The next level to integrate optics and electronics

12 April 2016

Researchers from UCL, the University of Sheffield and Cardiff University have demonstrated the first practical telecommunications wavelength quantum dot laser grown directly on a silicon substrate. The device shows a high efficiency and long lifetime in realistic conditions, and is a step towards breaking the current limits on data processing.

The demand for more and faster data and computation shows no sign of stopping, but a key bottleneck is the time taken to send signals between processing units. To remove this speed limit, designers would like to communicate with light instead of electronic pulses. The challenge is connecting to the silicon chips which do the computation. An ideal solution would be a semiconductor laser grown directly on silicon, as this is high efficiency, high-speed, connects directly to silicon electronics, and can carry a large amount of data.

However, growing laser structures on silicon is difficult, as the semiconductor compounds used to make them don’t fit on to, or expand at the same rate as, the silicon underneath them – this causes defects in the structure and lowers the efficiency. Teams from UCL Electronic and Electrical Engineering and the London Centre for Nanotechnology have previous success in fabricating this type of structure (link to previous work). In this work with Cardiff University and the University of Sheffield, published in Nature Photonics (link to paper) they demonstrate an increase in efficiency and lifetime, and a reduction in operating current, using ‘dislocation filtering layers’ combined with quantum dots.

Defects, or dislocations, occur when the perfect order of a crystal is broken. They often form when combining different elements that form crystals with different spacings, as the atoms do not fit together perfectly and the crystals are squished or stretched. This compression and tension is released by the breaking of order, creating a defect. The dislocation filtering layers are designed to release the strain in the system without making defects; in this case there were so few that they could not be detected by the electron microscope used.

With their nearly defect-free on-silicon laser, the team were able to produce continuous lasing up to 75C, a realistic operating temperature. At room temperature the laser output power was over 105mW, with a low current requirement for operation, and a projected lifetime of over 100,000 hours. This demonstrates the ability to grow uniform high-quality compounds of group III and group VI elements across a silicon substrate. Removing this obstacle opens up new possibilities for silicon photonics and for the direct integration of optical interconnects with silicon-based microelectronics.

Professor Huiyun Liu (UCL Electronic & Electrical Engineering), leader of the epitaxy research that enabled the creation of these lasers, said:

“Our work on defect filter layers and quantum dots has enabled us to create the first practical laser on a silicon substrate. The use of the quantum dot layer offers improved tolerance to dislocations, compared to alternative structures such as quantum wells. This work lays the foundation for reliable and cost-effective silicon-based photonic–electronic integration.”

Head of the Photonics Group in UCL Electronic & Electrical Engineering, Principal Investigator in the London Centre for Nanotechnology and Director of the EPSRC Centre for Doctoral Training in Integrated Photonic and Electronic Systems, Professor Alwyn Seeds, said:

“The techniques that we have developed permit us to realise the Holy Grail of silicon photonics- an efficient and reliable electrically driven semiconductor laser directly integrated on a silicon substrate. Our future work will be aimed at integrating these lasers with waveguides and drive electronics leading to a comprehensive technology for the integration of photonics with silicon electronics.”

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