The material concerned is the ‘improper’ ferroelectric Cu-Cl boracite Cu3B7O13Cl.
Within this material, even a single crystal of this material, multiple electrostatic domains – regions in which electric charges are aligned – can be created.
The boundaries between these domains – the domain walls, which are 2D sheets a few atoms thick – have their own electrical properties, and these walls are what the researchers have created, moved and erased.
Why is this important?
If sophisticated control could be gained over these domain walls, transistors could be written and erased into the solid at will.
In the simplest case, source and drain contacts on the surface of the solid could be connected together by shunting a domain wall under both of them, and just as simply disconnected, creating a ‘domain wall transistor’, Professor Marty Gregg told Electronics Weekly. This wall could be short, or “there is no reasonable upper limit on the length, it can be very long,” he said.
A more complex device could make use of the nature of domain wall conductivity, which can be p-type or n-type.
“Where they meet, you can make a p-n junction between walls,” said Gregg. “Devices can be made, and made to disappear. You can start with a completely blank system, and then you can start injecting devices and connections.”
There are two ways to manipulate the domain walls: piezoelectrically using mechanical force to distort the material, or using electric fields.
“The key is that, when a needle is pressed into the crystal surface, a jigsaw puzzle-like pattern of domains develops around the contact point,” said researcher Dr Raymond McQuaid. “We have also shown that these walls can then be moved using applied electric fields, suggesting compatibility with more conventional voltage operated devices. Our team has demonstrated that copper-chlorine boracite crystals can have straight conducting walls that are hundreds of microns in length and only nanometres thick.”
Results are published in ‘Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite‘ in Nature Communications.
To take the domain wall research further, the EPSRC has awarded a ‘Critical-Mass’ grant worth £2.3m across UK institutions including Universities of Cambridge, Warwick and St. Andrews.