Phase Change Materials and Their Application to Nonvolatile Memories

International audienc

Unravelling the interplay of local structure and physical properties in phase-change materials

As the chemical bonds in a covalent semiconductor are independent of long-range order, semiconductors generally have similar local arrangements not only in the crystalline, but also in the amorphous phase. In contrast, the compound Ge2Sb2Te5, which is a prototype phase-change material used in optical and electronic data storage, has been shown to undergo a profound change in local order on amorphization. In this work, ab initio ground state calculations are used to unravel the origin of the local order in the crystalline cubic and the amorphous phase of GeSbTe alloys and the resulting physical properties. Our study shows that this class of materials is characterized by two competing structures with similar energy but different local order and different physical properties. We explain both the local distortions found in the crystalline phase and the occurrence of octahedral and tetrahedral coordination in the amorphous state. Although the atomic rearrangement is most pronounced for the Ge atoms, the strongest change of the electronic states affects the Te states close to the Fermi energy, resulting in a pronounced change of electronic properties such as an increased energy gap

Structure of liquid Te-based alloys used in rewritable DVDs

We analyze the structure of Te-based chalcogenide compounds that are used as materials for rewritable DVDs by using a combination of neutron diffraction and ab initio computer simulation. We show that in the liquid, the atoms have a low average coordination number, as the result of a Peierls distortion. The partial pair correlation functions are obtained from the computer simulation data. © 2004 Published by Elsevier B.V

Three-dimensional nanomechanical mapping of amorphous and crystalline phase transitions in phase change materials

The nanostructure of micrometer sized domains (bits) in phase change materials (PCM) that undergo switching between amorphous and crystalline phases plays a key role in the performance of optical PCM based memories. Here we explore the dynamics of such phase transitions by mapping PCM nanostructures in three dimensions with nanoscale resolution by combining precision Ar-ion beam cross sectional polishing and nanomechanical Ultrasonic Force Microscopy (UFM) mapping. Surface and bulk phase changes of laser written sub-m to m sized amorphous-to-crystalline (SET) and crystalline-to- amorphous (RESET) bits in chalcogenide Ge2Sb2Te5 PCM are observed with 10-20 nm lateral and 4 nm depth resolution. UFM mapping shows that the Young’s moduli of crystalline SET bits exceed the moduli of amorphous areas by 11 ± 2%, with crystalline content extending from a few nm to 50 nm in depth depending on the energy of switching pulses. The RESET bits written with 50 ps pulses reveal shallower depth penetration, and show 30-50 nm lateral and few nm vertical “wave” like topography that is anti-correlated with the elastic modulus distribution. Reverse switching of amorphous RESET bits results in full recovery of subsurface nanomechanical properties, accompanied with only partial topography recovery resulting in surface corrugations attributed to quenching. This precision sectioning and nanomechanical mapping approach could be applicable to a wide range of amorphous, nanocrystalline and glass forming materials for 3-dimensional nanomechanical mapping of amorphous-crystalline transitions