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Si-CMOS compatible nanopillar LEDs promise accurate photonics integration

Technology News |
By Julien Happich


Published in the ACS Photonics journal under the title “Ultracompact Position-Controlled InP Nanopillar LEDs on Silicon with Bright Electroluminescence at Telecommunication Wavelengths”, the article reports the site-controlled growth of high-yield (90%) uniform arrays of InP nanopillars on silicon, grown under CMOS compatible conditions: low temperature and without catalysts.

Tilt-view low magnification SEM images
of arrays of site-controlled InP nanopillars
grown at 460°C. The scale bar in all images
corresponds to 10 μm and the growth
periods (pitch) are 1 μm, 4 and 40μm,
respectively.

The researchers started with a clean silicon wafer (111), depositing 140nm of silicon dioxide (at 350°C) into which they defined nanoscale apertures about 320nm in diameter to position the nanopillar nucleation sites, with a pitch varying from 1 to 40μm. After chemically roughening the silicon surface, the researchers grew the InP nanostructures in a MOCVD chamber at temperatures between 450 and 460°C. They found that the taper angle of the nanopillar was largely affected by growth temperature, yielding nanoneedles at 450°C but nearly vertical pillar-shaped structures at 460°C.

On the basis of these nanopillars and through a concentrical core−shell growth, the researchers incorporated five InGaAs quantum wells in the active region of a pn diode, forming electrically driven n-InP/InGaAs MQW/p-InP/p-InGaAs nano-LEDs.

Schematic of the nanopillar MQW LED device.

Thanks to the core−shell growth mode, the nanopillars grow out of their nucleation site and expand beyond the oxide opening to reach a final diameter of roughly 1μm. Hence, while the n-doped core of the nanopillar is in direct contact with the n-Si substrate, the p-doped shell grows over the oxide mask, eliminating the shunt path from the p-doped shell and n-Si substrate. 20/200nm of Ti/Au evaporated via angled electron beam evaporation onto a highly p-doped InGaAs contact layer completed the devices to form electrical contacts, with a small region of the nanopillars left bare and without metal as a window for the LED optical output.

The nanopillar LEDs where characterized to emit at 1510nm (a wavelength of interest of telecommunications) with a high internal quantum efficiency of circa 30%. Despite their small footprint, the nanopillar LEDs output 4μW of power which the researchers claim is the highest reported light output from a nanopillar/nanostructure-based LED. In this setup, since collection efficiency was only about 5%, the light output available was down to 200nW.

Another interesting aspect of this research is that the devices could be electrically biased to produce optical gain, they also demonstrate a strong photo-response under reverse injection, making them promising for photonics integration on silicon.

University of California, Berkeley – www.berkeley.edu

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