Authors:	Heinz, Sven
Advisor:	Jakob, Gerhard
Title:	Controlled Intermixing Superlattices of Thermoelectric Half-Heusler Materials
Online publication date:	26-Jan-2021
Year of first publication:	2021
Language:	english
Abstract:	This study investigates the further development of thermoelectric generators on the basis of half-Heusler materials. For this purpose two different routes are pursued: On the one hand the transfer of high-efficiency bulk (Hf,Zr)Co(Sb,Sn) materials to thin films as a p-type analogue to established n-type (Hf,Ti)NiSn and on the other hand the thermal optimization of established thin film systems in the framework of the superlattice-approach. A p-type analogue is a prerequisite for the transition from a one-legged to a advantageous two-legged generator design. In this study different growth-regimes of the p-type material are investigated and compared with corresponding bulk-materials. For certain sputter-conditions, they exhibit a negative instead of a positive Seebeck-coefficient, which is explained with the precipitation of acceptor-like Sn in nanometer-sized inclusions. However, by increasing the sputter power, hole-conducting thin films can be deposited reproducibly. All samples exhibit a bipolar behavior, that is also present in bulk, albeit to a lesser degree. This bipolar behavior manifests itself in a characteristic sign change of the Hall-coefficient at low temperatures. To explain this behavior, the electronic properties are discussed in the framework of different models. As a conclusion, hopping mechanisms in an acceptor-band are identified as a crucial part of the charge carrier dynamics in (Hf,Zr)Co(Sb,Sn), especially in thin films. In the investigation of superlattices, the dominance of interface-phenomena in the thermal properties for short period lengths are confirmed. Additionally, a close relationship between the interface quality and the thermal resistance is recorded. Through the controlled preparation of superlattices with different intermixing layers, a difference of 50% in the interface-related thermal resistance is achieved. To interpret the experimental results an analytical model is adapted to the HfNiSn/TiNiSn-system. It predicts a 95% transition probability of heat-carrying phonons for perfect interfaces. This value is significantly larger compared to other model systems like Si/Ge and AlAs/GaAs, which is caused by the relatively low acoustic contrast between TiNiSn and HfNiSn. At the same time the model predicts a strong decrease of transition probability down to »25% with the broadening of the boundary layer, which is accompanied by a signifcant uptake in thermal resistance. However, the experimental data confirms the predictions only partly. While the thermal resistance initially increases with a broadening of the intermixing layer, it ultimately decreases again below its initial value. As a consequence, an intermediate degree of interface intermixing maximizes thermal resistance. By reviewing a more extensive model, this behavior is explained with the function of the intermixing layer as an acoustic buffer. Because of the mediating acoustic properties of the mixed layer, the transition of heat-carrying phonons is promoted. The results are relevant for the thermal optimization of thermoelectric thin film systems specifically and nanoscale structures in general.
DDC:	530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-5587
URN:	urn:nbn:de:hebis:77-openscience-f3fe91b0-cb20-4a25-bd7e-9fcdaf3fd60e9
Version:	Original work
Publication type:	Dissertation
License:	CC BY-SA
Information on rights of use:	https://creativecommons.org/licenses/by-sa/4.0/deed.en
Extent:	xiii, 126 Seiten
Appears in collections:	JGU-Publikationen