to realize light emission from Si-based compounds. In the first route, the paper focuses on the fabrication of defect-free GeSn compounds. The growth mechanism is discussed, the structural properties are investigated by electron microscopy and atom probe tomography and the temperature dependent optical properties are studied. The second route concentrates on Si and Ge with a different crystal structure. Here, crystal structure transfer is employed, in which wurtzite GaP is used as a template to epitaxially grow SiGe compounds with the hexagonal crystal structure. With this method, defect free hexagonal SiGe shells and branches with tunable Ge concentration are gorwn. The structural and optical properties of these new crystal phases will be discussed. The author believes that these new 3-dimensional epitaxial nanostructures have great potential to integrate optical functionality in Si technology

Impurity and defect monitoring in hexagonal Si and SiGe nanocrystals

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