Transparent 2-terminal memories on flexible sheets could find use in head-up displays
Led by chemist James Tour and physicist Douglas Natelson, the Rice team is making highly transparent, nonvolatile resistive memory devices based on the revelation that silicon oxide itself can be a switch. A voltage run across a thin sheet of silicon oxide strips oxygen atoms away from a channel 5 nanometers wide, turning it into conductive metallic silicon. With lower voltages, the channel can then be broken and repaired repeatedly, over thousands of cycles.
That channel can be read as a "1″ or a "0," which is a switch, the basic unit of computer memories. At 5 nm, it shows promise to extend Moore’s Law, which predicted computer circuitry will double in power every two years.
The research details memories that are 95 percent transparent, made of silicon oxide and crossbar graphene terminals on flexible plastic. Manufacturers who have been able to fit millions of bits on small devices like flash memories now find themselves bumping against the physical limits of their current architectures, which require three terminals for each bit.
But the Rice unit, requiring only two terminals, makes it far less complicated. It means arrays of two-terminal memories can be stacked in three-dimensional configurations, vastly increasing the amount of information a memory chip might hold. Tour said his lab has also seen promise for making multi-state memories that would further increase their capacity.
The team is building memories out of silicon oxides sandwiched between graphene – one-atom-thick ribbons of carbon – and attached to plastic sheets. There’s not a speck of metal in the entire unit (with the exception of leads attached to the graphene electrodes).
The marriage of silicon and graphene would extend the long-recognized utility of the first and prove once and for all the value of the second, long touted as a wonder material looking for a reason to be, Tour said. He noted the devices not only show potential for radiation-hardened devices – several built at Rice are now being evaluated at the International Space Station – but also withstand heat up to about 700 degrees Celsius. That means they can be mounted directly atop integrated processors with no ill effects.
The lab is also building crossbar memories with embedded diodes to better manipulate control voltages, Tour said. "We’ve been developing this slowly to understand the fundamental switching mechanisms," he said. "Industries have flown in and looked at it, but we’re doing basic science here; we don’t package things nice and pretty, so what they see looks rudimentary. But this is now transitioning into an applied system that could well be taken up as a future memory system," he said.
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