Researchers at the National Institute of Standards and Technology (NIST) have cultivated many thousands of nanocrystals in what looks like a pinscreen or “pin art” on silicon, a step toward reliable mass production of semiconductor nanowires for millionths-of-a-meter-scale devices such as sensors and lasers.
NIST researchers grow nanowires made of semiconductors—gallium nitride alloys—by depositing atoms layer-by-layer on a silicon crystal under high vacuum. NIST has the unusual capability to produce these nanowires without using metal catalysts, thereby enhancing luminescence and reducing defects. NIST nanowires also have excellent mechanical quality factors
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-millimeter-wide wafer. The technique controlled nanowire location almost perfectly. Wires grew uniformly through most openings and were absent on most of the mask surface. All nanowires grew to about 1,000 nm tall over three days.
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—over 800 degrees Celsius—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
The location of GaN nanowires is controlled with essentially perfect selectivity using patterned SiNx prior to molecular beam epitaxy growth. Nanowire growth is uniform within mask openings and absent on the mask surface for over 95% of the usable area of a 76 mm diameter substrate. The diameters of the resulting nanowires are controlled by the size of the mask openings. Openings of approximately 500 nm or less produce single nanowires with symmetrically faceted tips.