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2014 MRS Fall Meeting


CC11.07 - Polarization Doping of Silicon Nanowire Arrays by Molecular Grafting: Impact on Thermoelectric Properties


Dec 4, 2014 11:45am ‐ Dec 4, 2014 12:00pm

Description

Silicon nanowires (NWs) are a well-assessed example of the beneficial role that dimensional constraints may impart to the thermoelectric properties of otherwise inefficient materials. Incoherent phonon scattering at NW walls leads to a decrease of the thermal conductivity while marginally affecting both the electrical conductivity and the Seebeck coefficient. Further improvements of ZT may be obtained if the power factor is increased by keeping the charge carrier density at its optimal value (≈ 1019−1020 cm-3 in Si) while increasing mobility. To this aim, polarization doping by molecular grafting at NW walls following an approach pioneered by Tour [1] in ultrathin silicon layers proves to be an interesting strategy. A top-down fabrication process for the massive production of silicon NWs, organized in large area arrays, was implemented. Using silicon-on-insulator substrates the simultaneous fabrication of more than 105nanowires/mm2 could be achieved by lithography, anisotropic etching, and oxidation [2]. Conventional optical lithography could be used for pattern definition, followed by NW width reduction by stress-limited oxidation. The NWs, smaller than 50 nm and several micrometers long, were interconnected in an array and then positioned between contacts for electrical and thermal measurements. For this experiment, virtually undoped NW arrays were fabricated and characterized. A resistivity of 0.012 Ω m was measured, reporting an equivalent doping of ≈ 1015 cm-3, higher than the nominal doping value possibly because of the band bending at the NW surface. Either 1-ethynyl-nitrobenzene or 4-ethynyl-aniline were grafted onto the NW walls by thermal hydrosilation in mesitylene (130 °C, 16 h) to impart p and n-type polarization doping, resp. [3, 4]. After removal of the unreacted (physisorbed) organics, thermoelectric characteristics were reassessed, showing in all cases a relevant increase of the electrical conductivity along with an expected decrease of the absolute value of the Seebeck coefficient. As in the pristine NW array, surface polarization leads to an increase of the carrier density (either holes or electrons) in the absence of dopants. As a result, no ionized scattering centers limit the carrier mobility, ultimately leading to enhanced electric conductivities and power factors.

References:

[1] T. He, D.A. Corley, M. Lu, N.H. Di Spigna, J. He, D.P. Nackashi, P.D. Franzon, and J.M. Tour, J. Amer. Chem. Soc.. 2009, 131, 10023.

[2] G. Pennelli, M. Totaro, M. Piotto, and P. Bruschi, Nano Lett., 2013, 13, 2592.

[3] A. Taffurelli and D. Narducci, Surf. Sci., 2007, 601 2840.

[4] G.F. Cerofolini, D. Narducci, and E. Romano, ‘Silicon Functionalization for Molecular Electronics’, in Dekker Encyclopedia of Nanoscience and Nanotechnology, 3rd edition, CRC Press: New York, 2014, p. 2663.

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