An electrically driven quantum dot laser grown directly on a silicon substrate with a wavelength suitable for use in telecommunications has been demonstrated, bringing a new generation of high-speed, silicon-based information technology one step closer. 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 difficult to realize 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. Quantum dot laser fabricated on a silicon substrate at UCL. (Image: University College London) 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. A group of researchers in the Department of Electronic and Electrical Engineering at University College London and at the London Centre for Nanotechnology has overcome these difficulties by developing special layers that prevent these dislocations from reaching the laser layer together with a quantum dot laser gain layer. This has enabled the team to demonstrate an electrically pumped 1300-nm-wavelength laser by direct epitaxial growth on silicon. The researchers report an optical output power of more than 15 mW per facet at room temperature. In related work, the investigators, working with device fabrication colleagues at the EPSRC National Centre for III-V Technologies, have demonstrated the first quantum dot laser on a germanium substrate by direct epitaxial growth. The laser is capable of continuous operation at temperatures up to 70 ºC and has a continuous output power of >25 mW per facet. “The use of the quantum dot gain layer offers improved tolerance to residual dislocations relative to conventional quantum well structures,” said Dr. Huiyun Liu, Royal Society University Research Fellow in the UCL Department of Electronic and Electrical Engineering. “Our work on germanium should also permit practical lasers to be created on the Si/Ge substrates that are an important part of the road map for future silicon technology.” “The techniques that we have developed permit us to realize the Holy Grail of silicon photonics — an efficient, electrically pumped, semiconductor laser integrated on a silicon substrate,” said professor Alwyn Seeds, from the UCL Department of Electronic and Electrical Engineering, the London Centre for Nanotechnology and the EPSRC Centre for Doctoral Training in Photonic Systems Development. “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.” For more information, visit: www.ucl.ac.uk