Chalcogenide phase-change materials (PCMs) are widely used as rewritable data storage media. Optical devices like DVD or Blu-ray players exploit the difference in properties between the amorphous and crystalline states of the material, but the writing speed is limited by the high currents and long pulse times needed to trigger melting and recrystallization. Interfacial phase-change materials, in which complete amorphization is not necessary, offer an efficient alternative.

Research on chalcogenide PCMs has been dominated by thin films of antimony–germanium–tellurium alloy (Ge2Sb2Te5), which still await the right combination of dopants to attain optimal material properties. Xilin Zhou and colleagues at Singapore University of Technology and Design (SUTD), and Shanghai Institute of Microsystem and Information Technology, take a different approach: "We aim to design materials to a specification rather than relying on serendipitous discoveries through Edisonian methods," says Zhou.

The principle that allows these researchers to adopt such a design-led strategy is their focus on iPCMs, in which phase transitions are confined to specific crystal interfaces. "The iPCM superlattice structure presents new degrees of freedom (strain, superlattice layer thicknesses, etc) to tailor the properties of the phase-change material," Zhou explains.

The iPCM studied by Zhou and his team, which is led by Robert Simpson of SUTD’s ACTA Lab, comprises cells of stacked 2D layers of Sb2Te3 and GeTe. This composite material’s crystal structure is such that each van der Waals (vdW) gap is sandwiched between two layers of Te atoms.

When a sufficiently large voltage is applied to the cell, phonons propagating perpendicularly to the plane cause Ge atoms to diffuse vertically through the Te layer into the vdW gap, in a process that the researchers call "GeTe interfacial premelt disordering". This rearrangement of Ge atoms, which leaves the Sb2Te3 superlattice intact, constitutes the disordered "reset state", and is accompanied by an increase in cell resistance of more than two orders of magnitude. Applying a lower energy electrical pulse triggers recrystallization and a phase change back to the ordered state.

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To understand the mechanism behind the process, Simpson’s group used density functional theory (DFT) to simulate Ge diffusion in an 81-atom model system. The researchers found that the ease with which each atom made the transition depended on how many had already gone before. "We showed that as more atoms switch, the energy required for subsequent atoms to follow is lowered. This leads to an exponential increase in the switching probability with the number of atoms participating," explains Zhou.

The height of the phase-change energy barrier was also affected by mechanical strain. As Sb2Te3 has a larger lattice constant, it imparts an in-plane tensile stress to the GeTe layer, which varies depending on the relative thicknesses of the two materials. The greatest discrepancy, in which a 4 nm-thick Sb2Te3 layer was coupled with a 1 nm-thick layer of GeTe, resulted in a biaxial strain of 2.2%, and led to the most pronounced effect on the phase-change dynamics. Larger thicknesses of Sb2Te3 would stress the GeTe layer even more, but would also inhibit the out-of-plane phonons that prompt the migration of each Ge atom.

A mystery explained

As well as contributing to the development of future rewritable storage media, the results also explain a property of Sb2Te3–GeTe PCMs that has been mysterious until now. "One of the most interesting aspects of the material is that at room temperature the disordered reset state is stable for years, yet at moderate temperatures the material crystallizes into the set state in nanoseconds," explains Zhou. "That is, the crystallization time changes by 15 orders of magnitude. The reason that Ge2Sb2Te5 PCMs can switch on short time scales is due to Ge atoms becoming exponentially more likely to switch as the temperature increases, and the activation energy being simultaneously lowered by the avalanche Ge switching effect."

Because crystallization is a self-reinforcing stochastic process, the chances of it occurring depend on the number of atoms locally available to initiate it. This means that the size of the memory cell must be chosen carefully to produce the appropriate sensitivity to temperature.

Full details of the work can be found in Nano Futures DOI: 10.1088/2399-1984/aa8434.