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.