Nano Arrays Are Step towards Mass Production of Nanowires

This is a colourised micrograph of semiconductor nanowires grown at NIST in a precisely controlled array of sizes and locations. Image courtesy of K. Bertness, NIST.

Researchers from the National Institute of Standards and Technology (NIST) report that they have grown nanowires made of semiconductors, gallium nitride alloys, by depositing atoms layer-by-layer on a silicon crystal under high vacuum. The scientist have produced these nanowires without using metal catalysts, thereby enhancing luminescence and reducing defects. The nanowires also have uniform mechanical quality properties.

The experiments, described in Advanced Functional Materials, maintained the purity and defect-free crystal structure of the nanowires while controlling diameter and placement better than has been reported by other groups for catalyst-based nanowires. Precise control of diameter and placement is essential before nanowires can be widely used.

The key trick in the NIST technique is to grow the wires through precisely defined holes in a stencil-like mask covering the silicon wafer. The NIST nanowires were grown through openings in patterned silicon nitride masks. About 30,000 nanowires were grown per 76-mm-wide wafer. The technique controlled nanowire location almost perfectly. Wires grew uniformly through most openings and were absent on most of the mask surface.

Mask openings ranged from 300 to 1000 nm wide, in increments of 100 nm. In each opening of 300 or 400 nm, a single nanowire grew, with a well-formed hexagonal shape and a symmetrical tip with six facets. Larger openings produced more variable results. Openings of 400 to 900 nm yielded single-crystal nanowires with multifaceted tops. Structures grown in 1000-nm openings appeared to be multiple wires stuck together. All nanowires grew to about 1000 nm tall over three days.

NIST researchers analysed micrographs to verify the uniformity of nanowire shape and size statistically. The analysis revealed nearly uniform areas of wires of the same diameter as well as nearly perfect hexagonal shapes.

Growing nanowires on silicon is one approach NIST researchers are exploring for making “nanowires on a chip” devices. Although the growth temperatures are too high — more than 800°C— for silicon circuitry to tolerate, there may be ways to grow the nanowires first and then protect them during circuitry fabrication, lead author Kris Bertness says.

More information on the research is available from NIST.

Tags: Advanced Functional Materials, gallium nitride alloys, nanotechnoloy, nanowires, National Institute of Standards and Technology, NIST, semiconductors