The resistence-switching mechanism within Crossbar's memory is based on the formation of a filament by the movement of silver ions from the top electrode within amorphous silicon. Source: Crossbar Inc.
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.
I don't quite understand why they use Ag in a circuit. Silver is known for its serous migration effects under an electrical field. The heat by current makes this even worse. I don't know how they solve the reliablity issue using such a material.
Resistion-I went back and had a look at K-H Kim's thesis. It states "..movement of Ag ions in the a-Si matrix..." Suggesting a belief that those were ions moving towards the cathode, with a positive charge. Belief because there was not much evidence provided to prove the point; perhaps I missed it. It is not clear to me what elements provide for the oxidation-reduction prcess to form a ECM cell and provide the positive charged ions that you suggested need to be neutralized. Describe that for me with the elements available and you can have your positively charged Ag ion.
The thesis also states ".. Current can only flow through the device with a positve bias.." By that I think it is the current for programming to the low resistance state making the Ag electrode positive. The reason for that is because of the non-linear element that is somewhere in structure. Current does flow in the reverse direction for erase.
Resistion-Not so much a complication, for me more a process of elimination of all possibilities. I tried to look at all the possible ways in which an electrochemical cell might be formed from the elements available in the Crossbar memory structure.
Resistion- Hydrogen should be the only other element that is available in a-Si and would have the possiblity of being mobile. (Remember a-Si is a sub hydride of silicon). I am not sure if it possible to make a silver-hydrogen ECM cell where oxidation-reduction takes place in the OIL RIG* sense. I did look at the possibilty that the movement of Ag into the a-Si lattice would displace hydrogen into the void at the depleted surface of the Ag electrode. I would think that would most likely trigger the crystalization of the a-Si and that would not be reversible.
For an Ag-aSi-xtalSi Crossbar memory structure I also looked at the possibility that solid state epitaxy on the single crystal surface might account for the diode/non-linear element (or even be a way of creating one). In the the distant past I did use palladium as a catalyst for low temperature solid state epitaxy of a-Si on xtal crystal after switching for use as a PROM. Some of the details are in my US patent No 4,174,521.
Sorry *OIL-RIG Oxidation Is Loss-Reduction Is Gain, of electrons