Aug 2, 2002
Semiconductors stride ahead
An efficient radiation emitter that could speed up cancer detection rates and nano-sized light sources that could lead to smaller optoelectronic devices were among the breakthroughs reported this week at the 26th International Conference on the Physics of Semiconductors. Other highlights of the Edinburgh meeting included the development of electrical components from nanowires, and a step towards semiconductor 'qubits', which could form the basis of a practical quantum computer.
Skin cancer changes the thickness of the layers of skin it affects, so measurements of this thickness can be used to spot tumours. Different layers of skin contain different levels of moisture, so skin thickness can be determined by measuring the amount of radiation each layer absorbs. Terahertz radiation - which has a wavelength of about a tenth of a millimetre - is ideal for this technique because it is absorbed by water and is much safer than X-rays.
Now Michael Johnston and colleagues from the universities of Cambridge and Sheffield have developed a semiconductor emitter that produces 20 times more terahertz radiation than existing devices, and could help tumours to be spotted more quickly. To achieve this, the researchers increased the emission of a semiconductor chip by attaching a prism to it.
In the US, a light source based on an atom or ion trapped in a semiconductor 'cage' has been developed by Rameshwar Bhargava's team at Nanocrystals Technology, New York. These 'quantum confined atoms' are trapped in crystals just billionths of a metre across. When the trapped atoms are stimulated by another laser, they can emit as much light as phosphorescent particles a thousand times larger. Bhargava and colleagues are optimistic that their research will lead to advances in optoelectronics and X-ray imaging.
Meanwhile, Lars Samuelson and co-workers at Lund University in Sweden have created some of the first nanowires made from layers of different semiconductors. Using this layer structure - on which conventional electronic devices are based - the team has successfully made a 'resonant tunnelling' device from a nanowire.
"Our group is, to the best of my knowledge, the only one that has made functional electronic devices and investigated their electronic properties," says Samuelson. The researchers hope that their growth techniques will allow them to build a range of electronic components using nanowires.
Semiconductors could also play a key role in the development of a quantum computer. In theory, such a machine could outperform conventional computers because it could carry out many processes at once. But a practical machine would require many identical quantum bits - or 'qubits' - that can take on two values at once.
"In a quantum gate, a controllable interaction is introduced between the qubits so that when the bits are addressed to perform a calculation, they are not affected individually like in a classical system, but all at once," says team member Manfred Bayer.
One candidate for such a system is a pair of linked quantum dots. A quantum dot is a tiny region of semiconductor embedded in a different semiconductor. This dot can trap a single electron, which can have one of two spin states. Now a team led by Gerhard Ortner of the University of Dortmund in Germany has connected two such dots and shown that the spin states of the two electrons were linked.
About the author
Katie Pennicott is editor of PhysicsWeb.