A US research team has written, read and erased polarised domains in a ferroelectric strontium titanate film on silicon.
"While theoretical calculations and spectroscopic data demonstrated that the distorted crystal behaved like a ferroelectric, proof of the ferroelectric functionality waited on the work led by Cornell University professor Darrell Schlom, whose team used a technique called piezoresponse force microscopy to write, read and erase polarised domains in the film." said US government lab NIST.
The work originated with Motorola researcher Hao Li who succeeded in depositing the metal oxide directly onto silicon with no intervening silicon oxide layer, producing coherency between the two crystal structures.
"This is a difficult trick both because silicon is highly reactive to oxidation and because the crystal spacing of the two materials does not normally match," said NIST. "Li developed a finely controlled method of depositing the strontium titanate in stages, gradually building up layers that were only a few molecules thick."
The result, showed X-ray analysis, was that the silicon atoms squeezed the cubic strontium-titanate crystals, distorting them and creating a structural instability that made the compound a ferroelectric.
NIST claims the films could be used to integrate temperature or pressure sensors as will as non-volatile memory on to silicon.
This is a large US military-funded project with researchers from Cornell, the University of Pittsburgh, NIST, Pennsylvania State University, Northwestern University, Motorola, Ames Laboratory, Intel, and Tricorn Tech contributing.
X-ray diffraction took place at the Advanced Photon Source, Argonne National Laboratory.
Atoms in a film of strontium titanate on a single crystal of silicon (left). When sufficiently thin, the strontium titanate can be strained to match the atom spacing of the underlying silicon and becomes ferroelectric.
On the right, the diagram has been written into such a film in the form of a pattern of re-writable electric polarisation.