• Branicio, Paulo S.
• Sullivan, Michael B.
• Ramanarayan, H.
• Bai, Kewu
• Srolovitz, David
• Wu, David T.

Abstract

The complete process of amorphization and crystallization of the phase-change material Ge<sub style="background-color: rgb(255, 255, 238);">2</sub>Sb<sub style="background-color: rgb(255, 255, 238);">2</sub>Te<sub style="background-color: rgb(255, 255, 238);">5</sub> is investigated using nanosecond <i>ab initio </i>molecular dynamics simulations. Varying the quench rate during the amorphization phase of the cycle results in the generation of a variety of structures from entirely crystallized (-0.45 K/ps) to entirely amorphized (-16 K/ps). The 1.5-ns annealing simulations indicate that the crystallization process depends strongly on both the annealing temperature and the initial amorphous structure. The presence of crystal precursors (square rings) in the amorphous matrix enhances nucleation/crystallization kinetics. The simulation data are used to construct a combined continuous-cooling-transformation (CCT) and temperature-time-transformation (TTT) diagram. The<i> nose</i> of the CCT-TTT diagram corresponds to the minimum time for the onset of homogenous crystallization and is located at 600 K and 70 ps. That corresponds to a critical cooling rate for amorphization of -4.5 K/ps. The results, in excellent agreement with experimental observations, suggest that a strategy that utilizes multiple quench rates and annealing temperatures may be used to effectively optimize the reversible switching speed and enable fast and energy-efficient phase-change memories.

Topics

• impedance spectroscopy
• amorphous
• phase
• simulation
• molecular dynamics
• annealing
• crystallization