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ReRAM Startup Bets on Silver

8/5/2013 11:02 AM EDT
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resistion
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Re: CeRAM
resistion   8/8/2013 2:00:54 AM
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Very interesting papers in Issue 109 of J. Appl. Phys (2011). So the NiO in insulating state has 4.3 eV bandgap?

nonvolatile
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Re: CeRAM
nonvolatile   8/8/2013 8:55:56 AM
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There is still a lot of controversy on the bandgap of NiO. But, this is the latest number. This is a photemission band gap which may represent the the lowest level 3d7 to 3d9. Not to be confused with a Band Insulator bandgap. If you look in the literature there are 4 types of insulators: Band Insulators (like SiO2 -wide band gap), Anderson Insulators (Heavily defective materials kills the mobility), Mott insulators (Extended states, like the Bands in a wide band device do not exist, only the coulombic interaction (Hubbard U) forms a gap) and the Charge Transfer type of Mott Insulators ( Same as Mott insulators in principle, but the energy gap is formed from the diference of a lower 2p state from the Oxygen and the upper 3d9 state - this gap is called Delta).

The controversy for NiO was resolved in the  90's when it was discovered by photoemission spectroscopy that it was a Charge Transfer type - this means that Delat is less than U. So for a true Mott insulator U>Delta. It is a fine detail that I mentioned before but did not elaborate. The density of states (DOS) shows a gap for delta around 1.2eV if there are not too many vacancies of oxygen. Thus, the dispersion in Vset and Vreset that is common in these NiO memories is because this Delta is localized and not a Typical bendgap of a band insulator or a semiconductor. 

It is very common to confuse the Band theory for independnet electrons (Semiconductors, band insulators) with the Localized "narrow bands" of an insulator with interacting electrons. This has been a very difficult point to explain this device. The correction of the oxidation number at every Ni site is done by fixing the coordination sphere to the required oxidation number that can be manipulated by the applied voltage to set or reset. This is not substitutional elment doping (called hydrogenic states) as it is done in Semiconductors, where the dopant takes a place in the lattice and loses its electron to the conduction band, This is very different, for a TMO usually is not a band insulator. So, the electron is localized and the bound state (forming delta) can be destroyed at Vset via the change in the background electron density increased by the injection from the cathode. It is a dynamic process that changes the NiO from an insulator to a Metal-like material. Undoing this by temperature is not easy -that is why 400 C retention is achieved. 

In summary: TMOs are not semiconductors, they switch due to the electron-electron part of the hamiltonian of a solid - that is set to zero for semiconductor because due to pn=ni(2) makes typically a free electron in the conduction band to occur at about every 10,000 atomic site. Thus, the "independent Electron Approximation" works. If your education is only semiconductors, you do not know and cannot appretiate the physics of electron-electron interactions. An example of e-e interaction that you may already know are the superconductors (BCS type) in which a Phonon and 2 electrons interact to form the superconducting state. For hi Tc, the Hubbard Hamiltonian is more in practice to model. In TMO case, the Hubbard Hamiltonian is a classic, and the kinetic energy to Potential energy ratio controls the metal to insulator transition.

My view is that semiconductor only people did not get educated in these fine parts of solid state - that is why to this date few understand the nuances of ferroelectric and now TMO. Brute force filament creation is not the way to go in ReRAM, however for more than 15 years that is all that is done. With this new "Coordination doping" strategy, a TMO is manageable without frying it first with electroforming.

---good tricky question CEO, but we've been there and done that. Careful science is a pre-requisite to real breaktrhoughs. And, data talks - so if we have no electroforming by materials design, we are so naive to confuse the band insulators with the other types.

Thanks for the challenge.

nonvolatile
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Re: CeRAM
nonvolatile   8/8/2013 8:58:47 AM
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Sorry for the typos- key correction:

Mott Insulator: U<delta

Charge transfer: Delta <U

I had it backwards in the last post.

resistion
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Re: CBRAM?
resistion   9/29/2013 6:33:21 AM
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Ron, just came across this today, thought back to this discussion. Although it seems the filament growing forward into Si seems different from the deposition "backward" toward silver, it looks like he (founder of Crossbar) is still classifying the Crossbar memory along with the other CBRAMs (as ECM). It is possible the electrochemical reduction is occuring in the silicon instead of a Pt electrode.

http://sites.ieee.org/sfbanano/files/2013/06/Lu_NVMT_2013.pdf

Ron Neale
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Re: CBRAM? ECM or EM good question
Ron Neale   9/29/2013 7:35:20 AM
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Resition-If you mean there is some form of interface layer at the Pt electrode that is responsible for the switching and the mecahnism is oxidation-reduction, then I think it is necessary to show the chemical/electro-chemical reaction involved. 

ECM or not, for the moment because Ag does not need much encouragement to move in silicon, amorphous or xtal, then for me heat and electromigration/electric field effects suffice. My worry with the Crossbar device is to create the filament, material must be moved from the silver electrode leaving what must be an Ag depleted spongy like contact region of nano filaments. Does that material return to the same place when the polarity is reversed, my view is that the material forms mutiple short filaments around the region from where the silver was originally removed. That is along the lowest resistance paths.

The nice thing about having an interface layer close to the passive electrode that switches and is responsible for the memory effect is the suspect contact at the Ag electrode depleted region once formed does not get modified.  However, I think memory devices that require a forming step different from normal operation do not have much of a future. from my experience with antifuses that used amorphous silicon threshold switching occurred on programming, for the Crossbar device will the first switching event in the virgin a-Si be the same a subsequent ones. 

I do have a number of slides that we prepared for a possible article that illustrate what I see as the problem for Crossbar, if you contact me I will send them to you. Also if you want what I think might be an example oxidation and reduction at a single site then perhaps CeRAM might be a better example-more on that later.

 

 

resistion
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Re: CBRAM? ECM or EM good question
resistion   9/29/2013 9:34:19 AM
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Thanks I would be interested. Earlier I meant the Ag+ ions might be neutralized in silicon instead of at the electrode. But the other effects you mentioned also could play a role.

Ron Neale
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Re: CBRAM? ECM or EM good question
Ron Neale   9/29/2013 2:03:30 PM
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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

resistion
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Re: CBRAM? ECM or EM good question
resistion   9/29/2013 9:00:24 PM
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I see, so the presence of mobile hydrogen could be an electrochemical complication.

Ron Neale
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Re: CBRAM? ECM or EM good question
Ron Neale   9/30/2013 9:13:43 AM
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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
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Re: CBRAM? ECM or EM good question
resistion   9/30/2013 1:00:58 PM
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So it's neutral Ag that's moving?

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