Summary

Hexagonal SiGe semiconductors are a promising material for optoelectronics - for seamlessly combining electron and photon technology. Under extremely high pressure and temperatures above 1,500 degrees Celsius, a team led by George Serghiou from the University of Edinburgh in Scotland and Hans J. Reichmann from the German Research Centre for Geosciences Potsdam (GFZ) succeeded in synthesising the material. To do this, they used laser-heated diamond stamp cells and multi-anvil presses, among other things, as are commonly used for geoscientific experiments. The publication ‘Chemistry - A European Journal’ was chosen as the cover story.

The challenge of optoelectronics: Materials for the connection of light and electricity

Electrons and photons, i.e. current and light, are the information carriers of today's IT systems. A central challenge in optoelectronics has long been to develop seamless communication between current and light conducting systems. A material must fulfil two decisive criteria so that such a seamless transition between electron and photon technologies can be achieved in components: The material should have a direct band gap in its electronic structure so that the conversion of light into current and vice versa takes place as efficiently as possible. And it should be chemically compatible with silicon, because semiconducting silicon with a cubic diamond lattice structure is still the most important basic material in the electronics and information industries. However, silicon itself does not have a direct but an indirect band gap in its electronic structure, which makes the conversion of electricity into light very inefficient. 

Promising approach: combine silicon and germanium in a hexagonal structure

For the development of semiconductors with a tailor-made direct band gap, different materials are combined with each other, among other things. Recent work shows the importance that hexagonal silicon-germanium (SiGe) semiconductors, for example, can have for combining electronics and photonics. Specifically, a study by Fadaly et al. (2020) fabricated 0.0005 millimetre thick nanowires of SiGe with a hexagonal symmetry that exhibit direct bandgaps. They were fabricated on a matrix of gallium arsenide (GaAs) nanowire cores. In the study, however, the researchers emphasise that it is necessary to fabricate matrix-free hexagonally crystallised SiGe. This SiGe should also be larger.

SiGe synthesis using high pressures and temperatures

This is exactly what the international research group led by George Serghiou from the University of Edinburgh in Scotland and Hans J. Reichmann from the German Research Centre for Geosciences Potsdam (GFZ) has been working on in recent years.  In a study published in the journal Chemistry A European Journal, they show how they succeeded in developing hexagonal, binary SiGe semiconductors.

Essential prerequisites for this were synthesis and characterisation methods under high pressures of 12 to 17 Giga-Pascal (120 to 170 Kbar) and temperatures above 1500 degrees Celsius (~1800 K). The material was produced in laser-heated diamond anvil cells and in multi-anvil presses as also used in experiments investigating planetary interiors. The crystalline and the chemical composition of the resulting materials can be investigated with state-of–the-art characterization methods including synchrotron-based angle dispersive X-ray diffraction, precession electron diffraction and electron microscopy.

Successful approach for the targeted manipulation of optoelectronic properties

The research team thus succeeded in synthesising new, matrix-free SiGe materials with a hexagonal lattice that are about 1 millimetre in size. The samples show different types of hexagonal SiGe crystals with different stacking orders of the crystal planes. This is also significant because the band gap can be possibly manipulated not only as a function of the chemical composition of the material, but also as a function of this stacking sequence. This could possibly be a new approach to tailor the optoelectronic properties of this material system according to the specific application needs.

Original publication: Hexagonal Si-Ge Class of Semiconducting Alloys Prepared Using Pressure and Temperature.
George Serghiou, Nicholas Odling, Hans Josef Reichmann, Gang Ji, Monika Koch-Müller, Daniel J. Frost, Jonathan P. Wright, Reinhard Boehler and Wolfgang Morgenroth Chem. Eur. J. (2021); DOI: 10.1002/chem.202102595