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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?

Figure 4 highlights the two regions of the thin-film CeRAM structure: the buffer layer and the central active layer. In those films, each nickel ion can be considered part of a small local switch that is on or off. The second expanded inset illustrates the reversible band splitting of the MIT that empowers the CeRAM. (The band structure is a localized effect and should not be confused with the band structure associated with single-crystal silicon, where the band structure linked to periodicity is fixed, and the density of states is not manipulated.)

Underpinning these changes is the split of the 3d8 band of the conducting state into the 3d7 and 3d9 bands of the insulating state (where 3 means the third atomic shell, and d8 refers to the d-orbital up to eight electrons).

Continuing with the simplified view, figure 5 is a conceptual view at the nickel-ion level of the difference between the insulating and conducting states. In the upper part of the figure, the material is conducting, and charge carriers can move freely in the material's local conduction band. When carriers become localized, the Coulomb repulsion inhibits electron flow, and the material acts as an insulator. The release of a localized electron during the set process brings the material back to the conducting state without the repulsive effect from the localized electrons.

The Ni(CO)4 doping technique, patented worldwide by Symetrix, creates a film of nickel oxide where all the nickel ions are in the single Ni+2 electronic state. Doping acts to clean up or stabilise the internal volume of the material while, as indicated earlier, the device structure design acts to clean up the surface. Without the benefit of Ni(CO)4 doping, free nickel, NiO traps, and oxygen vacancies would exist in the NiO, and the films would not be conducting. More importantly, the reversible MITs would not be possible. This means that the five 3d electron orbits of the nickel are in the 3d8 state.

Figure 4: Reversible band splitting.

Figure 5: A conceptual view at the nickel-ion level of the difference between the insulating and conducting states.

Ni(CO)4 doping
In the conducting state, all the positive ions are screened, and electrons have removed the effect of the potential of the positive charge, which, by definition, cannot exist in a metal. Aided by the Ni(CO)4, the screening is perfect. Even though there are many electrons to provide the screening, this does not mean the potential well (formed by the positively charged ion) has gone. For example, if the voltage applied to the device in its conducting state is increased, at about 0.6 V (in a region close to the anode and as a result of hole injection), the screening becomes less than perfect, and the carrier equilibrium is disturbed. The local band (previously for the 3d8 orbitals of all the nickel atoms) splits into two bands separated by an energy gap, where the upper band is now the Ni+1 band (corresponding to 3d9). This gap appears as a result of the drop in electron density as holes tend to reduce the number of screening electrons near the anode. The applied voltage is half that needed to inject electrons to screen the nickel cores. Thus, an electron entering the 3d9 orbital of the nickel ion becomes localized by interacting with the electron via a strong electrostatic potential, and it acts to trigger the transition to the insulating state. This transition propagates throughout the film as a quantum phase transition (see below), which is responsible for the sharp switching off of the conductive state.

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