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 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.
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 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, 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.”
1.3-?m InAs/GaAs quantum-dot lasers monolithically grown on Si substrates
Long-wavelength InAs/GaAs quantum-dot laser diode monolithically grown on Ge substrate
London Centre for Nanotechnology
SOURCE: University College London
