The underlying physics of interfacial phase change memory

Phase change materials can be rapidly and repeatedly switched between amorphous and crystalline phases with the application of an external voltage. Use of these structural changes to store information has led to the development of a new form of nonvolatile memory that has significant advantages over current memory standards.

The new article aims to better understand the switching process in interfacial phase change memory (iPCM), which switches between two different crystalline phases. iPCM was first demonstrated in 2011, but experts still debate the underlying physics of the switching mechanism and how the resistivity changes between the two crystalline phases.

The researchers used ab initio density functional theory simulations, incorporating structural models based on the latest experimental findings. Their calculations show that the material’s electronic structure strongly depends on the composition of its van der Walls (vdW) block constituents — the basic building unit of the material composed of stacked layers of GeTe and Sb₂Te₃. They also found that bi-layer flipping defects, which occur when two atomic layers detach from one block and attach to another, can cause a drastic change in the electronic structure.

Instead of the resistivity being determined by both charge mobility and carrier concentration, as with traditional semiconductor materials, the researchers discovered that the carrier concentration, which is dependent on the Fermi energy position determined by the vdW blocks, plays a much larger role.

The researchers propose a new switching mechanism for iPCM based on the change in the potential landscape of the band-gap, and that the presence of both Ge and Sb atoms on cation planes adjacent to vdW gaps, known as Ge/Sb intermixing is required for the resistance change to occur.

Source: “Origin of resistivity contrast in interfacial phase-change memory: The crucial role of Ge/Sb intermixing,” by Yuta Saito, Alexander V. Kolobov, Paul Fons, Kirill V. Mitrofanov, Kotaro Makino, Junji Tominaga, and John Robertson, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5088068 .