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Silicon oxide tested for next-gen ‘flash’ memory

RICE (US) — Chemists have built a 1-kilobit rewritable silicon oxide chip that shows potential for next-generation 3D memories for computers and consumer devices.

With gigabytes of flash memory becoming steadily cheaper, a 1k nonvolatile memory unit has little practical use. But as a proof of concept, the chip shows it should be possible to surpass the limitations of flash memory in packing density, energy consumption per bit, and switching speed.


Memory devices that use cheap, plentiful silicon oxide to store data have the potential to overcome the limitations of flash memory. (Credit: José Ramón de Lothlórien/Flickr)

The technique is based on previous work by James Tour and researchers at Rice University that shows when electricity passes through a layer of silicon oxide, it strips away oxygen molecules and creates a channel of pure metallic phase silicon that is less than five nanometers wide.

Normal operating voltages can repeatedly break and “heal” the channel, which can be read as either a “1″ or “0″ depending upon whether it is broken or intact.

The circuits require only two terminals instead of three, as in most memory chips. The crossbar memories are flexible, resist heat and radiation and show promise for stacking in three-dimensional arrays.

Rudimentary silicon memories made in the Tour lab are now aboard the International Space Station, where they are being tested for their ability to hold a pattern when exposed to radiation.

The diodes eliminate crosstalk inherent in crossbar structures by keeping the electronic state on a cell from leaking into adjacent cells, Tour says. “It wasn’t easy to develop, but it’s now very easy to make,” he adds.


Built like a sandwich

The device built by Rice postdoctoral researcher Gunuk Wang, lead author of the new paper in the journal Advanced Materials, sandwiches the active silicon oxide between layers of palladium. The silicon-palladium sandwiches rest upon a thin layer of aluminum that combines with a base layer of p-doped silicon to act as a diode.

Wang’s 32 x 32-bit test arrays are a little more than a micrometer deep with crossbar line widths of 10 to 100 micrometers for testing purposes.

“We didn’t try to miniaturize it,” Tour says. “We’ve already demonstrated the native sub-5-nanometer filament, which is going to work with the smallest line size industry can make.”


The team built crossbar memory chips. A diode made of silicon and aluminum makes a two-terminal memory cell of palladium and silicon oxide possible. (Credit: Tour Group/Rice University)

The devices have proven to be robust, with a high on/off ratio of about 10,000 to 1, over the equivalent of 10 years of use, low-energy consumption and even the capability for multibit switching, which would allow higher density information storage than conventional two-state memory systems.

The devices dubbed “one diode-one resistor” (1D-1R) worked especially well when compared with test versions (1R) that lacked the diode, Wang says.

“Using just the silicon oxide was not enough,” he adds. “In a (1R) crossbar structure with just the memory material, if we made 1,024 cells, only about 63 cells would work individually. There would be crosstalk, and that was a problem.”

To prove the 1D-1R’s capabilities, Wang isolated 3 x 3 grids and encoded ASCII letters spelling out “RICE OWLS” into the bits. Setting adjacent bits to the “on” state—usually a condition that leads to voltage leaks and data corruption in a 1R crossbar structure—had no effect on the information, he adds.

“From the engineering side of this, integrating diodes into a 1k memory array is no small feat,” Tour says. “It will be industry’s job to scale this into commercial memories, but this demonstration shows it can be done.”

The Boeing Corp. and the Air Force Office of Scientific Research funded the work.

Source: Rice University

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