Development of thin-film chalcogenide materials deposited by LPCVD for thermoelectric energy generation

Abstract

Thermoelectric generators have long been seen as a possible renewable energy source for both small scale and large scale applications. These devices use no direct fuel and therefore fossil fuels to produce power and are solid state so require little maintenance. However, efficiencies of these devices are currently insufficient to be seriously considered as primary power sources and are currently only considered for small scale applications, or where this is the only option such as in radioisotope thermoelectric generators for deep space probes. To improve these devices, two main approaches can be considered, one is to improve the thermal and electrical performance of devices by carefully optimised design, and the other is to improve the materials electrical conductivity, thermal conductivity and Seebeck coefficient. A new corrugated thin film thermoelectric generator design is considered and an analytical model for this is verified using finite element method simulations showing a maximum discrepancy of 15% over a wide range of parameters. The result of simulation and modelling shows that increasing the interconnect electrical conductivity and reducing the pitch of the device increases the power density. The power density is also increased by increasing the fill factor, and this thin film design can achieve higher fill factors compared to that of a conventional device at a specific minimum feature size. To evaluate thin film thermoelectric materials, methods for the measurement of thermoelectric properties are developed. For the measurement of the Seebeck coefficient and electrical conductivity, a Joule Yacht MRS-3L is used allowing for measurements from 100 - 600 K. The capabilities of this tool have been extended to allow for the more precise measurement of highly resistive films. A ω − 3ω system is developed for the measurement of thermal conductivity of films. This system is verified by the measurement of bulk silicon and thin films of bismuth telluride and show good agreement with literature values for both materials. Thin films of low pressure chemical vapour deposited (LPCVD) Bi₂Te₃ are optimised by the alloying of Bi₂Te₃ and Bi₂Se₃ to deposit ternary Bi₂Te₃-_xSe_x. The composition of the ternary films are tuned to optimise the combination of carrier concentration and mobility to give a three-fold enhancement of the thermoelectric power factor at 300 K, and six-fold enhancement at 500 K, with respect to Bi₂Te₃ . This improvement from the substitution of Te with Se is believed to be due to donor effects, as well as point defects caused by substitution. Pre-patterned substrates with open SiO₂ holes on TiN were used for selective deposition of Bi₂Te₃-_xSe_x on to the conductive TiN. This selective deposition behaviour allows for a reduction in fabrication steps for a thermoelectric micro-generator, and a reduction in wasted material. Deposition of Sb₂Te₃ by LPCVD is optimised by varying deposition temperature. The carrier concentration and mobility of the films can be optimised by reducing the deposition temperature to 364 ◦C, resulting in a power factor of 16.5 µW cm−1 K −2 at 350 K. The Sb₂Te₃ films also shows selective behaviour on the conductive TiN surface, which enabled the fabrication of a single-type thermoelectric micro-generator. The fabricated generator had a pitch of 400 µm, a fill factor of 25%, and 72 Sb₂Te₃ thermoelements. This prototype device was measured using a custom system and a maximum temperature difference of 0.11 K was achieved across the 500 nm thick thermoelements, leading to a voltage of 0.4 mV and current of 0.7 µA giving a power output of 280 pW. It is then shown by simulation that this power output is significantly limited by the interconnect resistance, and that by reducing the pitch down to 10 µm the power output could reach 500 nW. The thermoelectric properties of tin chalcogenides are investigated by comparing SnS, SnSe, and SnTe. It is found that the SnS and SnSe films deposited by LPCVD are much more resistive than the SnTe. The high resistivity is caused by low carrier concentrations, which also lead to high Seebeck coefficients of 650 and 790 µV K⁻¹ at 300 K for SnS and SnSe, respectively. Comparatively, the SnTe films show a resistivity of 4 orders of magnitude lower, due to a high carrier concentration and comparable mobility. Overall the SnTe films show the highest power factor of 8.3 µW cm⁻¹ K⁻² at 615 K. The SnTe films also show selective behaviour, but the SnS and SnSe do not.

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ORCID for Daniel W Newbrook: orcid.org/0000-0002-5047-6168

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