Last Friday, I wrote about a memristor development out of Jim Tour’s nano research group at Rice University. The development involves the observation of nonvolatile memristance-like behavior in silicon oxide, rather than the titanium dioxide employed by HP’s memristor. Silicon oxide is a well-understood substance used in every silicon-based IC device now made. Presently, it’s used strictly for insulation, so seeing memristance behavior in the material is revolutionary and exciting. The information for Friday’s blog entry came from a press release about the publication of a paper on the observed phenomena in the most recent American Chemical Society’s Nano Letters. Over the weekend, I obtained a copy of the article (thanks to the King Library, jointly run by San Jose State University and the city of San Jose) to delve further into the announcement. What I learned is that the paper covers the early observation of an interesting phenomenon. We’re pretty far from a working memristor memory chip made from silicon dioxide.
The paper published in ACS Nano Letters describes a test chip consisting of 200 50-micon circular pillars. Polysilicon electrodes sandwiching a 40nm-thick silicon oxide layer comprise each pillar. The 50-micron diameter allows each pillar to be probed with a microprobe for experiments. There is no active circuitry on the test chip. The paper’s description of silicon oxide sort of reset my basic understanding of the material, which was based on high-school and university chemistry courses from too long ago. I had always envisioned silicon dioxide as having two oxygen atoms bound to each silicon atom. My mistake. It’s important not to have this image of silicon dioxide when thinking about the silicon oxide in this experiment. Think more of a silicon lattice infused with oxygen atoms. Mobile oxygen atoms. The proper chemical formula is SiOx where x ranges from 1.9 to 2. Think of free-range oxygen.
An electric current can drive the free-range oxygen out of a localized region of silicon leaving pure silicon nanocrystals that conduct substantially better than the surrounding silicon oxide. With enough current applied for a long enough time, a chain of conductive nanocrystals forms along the current path and creates a relatively high conductivity (relatively low-resistance) silicon filament between the two electrodes. The paper proposes electron tunneling conduction as the mechanism for creating the initial current that forms the silicon nanocrystals.
Driving oxygen out of a material is called reduction (the opposite of oxidation) and the authors of the paper have included electron microscope images showing the filament of silicon nanocrystals formed by the electrically driven reduction process. Further experiments indicate that the reduction is a surface phenomenon and only occurs on the annular surface of the silicon oxide disk within the pillar sandwich in this experiment. Applying an oxygen atmosphere appears to prevent the formation of silicon nanocrystals, which will form under the same electrical conditions in a nitrogen atmosphere or in a vacuum. Together, the electron microscopy photos and the interfering nature of free atmospheric oxygen support the premise that the observed phenomenon is due to an electrically induced reduction reaction. The paper’s authors call this reduction process the “set” mode because it sets the “device” into the relatively low-resistance state. The reset mode, which places the device back into a high-impedance state, is “more likely” to be induced by electrically driven thermal heating, according to the theory published in the paper. The set mode requires 100nsec, 6-8V pulses and the reset mode requires 13V, 50nsec pulses. There’s a repeatable, 5-orders-of-magnitude difference in resistance between the set and reset states of the device—something that can easily be sensed on a chip.
So what we have here is the initial observation of a very interesting nanoscale phenomena based on silicon that’s still far removed from even a prototype device. There’s still a lot of engineering to be done here before we even start to work towards a commercial product based on this phenomenon.