Potentialities of silicon nanowire forests for thermoelectric generation

Abstract

Silicon is a material with very good thermoelectric properties, with regard to Seebeck coefficient and electrical conductivity. Low thermal conductivities, and hence high thermal to electrical conversion efficiencies, can be achieved in nanostructures, which are smaller than the phonon mean free path but large enough to preserve the electrical conductivity. We demonstrate that it is possible to fabricate a leg of a thermoelectric generator based on large collections of long nanowires, placed perpendicularly to the two faces of a silicon wafer. The process exploits the metal assisted etching technique which is simple, low cost, and can be easily applied to large surfaces. Copper can be deposited by electrodeposition on both faces, so that contacts can be provided, on top of the nanowires. Thermal conductivity of silicon nanowire forests with more than 10 nanowires mm have been measured; the result is comparable with that achieved by several groups on devices based on few nanowires. On the basis of the measured parameters, numerical calculations of the efficiency of silicon-based thermoelectric generators are reported, and the potentialities of these devices for thermal to electrical energy conversion are shown. Criteria to improve the conversion efficiency are suggested and described.

Seebeck coefficient of silicon nanowire forests doped by thermal diffusion

“…The Seebeck coefficient of the doped nanowires is difficult to evaluate because the doping is not uniform. There are experimental papers [2,[25][26][27] reporting the Seebeck coefficient S = S(n) measured on bulk silicon with uniform doping concentration n. As shown in our previous work [3], the best logarithmic fit of the reported experimental data is S(n) = 0.0077 -1.26 × 10 −4 ln(n) (n is the doping concentration in m −3 , S as absolute value). This relationship between S = S(n) and the doping concentration can be used to evaluate the Seebeck coefficient in our nanowires with nonuniform doping.…”

“…Also the electrical conductivity depends on doping, that is, σ(x,y) = σ(n(x,y)) It has been estimated using the formula by Arora [28], which is widely used in silicon device simulations. The Seebeck coefficient of the nanowires has then been calculated as: (3) This formula can be easily derived by considering the nanowires as many parallel thermoelectric generators S i ΔT, each with its resistance R i (conductance G i = 1/R i ). It is straightforward to obtain .…”

“…Nanowires with an average diameter of 80 nm and a length of several hundreds of micrometers can easily be obtained from MACE on n-and p-doped substrates, with concentrations up to 10 18 cm −3 . However, the maximum of the power factor is achieved for doping concentrations greater than 10 19 cm −3 [2,3]. Unfortunately MACE yields very unreliable results at such high doping concentrations.…”

“…However, a large-scale application of thermoelectric devices requires the development of materials that have good thermoelectric features and are, at the same time, of low cost, technologically affordable and sustainable. Silicon has a very high power factor S 2 σ [1][2][3][4] (S is the Seebeck coefficient and σ is the electrical conductivity). This, combined with the reduced thermal conductivity when nanostructured [5][6][7][8][9][10], makes it very suitable for thermoelectric applications.…”

“…There are two main requirements to the fabrication of a leg for a thermoelectric generator that is based on a large number of silicon nanowires perpendicular to a substrate: 1) Electrical contacts need to fabricated on the top of a silicon nanowire forest, which can be achieved by copper electrodeposition [18]. 2) The optimum doping concentration of the nanowires for the exploitation of the maximum power factor of silicon [3] needs to be found. Both the Seebeck coefficient and the electrical conductivity depend on the doping concentration.…”