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(a) (top) Crystallization times and (b) to (e) crystal structures (each 80 nm × 80 nm) for Ge2Sb2Te5 as a function of both temperature and electric field. The positions on the map in (a) to which the structures (b) to (e) relate are marked by the letters B, C, D, and E, respectively.
Schematic of the PCM “mushroom” cell used for the device simulations.
Simulated I–V curves for the PCM device of Fig. 2. Curve A in (a) is the I–V curve obtained by method I (i.e., amorphous phase conductivity is electric field dependent but the electric field does not contribute to the free energy); note that threshold switching occurs at just over 1 V, in line with published experimental results (e.g., see Refs. 5 and 6). Also shown in (a) is case B for which the field dependent conductivity has been removed from the simulation and no switching is evident even for high applied voltages. Case C in (b) is the I–V curve obtained by method III (i.e., the amorphous phase conductivity is not field dependent, but the field does contribute to the free energy); threshold switching is again evident but requires a significantly increased voltage (>2.5 V); indeed, smaller voltages than this produce no evidence of switching (see curve D). Also shown (curve E) for completeness is the I–V curve simulated using method II (i.e., field dependent conductivity and field term in free energy).
Simulated device temperatures and electric fields ((a) and (b)) during the SET pulse at the time of the maximum field (i.e., at 22 ns for (a) and 26 ns for (b)) and crystallization structure ((c) and (d)) at the end of the SET pulse (the white contour shows the spatial extent of the starting amorphous dome and different colors correspond to different crystallite orientations). Figures (a) and (c) were obtained using method I (i.e., switching is electronically driven as described in Ref. 6), whereas (b) and (d) were obtained using method III (i.e., switching is driven by field-induced nucleation). The SET pulse was rectangular and of 60 ns duration in all cases, but the pulse amplitude in (b) and (d) was higher than in (a) and (c) (2.6 V and 2 V, respectively). Movies of the switching/crystallization process with and without the electric field energy included in the Gibbs free energy (enhanced online). [URL: http://dx.doi.org/10.1063/1.4729551.1] [URL: http://dx.doi.org/10.1063/1.4729551.2]10.1063/1.4729551.110.1063/1.4729551.2
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