Ab initio simulations of crystallization of phase-change materials
ABSTRACT :
Phase-change materials (PCMs) are capable of switching very rapidly and reversibly between the amorphous and crystalline state at high temperature. Yet, the two phases are stable at room temperature and exhibit pronounced optical and electrical contrast. The ability of the glass to crystallize quickly at high temperature and yet to be extremely stable at low temperature has been linked to the pronounced fragility of the supercooled liquid phase of PCMs. These unique properties are exploited in rewritable optical devices (Blu-Ray Disc) and electronic non-volatile random access memories (PC-RAM), where heating is induced by laser irradiation and the Joule effect, respectively. Remarkably, the time and length scales relevant to state-of-the-art phase-change memory cells are almost within reach of first-principles methods based on density functional theory. Here we present ab initio molecular dynamics simulations of crystallization of two prototypical PCMs, namely Ge2Sb2Te5 and Ag,In-doped Sb2Te, as well as a recently discovered ultrafast PCM, Sc0.2Sb2Te3. It is shown that high-temperatures simulations of Ge2Sb2Te5 and Ag,In-doped Sb2Te yield crystal growth velocities in good agreement with experimental data. They indicate that fast crystal growth stems from the large diffusion constants and sticking coefficients and the presence of a sharp crystalline-liquid interface. It is also discussed how metadynamics, an enhanced-sampling method capable of accelerating the occurrence of rare events, can be effectively used in combination with ab initio molecular dynamics to create crystalline nuclei within amorphous models of Ge2Sb2Te5.
In the last part of the talk, an alloying strategy to tune the crystallization kinetics of PCMs is introduced. It is shown that ab initio simulations enable efficient screening of transition-metal alloys to select those that best speed up the high-temperature nucleation rate of Sb2Te3, a parent compound of Ge2Sb2Te5. These simulations predict that Scandium-alloyed Sb2Te3 displays the fastest crystallization kinetics. Subsequent experiments on phase-change memory cells containing Sc0.2Sb2Te3 confirm these predictions by showing a record-breaking writing speed of 700 picoseconds without preprogramming.