Control and characterization of vacancy disorder and related properties in sputter deposited GST124 thin films

Dück, Matthias Maximilian; Wuttig, Matthias (Thesis advisor); Kooi, Bart J. (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Chalcogenides are a versatile and widely useful material class. They generate a lot of interest due to their rich property set, which makes them strong candidates for applications, e.g. as phase-change materials, topological insulators and thermoelectrics. They are fueled by a recently proposed novel chemical bonding mechanism, metavalent bonding (MVB). Hence, this material class is also of profound fundamental interest to grasp the nature of solids, and establish new approaches to materials design based on this understanding. In addition to the abovementioned properties, a subgroup of MVB chalcogenides, most prominently alloys of Ge, Sb and Te (GST), display a highly unconventional response to disorder. While disorder manifests in several ways, specifically the distribution of the large number of stoichiometric vacancies - 25% on the cation sublattice in GST124 - was observed to have a severe influence on thermal and charge transport. In fact, a genuine metal-to-insulator transition (MIT) is observed upon annealing-induced ordering of the vacancies. This MIT is understood to be driven by disorder, as doping-effects can be excluded, dictating a close connection between vacancy distribution, electronic structure and electrical transport properties. A holistic study encompassing all three of these areas of interest and combining them with a viable approach to controlled disorder variation has yet to be performed. The present thesis aims to fill this void by first establishing a well-optimized sputter deposition method, which enables the creation of textured thin film samples with a high degree of disorder control. These sample properties are utilized in a next step to extract highly dense information derived from a large number of measurement techniques performed on one and the same sample for each state. The utilized methods include X-ray diffraction (XRD), photoemission spectroscopy (PES) and a low-temperature electrical transport characterization. After achieving an unprecedented degree of control over the vacancy distribution and texture, the well-established transport behavior could be reproduced and characterized with respect to structural metrics. Additionally, disorder-related signals were observed and quantified in the PES measurements, completing the approach to characterize disorder from all relevant perspectives. Lastly, the present thesis lays a foundation for the experimental connection of vacancy disorder and MVB, which was strongly suggested in the previously available data.