ScienceWise - Mar/Apr 2007

Lollipops in Nanospace

Article Illustration
Electron microscope image of gallium antimonide growing on top of nanowires.
Article Illustration
Dr Michael Gao with wafers on which the nanostructures are grown.

Exotic structures eliminate lattice mismatch problems

Dr Michael Gao

The alien scene pictured below shows crystals of pure gallium antimonide sitting on top of spindly stalks of gallium arsenide. The stalks are around 30 nm in diameter and the formations were grown by Dr Michael Gao, a postdoctoral fellow in the Department of Electronic Materials Engineering (RSPSE). They represent new efforts to work with the semiconductor gallium antimonide. “There’s a lot of interest in working with gallium antimonide,” says Dr Gao. “It possesses a band gap that makes it an excellent material for many optoelectronic devices such as lasers and infrared detectors and LEDs. Unfortunately it’s very expensive and it’s difficult to deposit on other more commonly used semiconductor wafers like gallium arsenide.

“That’s because gallium antimonide has a larger lattice constant than gallium arsenide. When it crystallises on top of a wafer of gallium arsenide, large amounts of tension develop in the crystal structure of the gallium antimonide as the two crystal structures attempt to match. This tension prevents the gallium antimonide from forming smooth layers or growing into usable shapes.” “However, we have discovered that if you grow gallium antimonide on top of a nanowire of gallium arsenide that the gallium antimonide crystal can grow without large amounts of tension. We believe this is because the nanowirebase from which the gallium antimonide is growing is small enough not to impose its crystal structure on the newly forming crystal.”

Dr Gao’s work is part of the research of Professor Chennupati Jagadish’s Semiconductor Optoelectronics and Nanotechnology Group on nanowires. The Group has been growing nanowires by placing nanoparticles of gold onto semiconductor wafers of gallium arsenide. The sample is then placed in their MOCVD (Metal Organic Chemical Vapour Deposition) reactor and heated causing the gold nanoparticles to melt. Gallium and arsenic atoms are then passed over the sample in the form of a vapour and a nanowire of pure gallium arsenide starts to grow under the gold droplet.

Having become confident in their capacity to grow simple gallium arsenide nanowires the researchers are now exploring a variety of ways of modifying the wires, including the incorporation of different elements.

The hetero-nanowires that Dr Gao has been growing have gallium antimonide on top of gallium arsenide. Because the gallium antimonide has the larger lattice constant it grows out rather than tapering in. In so doing, it forms nanoscale lollipops, popsicles or match heads depending on the particular growth treatment being applied.

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