So it all boils down to the key question: how does this Ag-aSi system conduct vs. temperature? Is there a Schottky fit? Is there an activation energy? Could there be a crystallization temperature involved?
Yjdong:-Looking at other metals and Shottky formation, I suppose the question is how much is the original amorphous silicon structure disturbed by the movement of metal into and out of that structure, the ideal situation would be not at all. My view is that would only occur if the movement was electro-migration/current density/diffusion driven, candidates Ag, Cu and even Gold.
In which case the metal poly/single crystal contact would most likely be considered a Shottky. I would suggest that trying to detect how much the amorphous structure has changed would be the basis of the analytical technique.
As one moves away from that ideal situation the alloying of the metal with the amorphous silicon has to be considered. In the extreme case that would be say a VIA formed using a refractory metal and a-Si where say an irreversible W-Si alloy link is formed. In the intermediate case if in its molten state the Si-X alloy can form an electrochemical cell that would provide the needed reversibility for an NV memory. In all three cases the threshold switching effect displayed by a-SI provides the means for localizing the current and raising the temperature.
I agree that the most possible choice of the non-linear element location is the contact between silver filament tip and the polysilicon, but not sure whether it is a Shottky or not.
Just for your information, a good theory has to be able to explain the observed metal differences.
Some examples of other metal/a-Si/c-Si switching behavior can be found in fig12 of patent publication #:US20110001117 A1 http://www.google.com/patents/US20110001117?cl=en
Metal dependence was also observed in previous work on metal/a-Si/metal switches (like summarized in this reference: Owen, A. E. et al. Int. J. Electron. 1992, 73, 897.) It was this paper, also your classical 1970 paper, that made me realize the origin of the switching could be the amorphous semiconductor and motivated me to try different metals, which led to the discovery of the Ag/a-Si/c-Si switch in Nanowires.
The work at Harvard was based on bottom up nanowires, while UMichigan work by my previous colleague was to apply this discovery to CMOS compatible planar structures, which wasn't of interest to my advisor at that time.
As to why other metals didn't work as well, the diffusivity difference could be one reason. HRTEM observation of the switching process might provide the ultimate experimental explanations of the metal difference. Which I believe Umich are still working on. But there might be some technical difficulty in observing Ag/a-Si/c-Si structures in TEM and the video they presented on Crossbar website may not be clear enough yet.
yjdong: Yes other fast diffusing elements or alloys may be possible and certainly should be tried (Cu or as you suggest an alloy, e.g. CuAg ?). For the paper I was preparing I was going to suggest three locations for the diode or non-lnear element. The first choice is the contact between the tip of the silver column and the poly silicon, forming a Shottky. If in that case the actual growth and removal of the bridge occurs at its centre away from both electrodes, it would mean the the diode and the contact between the silver and the column remain fixed once "formed" If a diode in that location must be formed and remade for each write/erase event that adds another variable to my concerns about the contact between the Ag electrode and the silver column or dendrites
If you are prepared to accept that the bridge leaves a small tunneling gap at its tip and then I suppose there is just the possibility that a diode could be formed between all the interfaces between the Ag and the polysilicon or at the polySi-to-aSi interface. I think the diagram the accompanies this article does not attempt to account from where the ions shown floating in the a-Si have come. If you contact me through EETimes your certainly welcome to my diagrams/figures. Is silver essential don't know.
PhyandEE: I was not trying to minimize the potential problems of mixing Ag and the silicon fab process but I think it should be possible to create an Ag based memory pore structure that keeps the silver isolated, in the same way as in the case of copper-silicon. However, I think the fact that silver is a fast diffuser in silicon raises a potential reliability problem at the memory cell level. The filament represents a silver concentration gradient in the a-Si so I would think there is a distinct possiblility that with the device in its on state the filament might just diffuse away in a radial direction with time-temperature, resulting in failure. If that is what you were trying to imply then I agree with you. When we see the reliability test results for representative devices (20nm) that should give us a good idea of the magnitude of the problem, "big" or otherwise.
Copper might be a different story. To solve this problem, for example, additional diffusion barriers are used.
But in their work, they seem to use silver as the conducting bridge (filament). The silver atoms diffuse through the amorphous silicon and form the filament for switching. The problem comes. Silver atoms can diffuse through the whole silicon layer. The silicon layer can not be perfect. With the electrical field applied, with the heat generated, and with the time, it is a big problem.
PHYandEE: I think you will find I made the same point in my comment to Peter Clarke that you will find in the last page of his piece above. I think the fact that silver is a fast diffuser in silicon and needs very little encouragement (electric field, current, heat) to move is the very reason the Crossbar device "works".
However, remember there was a time, for similar resons, when if you suggested that you wanted to use copper as a conductor on silicon people would have advised you it was not a wise step, they might have used stronger language, now its use is common.
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