Spotlight on “Nanolasers grown on silicon-based MOSFETs”

A laser source on the head of a silicon transistor? Don’t worry, they both work… Data transport in the optical domain is believed today to be the most promising technology, or more likely the only viable one, to unchain the capacity of high speed data links without crashing into unsustainable energy consumption. Cooling is the main cause of power dissipation in data centers, so that a further increase in the speed of data transmission is hardly attainable without a dramatic energy-per-bit reduction. Since copper wires are close to their transmission limit, in this game electric connections are likely to be offside; as a valuable alternative, photonics offers a unique opportunity to become the forefront technology for high speed interconnects in the near future. In order for communication between data servers, modules, boards, or even processors to transfer into the optical domain, the integration of photonic transmission systems on conventional electronic platforms becomes a primary issue. This motivation has strongly boosted the search for CMOS-compatible optical technologies that could coexist with electronics within the same silicon chip. Optical waveguides have been the first building blocks to be realized on a silicon platform, and today they combine very low loss and strong light confinement, enabling an impressive miniaturization of photonic integrated circuits. Little by little, silicon-based platforms have been enriched with a wide variety of passive and active devices, including for instance high-speed modulators and detectors. In this scenario, light generation has been always considered the missing piece of the jigsaw, since the indirect bandgap of silicon, similarly to other group IV materials, naturally provides poor radiation efficiency. A successful approach to make group IV lasers directly on silicon exploits the engineering of strain and doping concentration, and has been recently employed to realize the first electrically pumped Ge laser monolithically integrated into a CMOS process. Direct gap III-V semiconductor lasers are typically much more efficient, but all efforts to monolithically integrate them on a silicon platform have had very limited success. The main technological challenges here are the mismatches of both lattice constant and thermal expansion coefficient, and the high growth temperature of III-V bulk materials, which cannot be tolerated by CMOS chips.