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Growing whiskers that won`t need shaving

July 23, 2002

As manufacturers try to incorporate more and more functions into electronic gadgets like mobile phones and laptop computers, and at the same time decrease their size, the need for smaller electronic circuit components increases. At the 26th International Conference on the Physics of Semiconductors in Edinburgh on Thursday 1 August, Prof Lars Samuelson from Lund University in Sweden will describe how his team are building minute electronic devices within wires just 20 billionths of a metre wide, then studying how well they operate.

Wires this small are known as nanowires, or nanowhiskers. Scientists have been able to grow semiconductor nanowires for about 10 years, but until recently had no way of mixing different materials together within one wire. However, Prof Samuelson`s team was one of three groups who simultaneously reported the ability to grow single nanowires made from layers of different semiconductors earlier this year. Since conventional electronic devices are made from layers of different types of semiconductor, this breakthrough in growing methods meant tiny electronic devices could potentially be made as an integral part of a nanowire.

By combining the semiconductor materials indium arsenide and indium phosphide in layers of particular thicknesses, the Swedish scientists have now taken that work one step further and produced an electronic device known as a double-barrier resonant tunnelling device. This device is commonly used in electronic circuits, and the researchers are currently testing how well their nanowire version works. "Our group is, to the best of my knowledge, the only one that has made functional electronic devices and investigated their electronic properties," says Prof Samuelson.

The nanowire devices are made using a variation of a standard technique for forming semiconductor devices known as chemical beam epitaxy. A substrate (base) for the nanowire to grow on is put inside an ultra-high vacuum chamber. This substrate is heated up, and small particles containing gold atoms whose is size carefully controlled, are placed on its surface. The semiconductor materials that the wire will be made from are then injected into the chamber in the form of gases. These gases react with the gold atoms and condense into a solid crystalline structure. Layers are formed by introducing each gas separately and in turn - the more gas let in at any one time, the thicker that particular layer will be.

The first solid layer forms between the gold atoms and the substrate, while each subsequent layer builds up between the substrate and the last layer grown. This means the gold atoms end up sitting at the end of the nanowire that is furthest from the substrate surface. Growing the nanowires in this way allows the atoms of each layer to line up perfectly with those of the layer below, creating an ideal heterostructure (composite material made from layers of different types of semiconductor).

As well as enabling tiny versions of existing electronic devices to be manufactured, entirely new types of devices could be developed using this technology. This is because the nanowires are so small that they only allow electrons - which produce electric currents in metals and semiconductors by moving steadily through them - to move in just one direction. "In general, all heterostructure devices have their specific functions because we can design the `landscape` in which electrons can move. The one-dimensionality of nanowire devices offers many more exciting opportunities, such as creating ultra-small light sources for the safe transfer of information via quantum cryptography" says Prof Samuelson. Nanowire devices could also be used in displays and solar cells.

Institute of Physics