Vertical nanolasers open optical ports out of silicon
This opens up new optical ports for the integration of photonic components on top of CMOS circuits.
Because the materials have different lattice parameters and different coefficients of thermal expansion, growing a III-V semiconductor onto silicon leads to strain and typically yields a large number of defects, which makes the layers unsuitable to create operational devices.
The TUM team solved this problem by first depositing GaAs nanowires freestanding on silicon, with a footprints only about 40 to 50nm in diameter, corresponding to the diameter of seeding pinholes in a 250nm thick SiO2 interlayer. They then used molecular beam epitaxy (MBE) to grow the inner core nanowire up to around 10um in length, before widening selectively the diameter of the GaAs nanowire through a controlled lateral growth, into coaxial laser structures.
Fig. 1: Scanning electron microscopic image of the GaAs/AlGaAs NWs (a), size measurement (b) and lasing experimentation (c) through external optical pumping.
By altering their chemistries, the researchers managed to build multiple layers of Quantum Wells (QW) through depositing multiple hexagonal GaAs-AlGaAs core-shell structures. They demonstrated a 8nm thick GaAs QW sandwiched between 75nm thick AlGaAs barrier layers, but also a multiple quantum well laser structure consisting of seven 8nm thick GaAs QWs separated by 10nm thick AlGaAs barriers.
Fig. 2: (a) Schematic illustrations of the coaxial GaAs-AlGaAs MQW NW heterostructure; (b) SEM image of the laser structure as grown on Si; (c) Cross-sectional HAADF-STEM image of the same structure showing the GaAs layers in bright and the AlGaAs regions in dark; (d) Magnified image as well as Z-contrast function across a section of adjacent GaAs QWs and AlGaAs barriers.
In both designs, the core−shell GaAs−AlGaAs nanowires remain connected to the silicon substrate via the inner core that extends throughout the SiO2 interlayer, the later acting as one mirror in the lasing operation.
In previous work, the researchers had investigated detached GaAs-AlGaAs core-shell NW lasers to characterize and probe their intrinsic properties and performance.
In their latest paper, "Coaxial GaAs-AlGaAs core-multishell nanowire lasers with epitaxial gain control" just published in the Applied Physics Letters, they proved near-infrared lasing operation (through external optical excitation using a Ti:sapphire laser tuned to 780nm) with the nanowires standing on top of their silicon base, at room temperature.
Next the researchers want to improve the NW-laser performance by improving the end-facet reflectivity via chemical polishing or by depositing dielectric Bragg mirrors. Also, for a practical use, the lasers should emit unidirectionally into the underlying silicon where patterned photonic hardware could be nested, including on-chip waveguides. For a true integration, they will also require electrical injection.
"Our current research goals focus on developing methods to realise electrically pumped NW lasers on silicon and also integrated lasers emitting into underlying photonic circuitry. Until now the devices are optically pumped but they are grown site selectively on the silicon substrate" commented Dr. Jonathan J. Finley, Professor at the Technical University of Munich Walter Schottky Institute.
"We have already filed patents for the basic technologies for the NW laser on silicon and we are certainly interested to discuss licensing of IP to interested parties" he concluded.
Read more at https://pubs.acs.org/doi/full/10.1021/acs.nanolett.5b03404
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