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CeRAM gains ARM's attention

Posted: 20 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:nonvolatile memory? semiconductor? correlated electron random access memory? CeRAM? metal-to-insulator transition?

In the insulating state at low applied voltages, the conduction mechanism is dominated by thermionic emission carriers crossing interface barriers. As the voltage is increased, the device behaves like a metal-insulator-metal diode, and tunnelling becomes the dominant method of conduction (the steep part of the I-V curve in figure 1). At about 1.6 V, the electron density reaches a level where the potential well of the nickel ion (which holds the localized electron) is so narrowed that it allows the electron to escape by tunnelling out. This kills the bound state responsible for the intra-site Coulombic repulsive electrostatic potential, thus returning the material to its original metallic state.

Simultaneous oxidation and reduction
Figure 6 provides a view of the valence orbits and the band structure of the nickel ions with a sequence from left to right of the transition between the as-born conducting state of the NiO, CeRAM, and its insulating state. Initially, the doping process has ensured that all nickel ions in the conducting material are in the Ni+2 or 3d8 state, with the valence and conducting bands overlapping. Injecting holes creates a situation where an electron can move from the conduction band of one 3d8 to double occupy the conduction band of a second 3d8 and create a 3d9. This results in the band split shown in the lower part of figure 6. The material becomes insulating. More simply, the 3d7 does not have any conducting electrons, because there is now a gap between the bottom of the conduction band and the lower bands where electrons are available.

At the core of these many body effects is a reversible disproportionation reaction that can be used to store data at temperatures as high as 400C, with the transitions possible over a temperature range from 4 K to 150C. Phase transitions are independent of temperature, and they might be considered strong evidence that quantum phase transition is involved.

Figure 6: View of the valence orbits and the band structure of the nickel ions.

The situation is complicated because the p-band of the oxygen in the NiO overlaps the conduction band of the nickel after the band split. Only a few ions are shown in figure 1, but the band splitting is occurring locally throughout the volume of material. The reversibility of this transition is the basis of its use as a memory.

This article has not been intended as critical analysis of CeRAM. Instead, the goal is to provide the highlights of the operation and structure on which CeRAM claims are based. Readers have to accept there is always a danger: Relying on particle electronics descriptions for a device that has its roots in quantum mechanical effects might lead to omissions and misunderstandings.

The formal publications from the University of Colorado team will provide the scientific detail, while third-party evaluation of CeRAM devices will establish the validity of the manufacturability and performance claims for the devices. One can speculate that the reason the bulk switching effects claimed as the basis of CeRAM operation have not been observed in other oxides where filamentary switching is observed (i.e., HfO and Ta2O5) rests with the difference between n-type and p-type conduction.

About the author
Ron Neale is an independent electrical/electronic manufacturing professional.

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