Dislocation loops as a mechanism for thermoelectric power factor enhancement in silicon nano-layers

Abstract

A more than 70% enhancement in the thermoelectric power factor of single-crystal silicon is demonstrated in silicon nano-films, a consequence of the introduction of networks of dislocation loops and extended crystallographic defects. Despite these defects causing reductions in electrical conductivity, carrier concentration, and carrier mobility, large corresponding increases in the Seebeck coefficient and reductions in thermal conductivity lead to a significant net enhancement in thermoelectric performance. Crystal damage is deliberately introduced in a sub-surface nano-layer within a silicon substrate, demonstrating the possibility to tune the thermoelectric properties at the nano-scale within such wafers in a repeatable, large-scale, and cost-effective way.

Hierarchically nanostructured thermoelectric materials: challenges and opportunities for improved power factors

“…It is shown, however, that this is the most probable and realistic case if the well region sizes are in the order of a few tens of nanometers. Power factors beyond 30 mW/mK 2 can be reached, and we pointed out to some experimental evidence that supports this [76,77,79,80].…”

“…Specifically, we describe theoretical and experimental findings that can lead to designs in which simultaneous improvements in both the electrical conductivity and the Seebeck coefficient are achieved in order to largely improve the power factor σS 2 . Experimental works have indeed verified that it is possible to achieve very high power factors (PFs > 15 W/mK 2 , 5× compared to bulk values) in nanostructured Si-based materials [76][77][78][79][80]. Our simulations show that design optimization can allow for even higher, PFs > 30 W/mK 2 [81].…”

“…Further work on a hierarchical nanostructured extension of the polycrystalline material with pores within the grain domains measured an even higher PF (~22 W/mK 2 ) as a consequence of a further improvement in the Seebeck coefficient [79]. Annealing of highly doped Si nanocrystalline materials were also reported in systems with dislocation defects to allow for PF improvements up to 70% compared to the pristine material [80]. A theoretical model based on a combination of energy filtering by the barriers formed at the grain boundaries [226,227,228] and dopant segregation was used to explain the behavior [76], and such models also predict that this design direction can provide even higher PFs [81].…”

“…Nanocrystalline materials in which carrier flow happens between grains and potential barriers at grain boundaries can also be thought of as 'effective' superlattices at first order. Such nanostructures allow increases in the Seebeck coefficient [174,175], and interestingly, in some cases cause increase in the PF as well [76,80,81,94,176,177]. Initially, however, the primary reason for considering such structures was the reduction of the thermal conductivity as a result of extensive phononboundary scattering [178,179].…”

Nanostructured potential well/barrier engineering for realizing unprecedentedly large thermoelectric power factors

“…Recent efforts by us and others, however, both theoretical and experimental, have revisited the energy filtering concept, and targeted designs that provide simultaneous improvements in both the electrical conductivity and the Seebeck coefficient in order to largely improve the power factor σS 2 . Experimental works have indeed verified that it is possible to achieve very high power factors in nanostructured Si-based materials after undergoing specific treatment [48,49,50,51,52]. Measured data for PF improvements of over 5× compared to bulk values, were adequately explained using Boltzmann transport theory [48,49].…”