In today’s microelectronics industry, the movement of an electron’s charge has been widely exploited to create a plethora of integrated circuits, which has continued to shrink in size and increase in density year after year.
However, anticipating that this trend might not last, engineers are now looking at alternative means from which to create the microelectronic components of the future. One way they might do so is to develop electronic circuitry that uses the intrinsic spin of electrons.
Physicists at Surrey University, led by Prof Ben Murdin and Dr Steven Clowes, are trying to create such spintronic, or spin-based electronic, devices based around existing semiconductor materials.
The spin of an electron can take one of two states: either up or down. So just as the movement of the electron charge has been used to create circuits that encode data in a binary fashion, so too could future circuitry that exploits the values of the electron’s spin.
The electronics industry’s vast technical knowledge is built almost solely around silicon. However, the realisation of a spintronic device using silicon is extremely challenging as it does not interact strongly with magnetic fields.
However, with funding from the Engineering and Physical Sciences Research Council (EPSRC), that is exactly what Murdin and his team want to do. Working in conjunction with colleagues at the London Centre of Nanotechnology and Peking University, who are experts in silicon device fabrication, they aim to work towards spintronic devices using atoms trapped in silicon.
Their new spintronic device would be made of a trimer: a three atom system — comprising two bismuth atoms separated by one phosphorus atom — that sits within the bulk silicon itself.
The atoms could be positioned so that, by exciting the electrons in the bismuth and the phosphor’s outer shells, their quantum mechanical wave functions would overlap. When they do, Pauli’s exclusion principle would come into play. The principle states that electrons cannot exist in the same state with the same quantum properties, one of which is spin. In this way, the bismuth atoms become quantum mechanically linked via the central phosphor atom. This exchange interaction allows the possibility of complex quantum operations.
‘When the electrons are excited in the bismuth and the phosphor’s outer shells, their quantum mechanical wave functions overlap and the two bismuth atoms essentially “see each other”. But that will only occur if the middle phosphorus atom is excited too, which would then link the three states in a bridge,’ said Clowes. So by controlling the state of the phosphorus atom sandwiched in between the bismuth pairs, it is theoretically possible to control the spin on the electrons in the device.
To create such an atomically based device will, he claims, demand that the researchers use isotropically pure silicon. This is where the link with China comes in. Unless a device is made from such pure silicon, the spins of the donor atoms will be affected by any other spin in the system.
‘The great thing about silicon is that it does not have any nuclear spin,’ said Clowes. ‘So as long as you have pure silicon, nothing will interact with the spin created on the donor atoms. But if you used normal silicon, all the impurities in there would kill the spin.’
One of the challenges to the realisation of such a spintronic device is that the atoms would have to be positioned very precisely in the silicon. Clowes said that a hydrogen lithography technique is one way that the donors could be precisely dropped into the silicon, although that technique is a long way from commercial production.
Another challenge is the development of a means to electrically excite and then detect the state of the quantum mechanical interaction between the atoms. This, he believes, can be achieved by optically exciting the atoms and then electrically detecting their state using the established technique of electrically detected magnetic resonance (EDMR).
Researchers in England and China are working on a EPSRC-funded project to create spintronic devices using atoms trapped in silicon