I would like to first answer the question on Temperature dependence of the Mott transition.
Yes, in VOx that is the case - but the true definition of a Mott transition is quite complex and is still an area of major study in condensed matter physics. That being said, it is better to consider this without the specific lable "Mott Transition" to the more general label of "Metal Insulator Transition (MIT)" and the reverse IMT. Now, the well known formula of the relationship between electron density and lattice length -a - is 1/n = (4/3)(pi)r^3, in this sense (in a very rough manner), we can see that r is the radius of the ion-core potential. So, as you can see, if r=bohr radius we have no bound state and the material is a metal. If r=screening length which is related to 1/n and the density of states, a reduction of electron density is just enough for the screening length capture an electron and form a bound state. Subsequent to that, we then have electrode polarization that injects electrons in the cathode side at Vset, and when at a lower voltage it starts the on mode (metal side), it injects holes and attract electrons in the anode side . As the quantum phase transition is of the order of femtoseconds, we have the ability to shut off electron flow because (1) the barrier for electrons in the cathode is high now, as Vapplied is smaller, and (2) The electron deficit near the electrodes trigger the bound state to be create faster than the drift velocity of the electrons that may still jump the cathode barrier. This lack of thermal equilibrium is the reason why the insulating state now comes into play and the device is off. Stability of Vset and Vreset, together with Low on current are key for device reliability, Also, it is necessary to really model this device using the tools of many body physics, where the electron-electron interaction is dominant. Such electron localization is not used in semiconductors and common materials because the bloch electron does not interact with other electrons when it is in the conduction band. So, textbooks in semiconductors and simplistic filament or not brute force devices will never be reliable, as we have a physical phenomena embedded in a sea of defects. To summarize, This is a complex device - it is controlled by a baseline MIM diode with a quantum phase transition that is electron density driven. Without concepts from many body theory, such as Self-energy etc. it is a real pain to explain the phenomenon and "sell" the idea.So Data talks and the rest walks - and for this reason we kept quite.
My company has its own financial resources and it is privately owned. Now, we may have to open for capital infusion but we may not have to.
We do not follow the capital raising path and we are 27 years old with a portfolio of over 200 patents. We sold many licenses, and the royalty stream is enough to continue innovation. We are responsible to about 1.3 Billion devices in the market in many areas. Eliminating the influences of chasing money, we believe that true innovation must be fundamental and game changers. So, we took good care of this discovery and we are working in materials beside NiO already for magnetic oxides and light switches, This is truly a magnificant chance to innovate electronic devices - there is no nonvolatile memory without some form of hysteresis. Even flash has a threshold voltage hysteresis due to floating gate charge trap and reflection of that to the channel. So, since all electrodes from 65 nm down are nickel silicide, the low temperature NiO is key, and the elimination of Pt as an electrode is just right, Imagine, we have NiO with Al electrodes, talk about a post process memory for the embedded guys. So we have designed a 64M device for the 22 nm node and we are doing TEGs to get full design parameters etc.
We are the guys that many years ago made front page with nonfaigue FeRAM. By using SBT instead of PZT, we have the world's most used FeRAMs coming out of Panasonic. Yes, it had an embedded microcontroller in every SUICA and it is in playstations, drivers licences and a myriad of devices. It works at 1.1 Volts in the 0.18 micron node and can be made as this as 25 nm for future nodes. Unfortunately, since 65 nm and below, annealling cannot go over 400 C, ferro is not going to large scale. So, we quietly switched to do ReRAM, but came out as CeRAM, so that we work in the realistic world based on solid physics.
My Company is Symetrix Corporation - Colorado Springs.
I recieved in 2006 the IEEE Daniel Noble award for FeRAMs and I am an IEEE fellow, so now you can discount exuberant enthusiasm as found in many memory house start ups.
You are correct, but such a preliminary analysis you should understand that we have already done. This was 6 years ago.
The key issue is that area dependence is almost zero in the conductive sice, but the non-conductive side, is really an MIM diode-like device. Such a device would have area dependence whether you have filaments or not. This area dependence of the high resistive side is beneficial because in fact it enhances the read noise margin. The problem is that in filamentary devices, the random connect/disconnect does not afford good and systematic stability of what the diode induced area dependence is. Think of it as a "soft broken " insulator.
