Magnetic touch

The principles governing increases in computing power could be sustained into the foreseeable future thanks to UK-based research that is applying advanced metallurgical techniques to magnetic semiconductors.

Moore's law is the observation that the density of transistors on an integrated circuit doubles every two years. To achieve this, components have shrunk over time, but scientists anticipate that when nanowire parts go down to around a tenth of a nanometre, heating and quantum effects will become so severe that they will not be of practical use.

Dr Olga Kazakova, senior research scientist, at the National Physical Laboratory (NPL) looked at solutions to this problem as part of a project into the metallurgy of nanomagnetism. 'The solution lies in changing not only the material but also the structure of our transistors,' she said.

'We work mainly with germanium nanowires, and our idea is to add another degree of freedom of material by making this material magnetic. Magnetic semiconductors don't exist in nature, so they have to be artificially engineered.'

Germanium is a good candidate to be used as what is known as a diluted magnetic semiconductor (DMS) as is it closely compatible with silicon, meaning it can easily be used with existing silicon electronics without further redesign.

Germanium has a much higher intrinsic 'hole mobility' — a factor that influences the speed of a semiconductor device — than silicon or gallium arsenide. But introducing magnetic properties causes further problems that have to be overcome.

'We're trying to dope germanium with magnetic materials, but even at a very small concentration of magnetic impurity — less than five per cent — the magnetic materials start to precipitate from the semiconducting magnet,' said Kazakova. When precipitation occurs, it forms various metallic germanium alloys which cause problems in circuits.

A further problem with magnetic semiconductors is that nearly all can be made magnetic, but most have a low Curie temperature — at which materials become ferromagnetic.

'The majority will be ferromagnetic at around 100 Kelvin (-173ºC) which is useless for practical applications,' said Kazakova. 'For the IT industry, it should be ferromagnetic at room temperature and up to 400ºC to compete with current technology.'

Until now, few materials have been shown to display ferro- magnetic materials at room temperature, and regular germanium manganese materials can only be made magnetic at around 120 Kelvin.

'Surprisingly, when we began to grow germanium manganese nanowires, we found some combinations of the growing environment parameters where the material was ferromagnetic at room temperature and free of metallic precipitation,' said Kazakova.

The nanowires the team uses normally have a diameter of 60nm and a 60-micron length — a ratio of 1:1000 — which means there is a huge surface-to-volume ratio, which positively influences the nanowires' magnetic and electronic properties. They are specially grown for the project by a microbiologist in University College Cork in Ireland.

The researchers are also looking into creating a nanocable consisting of a germanium manganese core with a silicon sheath, a system which gives unique properties.

'The electronic structure of germanium and silicon is such that a one-dimensional electron hole is formed at the border between these two materials,' said Kazakova. 'This is good as it gives a greater length of free movement without scattering or filling neighbouring holes, even at room temperature.'

This solution also addresses another problem for the semiconducting industry. All electric contacts are usually metal, and when a semiconductor and metal are in contact, it reduces normal current flow and increases resistance of the system.

'In our solution, when we build a germanium manganese silicon nanocable and put it in contact with a standard gold electrode, we don't fill electron holes at the barrier. We're just beginning research in this area, but it's an interesting effect, which could have a lot of impact in terms of applications,' said Kazakova.

Another part of the project is developing ultra-sensitive sensors to detect the tiny magnetic moments produced in a single magnetic nanowire.

'We can currently detect stronger magnetic particles in conventional ferromagnets, but we haven't succeeded yet with diluted magnetic semiconductors the sensitivity needs to be improved for nanowires,' said Kazakova

She said that transistors based on NPL's germanium nanowire technology, which could revolutionise computing and electronic devices, could realistically be 10 years away.

Berenice Baker