PORTLAND, Ore. Graphene sheet memories at least twice as dense as flash and using a novel two-terminal architecture are being developed by researchers at Rice University.
Graphene, which is composed of thin films of pure carbon atoms not rolled into nanotubes, have been used for ultra-fast transistor prototypes at IBM and elsewhere, but a group led by Rice professor James Tour is the first to report fabricating simpler, two-terminal memory devices using graphene.
Instead of a three-terminal device like a transistor, "our graphene-based memory cell just has two terminals," Tour said. "Depending on the voltage you put across it, you can write it or erase it or read it."
In the laboratory, 3.5 volts erased the memory by breaking the connection pathway through the graphene sheet. Using a 3-volt signal, however, repairs the break, allowing the device to again conduct. A 1-volt signal allows circuitry to interogate the memory cell to determine whether it is "on" or "off."
The precise mechanism of operation is not yet known, but Tour's best guess is that a mechanical open circuit is being made by the erase voltage, which is then repaired by the write signal.
The experimental devices were deposited as thin films only 5 to 10 atoms thick, with bit cells measuring just 5 nanometers on a side. Since the graphene sheets do not have to be continuous, Tour said the size of their bit cells will be determined almost entirely by how fine the electrode lines can be made. Since the device uses just two terminals, they can be sandwiched between crossbar switches with ultrafine geometries.
"We put down the graphene by chemical vapor deposition, but we don't have to form perfect monolayers," said Tour. "They can even be discontinuous, because it doesn't seem to matter. Its a very, very forgiving process," said Tour.
The on-off current ratio ranges from 10,000 to 1 million to one, compared to just 100 to 1 or even 10 to 1 for other nonvolatile memory technologies. This allows bit cells to be very tightly packed with very little of the leakage current that causes heat build up. Low-power operation and the two-terminal architecture should also usher in three-dimensional structures in future devices.
Early ultra-dense nonvolatile memory research was funded by NASA, but Tour's group now has backing for future research from an unnamed semiconductor maker.