Researchers create graphite memory only 10 atoms thick
Graphene memory is impervious to radiation
The technology could potentially provide many times the capacity of current flash memory and withstand temperatures of 200 degrees Celsius and radiation that would make solid-state disk memory disintegrate.
The team, lead by professor James Tour, included postdoctoral researchers Yubao Li and Alexander Sinitskii. In an interview, Tour said laboratory tests started a year and a half ago but his team only recently published a paper on the results.
Laboratory tests showed that they were able to grow graphene, which technically is 10 or fewer layers of graphite, atop silicon and use it to store a bit of data. The sheets were roughly 5 nanometers in diameter. Graphene is a form of carbon.
"Though we grow it from the vapor phase, this material is just like graphite in a pencil. You slide these right off the end of your pencil onto paper. If you were to place Scotch tape over it and pull up, you can sometimes pull up as small as one sheet of graphene. It is a little under 1 nanometer thick," Tour said.
The new solid-state memory is one of many next-generation technologies that could someday replace NAND flash memory at the 20 nanometer (nm) node size. Others include race track memory and phase-change memory. Currently, NAND flash memory can be as small as 45nm in size, but projections show the technology will reach its limit of 20nm by around 2012. By using graphene, Tour said, bits could be made smaller than 10nm in size.
Unlike NAND flash memory, which is controlled by three terminals or wires, the graphene memory requires two terminals, making it more viable for three-dimensional or stacked graphene arrays -- multiplying a chip's capacity with every layer, according to Tour. But like flash memory, chips made with graphene will consume virtually no power while keeping data intact. Tour also said graphene generates little heat, making it more suitable to three-dimensional or stacked memory.
Graphene also distinguishes itself from future storage mediums through its "on-off ratio" or the amount of electricity a circuit holds when it's on compared with when it's off. "It's huge -- a million to one," Tour said. "Phase change memory, the other thing the industry is considering, runs at 10-to-1. That means the 'off' state holds, say, one-tenth the amount of electrical current than the 'on' state."
In a statement, Tour explained that electrical current tends to leak from an "off" that's holding a charge. "That means in a 10-by-10 grid, 10 'offs' would leak enough to look like they were 'on.' With our method, it would take a million 'offs' in a line to look like 'on,' " he said. "So this is big. It allows us to make a much larger array."
One thing Tour has yet to demonstrate, but is confident he can, is laying down multiple bits on a single layer of graphene. Currently, multi-level cell (MLC) NAND flash has been demonstrated by Samsung to store up to four bits of data per cell. Commercially sold MLC flash drives can store 3 bits per cell.
The material has so far been tested from minus 75 degrees Celsius to 200 degrees Celsius. "In places where flash would be history, this would be fine," Tour said.
When it comes to speed, Tour said he's only tested the graphene memory's access time at 100 nanoseconds, "and we're certainly faster than that." MLC flash memory sports 50-nanosecond access times, and SRAM boasts 10-nanoseconds access times. "We'd like to be in the 10-nanosecond realm. If we can get this into nanosecond regime, say 1 or 10, that would be much better," he said.
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