Wednesday, September 01, 2010

Silicon oxide and all that.

It's been a busy week work-wise; hence the low rate of blogging.  However, I would be remiss if I failed to talk about the science behind a paper (on which I am a coauthor) that was mentioned on the front page of the New York Times yesterday.  A student, Jun Yao, co-advised by my colleagues Jim Tour and Lin Zhong, did a really elegant experiment that has gotten a lot of attention, and the science is pretty neat.  Here's the deal.  Lots of people have done experiments where they've seen what appears to be nonvolatile switching of the electrical resistance in various nanoscale systems (e.g., junctions in nanotubes and other nanomaterials).  That is, what is observed is that, with the use of voltage pulses, the electrical resistance of a device may be programmed to be comparatively high or comparatively low, and that state is preserved for a looooong time.  Long story short:  sometimes this behavior has nothing in particular to do with the nanoscale system being studied, and really results from the properties of the underlying or nearby silicon oxide, which is generally treated as inert and boring.  Well, as people in the Si industry can tell you at length, it turns out that silicon oxide isn't necessarily inert and boring.   What Jun showed via some elegant cross-sectional transmission electron microscopy is that when big voltage pulses are applied across small distances, it is possible to modify the oxide, effectively doing electrochemistry, and turning some of the oxide back into Si nanocrystals.  When those nanocrystals give a hopping path from one electrode to the other, the device is "on".  When that path is broken, the device is "off".  The resulting nanocrystals themselves are quite small, on the order of a few nm.  Hence the excitement about possibly using this approach for very dense, nonvolatile memory.  There are, of course, a great many engineering issues to be overcome (there's no need to tell me about that in the comments....), but it is definitely a pretty science result. 

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