Important step towards the long-sought goal of a silicone-based laser

When it comes to microelectronics, there is one chemical element like no other: silicon, the working cell of transistor technology that drives our information society. The countless electronic devices we use in everyday life are proof of how very high volumes of silicone-based components can be manufactured at very low cost. It therefore seems natural to use silicon in other areas as well, where the properties of semiconductors, such as silicon one, are technologically exploited, and to explore ways to integrate different functions. Of particular interest in this context are diode lasers, such as those used in bar code scanners or laser pointers, which are usually based on Gallium Arsenide (GaAs). Unfortunately, the physical processes that create light in GaAs do not work as well in silicon. It therefore remains an excellent and long-term goal to find an alternative way to realize a ‘laser on silicone’.

Sign up today Applied Physics Lettersan international team led by professors Giacomo Scalari and Jérôme Faist of the Institute for Quantum Electronics offers an important step towards such a device. They report electroluminescence – generation of electric light – from a semiconductor structure based on silicon germanium (SiGe), a material that is compatible with standard manufacturing processes used for silicon devices. The emission they observed is, moreover, in the terahertz frequency band, which sits between that of microwave electronics and infrared optics, and is of great current importance with a view to different applications.

Let silicone shine

The main reason why silicone cannot be used directly for building a laser following the GaAs template has to do with the different nature of their band gaps, which are directly in the latter but indirectly in the former. In a nutshell, in GaAs, electrons combine with holes across the bandgap that produce light; in silicon they produce heat. Laser action in silicon therefore requires a different path. And exploring a new approach is what David Stark, a doctoral researcher at the ETH, and his colleagues are doing. They work in the direction of a quantum laser laser (QCL) on silicon. QCLs achieve light emission not by electron-hole recombination across the band gap, but by tunneling electrons through repeated stacks of precisely engineered semiconductor structures, during which process photons are emitted.

The QCL paradigm was demonstrated in a number of materials – first in 1994 by a team, including Jérôme Faist, who then worked at Bell Laboratories in the US – but never in silicone-based, despite promising predictions. The conversion of these predictions is the focus of an interdisciplinary project funded by the European Commission, bringing together a team of leading experts in the growth of the highest quality semiconductor material (at the Università Roma Tre), which characterizes them (at the Leibniz Institute for Innovative Microelectronics in Frankfurt an der Oder) and manufactures it into devices (at the University of Glasgow). The ETH group Scalari and Faist is responsible for performing the measurements on the devices, but also for the design of the laser, with numerical and theoretical support from partners in the nextnano business in Munich and at the universities of Pisa and Rome.

From electroluminescence to lasing

With this accompanying knowledge and expertise, the team designed and built devices with a unit structure made of SiGe and pure germanium (Ge), less than 100 nanometers high, which is repeated 51 times. From these heterostructures, fabricated with essentially atomic precision, Stark and co-workers detected electroluminescence, as predicted, with the spectral properties of the emerging light that match well with calculations. Further confidence that the devices work as intended comes from a comparison with a GaAs-based structure manufactured with identical device metrics. While the emission of the Ge / SiGe structure is still significantly lower than that of its GaAs counterpart, these results clearly indicate that the team is on 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 the action of a QCL on silicone at room temperature.

Such an achievement will be important in several respects. It will eventually not only realize a laser on a silicon substrate, thus boosting the silicon photonics. The emission of the structure created by Stark et al. is in the terahertz region, for which compact light sources are currently in short supply. Silicone-based QCLs, with their potential for versatility and lower manufacturing costs, can be a boon to the large-scale use of terahertz radiation in existing and new fields of application, from medical imaging to wireless communication.