Milestone achieved towards the long-sought goal of a silicon-based laser

When it comes to microelectronics, there is a chemical element like no other: silicon, the workhorse of transistor technology that drives our information society. The countless electronic devices that we use in everyday life bear witness to how today very large volumes of silicon-based components can be produced at very low cost. It therefore seems natural to use silicon also in other fields where the properties of semiconductors – such as silicon is one – are technologically exploited, and to explore ways of integrating different functionalities. Of particular interest in this context are diode lasers, such as those used in barcode scanners or laser pointers, which are typically based on gallium arsenide (GaAs). Unfortunately, the physical processes that create light in GaAs don’t work as well in silicon. Finding an alternative way to produce a “laser on silicon” therefore remains an exceptional and long-standing objective.

Write today in Letters of Applied Physics, an international team led by professors Giacomo Scalari and Jérôme Faist from the Institute of Quantum Electronics presents an important step towards such a device. They report electroluminescence – the generation of electric light – from a semiconductor structure based on silicon-germanium (SiGe), a material compatible with standard manufacturing processes used for silicon devices. In addition, the emission they observed is in the terahertz frequency band, which falls between those of microwave electronics and infrared optics, and is of high current interest in view of a variety. applications.

Shine silicon

The main reason why silicon cannot be used directly to build a laser according to the GaAs template is related to the different nature of their bandgaps, which is direct in the latter but indirect in the former. In a nutshell, in GaAs, electrons recombine with holes across the bandgap producing light; in silicon, they produce heat. Laser action in silicon therefore requires another path. And exploring a new approach is what ETH doctoral student David Stark and his colleagues are doing. They are working towards a silicon-based quantum cascade laser (QCL). QCLs achieve light emission not by electron-hole recombination across the bandgap, but by tunneling electrons through repeated stacks of precisely engineered semiconductor structures, during which process photons are emitted.

The QCL paradigm has been demonstrated in a number of materials – for the first time in 1994 by a team including Jérôme Faist, then working at Bell Laboratories in the United States – but never in silicon-based materials, despite promising predictions. . Turning these predictions into reality is at the heart of an interdisciplinary project funded by the European Commission, bringing together a team of leading experts in the cultivation of the highest quality semiconductor materials (at the Università Roma Tre), characterizing them (at the Leibniz-Institut für innovant Mikroelektronik in Frankfurt-on-the-Oder) and transforming them into devices (at the University of Glasgow). The ETH group of Scalari and Faist is responsible for carrying out the measurements on the devices, but also for the design of the laser, with the numerical and theoretical support of the partners of the company nextnano in Munich and the universities of Pisa and Rome.

From electroluminescence to laser

With this combined knowledge and expertise, the team designed and built devices with a unitary structure of SiGe and pure germanium (Ge), with a height of less than 100 nanometers, which repeats 51 times. From these heterostructures, fabricated with essentially atomic precision, Stark and his colleagues detected electroluminescence, as expected, with the spectral characteristics of the emerging light agreeing well with the calculations. Further confidence that the devices will perform as expected has come from a comparison with a GaAs-based structure which was fabricated with identical device geometry. While the emission of the Ge / SiGe structure is still significantly lower than that of its GaAs-based counterpart, these results clearly indicate that the team is on the right track. The next step will now be to assemble similar Ge / SiGe structures according to a laser design developed by the team. The ultimate goal is to achieve room temperature operation of a silicon based QCL.

Such an achievement would be important in several respects. Not only would he finally realize a laser on a silicon substrate, thus bringing a boost to silicon photonics. The emission of the structure created by Stark et al. is located in the terahertz region, for which compact light sources are currently largely absent. Silicon-based QCLs, with their potential versatility and reduced manufacturing cost, could be a boon for the large-scale use of terahertz radiation in existing and new application areas, from medical imaging to communication wireless.