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Nano snow-blowers carve straight channels in III-V surfaces

NIST, IBM researchers report new addition to toolkit of semiconductor 'self-assembly' methods

Scientists from the National Institute of Standards and Technology (NIST) and IBM in the US have shown that nano particles of gold can operate like snow blowers, churning through surface layers of compound semiconductors to dig straight paths. The trenching capability, reported in Nano Letters could be a new addition to the toolkit of 'self-assembly' methods that researchers aim to harness for making useful devices.

Foreseeable applications include integrating lasers, sensors, wave guides and other optical components into so-called lab-on-a-chip devices now used for disease diagnosis, screening experimental materials and drugs, DNA forensics and more. Easy to control, the new gold-catalysed process for creating patterns of channels with nanoscale dimensions could help to spawn entirely new technologies fashioned from ensembles of ultra-small structures.

Preliminary research results began as a contaminant-caused failure that impeded the expected formation of nanowires."We were disappointed, at first," says NIST research chemist Babak Nikoobakht. "Then we figured out that water was the contaminant in the process - a problem that turned out to be a good thing."

That's because, as determined in subsequent experiments, the addition of water vapour served to transform gold nanoparticles into channel diggers, rather than the expected wire makers. 

Beginning with studies on the semiconductor InP, the team first patterned the surface of the semiconductor by selectively coating it with a gold layer only a few nanometers thick. On heating, the film breaks up into tiny particles that become droplets. The underlying InP dissolves into the gold nanoparticles above, creating a gold alloy. Then, heated water vapour is introduced into the system. 

At temperatures below 300degC, the tiny gold-alloy particles, covered with water molecules, etch nanoscale pits into the InP. But at 440degC and above, long V-shaped nanochannels formed. The channels followed straight paths dictated by the regularly repeating lattice of atoms in the crystalline semiconductor. During the process, indium and phosphorous atoms interact with oxygen atoms in the water molecules on the surface of the gold alloy droplet. The oxidised indium and phosphorous evaporate, and the droplet advances, picking up more semiconductor atoms to oxidise as it goes.

The result is a series of crystalline groves. The dimensions of the grooves correspond to the size of droplet, which can be controlled.

In effect, the droplet is the chemical equivalent of the auger on a snow blower that, instead of snow, burrows through the top portion of the semiconductor and ejects evaporated bits, Nikoobakht explains.

The team observed the same phenomena in the compound semiconductors GaP and InAs. Nikoobakht believes that, with adjustments, the etching process might also work for creating patterns of channels on silicon and other materials.

Controllable, fast and flexible, the "bottom up" channel-fabrication process shows promise for use on industrial scales, the researchers suggest. In their article, the teams describe how they used the process to etch patterns of hollow channels like those used to direct the flow of liquids, such as a blood sample, in a microfluidic device, or lab on a chip.

'Vapor-Liquid-Solid Etch of Semiconductor Surface Channels by Running Gold Nanodroplets' by B. Nikoobakht et al, Nano Letters, vol. 15, issue 12, Dec. 9, 2015.

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