Gold-etched channels, a cut above

Gold nanoparticles can mow their way through the surface layers of an important class of semiconductors to create perfect striped lawn according to work by scientists from the US National Institute of Standards and Technology (NIST) and IBM. The technique could be exploited to pattern materials for integrated lasers, sensors, waveguides and other optical components for lab-on-a-chip devices. [DOI: 10.1021/acs.nanolett.5b04051]

NIST research chemist Babak Nikoobakht remarks that initially the team was hoping to make nanowires on the surface but scanning electron microscope (SEM) images revealed they hadn't created wires, but long, straight channels. "We were disappointed, at first," he says. "Then we figured out that water was the contaminant in the process - a problem that turned out to be a good thing." The presence of water vapor during the fabrication process transformed the gold nanoparticles into channel diggers, rather than the expected wire makers. Preliminary experiments with indium phosphide allowed them to extract the chemical mechanism and optimal conditions for this new approach to surface etching.

The process involved first patterning the surface of the semiconductor by selectively coating it with a gold layer only a few nanometers thick. They then heated this to form "droplets" and the underlying indium phosphide dissolves into the gold nanoparticles above, creating a gold alloy. Heated water vapor introduced into the system at temperatures above 440 degrees Celsius leads to the gold-alloy particles, now bathed in water molecules running on the surface and etching nanoscopic channels with a V-shaped profile on the semiconductor surface.

The team reports that these V-channels are very straight following the crystal lattice of the semiconductor, their width depends on the size of the gold droplets formed. The etching mechanism involves both indium and phosphorus atoms being oxidized by the oxygen from the water and vaporizing. Nikoobakht explains the process by analogy with a snow blower rather than a lawnmower, picturing the gold-alloy droplet as acting like the auger of a snow blower burrowing through the top layers and ejecting "evaporated" debris.

The same process is possible with gallium phosphide and indium arsenide; both technologically important semiconductors. Nikoobakht also points out that fine tuning the process might make it viable for etching silicon and other materials.

The researchers say that such a controllable, fast and flexible, "bottom up" channel-fabrication process could be useful on an industrial scale for creating channels and other features in lab-on-a-chip and microfluidic devices.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".