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Phase change memory advances: Discontinuity, melting

Posted: 11 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:phase change memory? PCM? 3DXPoint? GST?

In a recent article [Ref 1], we brought up a couple of phase change memory (PCM) questions and left them unresolved. One was the degree to which localized melting might be involved in threshold switching. With a possible discontinuity in electrical conductivity on melting able to provide provide a more powerful feedback mechanism than would be the case with Joule heating and Poole-Frenkel conduction alone; the thermistor-like model proposed by IBM. The second was the question regarding the possible existence of a separate post-threshold switching conducting state, melting or otherwise, and prior to crystallisation for a PCM and by definition the only post-switching state in a threshold switch.

There has been a considerable amount of follow up interest on the thermal aspect of threshold switching. Especially with Intel confirming [Ref 2] that the basic element in 3DXPoint is a stack of four devices where it now appears threshold switching for each will make a significant contribution to the thermal budget. Figure 1 illustrates a PCM memory stack with the option of the threshold switch filament serving as heater electrode.

Figure 1: A possible double stack of PCM-threshold switch cells with the hot threshold switch filament serving as the heater electrode and some questions.

A more powerful feedback mechanism driving the switching transition would result in a faster transition, required for shorter read and write times; especially in those cases where the memory cell consists of a threshold switch stacked above a memory cell. Melting would only be of an additional value to the thermal hysteresis effect alone as described in [Ref 1], if there was a discontinuity in conductivity towards a higher value at the melting point. The possible downside with melting and current localisation in a hotspot is the possibility that even at low device currents damaging levels of current density and electro-migration could be experienced.

Although there remains some disagreement as to exactly when melting occurs during the memory SET operation, figure 2 brings together in one set of I-V characteristics what might be considered a 2016 enlightened view of threshold switching. This for illustrative purposes is an example threshold switching for a pulse with insufficient time for any significant crystallisation to occur in the case of (PCM) memory device.

Four types of characteristics are shown: the first (yellow) is what must now be called the LeGallo [Ref 1] hysteresis a purely thermal threshold switching effect; the second (blue) when there are no reactive components and after reaching the threshold voltage the I-V characteristics rapidly follow a transition path determined by the series resistor and the third (light blue dashed) is when stray capacitance is present and a current i = CdV/dt is determined by the value of any reactive component. The fourth (purple) is a direct tunnelling current. If the IBM results are generally applicable to all threshold switching, three of these characteristics will have one thing in common it is what I have designated as the thermal trigger point at the threshold voltage. Vts and Vtl are the threshold voltages for pulses of short and longer duration respectively.

Quantitative values will be structure and material composition dependant. In [Ref 3] it was suggested that at some voltage either by thermal means or tunnelling from the traps all carriers will have contributed to the conductivity. Beyond that point (marked with a blue dot in figure 2) and assuming the pulses were of short enough duration direct tunnelling might be observed, the purple curve.

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