A new generation of high speed, silicon-based information technology has been brought a step closer by researchers in the Department of Electronic and Electrical Engineering at UCL and the London Centre for Nanotechnology. The team's research, published in next week's Nature Photonics journal, provides the first demonstration of an electrically driven, quantum dot laser grown directly on a silicon substrate (Si) with a wavelength (1300-nm) suitable for use in telecommunications.
Silicon is the most widely used material for the fabrication of active devices in electronics. However, the nature of its atomic structure makes it extremely hard to realise an efficient light source in this material.
As the speed and complexity of silicon electronics increases, it is becoming harder to interconnect large information processing systems using conventional copper electrical interconnects. For this reason the field of silicon photonics (the development of optical interconnects for use with silicon electronics) is becoming increasingly important.
The ideal light source for silicon photonics would be a semiconductor laser, for high efficiency, direct interfacing with silicon drive electronics and high-speed data modulation capability. To date, the most promising approach to a light source for silicon photonics has been the use of wafer bonding to join compound semiconductor laser materials from which lasers can be made to a silicon substrate.
Direct growth of compound semiconductor laser material on silicon would be an attractive route to full integration for silicon photonics. However, the large differences in crystal lattice constant between silicon and compound semiconductors cause dislocations in the crystal structure that result in low efficiency and short operating lifetime for semiconductor lasers.
The UCL group has overcome these difficulties by developing special layers which prevent these dislocations from reaching the laser layer together with a quantum dot laser gain layer. This has enabled them to demonstrate an electrically pumped 1,300 nm wavelength laser by direct epitaxial growth on silicon. In a recent paper in Optics Express (Vol. 19 Issue 12, pp.11381-11386 (2011)) they report an optical output power of over 15 mW per facet at room temperature.
In related work the group, working with device fabrication colleagues at the EPSRC National Centre for III-V Technologies, have demonstrated the first quantum dot laser on a germanium (Ge) substrate by direct epitaxial growth. The laser, reported in Nature Photonics , (DOI: 10.1038/NPHOTON.2011.120, 12 June 2009) is capable of continuous operation at temperatures up to 70 deg. C and has a continuous output power of over 25 mW per facet.
Leader of the epitaxy research that enabled the creation of these lasers and Royal Society University Research Fellow in the UCL Department of Electronic and Electrical Engineering, Dr Huiyun Liu, said: "The use of the quantum dot gain layer offers improved tolerance to residual dislocations relative to conventional quantum well structures. Our work on germanium should also permit practical lasers to be created on the Si/Ge substrates that are an important part of the roadmap for future silicon technology."
Head of the Photonics Group in the UCL Department of Electronic and Electrical Engineering, Principal Investigator in the London Centre for Nanotechnology and Director of the EPSRC Centre for Doctoral Training in Photonic Systems Development, Professor Alwyn Seeds, said: "The techniques that we have developed permit us to realise the Holy Grail of silicon photonics - an efficient, electrically pumped, semiconductor laser integrated on a silicon substrate. Our future work will be aimed at combining these lasers with waveguides and drive electronics leading to a comprehensive technology for the integration of photonics with silicon electronics."
Notes for editors
Contact details: For more information, please contact Dr Huiyun Liu (tel: +44 (0)20 7679 3983 e-mail: h.liu@ee.ucl.ac.uk) or Professor Alwyn Seeds (tel: +44 (0)20 7679 7928 e-mail: a.seeds@ee.ucl.ac.uk)
Images: Images of quantum dot lasers fabricated on a silicon substrate at UCL can be obtained by calling the Department of Electronic and Electrical Engineering on +44 (0)20 7679 3983 or by emailing h.liu@ee.ucl.ac.uk
About the UCL Department of Electronic and Electrical Engineering The Department of Electronic and Electrical Engineering at UCL was the first department of Electrical Engineering to be established in England and now comprises some 200 researchers working on topics in communications and information systems, electronic materials and devices, optical networks, photonics and sensors, systems and circuits, with turnover exceeding £11 million. It has consistently been rated among the top ten UK Departments in its subject area in the UK Government's Research Assessment Exercise. Website: www.ee.ucl.ac.uk
About the London Centre for Nanotechnology The London Centre for Nanotechnology, is a UK-based, multidisciplinary research centre forming the bridge between the physical and biomedical sciences. It was conceived from the outset with a management structure allowing for a clear focus on scientific excellence, exploitation and commercialisation. It brings together two world leaders in nanotechnology, namely University College London and Imperial College London, in a unique operating model that accesses the combined skills of multiple departments, including medicine, chemistry, physics, electronic and electrical engineering, biochemical engineering, materials and earth sciences, and two leading technology transfer offices. Website: www.london-nano.com
About UCL Founded in 1826, UCL was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender, and the first to provide systematic teaching of law, architecture and medicine. UCL is among the world's top universities, as reflected by performance in a range of international rankings and tables. Alumni include Marie Stopes, Jonathan Dimbleby, Lord Woolf, Alexander Graham Bell, and members of the band Coldplay. UCL currently has over 13,000 undergraduate and 9,000 postgraduate students. Its annual income is over £700 million. Website: www.ucl.ac.uk
Journal
Nature Photonics