Unravelling the Structure and Crystallization Mechanism of Amorphous Nanoparticle Phase-Change Materials

Chalcogenide-based phase change materials such as GeTe and Ge-Sb-Te are commonly used for phase-change memory (PCM) applications, where information is stored using a high contrast in the resistivity or refractive index of the amorphous ‘0’ and crystalline ‘1’ state. Data is written to the memory cell using electrical or optical pulses, which crystallize or amorphize the structure through induced Joule heating. As demand for faster memory and higher data density increases, PCM nanoparticles are well suited to act as the building blocks for future ultrasmall memory devices. To realize this and to optimize memory properties, the structure and switching mechanism of nanoscale PCM materials need to be well understood. To this affect we synthesize 5 nm GeTe nanoparticles, coat them in a ZnS shell isolating the effects of coalescence, and compare their structure and crystallization to sputtered ‘bulk’ GeTe.<br/><br/>The reversible phase transitions in PCM devices are extremely fast, strongly suggesting a close structural resemblance between the amorphous and crystalline phases. In contrast to this, the amorphous structure is generally assumed to consist of a highly random ordering of atoms, which is generally quite distant to the crystalline counterpart. This work uses ultrafast in-situ X-ray absorption spectroscopy (XAS) and theoretical calculations to study and quantify the amorphous structure of bulk and nanoscale GeTe. Through a series of high-temperature XAS measurements, we can slow down the nanosecond crystallization process and consequently provide a detailed crystallization mechanism, which is validated using molecular dynamic (MD) simulations. We argue that crystallization phase transition is diffusionless for Te atoms, while Ge atoms order via cleavage of Ge-Ge homopolar bonds forming Te-Ge-Te intermediate ‘bridge’ states. In parallel to experimental measurements, we developed a theoretical model of the amorphous structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge in tetrahedral coordination. This structure is relaxed using density functional theory (DFT) calculations and strongly matches our experimental data and previous literature on amorphous GeTe.<br/><br/>We apply our new knowledge about amorphous PCM materials to quantify differences between bulk and nanoscale GeTe. Finally, we extend our model to ternary X-Ge-Te systems and study composition effects on their phase-change properties. Our work provides a high-throughput pathway to model and optimize chalcogenide materials for PCM applications and to design scaling rules for sub-10 nm PCM devices.