A Light Detector Built Into Silicon Runs at 50 Gigabits a Second

A group at the University of Southampton and the photonics company Rockley Photonics has built a light detector onto the same 200-millimeter silicon wafer as the on-chip channels that feed light into it, and reports running it at 50 gigabits per second. The device and the measurements were published Aug. 22 in Communications Engineering, a Nature Portfolio journal, under an open license.

The detector is made from stacked germanium and silicon-germanium quantum wells, thin layers whose light absorption shifts when a voltage is applied across them. It sits on silicon-nitride waveguides, the channels that route light around a chip. The authors report bandwidths above 25 gigahertz, measured at the standard 3-decibel point, under a bias of minus 5 volts; eye diagrams at 50 gigabits per second in avalanche mode, with responsivities above 4 amps per watt; and dark-current densities of 0.03 amps per square centimeter at minus 3 volts. Each figure is the group's own measurement of its own devices.

Because the process is compatible with standard CMOS chipmaking, the authors write, it allows "the integration of multiple quantum-well stacks with SiN for both modulation and detection, all within a single wafer-scale fabrication run on silicon." The paper claims no first and no priority, and describes the work as "laying the foundation for future photonic integrated systems for both access and short-reach communication."

The devices were fabricated at the Southampton Nanofabrication Centre, and two of the six authors are at Rockley Photonics in Pasadena, California. The paper credits the UK Engineering and Physical Sciences Research Council, through a Southampton partnership grant with Rockley Photonics and a project on quantum dots on silicon, and the European Union's Horizon 2020 program. Communications Engineering says it is releasing the peer-reviewed accepted manuscript early, and that it will be replaced by the final version of record.