Thin film thermoelectric materials and generators deposited by chemical vapour processes

Abstract

At present, a huge expansion of the Internet of Things (IoT) market is taking place, with issues stemming from potential power methods for integrated wireless sensor networks, which operate in the range of 1μW- 10mW. Common power methods, such as batteries, are impractical due to the sheer number of IoT nodes that are projected to be in operation. This has caused a shift to alternative energy harvesting methods to be explored. One that has shown great promise is thermoelectric power generation realised by thin-film thermoelectric generators. These devices directly convert heat into usable electrical energy and are comprised of an array of n-type and p-type semiconductors wired electrically in series and thermally in parallel. Enclosed within this thesis is an exploration of alternative thin film thermoelectric materials, which alleviate the need for scarce and expensive materials. The thin films in this work were deposited via chemical vapour processes, due to superior conformity, coverage and stoichiometry when compared to industrially established physical vapour processes (i.e sputtering, thermal evaporation, and electrodeposition). The successful deposition of binary and ternary thin films has been reported with the negation of the use of multiple sources which often require calibration, which is unfavourable for supply chains. The use of chemical vapour processes has also led to the formation of nanostructured films which display improved material properties, beneficial for thermoelectric applications. Exploration of the thermoelectric performance of highly stoichiometric GeTe thin films deposited by low-pressure chemical vapour deposition (LPCVD) using a novel single source precursor (SSP) is conducted. The potential of tuning thermoelectric performance will also be presented through the alteration of deposition conditions which suggests the potential to control crystallite size and majority carrier concentration. A competitive power factor of 40 μW/cm·K² at 629 K was achieved, attributed to a high electrical conductivity driven by the formation of Ge vacancies. A prototype device was also fabricated with an encouraging specific power generation density of 175.4 μW/cm²K². A range of binary and ternary WS_2xSe_2−2x (0 ≤ x ≤ 1) films were deposited via LPCVD using a range of SSPs containing different configurations of S and Se. The successful deposition of stoichiometric binary films and ternary films with a stable composition is encouraging, along with evidence of Janus binding which has been suggested to exhibit outstanding thermoelectric performance. Interestingly the films all displayed semi-metallic p-type conduction, this is unexpected for WS₂ which generally favours n type conduction. This behaviour is believed to be linked to the formation of S vacancies acting as trapping centres for electrons. The deposited WS₂ films achieved a power factor of 6 μW/mK² at 553 K. Improved performance was also noted with a simple annealing process which enhanced the room temperature power factor from 1.9 to 3.9 μW/mK². Al-doped ZnO (AZO) thin films were deposited via plasma-enhanced atomic layer deposition (PE-ALD) using an in-situ O₂ plasma treatment. The thin films were deposited with a 4% Al doping concentration. The deposition process displayed the ability to grow AZO nanopillars perpendicular to the substrate, which is supported by SEM, AFM and XRD measurements. The formation of this led to the lowest reported cross plane thermal conductivity for AZO thin films, with a cross-plane thermal conductivity of 0.16 W/mK. The deposited thin films also yielded an encouraging power factor of 2.94 μW/cmK² at 563 K. Furthermore, the fabrication and testing of a prototype thin film μ-TEG, without the use of scarce and expensive materials is reported. The device achieved a specific power generation density of 88.1 nW/cm²K², with a peak power output of 1.08 nW with an applied temperature difference of 16.9^◦C.

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Identifiers

ORCID for Vikesh Sethi: orcid.org/0009-0002-2711-1889

ORCID for Ruomeng Huang: orcid.org/0000-0003-1185-635X

ORCID for Kees De Groot: orcid.org/0000-0002-3850-7101

ORCID for Otto Muskens: orcid.org/0000-0003-0693-5504

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