The Semiconductor Devices of the Future

When physicists sandwiched together different types of semiconductor to create the first transistor in 1947, they made bulky vacuum valves obsolete and so revolutionised the electronics industry. Since then researchers have been pushing the boundaries of semiconductor technology hoping for another revolution. At the 26th International Conference on the Physics of Semiconductors in Edinburgh from 29 July to 2 August, progress towards ultra-high density magnetic recording, and a new branch of nanotechnology are among the highlights of the cutting-edge semiconductor research being presented.

Nanotechnology is the science of making new materials – and structures like minute electronic devices less than one millionth of a metre big – atom by atom. On Friday 2 August, Dr Rameshwar Bhargava will describe how his team at Nanocrystals Technology in the USA have developed a structure known as a `quantum confined atom` (QCA) that they hope will become the building block for a range of new semiconductor devices.

A quantum confined atom is an atom or an ion (atom with an electric charge) trapped within a `nanocrystal cage` (a tiny cage made from the atoms of a semiconductor). In conventional semiconductor technology, most electronic devices are made from layers of different types of semiconductor. The semiconductor materials used are `doped` with atoms of different elements, which alter their properties so that each layer will have the particular electrical characteristics needed for that type of device to work. QCA technology however is the direct opposite of this. Instead of the trapped atom altering the properties of its semiconductor host, the semiconductor atoms of the cage modify the properties of the atom they are confining.

Using a patented mixing process, the Nanocrystals team can trap atoms in spaces between 2 and 10 billionths of a metre big. At present they are concentrating on trapping phosphorescent ions (ions that emit visible light of a certain colour when light of a different colour or invisible ultraviolet (UV) light is shone at them). By reducing the size of these cages from 10 down to between 2 and 5 billionths of a metre, the researchers found the ions could generate 20 times more light. This meant they were emitting as much light as conventional phosphor particles 1000 times larger.

“This dramatic enhancement of luminescence efficiency is expected to have an impact all optical devices such as light emitting diodes (LEDs), lasers, displays and fluorescent lamps” says Dr Bhargava, who predicts products based on QCAs – such as ultra-efficient energy saving lamps – will be available commercially within five years. In fact, his nanophosphors could find their first application as early as next year, replacing existing phosphors that convert the X-rays used in medical imaging into light so a picture can be recorded. Because they are so small and efficient, the new nanophosphors can improve the resolution of X-ray images, and allow a smaller dose of X-rays to be used while still obtaining a clear picture. “In the future, we also expect to modulate and enhance other properties such as magnetic properties of the nanocrystals,” adds Dr Bhargava. This would mean QCAs could act as storage materials for data in ultra-high-density magnetic recording systems – which will soon be required.

“There is an ever increasing demand for greater data storage capacity, not only for computers, but for a variety of other devices such as MP3 players and cell phones” explains Prof Stuart Solin from the NEC Research Institute in the USA. Prof Solin`s team, which includes researchers from the University of Oklahoma in the USA and from the NEC Fundamental Research Laboratories in Japan, has been developing a semiconductor-metal composite material that could be used to make read-heads for the ultra-high-density magnetic recording systems of the future.

Read-heads are a type of magnetic sensor that detects when a magnetic field is present via changes in its resistance to the flow of electricity – an effect known as magnetoresistance. In magnetic recording systems, data is stored as variations in the amount of magnetisation on a disk or tape surface. A read-head can decode the stored information because the stronger the magnetisation is in a particular area, the stronger the corresponding magnetic field is around that point and the greater the change in read-head resistance it produces. Because so much more data would be squeezed into the same space, an ultra-high-density magnetic recording system would need read-heads that were not only much smaller and more sensitive than those currently available, but that could work much faster as well.

Magnetoresistance (MR) can occur in both metals and semiconductors, and on Thursday 1 August, Prof Solin will describe the material that his team is developing for use as an MR sensor. The new material is a combination of metal (gold) and semiconductor (indium antimonide). Although both the metal and semiconductor are non-magnetic, their composite structure shows the largest magnetoresistance – dubbed extraordinary magnetoresistance (EMR) by the researchers – ever recorded from a magnetic sensor material at room temperature. The team have already built a prototype read-head from this material, and on the strength of the performance this is showing, think EMR read-heads for ultra-high density recording could become a reality within the next three years. Meanwhile, “for applications such as car ignition timing sensors, we believe that EMR devices are commercially viable now,” says Prof Solin.

Before higher density magnetic recording systems can hit the market however, new write-heads will also need to be developed, and better materials on which to store magnetic data found – a possible use for Dr Bhargava`s QCAs. If this can be achieved, the amount of data we can store on disks will be dramatically improved. “One could expect a single three-and-a-half inch disk that can hold five feature length films today would have the capacity to hold a personal library of multimedia data in the near future” says Solin.

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Michelle Cain AlphaGalileo

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