Lattice mis-match makes growing GaAs on silicon difficult, which is why Dr Gregor Koblmüller and Professor Jonathan Finley are building the lasers vertically.
“The two materials have different lattice parameters and different coefficients of thermal expansion. This leads to strain. Planar growth of GaAs onto a silicon surface results in a large number of defects,” said Koblmüller.
By growing them upwards, mismatch only has to be dealt with over a few square nanometers, precluding emerging defects in the GaAs.
To make the wires into laser cavities, they needed mirrors at each end.
“The interface between GaAs and silicon does not reflect light sufficiently. We built in a mirror – a 200nm thick silicon oxide layer evaporated onto the silicon,” said researcher Benedikt Mayer.
The wires start life as tiny holes etched through the mirror layer.
Using epitaxy, wires are grown upwards though the holes. Eventually, the intention is that these holes will be over waveguides in the silicon, allowing light to couple between laser and waveguide.
These wires, which are hexagonal in cross-section, are then fattened with alternate layers of GaAs and AlGaAs forming coaxial quantum wells along their length. Fattening also extends the fibre end out over the SiO2 layer, allowing it to act as a mirror for all but the centre of the fibre where the original hole remains.
In final form, one type with seven quantum wells is ~11μm long and 350nm in diameter, constructed from alternating layers around 9nm thick.They emit infra-red.
The team has yet to report a way of electrically stimulating the lasers into action and relies on exciting emission using an external pulsed laser.
“We want to modify the emission wavelength and other laser parameters to better control temperature stability and light propagation under continuous excitation within the silicon chips,” said Finley.
The work is published as Monolithically integrated high-β nanowire lasers on silicon in Nano Letters and Coaxial GaAs-AlGaAs core-multishell nanowire lasers with epitaxial gain control in Applied Physics Letters.