technology integration
rhombus4
Instead, the micro-tubes are directly transferred from the example, the tube diameter is determined by layer
GaAs substrate to a silicon platform with the aid of a composition, and the wall thickness by the etching
solvent. According to Mi, removal of the GaAs substrate process.” The fabrication process is also simple and
creates freestanding micro-tubes that preferentially stay controllable, says Mi, and it can produce incredibly
on silicon due to the gravitational force induced by the smooth surfaces with a roughness that’s determined by
solvent in and around the tube. Solvent evaporation leaves MBE growth.
the micro-tubes bound to silicon via a Van De Waals
attraction. Mi believes that his micro-tube laser fabrication From emission to lasing
process has several strengths. “You combine the benefits Mi has made rapid progress since he started developing
of top-down and the bottom-up up processes. For micro-tube lasers following an appointment at McGill’s
Department of Electrical and Computer Engineering in
September 2007. Optically pumped emission from his
microcavities at 77K was realized in late 2008, and this
year he has progressed to coherent emission at room
temperature and finally lasing from these structures.
Advances are the fruits of processing improvements
resulting from ever greater familiarity with device
fabrication.
The emission wavelengths produced by these tubes are
governed by the lateral dimensions of the waveguide, and
its associated whispering-gallery modes. This leads to
multiple emission modes. The dominant lasing
wavelengths produced by one of the most recent
structures occur at 1194 nm, 1217 nm and 1241 nm, and
there are also weaker, subsidiary peaks (see Fig 3).
If Mi’s micro-tubes were perfect ring resonators, emission
would be produced from every point in the tube. But
these tubes have a starting and stopping edge that
scatter light, and this dominants the emission because the
rest of the structure is incredibly smooth. Light that
emanates from these points rapidly diverges.
Figure 2:
Scanning One of the promising aspects of Mi’s approach is its very
electron high yield. This is partly thanks to the simplicity of the
microscopy process, which involves just one photolithographic step.
reveals the Emission from the tubes may not be in the preferred
free-standing direction, but this can be remedied by simply rotating
quantum dot them.
micro-tube, and
the etched If these micro-tube lasers are to kick-on and enjoy
GaAs mesa (a). commercial success, then they will have to be driven by
This can be an electrical source. “It is difficult to achieve high
transferred to a efficiency, electrically injected micro and nano-scale
silicon lasers, and making an electrical contact directly onto the
substrate (b). free-standing laser without adversely affecting device
Closer performance is particularly challenging,” admits Mi.
inspection of “However, we have devised an approach and expect to
the micro-tube achieve electrically injected devices within one year.”
reveals the
sinusoidal The spectral emission profile from these electrically
geometry of its pumped lasers is expected to feature multiple-emission
surface (c). lines that are already seen in optically pumped
20 www.compoundsemiconductor.net November / December 2009
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