ScienceWise - Mar/Apr 2006

Growing Nanowires

The electronics and photonics industries are largely based on a top-down approach to building things. By this we mean you begin with a chunk of material – a wafer of silicon for example – and carve it down to create the circuits you need. These days industry can produce circuits with features only 90 nanometres apart but a top down approach has major problems going smaller than this.

“When you etch out circuits on semiconductors, you create a lot of defects — there are missing atoms which can trap the electrons,” explains Professor Chennupati Jagadish. “This isn’t a problem when the circuits are big but it becomes a limiting factor when you’re working at scale of tens of nanometres. Defects destroy these structures and the notion of carving out intricate three dimensional forms is simply out of the question.”

Professor Jagadish is Head of the Semiconductor Optoelectronics and Nanotechnology Group in the Department of Electronic Materials Engineering. For more than a decade his group has been building up expertise in working with compound semiconductors using a variety of techniques to create a range of devices including lasers, quantum dots and photodetectors. Now he has turned his attention to growing nanowires.

Nanowires have become the focus of several labs around the world in recent years. These are columns of semiconductor material that are only tens of nanometres in diameter that can be used to make circuits or serve as components in a range of devices.

“While you can’t carve out working nanoscale structures, it is possible to grow them from the bottom up,” says Professor Jagadish. “Our understanding of how to grow nanowires is now proceeding in leaps and bounds and this could prove to be the gateway to a new age in optoelectronics.”

So, how do you grow a nanowire?

“To build nanowires we deposit nanoparticles of gold onto a semiconductor wafer of gallium arsenide,” explains Professor Jagadish. “We then place the sample into the metal organic chemical vapour deposition chamber and heat the sample to around 600º C. This removes any surface contaminants and melts the gold so it forms an alloy with the gallium from the gallium arsenide wafer. This is a eutectic alloy in which the melting point of the alloy is lower than gold by itself.

“Next we lower the temperature of the chamber to below the melting point of gold but above that of the gallium/gold alloy so it remains molten. At the same time we introduce gases into the chamber containing gallium and arsenic atoms.

“And then the most amazing phenomenon takes place – a tower of pure crystalline gallium arsenide begins to grow under the molten droplet of gold giving you a self-assembled nanowire.

The researchers have learnt through trials that the critical variables involved in growing nanowires under gold nanoparticles is the size of the particles, the density and distribution of the particles on the wafer, and the heat at which the nanowires form.

By using different reactive species in the vapour such as indium, gallium and arsenic, it’s possible to incorporate different compounds into the nanowire thereby giving it different electro optical properties.

And where will these nanowires be used?

“It’s possible that the nanowires we are growing might be used as wires connecting circuits and devices,” says Professor Jagadish. “By breaking them from the wafer on which they’ve grown using ultrasonic treatment they can then be placed into circuits.

“However, we suspect their more significant value will come using them where they grow. They can serve as nanowire lasers, nanowire photo-detectors, photonic crystals and quantum dot lattices. Our next challenge, therefore will be to work out different ways of growing the nanowires in ordered arrays and we’re currently experimenting with different ways of achieving this.

“It might take a few years but as our mastery of self-assembled nanostructures begins to match our traditional top-down mass-manufactured capacity it’s likely a new world of possibilities will open. It’s impossible to say exactly how it will progress. What’s important is that Australia build a capacity to engage with these emerging technologies,” stresses Professor Jagadish.

More info: Professor Chennupati Jagadish

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