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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?

To obtain the filaments shown in Figure 1 requires an initial forming step, the ReRAMs can be uni-polar operated in the first quadrant of the I-V characteristics or bidirectional in the first and third quadrant.

During forming the initial dislocations on the column surfaces act as nuclei, as illustrated in figure 2(a), and by the movement of oxygen form into chains of vacancies that increase in length with applied field until at a critical point conducting percolation pathways are formed along the column boundaries. As illustrated in figure 2(b) current transport in devices in the ON state is by trap assisted tunnelling and in any of the possible intermediate resistance states before the full OFF high resistance state, supporting the case for discontinuities in the conducting path. At high fields some evidence of Fowler-Nordheim conduction was observed. Figure 2 (c) is a top down illustration of the CAFM view of enhanced conductance at the edges of the columns, with many actual examples in [Ref 2].

Probing the filament and 3D views
The technique used by the UCL team to probe the ReRAM filament and to produce the tomographs of figure 1 uses the probe of the conductance atomic force microscope (CAFM) in a number of roles. Initially, after the upper electrode has been removed, it is used to locate the filament, then for the progressive removal of thin horizontal slices to produce and to provide a conductance map of each new surface as it is exposed, illustrated in figure 3.

Figure 3: (a) Building the 3D filament image (a) the CAFM used initially to locate the filament then in contact (b), (c) and (d) used to scrape away surface layers for conductance measurement.

The conductance results are then electronically reassembled to produce the image as shown in Figure 1, where shown alongside are some examples of some of the conductance planes.

It is claimed that these are the first for three-dimensional filament images of uni-polar sub-oxide ReRAMs, although in the past [Ref 4] imaging of conductive filament involving metallic ions (CbRAMs) have been observed.

While the devices for much of the earlier UCL ReRAM work utilized single crystal and poly-crystal silicon as the electrodes this latest work used ReRAMs with TiN for both electrodes and employed unipolar operation.

In both examples in figure 1 the column like sub-structure is visible as the building blocks of the larger filament. A second feature is initially a number of filaments start to grow during the forming process with one becoming dominant as it approaches the upper electrode and the others are most likely robbed of current.

Emulating brain functions
For brain-like functions the UCL team made their target the use of the SiOx-based ReRAM for both the synapse and the neuronal electro-physiological conductance/voltage response. A step which would in the long term make neural networks much simpler than hybrid analogue/digital CMOS silicon neurons that have characterised earlier attempts. The UCL group have used their SiOx ReRAMs to demonstrated this capability for operation in the non-volatile memory and volatile threshold switching modes

Figure 4: (a) the leaky integrate and fire schematic, (b) the characteristics of the ReRAM with the appearance of spikes with DC current applied with a value close to the compliance level, (c) pulses applied, (d) the resistance of the ReRAM with spike as a function of pulse repetition rate.

The technique capitalizes on what in some quarters would be described as unwanted noise or the spikes that occur when SiOx ReRAMs are switched with currents and voltages close to minimum values rather than hard switching between the two resistance states.

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