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Probing ReRAMs: 3D filaments, brain-like functions

Posted: 16 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:ReRAMs? memory? filament? CMOS?

ReRAMs based on the sub-oxides of silicon are attractive because SiO2 is already present in the fabrication process of the solid state circuits with which the ReRAMs will almost inevitably need to be integrated. More so if the two or more memory states can be achieved without needing to introduce and use the movement of foreign materials like silver and copper to build and remove the filaments.

ReRAMs based only on the sub-oxides of silicon have been the focus and are part of the ongoing work at University College London (UCL). Building on the foundation of earlier work, in their latest paper [Ref 1] UCL Research Associate Adan Mehonic now makes the case for the suitability of their devices for use as emulators of brain-like neural functions. He with UCL Prof. Tony Kenyon have linked an interesting feature of ReRAM conduction at switching to conductance-based models of the neural membrane and the so-called leaky integrate-and-fire Hodgkin-Huxley models. Where in the brain a train of closely-spaced current pulses builds up a potential across the neural membrane until, at a specified threshold the neuron generates a voltage transient.

Along the way [Ref 2] the UCL team have provided the memory community with what are claimed to be some of the first 3D views of a silicon unipolar sub-oxide ReRAM filaments shown in Figure 1.

These unique views of unipolar SiOx filaments are one of the essential steps required if this type of ReRAM is ever to be able to operate reliably emulating neurons or in any memory applications.

In the UCL ReRAM project the first step was to try and establish the true nature of the electrical conduction processes in sub-oxide SiO sputtered films. In a joint EU effort with colleagues from France and Spain,[Ref 3] the team linked the operation of the ReRAMs to the naturally-forming column structure in sputtered sub-oxide films, which provided a matrix of discontinuities. As the sub-oxide is an amorphous film without structure the edges of the columns are formed from structural dislocations.

Figure 1: The first three-dimensional views of uni-polar SiOx ReRAM filaments with alongside (b) to (e) a sample set of conductance slices used to reconstruct the filament.

Electrical and conductance atomic force microscope analysis combined with the ability of the sub-oxides of silicon to easily precipitate silicon inclusions gave rise to an initial hypothesis based on oxygen vacancy electro-migration. A reasonable interpretation of those results suggested that inclusions of silicon decorating the sides of the columns with at least one of the insulating regions between the silicon inclusions responsible for the ability of the ReRAM to provide the changes in resistance required for brain emulation or more conventional memory applications.

From ongoing work this earlier view of the conduction and switching process has been modified, Professor of Nanoelectronic & Nanophotonic Materials Tony Kenyon in the Faculty of Engineering Sciences at UCL provided EE Times with this very latest update:

"We believe conduction to be by a chain of oxygen vacancies bridging the gap between the electrodes. These aren't really silicon nanoinclusions in the sense of nanometre-scale silicon nano-clusters, but are extended chains of defects, which are generated by the movement of oxygen within the switching layer under the influence of an external electric field. They form an electronic sub-band in the oxide, which may not be continuous, hence the appearance of trap assisted tunnelling transport. It is a subtle point, but I think it's important to be clear on this, because we have no specific evidence for silicon nano-clusters in our electron microscopy results."

Figure 2: (a) The column structure of the deposited SiOx film,(b) illustration of the formed filament and the conduction processes, (c) illustration of increased conductance at column boundaries.

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