Vertical nanolasers open optical ports out of silicon

February 12, 2016 // By Julien Happich
Whilst today's silicon-based photonics chips require complex manufacturing processes to connect the light sources to the silicon, involving wafer-level stacking, physicists at the Technical University of Munich (TUM) have managed to grow vertical nanolasers, only 360nm in diameter, directly onto 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