May 29, 2007 – Researchers from the National Institute of Standards and Technology (NIST), with help from the U. of Maryland and Howard U., have devised a fabrication method that creates tiny ultraviolet light-emitting diodes from nanowires, and NIST says the technique is “well-suited” for scaling to commercial production.
Direct bandgap group III-nitride (AlN/GaN/InN) semiconducting nanowires are seen as promising candidates for small LEDs to be used in sensors, data storage, and optical communications. But making nanowire LEDs typically involves a series of manufacturing techniques that don’t easily translate into commercial production — i.e., crossing n-GaN and p-GaN nanowires, crossing n-GaN and n-Si nanowires, n-GaN core and InGaN/Gan/p-AlGaN/p-GaN multishell structures — “tedious nanowire manipulation methods,” NIST noted — and a series of one-by-one fabrication techniques such as electron-beam lithography and focused ion beam etching.
For their study, the researchers grew a 4µm-thick p-GaN epilayer via hydride vapor-phase epitaxy on a c-plane sapphire substrate (Mg was the p-type dopant (hole concentration = 2.5 x 1018 cm3), onto which was first deposited a 65nm thick SiO2 layer using PECVD, then a Ti/Al/Ti layer (thicknesses 30-100-30nm) using e-beam evaporation. Photolithography and wet etching yielded 300µm-dia circular, insulated metal contact pads with 500µm pitch on the surface of the p-GaN epilayer. A suspension of GaN nanowires in isopropanol was then dispersed onto the substrate with the insulated metal contact pads; the nanowires were aligned dielectrophoretically by applying a 10 V peak to peak, 1 kHz sinusoidal voltage to one post.
A second photolitho/etch step resulted in 250µm-diam. circular contacts on top of the first level contacts; deposited Ti/Al/Ti/Au was followed by a lift-off to form the top contact to the nanowire. For contact to the p-GaN layer, a Ni/Au (30nm/30nm) layer was deposited at the four corners of the sample by a shadow mask.
NIST claims the new GaN LEDs emit a 365nm light wavelength with 25nm full width half maximum FWHM at an applied voltage of 50µA, “squarely in the UV range.” Higher emissions of 385nm were obtained with 65µA injection levels, though possibly due to GaN-oxide interface related recombinations. The UV LEDs also showed “excellent thermal stability” up to 750 degrees C and operational stability after two continuous hours of operation at room temperature.
“The present technique can be applied to other nanowire systems, and is suitable for applications requiring large area nanoscale light sources,” NIST said, in its paper. Using passivations such as AlN with higher dielectric constant could improve the extraction efficiencies, the group added.
The work from NIST and university researchers was been published in the May 29 edition of Applied Physics Letters.
IMAGE BELOW: Schematic representation of the process of aligning and fabricating n-GaN nanowire/p-GaN epilayer LED. a) deposition of SiO2 and Ti/Al/Ti layers; b) nanowires aligned on the pads and GaN layer, forming the p-n junction; c) more SiO2 deposited via PECVD; d) 250µm contacts built on the first level contacts, followed by Ti/Al/Ti/Au and Ni/Au deposited layers. (Source: NIST)