Now, your suggestion about a thermal effect creating filaments and we just missed them has been deeply studed, specially in the case that the active region is in the metal and of only 5-20 nm. The doped NiO of the outside buffer layers are always conducting as the pi bond of the CO:Ni is a strong electron donor. So, there is a direct short from electrode to the active region. Within that region the Ions have just enough electrons to switch on or off the coulombic interaction U. So, there is no possibility that a large highly conductive region connecting the electrode to the active region to form filaments. And, a thermal filament formation would show variations as the 20 nm layer would be stuck in an always on connected filament. Very high Resolution TEM and EELS show the right mechanism, but if you missed the point of the doping and the octahedral crystal field control of the oxidation number, I can see that it is easy to look for other explanations.
Perhaps you could take a look at a wikipedia article on Mott Insulators and see that an enormous effort in the last 60+ years, found Metal to Insulator transitions in TMO without ever mentioning filaments.
Finally, let me add that one of the most difficult problems with ReRAMs is the high on current - filaments in the majority of the case are metallic and follow the patch surrounding grain boundaries - so, how to lower the on current. In our case, the CO doping and element doping are independent mechanisms, so we can dope for coordination correction with CO and for scattering increase to lower the on current 5 orders of magnitude. If we had filaments, such a control of the on current would be impossible.
Interesting claims that seem to be based on the Mott transition in such materials as nickel oxide and vanadium oxide.
We do wish you will tell us more about what you have done, for what company and how you have been funded.
Is it not the case that there is a temperature associated with Mott transitions which can be in the room temperature range? Is it necessary to somehow remove that temperature dependence to avoid reseting of memory?
I am intrigued. To prove your memory is non-filamentary, it must show resistance that has weaker than inverse proportion to area dependence. In addition, although not commonly shown, the burden of electroforming may be significantly relieved thermally. I.e. the formation of defect-based RRAM may proceed with a similar process to CeRAM. For different reasons, of course.
Manager - I just noticed your question about the diode.
We consider using the diode only in 3D, not in the planar mode - although there is nothing impeding that. The diode for us is aspecial layer within our doping scheme, but not inside the two electrode areas of the device. It is under the bottom electrode. The window from -V to +V is about 1.4 volts, clearly allowing the -V/2 to +V/2 array pulsing without cross talk for the array-only large scale memory. As we write at 0.6,1.2 and read around 0.1-0.2 volts, this is not a problem, as both of these voltages divided by 2 are within the non-disturb window. Our device would look like the 3D version of Samsung, in which they use the diode made of Vanadium Oxide as a varistor. But, we do not have anything foreign to add to our process like that. Besides NiO, all materials are already in the fab.
Manager - I clearly understand that these are bold claims. And for that reason we only published 3 papers on this as we were awaiting for patents to issue. Thank you for your references on TMO - we have a library with about 7000 articles within our group.
I will try to answer your questions and clarifications, but if they do not satisfy you, then, write me again.
