Robust Simulations of Nanoscale Phase Change Memory: Dynamics and Retention

A robust simulation framework was developed for nanoscale phase change memory (PCM) cells. Starting from the reaction rate theory, the dynamic nucleation was simulated to capture the evolution of the cluster population. To accommodate the non-uniform critical sizes of nuclei due to the non-isothermal conditions during PCM cell programming, an improved crystallization model was proposed that goes beyond the classical nucleation and growth model. With the above, the incubation period in which the cluster distributions reached their equilibrium was captured beyond the capability of simulations with a steady-state nucleation rate. The implications of the developed simulation method are discussed regarding PCM fast SET programming and retention. This work provides the possibility for further improvement of PCM and integration with CMOS technology.

Occurrence of Dynamic Alpha-Phase Nucleation in Ti-5553 during Hot Deformation

This paper discusses the microstructural changes during hot deformation, mainly dynamic nucleation and growth of a precipitates, occurring in the b metastable titanium alloy Ti-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe). The effect of process variables on flow response and microstructure evolution during hot working of the alloy with initial bimodal microstructure was established using isothermal hot-compression tests. Testing was conducted at 4 strain rates between 0.001 and 1 s−1and 5 temperatures between 720 °C and 850 °C, on material with prior b grain size of 450 μm, a nodular size of 3 µm and a known primary a-phase fraction. All flow curves exhibited a peak stress followed by moderate flow softening in the two-phase domain. Flow softening was interpreted in terms of deformation heating and substructure or texture evolutions. The dependence on strain rate and temperature of the kinetics of dynamic a-phase nucleation during straining is complex and appears to be of second-order importance compared to the effects of strain. This suggests that the nucleation and growth of a phase in the temperature range between 720 °C and 990 °C results from a mixed-mode displacive-diffusional transformation, similar to the austenite / ferrite transformation above the Ae3 temperature reported by some authors.