Research Highlights X-Ray Group.

Direct-bandgap emission from hexagonal Ge and SiGe alloys, Nature 580, 205–209 (2020)

Elham M. T. Fadaly, Alain Dijkstra, Jens René Suckert, Dorian Ziss, Marvin A. J. van Tilburg, Chenyang Mao, Yizhen Ren, Victor T. van Lange, Ksenia Korzun, Sebastian Kölling, Marcel A. Verheijen, David Busse, Claudia Rödl, Jürgen Furthmüller, Friedhelm Bechstedt, Julian Stangl, Jonathan J. Finley, Silvana Botti, Jos E. M. Haverkort, Erik P. A. M. Bakkers.



Emitting light from silicon has been the ‘Holy Grail’ in the microelectronics industry for decades. Solving this puzzle would revolutionize computing, as chips will become faster than ever. A multi-national research group lead by scientists from Eindhoven University of Technology and with contributions from JKU Linz now succeeded: they have developed an alloy with silicon that can emit light. The results have been published in the journal Nature.

Every year we use and produce significantly more data. But our current technology, based on electronic chips, is reaching its ceiling. The limiting factor is heat, resulting from the resistance that the electrons experience when traveling through the copper lines connecting the many transistors on a chip. If we want to continue transferring more and more data every year, we need a new technique that does not produce heat. Bring in photonics, which uses photons (light particles) to transfer data.In contrast to electrons, photons do not experience resistance. As they have no mass or charge, they will scatter less within the material they travel through, and therefore no heat is produced. The energy consumption will therefore be reduced. Moreover, by replacing electrical communication within a chip by optical communication, the speed of on-chip and chip-to-chip communication can be increased by a factor 1000. Data centers would benefit most, with faster data transfer and less energy usage for their cooling system. But these photonic chips will also bring new applications within reach. Think of laser-based radar for self-driving cars and chemical sensors for medical diagnosis or for measuring air and food quality.To use light in chips, you will need a light source; an integrated laser. The main semiconductor material that computer chips are made of is silicon. But bulk silicon is extremely inefficient at emitting light, and so was long thought to play no role in photonics. Thus, scientists turned to more complex semiconductors, such as gallium arsenide and indium phosphide. These are good at emitting light but are more expensive than silicon and are hard to integrate into existing silicon microchips.To create an all-silicon laser, scientists needed to produce a form of silicon that can emit light. That’s exactly what researchers from Eindhoven University of Technology now succeeded in. Together with researchers from the universities of Jena, JKU Linz and Munich, they combined silicon and germanium in a hexagonal structure that is able to emit light. A breakthrough after 50 years of work.

The work received major response from the international scientific-press:
15.04.2020 - Light-emitting hexagonal SiGe promises integrated photonics breakthroughy; (
10.04.2020 - Direct-bandgap emission achieved from hexagonal Ge and SiGe alloys; (
09.04.2020 - Light-emitting SiGe alloys developed; (
09.04.2020 - SiGe alloys usable in silicon photonics emit light via direct bandgap; (
09.04.2020 - Durchbruch für Photonik-Chips; (
08.04.2020 - Nanostructured alloys light the way to silicon-based photonics; (
08.04.2020 - Silicon-based light emitter is ‘Holy Grail’ of microelectronics, say researchers; (

Self-Seeded Axio-Radial InAs–InAs1–xPx Nanowire Heterostructures beyond “Common” VLS Growth

Bernhard Mandl, Mario Keplinger, Maria E. Messing, Dominik Kriegner, Reine Wallenberg, Lars Samuelson, Günther Bauer, Julian Stangl, Václav Holý, Knut Deppert

Semiconductors are essential for modern electronic and optoelectronic devices, and the ability to fabricate increasingly complex semiconductor nanostructures is of utmost importance. Nanowires offer excellent opportunities for new device concepts; heterostructures have been grown in either the radial or axial direction of the core nanowire but never along both directions at the same time, a consequence of the common use of a foreign metal seedparticle with fixed size for nanowire heterostructure growth. In this work, we present a growth method to control heterostructure growth in both the axial and the radial directions simultaneously while maintaining an untapered self-seeded growth. This is demonstrated for the InAs/InAs1−xPx material system. We show how the dimensions and composition of such axio-radial nanowire heterostructures can be designedincluding the formation of a “pseudo-superlattice” consisting of five separate InAsP segments with varying length. The growth of axio-radial nanowire heterostructures offers an exciting platform for novel nanowire structures applicable forfundamental studies as well as nanowire devices. The growth concept for axio-radial nanowire heterostructures is expected to befully compatible with Si substrates.

Growth Mechanism of Self-Catalyzed Group III−V Nanowires

Bernhard Mandl, Julian Stangl, Emelie Hilner, Alexei A. Zakharov, Karla Hillerich, Anil W. Dey, Lars Samuelson, Günther Bauer, Knut Deppert, Anders Mikkelsen

Group III-V nanowires offer the exciting possibility of epitaxial growth on a wide variety of substrates, most importantly
silicon. To ensure compatibility with Si technology, catalyst-free growth schemes are of particular relevance, to avoid impurities from
the catalysts. While this type of growth is well-documented and some aspects are described, no detailed understanding of the nucleation
and the growth mechanism has been developed. By combining a series of growth experiments using metal-organic vapor phase
epitaxy, as well as detailed in situ surface imaging and spectroscopy, we gain deeper insight into nucleation and growth of selfseeded
III-V nanowires. By this mechanism most work available in literature concerning this field can be described.

Strain distribution in single, suspended germanium nanowires studied using nanofocused x-rays

Mario Keplinger, Raphael Grifone, Johannes Greil, Dominik Kriegner, Johan Persson, Alois Lugstein, Tobias Schülli, Julian Stangl

Within the quest for direct band-gap group IV materials, strain engineering in germanium is one promising route. We present a study of the strain distribution in single, suspended germanium nanowires using nanofocused synchrotron radiation. Evaluating the probed Bragg reflection for different illumination positions along the nanowire length results in corresponding strain components as well as the nanowireʼs tilting and bending. By using these findings we determined the complete strain state with the help of finite element modelling. The resulting information provides us with the possibility of evaluating the validity of the strain investigations following from Raman scattering experiments which are based on the assumption of purely uniaxial strain.

Hexagonal Silicon Realized

Håkon Ikaros T. HaugeMarcel A. VerheijenSonia Conesa-BojTanja EtzelstorferMarc WatzingerDominik KriegnerIlaria ZardoClaudia FasolatoFrancesco CapitaniPaolo PostorinoSebastian KöllingAng LiSimone AssaliJulian StanglErik P. A. M. Bakkers

Silicon, arguably the most important technologicalsemiconductor, is predicted to exhibit a range of new andinteresting 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.