A1 From structure – property correlations to tailor-made chalcogenide materials

Project A1 (2019-2023) ​seeks to probe the fundamental microscopic mechanisms responsible for the properties of PCMs, both by careful quantum-theoretical simulations and by experimentation. Recent work in our groups reveals that the bonding in crystalline phase-change materials, which we coin metavalent bonding, is located between metallic, covalent, and ionic bonding. This finding provides us with the necessary tools to begin the rational design of new PCMs. To explore which materials fall into the realm of metavalent bonding, we first need to define the domain in which metavalent bonding occurs, hence answering the question: Where are the borders of metavalent bonding with respect to ionic and covalent bonding? Within this project, we will determine these boundaries with a combined experimental and theoretical approach. To this end, we are developing new quantummechanical descriptions for solids, which will enable hitherto inaccessible predictions of physical properties in crystalline PCMs.

Based on a fundamental understanding of structural relationships in PCM, we will extend our studies to the origin of potential failure modes in resistively switching chalcogenides. We expect this project to disclose the unclear nature of defects in PCMs and their influence on physical properties as well as chemical bonding. To this end, we are investigating stacking faults, point defects, and disorder in layered chalcogenides by means of a combined experimental and theoretical approach.


Understanding phase-change materials
(PCMs) on the atomic scale is the first and most fundamental step
towards better memory devices – one hopes to improve existing materials
or even design new ones from scratch. In this project, we merge
first-principles modelling and careful experiments to achieve a
microscopic understanding of complex chalcogenides. In particular, we
are interested in how charge transport through crystalline PCMs can be
tailored by the interplay of stoichiometric modifications and
self-doping. Furthermore, we will extend our scope from crystalline to
amorphous PCMs (“zero bits”); in the long term, we aim to understand and
control the chemical nature in both.

Structure – property relationship for chalcogenides:
The role of chemical bonding and defects

Phase-change materials are
characterized by a unique property portfolio which includes a pronounced
change of optical properties upon crystallization of the amorphous
state. This finding is a unique identifier for this class of materials.
Recently this observation has been attributed to the “resonance bonding”
description of covalency in crystals. We intend to derive a microscopic
understanding of the chemical bonding and how its strength depends upon
the composition and atomic arrangement of the crystalline phase. We are
investigating the electronic structure of phase change materials both
theoretically and experimentally and will relate it to the material
composition with the ultimate goal to design phase change materials by
choosing materials with suitable composition which feature resonance
bonding. Particular emphasis is devoted to understand the nature of
defects and their influence on material properties.

Principal investigators:

Prof. Dr. rer. nat. R. Dronskowski
Institute für Anorganische Chemie
RWTH Aachen University
Phone: +49 (0)241 80 93642
E-mail: drons@HAL9000.ac.rwth-aachen.de

Prof. Dr. rer. nat. M. Wuttig
I. Physikalisches Institut IA
RWTH Aachen University
Phone: +49 (0)241 80 27155
E-mail: wuttig@physik.rwth-aachen.de