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PCM progress report no. 3: New direction with polyamorphic states

Posted: 09 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:phase change memory? polyamorphous chalcogenide? Ohmic resistor?

Moving back to the SM characteristics, if the switched material of a PAC SM is constrained by sidewalls of the device and if the material in the conducting state has a positive temperature coefficient of resistance, the resulting dynamic characteristics would extrapolate to a negative voltage axis intercept, as illustrated in figure 4. Without the actual electrical conductivity as a function of temperature for the PAC materials used in SMs it is difficult to comment further.

Filament or bulk switching in SMs
The "S" shaped negative resistance I-V threshold switching characteristics are indicative of a the post threshold switching current having collapsed into a narrow filamentary path, a view with which Savransky [1] agrees [2] and suggests that in the ideal SM device the initial post-conducting filament would expand to include all the material contained in the necessary "pore" like structure.

In considering the operation of an SM, this will be of special importance if the material is initially deposited in the low threshold voltage state, as illustrated in figure 5.

Figure 5: Filamentary operation is impossible with PAC material in low Vth state from fab.

If in that state the SM device operates in a central filamentary region, then any attempt to switch the central region to the high threshold voltage state would leave the surrounding region in the low threshold voltage state, compromising the intended two terminal data state at the next read operation. No specific information was provided [1] on the threshold voltage characteristics of the PAC material in its virgin, as deposited, state. Savransky claims a threshold voltage range of a factor 3 is possible; if so, provided the as-deposited material is somewhere in that range the device will respond to a first attempt to write.

I raised the question of a first switching threshold voltage that might be different from subsequent switching events, the answer was that with modern processing techniques and clean contact interfaces there would be no "first firing" threshold voltage effect. At this time it is necessary to leave to others the verification of that claim and that completely stress-free chalcogenide glasses can be deposited. The actual state of the as deposited material is important because it raises the possibility that the first attempt to write an SM would always need to switch the device with a high threshold voltage pulse, just in case the as-deposited material had a threshold voltage with a value somewhere between the two intended threshold voltages and higher than the applied read voltage, a different type of first switching effect.

There are those who consider that the conducting filament is essentially at a constant temperature, for a fixed current. Its constant voltage characteristics are the result of a narrow annulus around the filament undergoing a small increase in temperature to become part of the filament and carry any increases in current. That is the lowest energy solution to accommodate increases in current when compared with the alternative of raising the temperature of the whole volume of the conducting filament. If this is the case, in order to raise the temperature of the filament to access the polyamorphic states, the filament will need to be constrained, or write pulses that exceed the filament response time (figure 4) will be required.

Another important aspect that will limit the possible maximum operating temperature of PAC-based SMs relates to the temperature coefficient of threshold voltage. Without need to resort to read design techniques that compensate for temperature changes, the operating temperature range cannot exceed the point where the high threshold voltage of a PAC-based SM equals the low threshold voltage. Specific quantitative information with respect to the way in which the threshold voltages of the two polymorphic states change with temperature and if they have the same temperature coefficient was not provided at this time by [1].

Thermal stability of polyamorphic states
If the transformation between polyamorphic states uses the post threshold switching power associated with crystallization in PCM devices, how stable will the devices be at elevated device operating temperatures? Claimed as the key to the possible success of NV memory devices based on PACs, is the energy of crystallization for a polyamorphic glass is high about 5 eV while that of the materials that are used for conventional phase change memory devices is low ~ 2.5eV. Furthermore the polyamorphic states have a potential energy of formation close to that of the amorphous-to-crystal transformations utilized in conventional PCMs. This suggests that the polyamorphic transformations occur at temperatures about 200 C.

Savransky used evidence from differential thermal analysis (DTA) to support the claim that it was possible to maintain PAC material at temperatures close to the PAC glass transition temperatures for extended periods of time without any evidence of crystallization. That is an absence of the exothermic peak in the DTA measurement normally associated with crystallization. In relation to memory device parameters what does that very condensed piece of solid state physics translate into? It is that access to the different polyamorphic states can be achieved with the level of power, current and temperature that is associated with the set (crystallization) process of the more conventional PCM and without the need of a reset pulse.

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