Design & optimisation of nanostructured silicon by ion-modification for thermoelectric devices

Abstract

Energy is a determining factor in the prosperity of societies all around the world. Electricity is the most modern form of energy, with rapidly expanding market applications, and is mainly produced by combustion-based technologies. These, combined with wide-spread consumer electronics, produce vast amounts of ‘low-grade heat’ whose temperature is too low to efficiently recover by conventional means. Thermoelectric generators (TEGs) have received renewed interest as they can partially recover these losses using special materials, exhibiting the thermoelectric effect, to convert heat-to-electricity directly with no moving parts. The key drivers to this are; (i) strong political will for energy recovery, (ii) new attentions to space exploration, an industry with historical ties to TEGs, and (iii) technological advancements in microelectronic manufacturing. Relatively recent discoveries (silicides) and these modern techniques (ion-implantation) are combined to form nano-engineered silicon compounds (primarily β-FeSi2 nanoinclusions) and investigate their thermoelectric properties. Highly doped p-type silicon wafers (3 ± 2 Ω cm), some of which silicon-on-insulator (SOI) (18 ± 4 Ω cm), were implanted with either xenon or iron either as wafers or microscopic lamellae (manufactured using a Focussed Ion Beam). The resulting samples were annealed at an array of temperatures (300-1000°C) and exposure times (102 -105 s) for characterisation. The implants were performed at the University of Surrey IBC and University of Huddersfield MIAMI-2 with some samples annealed in-situ (in implanter holder) and most ex-situ (Heriot-Watt University custom-built furnace) The samples were characterised for different properties, including; (i) electrical (van der Pauw resistivity, Seebeck), (ii) thermal (2ω, Raman spectroscopy), and (iii) morphological (XTEM, EFTEM-EELS, SEM, XRD, Raman) properties. Salient results are reported, such as the very effective thermal conductivity reduction using implant amorphisation (lowest recorded value of 1.4 ± 0.1 W m-1K-1 at 25°C for pre-annealed Xe-implanted SOI samples), an extensive study on the electrical resistivity of Fe-implanted SOI samples (best value measured for pre-annealed samples, with 9.8 ± 0.2 mΩcm at 150°C, although unstable), and correlations between the properties and relative amounts of β-FeSi2. A Raman thermal conductivity method, developed by Wight et al. (2019), successfully showed that the as-manufactured lamellae are 2.17 ± 0.33 W m-1K-1 and iron-implanted (5×1016 ions cm-2 , annealed at 950°C), are 6.54 ±1.40 W m-1K-1 . A study on Seebeck has been carried out with up to 355 μV K-1 found for an Fe-implanted (300°C for 103 s) sample at 75°C, although most were not responsive to the equipment. The experimentation has successfully shown that a drastic reduction in thermal conductivity is possible through the implantation using both xenon and iron on different structures, SOI wafers and silicon lamellae, and that electrical properties can be boosted with iron silicide nanoinclusions under certain conditions. Silicon-based thermoelectrics are highly compatible with the versatile technique of ion-implantation and can bring about potentially viable thermoelectric materials. Further development in this area is hopeful and represents an opportunity to serve new consumer and industrial applications by portable energy harvesting techniques.