Hexagonal SiGe promises direct integration of photonics on silicon

April 09, 2020 //By Julien Happich
Hexagonal SiGe
Growing hexagonal SiGe alloys on top of nanowires with an hexagonally-arranged crystal lattice, a team of researchers has reported direct bandgap light-emission from SiGe alloys.

What’s more, as they reveal in a paper “Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys” published in the Nature journal, they were able to tune continuously the emission wavelength over a broad range by simply changing the composition of the hexagonal SiGe alloy. These new findings, they say, could enable the design of photonic chips directly into silicon-germanium integrated circuits.

A look inside the Metal Organic Vapor Phase Epitaxy
(MOVPE) equipment used to grow nanowires with hexagonal
silicon-germanium shells. Credit: Nando Harmsen, TU/e

The key to turning SiGe alloys into direct band gap emitting materials was to produce germanium and alloys from germanium and silicon with a hexagonal crystal lattice structure.
Together with colleagues from the Technical University of Munich and the universities in Jena and Linz, researchers at the Technical University of Eindhoven used nanowires made of another material as a template for hexagonal growth.

The nanowires then served as a template for a germanium-silicon shell on which the underlying material imposed its hexagonal crystal structure.
Initially, however, these structures could not be stimulated to emit light. Through the exchange of ideas with colleagues at the Walter Schottky Institute at the Technical University of Munich, who analyzed the optical characteristics with each successive generation, the production process was finally optimized to a grade of perfection where the nanowires were indeed capable of emitting light.

“In the meantime, we have achieved properties almost comparable to indium phosphide or gallium arsenide,” says Prof. Erik Bakkers from TU Eindhoven. As a result, it could be just a matter of time before a laser made from germanium-silicon alloys and capable of integration into conventional production processes is developed.

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