Epitaxial MOCVD deposition of thermoelectric material films and determination of thermoelectric properties including thermal conductivity

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The goal of this project is the epitaxial MOCVD deposition of thin films of thermoelectric materials for technical application in the low (<350 °C) and medium (400 - 700 °C)) temperature range and the determination of their transport properties. Tailor-made metal organic group 15 and 16 precursors such as low-valent compounds E2R4 and E'2R'2 (E = Sb, Bi; E' = S, Se, Te) and single-source precursors R2EE'R', (R2E)2E', RE(E'R')2 und E(E'R')3 (E = Sb, Bi; E' = S, Se, Te), which cleanly decomposed at rather mild temperatures, will be investigated. Binary (Sb2Se3, Sb2Te3, Bi2Se3, Bi2Te3) and ternary material films (SbxBi1-x)2Te3, Sb2(SexTe3-x), Bi2(SexTe3-x) as well as complex multilayer structures will be epitaxially deposited in systematical MOCVD studies and a fundamental understanding of the influence of different chemical compositions on the thermoelectric properties, e.g. electrical and thermal conductivity and Seebeck coefficient will be developed. The characterization of the material films is essential in order to understand the specific influence of the precursor, deposition temperature, and substrate material on the thermoelectric properties of the resulting material films. The films will therefore be pre-characterized in the Schulz group (crystallinity, surface morphology, chemical composition). Promising films are then investigated in detail in the DFG- research infrastructure center ICAN (Interdisciplinary Center for Analytics on the Nanoscale, University of Duisburg-Essen; SAM, XPS, TOF-SIMS) and by our collaborators Dr. Gabi Schierning (Seebeck coefficient, power factor) and Prof. Axel Lorke (electrical conductivity). The determination of the thermal conductivity of the binary and ternary material films, which is crucial for the current project, will be done by Prof. C. Jooss using the 3-omega method. Careful investigation of the temperature-dependence of the thermal conductivity, which will be done in the temperature range from 30 K to 900 K, gives valuable information on the influence of phonon scattering at point disorder, grain boundaries (low temperatures) as well as phonon-phonon scattering (high temperatures). In addition, the defect and microstructure of selected samples will be investigated using high-resolution and analytical TEM. Non-stoichiometries at grain boundaries as well as point defects and defect clusters as function of synthesis parameters and doping levels are of particular interest and will be investigated using electron energy loss spectroscopy (EELS) and Annular Dark Field (ADF) techniques. In-situ TEM experiments will give information on behavior under thermal stress. The potential of superlattices on the further optimization of the thermoelectric properties of selected material systems will be studied. The in plane and cross plane coefficients of the thermal conductivity will be separated out for selected samples.