Topical area A:
Structure and material complexity

In this topical area, we focus on design rules for materials that can be utilised as resistively switching chalcogenides. To do so, our goal is to identify rational design strategies for VC systems and to further elaborate those found for PC systems.

We have identified a larger number of new phase change materials, all characterised by resonance bonding in the crystalline state. In addition, it has become clear that for a subgroup of chalcogenides, even in the crystalline state, pronounced disorder exists. This disorder can govern the material properties, in particular the transport of charge and heat. Finally, there is strong evidence that closely related compounds can be superconductors, thermoelectric materials and topological insulators. We would like to unravel and exploit this remarkable property combination. As can be seen in the figure below, phase change materials can only exist for low levels of iconicity and s-p hybridisation. This implies that a stoichiometry range exists, where resonance bonding and hence the attractive features of phase change materials can no longer be realised. One of the questions we want to address hence deals with the disappearance of resonance bonding at this boundary. Will resonance bonding gradually decrease until a critical value is reached or will it vanish smoothly? Will the disappearance of resonance bonding also be characterised by a change in atomic arrangement? Will these two changes occur at the same stoichiometry? In most phase change materials, the Fermi level lies in the valence band, giving rise to prevalent p-type conductivity. If we move towards the border for resonance bonding, will the Fermi level rise gradually out of the valence band? This would imply that we can tune the position of the Fermi level by control of the stoichiometry. Indeed, this is one of the working hypotheses we want to explore in project A1.

Topical area A - Fig. 1

Because of the complexity of the materials and the underlying mechanisms, the design strategies for VCM systems are much less advanced as in the case of the PCM systems. Therefore, we will address this point by comparing selected VCM materials. Besides SrTiO3, TiO2 and GaOx, we will extend our sys-tems to HfOx and Ta2O5. These two substances have been identified by the industry as the most promising materials for ReRAM applications. In order to identify the role of specific defects, we will compare stoichiometric samples to samples with oxygen deficiency as well as crystalline systems with polycrystalline and amorphous materials.

We have revealed that switching in VCM systems might take place in different dimensionalities, either within a single filament, on a more extended interface, or even within the whole bulk of a device. This will have significant impact on the scaling behaviour and the switching kinetics. By employing scanning probe as well as spectroscopic techniques, we would like to clarify the impact of the atomic and electronic structure on the tendency towards a specific dimensionality of the switching process. Considering systems with systematic differences in electronic structure, defect formation energies (anion and cation site) and energy barrier for ionic transport, we are aiming at the development of design strategies for VCM systems with predictable switching and scaling behaviour.

Since TC systems have lost their relevance for ReRAM applications due to poor energy efficiency, we will shift our focus to VC and PC systems. Because of the phase transformations observed in VCM systems we will further elucidate the similarities between the two systems. Furthermore, we plan to include a new link which has recently emerged between VCM and PCM systems. Hence, we will extend the material systems of the SFB by doped VO2 oxide that is under consideration to exhibit a Mott-type metal-to-insulator transition (MIT) switching instead of or in addition to a redox-based one.

We have identified valence as well as phase changes in VC systems taking place during electroforming and switching. We will strive to quantify the electrically induced changes of the electronic structure, the role of the mobile donors (O vacancies and/or cation interstitials), as well as the phase separation processes by employing in situ characterisation techniques. These techniques will also enable us to elucidate the microscopic mechanisms of device operation and of device failure like retention loss and reduced endurance. Based on experimental hints gained, we will explore the role of the phase boundary between the ion supply and the switching layer for the long-term stability of the device.

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