Molecular Foundry Seminar

"Phase Change Materials: A Challenge for Our Understanding
of the Amorphous and Crystalline State"

Professor Matthias Wuttig, Department of Materials and Interfaces,
Department of Physics, RWTH Aachen University, Aachen, Germany,
Tuesday, July 20th at 1:30 pm, Bldg. 67-Room 3111

Abstract:

This talk will discuss the unique material properties which characterize phase change materials. These compounds are among the most promising materials in information technology. They are already employed in rewriteable optical data storage, where the pronounced difference of optical properties between the amorphous and crystalline state is used. This unconventional class of materials is also the basis of a storage concept to replace flash memory.

The focus of this presentation will be on a discussion of the unconventional material properties of phase change materials and their explanation. In particular, it will be shown that the crystalline state of phase change materials is characterized by the occurrence of resonant bonding, a particular flavour of covalent bonding [1]. The concept of resonant bonding leads to an intuitive understanding of the observed material properties, e.g. the approximately octahedral coordination and the polarizability enhancement in the crystalline state, as well as the impact of stoichiometry variations. In order to quantify resonance bonding and to identify trends, both infrared spectroscopy experiments, i.e., a combination of FTIR-reflectometry and ellipsometry, and density functional theory calculations have been performed. Here, the results of these investigations will be presented. It will be shown that a map can be developed [2] which shows phase change behaviour for a selected range of chalcogenides with a well defined structure and unique bonding properties. The consequences of resonance bonding for the electronic properties are particularly interesting. It will be shown, that certain crystalline phase change materials employ a highly unusual charge transport mechanism, which employs Anderson localization [3]. This finding is in striking contrast to the behaviour of ordinary semiconductors and proves that crystalline phase change materials form an extraordinary quantum state of matter.

[1] K. Shportko et al., Nature Materials 7, 653 (2008)

[2] D. Lencer et al., Nature Materials 7, 972 (2008)

[3] T. Siegrist et. al., Nature Materials, submitted.