Materials change may lead to more efficient superconductors

Dr Issam Qattan, an assistant professiror of physics at Khalifa University, is researching how semiconductors perform through careful analysis and mathematical calculations of the materials used in manufacture.

Dr Issam Qattan, an assistant professiror of physics at Khalifa University, is researching how semiconductors perform through careful analysis and mathematical calculations of the materials used in maShow more

All around us, the most mundane everyday objects rely on semiconductors.

Found in microprocessors and electronic circuits, they are crucial to the working of lasers, computers, cars, satellites, radios and countless other items we take for granted.

Semiconductors conduct electricity better than insulators, but not as well as metals. It is precisely this property that makes them useful for converting or amplifying electrical inputs in electronic devices.

Over the past eight decades or so, the collective findings of thousands of researchers and engineers have made them cheap, reliable and efficient.

Now, a scientist in Sharjah could add his name to the list of those who have made breakthroughs in the development of these vital components.

Dr Issam Qattan, a researcher at the Sharjah branch of Khalifa University of Science and Technology, has published results that are likely to spark considerable interest in the field.

They could do nothing less than change the underlying chemistry of some of the electronics around us.

He and Dr Ahmad Alsaad, a scientist at Jordan University of Science and Technology, designed a computer programme that simulates semiconductors’ inner workings.

They used it to see what would happen when they tweaked the semiconductors’ chemical composition.

Many semiconductors are made largely from one type of material, such as silicon, with small quantities of a different material added.

This is done because a semiconductor made of, for example, silicon alone is typically a modest conductor of electricity – hence the name semiconductor – because it is balanced in electrical terms.

That means that all its electrons – the negatively charged particles found outside the nucleus of atoms – are tied up in bonds with neighbouring atoms. When they are forming bonds, the electrons are not free to move. No movement, no conduction of electricity.

But add a little of another substance, a process known as doping, and the electrical conductivity can be improved by many hundreds of thousands of times.

It has been more than eight decades since scientists began to realise how important this doping process could be. It can cause a semiconductor to have a big surplus or deficit of electrons – and that is crucial in letting electrons pass through the material, and so in letting the material conduct electricity.

But merely letting the electrons move is far from the whole story. Over the past decade attention has become focused on the science of “spintronics” – the alignment of the spin of those electrons.

In very approximate terms, spin can be thought of as the way the electrons are rotating.

The spin gives the material a magnetic field. And the direction of the spin – the “spin polarisation” – determines the shape and nature of the magnetic field – its “magnetic moment”.

Working out how to manipulate those offers another way in which the semiconductor can generate signals and transmit information.

In the journal Physica B: Condensed Matter, Dr Qattan has announced a discovery linked to this which he describes as counter-intuitive.

“Scientists had believed that enhancing semiconductors’ magnetic properties and hence magnetic moments is best done by doping a magnetic material with a magnetic impurity,” he said.

But Dr Qattan and Dr Alsaad’s simulations showed that, in fact, an even larger magnetic moment is generated by semiconductors made up of a non-magnetic compound doped with a magnetic impurity.

This result was not a complete surprise – previous experiments had found that when non-magnetic materials were doped with magnetic impurities, they could, under the right conditions, emit light.

That suggested that some kind of interaction was going on within the material – energy was being exchanged between the host material and the magnetic impurity.

Such energy exchanges can be significant because when they are associated with the release of light particles – photons – a large magnetic moment is often created.

Dr Qattan’s study looked at cases where a magnetic impurity such as chromium is doped into a magnetic host material such as iron.

It also considered the effect of doping “rare earth” magnetic ions such as samarium and gadolinium into non-magnetic semiconductor materials such as gallium nitride.

The results are, said Dr Qattan, nothing less than striking. A higher magnetic moment means a better semiconductor.

“People were thinking that to enhance a semiconductor, you had to introduce a magnetic impurity [to a magnetic material],” he said.

“But you can start with a non-magnetic material and dope with a magnetic material such as rare earths, and you will get a larger magnetic moment.”

As well as being highly efficient, these non-magnetic-based semiconductors would also be relatively easy to make.

“This should be of potential to a lot of people in the industry. People were not thinking about this type of material before,” he said. “Maybe they can change gear a bit and start looking at different materials.”

In particular, the material could be used to make more efficient magneto-optical devices – the successors to today’s writable DVDs.

DVDs are coated with a substance that can have its magnetic field altered by a laser. Variations in that magnetic field across the surface of a disc hold the information stored on it.

But there is still much to do. The two researchers now plan to take a broader look at how much magnetic “dope” produces the best semiconductors.

“We have to see how far we can go by increasing the concentration,” said Dr Qattan.

And in theory the result, eventually, could be a bigger, better DVD.