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Silicon ReRAM beats graphene, say Rice researchers

Posted: 06 Sep 2010  Print Version  Bookmark and Share

Keywords:memristors  resistive RAM  ReRAM 

Researchers at Rice University say that memristors made from pure silicon can be used for resistive random access memory (ReRAM) that would be simpler and cheaper to make than Hewlett-Packard Co.'s titanium-based devices. Working with fabless chip design house PrivaTran Inc. they demonstrated a proof-of-concept ReRAM. It packs only 1Kb, but they claim it can be scaled beyond the densities of flash.

"Our memristors are made out of silicon instead of titanium like HPs," explained James Tour', a professor at Rice University. "In its patent application, HP listed many oxides, but not silicon-oxide, which we have now turned into a bit cell for resistive RAMs."

Silicon oxide, the most common insulator today, is used in all CMOS chips. Since its initial characterization in the 1960s, engineers have intentionally deposited silicon oxide in thicknesses that prevent its breakdown. However, by carefully crafting the voltage pulses going through it, thin layers of silicon dioxide can be made to change their resistance from near infinite to near zero, according to Rice and PrivaTran. This ohenomenon has been used by SanDisk Corp. to create write-once memories, but now Rice and PrivaTran claim to have made the process reversible, thereby enabling pure silicon ReRAMs.

In 2009 Tour's lab demonstrated how thin-films of carbon—graphene—could be made into a memristor-like bit cell that could double the typical memory densities of flash, But during characterization of that prototype, Tour noticed that the bit cell, which had been using silicon oxide as an insulator, seemed to work even without the graphene. Careful observations of the phenomena by Tour and fellow professors Douglas Natelson and Lin Zhong led to the discovery of a reversible "soft" breakdown of silicon oxide, allowing its use as a bit cell.

"We had noticed in our work with graphene that the silicon oxide was breaking down, but we did not understand the mechanism—now we do," said Tour. "By applying the right voltage pulses we can cause a reversible soft breakdown in silicon oxide where oxygen atoms move out leaving a silicon filament between the source and drain electrodes, allowing current to flow—turning it into a memistor that can be used as a bit cell. All you need besides the silicon oxide is a crossbar and a vertical diode to build 3D resistive RAMs."

According to Tour, the silicon filaments are composed of <5nm nanocrystals, which should allow the bit cells to scale beyond the densities of flash which loose functionality beyond 20nm. In their prototype, the crossbar electrodes were made from polysilicon, enabling the entire memory array to be cast in silicon. In characterizing the all-silicon memories, the researchers found that they could switch in under 100ns and could withstand 10,000 read/erase/write cycles, similar to flash memories, but with the possibility of going to much higher densities than flash.

Funding for the research was provided by the Army Research Office, National Science Foundation, Air Force Office of Scientific Research and the Navy Space and Naval Warfare Systems Command Small Business Innovation Research (SBIR) and Small Business Technology Transfer programs. Jun Yao and Zhengzong Sun, doctoral candidates at Rice, performed much of the lab work.





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