Silicon conducts an electrical surprise

Silicon can conduct electricity when experts assumed it couldn’t, sparking a surprising direction in silicon electronics.

Max Lagally and colleagues at the University of Wisconsin-Madison, US have overturned predictions that silicon layers on the widely used silicon-on-insulator (SOI) platform do not conduct electricity.

It was generally accepted that thin layers of silicon should not conduct electricity because a very thin surface does not have enough charge carriers, and hence would not give an image in a scanning tunnelling microscope (STM). But Lagally’s group got a clear STM image from its SOI sample in a vacuum, suggesting conductivity and leading them to investigate a possible mechanism.

The researchers claim that the conduction mechanism at the silicon surface comes from the bulk, but only because the surface silicon has the right energy levels in its so-called band structure to allow the bulk electrons to move. ’The difference between the old measurements and the new measurements is that people thought the surface was doing all the conducting, but it isn’t,’ Lagally told Chemistry World. ’It doesn’t need to conduct, it just needs to be there.’

Lagally is now developing silicon membranes for different applications, including sensors. This is where the technology will have a big effect, he predicts. The membrane surface can be free or can be functionalised - to become pH sensitive or biologically sensitive, so it could bind DNA. 

Details of Lagally’s devices remain closely guarded, but he hinted that the next step would take advantage of a third dimension in silicon devices, rather than 2-D used in the industry today.  

’There’s a totally different paradigm coming where you don’t use rigid silicon. You might be able to find a way to do things with thin layers where the thin layers are more useful than the bulk rigid substrate,’ said Lagally. ’We can do in a membrane what the device industry does on a rigid piece of silicon.’

Gareth Parry, an electronic materials expert at Imperial College London was surprised by the results. ’The conductivity is high, it isn’t a small effect,’ he said. Parry said it is too early to tell what impact Lagally’s work will have.

Lagally cautions against hailing the discovery as the saviour of microelectronics, where silicon’s capabilities will be limited as devices get smaller and smaller. ’The fact that it’s thin and flexible does not add to the microelectronics industry’s present paradigm,’ said Lagally. To incorporate his discovery would take a leap that the industry does not yet need to take, he said.

Katharine Sanderson