(1) TMOs are materials with incomplete 3d energy levels. In NiO, the NaCl lattice in fact has a unit cell that is Octahedral of the MO6 coordination. That is 1 Nickel anion and six oxygen cations, creating the NiO complex. Since 1937 that it is well known that NiO and transition metals oxides in general do not follow the Bloch-electron/Wilson's band theory. The reason is that in the case of Ni, the poster child of this observation, the next shel, 4s, in this case actually has no effect in bonding and leaves the 3d8 orbitals to interact with the oxygen 2p states. What then happens is a nature driven "disproportionation" a big word that means that the material goes simultaneously through an oxidation-reduction reaction in a single site. In the case of NiO, this is the reason why typically the native state of NiO is an insulator, when in fact, according to band theory, it should be a metal. Sir Neville Mott usesd NiO as the example of a electron density driven phase transition. The simple way of showing this is to show this reactio (the disproportionation): 2Ni(+2)O)-2)---Ni(+3)O + Ni(+1)O. In simple terms, the native "NiO" is the equation's right side, which splits the 3d8 state into two separate states "3d7 and 3d8" - a gap caused by this splits is called a Hubbard U gap (technically and specific to NiO this is not exactly the key gap, but for this discussion it surfices). For many years since 1957, studies of all TMOs for metal-to-insulator phase transitions, including a zillion theoretical paers entered the literature. Now, what is important here is that the Ni species that I showed above are in the lattice, and not a bunch of different oxidation numbers that do not participate in the disproportionation. Let me explain, Take any reference on NiO ReRAM, (or other TMO), and you will see the XRD spectra with Ni(0) and Ni(+4). These become "dead" clusters that sit there and together with oxigen vacancies make the mess that kilss reliability. When vacancies move, atoms also move etc. This is the idiotic TiOx "memristor" which is truly the parasitic device in all ReRAMs. Carefull look at "Coordination Chemistry" will show that oxigen is not the only Ligand of a Transition metal. Many others occur, and the most common one and easily incorporated in the sources (proprietary part-can't say of what deposition equipment) is Ni(CO)4 which at 60 C makes Ni5(CO)10 stable complexes. Loking at this the "CO" doping is novel and we patent it in every important country - including china. All patents now issued so we are starting to transfer this to industry with example devices. So, the trick in all TMOs is to control the "coordination sphere around the metal anion" to force the oxidation number to stay ameanable to the phase transition from metal to insulator and vice versa. This is the "Novel" doping technique.
(2) All complex oxides need to be annealed - this is not to be confused with "forming, such as Electroforming", but it is not wrong to say "forming" in the sense that annealing forms the proper crystalline phase. The annealing temperature for us is between 390-450" - typically 400. A key point is that the device is 60 nm or less in thickness, but done in 3 layers of different conductivity(doping levels) such that the possible schottky barriers at the contacts are screened and space charge is reduced. Then, the active area in the middle is 20 nm thick or less.
(3) The "electroformed ReRAMs" out there, start with a sputtered and then partially oxidized TNO. They use Platinum because the connect/disconnect of a filament uses the myriad of oxidation numbers within the space chrge region. Because the definition of a "Mott insulator" is that each ion has an independent transiton (In the language of the trade it means "The self-energy is position independent"), eventually a sporadic reaction occurs in the highly deffective surface that closes or open the contact to the filament - this also happens near grain boundaries internal to the device. Now, the off state of such a device is very sensitive to temperature and dwell time of the write pulse. If tested by pulsing and then raise the temperature and repeat, all these ReRAMs will show an activation enerfy of around 0.43 eV (NiO) which is exactly electron detrapping. Thus, ReRAM with filaments are really a charge trap device. We call our device CeRAM - Correlated Electron RAM, to separate ourselves from the filament crowd. The electron-electron repulsion inside 3d is found in the right side of the chemical equation and the metal-like in the left side. Papers using pressure to achieve the Metal/insulator transition are all over the literature. But, pressure corms the metal side, due to wavefunction overlap, and releasing it forms the insulating phase. The same can be accomplished by mesoscale devices and electron injection - too much detail to explain here. In any case, devices that are "ligand Compensated" work beautifully right out of the oven - No electroforminf, no filaments.
(4) Your reference to IBM is partially correct. In their case, they use the field effect to do what is called "electrostatic doping" But, this FET is full of problems and I do not want to bore you.
I am not in a position to give our company name at this time, but you can use my personal email - email@example.com
Jaybus0-When trying to locate (guess) where the built-in diode is I looked at the case of bridging filaments. In that case I think you come to the conclusion that the diode would have to be formed at the touching junction of the tip of the silver filament and the polycrystalline electrode and that would be a Shottky barrier diode. However if you assume the bridge is not complete then you have a couple of options either again a Shottky along the surface of the silver electrode and the tip of the filament, or a conventional pn aSi-polysilicon diode at the crystal electrode interface. Or if a doped amorphous silicon involved in a two part a-Si structure that is another possible location formed by an amorphous pn diode. We will see.
Blog That A-Ha Moment Larry Desjardin 3 comments Have you ever had an a-ha moment? Sure, you have. The Merriam-Webster dictionary defines it as "a moment of sudden realization, inspiration, insight, recognition, or ...