Researchers from ETH Zurich, the Paul Scherrer Institute (PSI) and the Politec-nico di Milano have jointly developed a manufacturing technique to render the semiconductor germanium laser-compatible through high tensile strain. In their article recently published in "Nature Photonics", they reveal how they can gen-erate the necessary tensile strain efficiently. The scientists demonstrate how to use their method to effectively alter the optical properties of germanium, which is naturally unsuitable for lasing: "With a strain of three per cent, the material emits around twenty-five times more photons than in a relaxed state," explains Martin Sess, a doctoral student at the Laboratory for Nanometallurgy headed by Ralph Spolenak and the EMEZ at ETH Zurich. "That's enough to build lasers", adds his colleague Richard Geiger, a doctoral student at the Laboratory for Micro- and Nanotechnology at the PSI and the Institute for Quantum Electronics at ETH Zurich under Jrme Faist.
High tension through microbridges
With the new method, the researchers use the slight tension generated in ger-manium when it evaporates on silicon, to bring the germanium into a laser-compatible, strechted form. This prestrain is enhanced with so-called micro-bridges: the researchers centrally notch the sides of exposed germanium strips, which remain attached to the silicon layer at both ends. The two halves of the strip thus remain connected solely by an extremely narrow bridge, which is for physical reasons precisely where the strain in the germanium grows so intense that it becomes laser-compatible.
"The tensile strain applied to the germanium is comparable to the force exerted on a pencil as two lorries pull upon it in opposite directions," says Hans Sigg, the project manager at the PSI, explaining the feat on a micrometre scale in everyday proportions. Through the expansion of the material its properties change because the individual atoms slightly move ap
|Contact: Martin Sueess|