Assessing correlated electron memory claims by 4DS
For this latest presentation, data for write/erase cycle lifetimes of 106 cycles were shown with claims that 109 cycles are possible, supported by impressive read stability data, all without any indication of the number of devices evaluated or the important statistical distribution of those parameters across an array or population. The elevated temperature retention is cited as greater than 10 years at 85OC, supported with what was described as thermal relaxation data up to 130 OC, again without the number of device tested. Typical device programming voltages ranged from +4Volts for low to high and -5Volts, for high to low reset and set resistances respectively with on-to-off resistance ratios of about 100 over a cell size range of ~20 to 400nm. No details of the 16k-bit array interface were provided nor any detail of the memory matrix isolation devices. It is possible that the asymmetric I-V characteristics of the individual memory devices are serving in that latter role.
Figure 1: Desirable features of NV memory.
4DS Inc. now claims that the operation of its memory is based on strongly correlated electron-effect Mott-like transition, and, in so doing, they become the second company with a memory device that lays claim to correlated electron effects, following the CeRAM of Symetrix. The 4DS Inc. trade mark for the memory device is MOHJO, the active material is an alloy of PrCaMnO3, with the acronym PCMO, for praseodymium calcium metal oxide memory. Or, more simply, it is PMO doped with calcium, and it belongs to the family of perovskite mangantite materials with composition R(1-x)AxMnO3, where R is a rare earth metal and A is a divalent element.
If, as is claimed, the PCMO memory is a true interface device, it rates highly on my NV memory desirable features list (figure 1), a list which for the moment ignores any undesirable features such as the need for forming and bidirectional operation.
One very positive aspect of any interface device is it removes as a first order effect the control of film thickness from the fabrication process and any associated variables from the device characteristics.
The PCMO structure and operation
The PCMO memory device structure is shown in figure 2. The lower electrode is platinum (Pt), a metal that does not react with the PCMO alloy. The top electrode is titanium overlaid with a suitable good electrical conductor, such as gold. While Ti and the formation of α-TiO is apparently the preferred implementation, 4DS has evaluated many other contact metals.
Figure 2: The PCMO memory device structure.
Many companies and organisations have explored the resistance switching properties of various compositions of PrCaMnO3 materials. Perhaps the best starting point for understanding the 4DS memory (and the evolution of its claimed mechanisms of operation) is an investigation  of the switching effect in Ti/Pr(1-x) Cax MnO3 structures; especially as a significant part of this work was included by 4DS in their most recent presentation. That earlier investigation uncovered and confirmed a number of important features. For instance, as a result of switching (with titanium in contact with the PCMO material), a layer of amorphous titanium oxide (α-TiO) is formed at the interface by what was reported as the electrochemical migration of oxygen ions into the original (Ti) electrode.
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