PORTLAND, Ore.—Electrical engineers who expressed skepticism that Hewlett Packard Co.'s memristors could switch as fast as DRAM and yet retain their memories millions of times longer than flash can now rest easy, according to their inventor, senior HP Fellow Stanley Williams.
"What we have discovered is that an electric field and a current act together to enable a memory device that can both be switched very rapidly and hold its state indefinitely," said Williams. "Not only does an applied voltage drive the migration of oxygen vacancies in the device, but at the same time there is a current that heats it up to about 300 degrees Celsius—just enough to turn the amorphous film into a crystalline film."
Memristors are touted as the future "universal memory" device because they are as fast as DRAM, as small as flash, and as durable as read-only-memories, according to HP. As the fourth fundamental passive circuit element—after resistors, capacitors and inductors—memristors retain either a high- or low-resistance state by virtue of introducing or removing oxygen vacancies in oxide thin films.
Synchrotron x-rays probed the memristor in a 100 nanometer region with concentrated oxygen vacancies (right, shown in blue) where the memristive switching occurs. Surrounding this region a newly developed structural phase (red) was also found to act like a thermometer revealing how hot the device becomes when read or written.
Using their favorite formulation—titanium oxide—HP recently used high-energy synchrotron x-rays to correlate the device's electrical characteristics with its atomic structure, chemistry, and temperature in three dimensions. The until now unforeseen conclusion was that a hot spot near the bottom electrode heats enough during switching to induce a crystallization of the oxide. After driving out vacancies (for a 1) or introducing them (for a 0) in one-to-two nanometers thick region, the film cools in an annealing-like like process which leaves the film in a fixed crystalline state that should remain that way indefinitely.
"In testing, we have switched these devices over 30 billion times and counting, with no degradaton in their ability to retain information," said Williams.