Effect of Nanocavities on the Thermoelectric Properties of Polycrystalline Silicon
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Effect of Nanocavities on the Thermoelectric Properties of Polycrystalline Silicon
Ekaterina Selezneva1, Andrea Arcari1, Gilles Pernot2, Elisabetta Romano1, Gianfranco Cerofolini1, Rita Tonini3, Stefano Frabboni3, Giampiero Ottaviani3, Ali Shakouri2 and Dario Narducci1 1 Department of Materials Science, University of Milano-Bicocca, via R. Cozzi 53, 20125 Milano, Italy 2 Department of Electrical Engineering, University of California, 1156 High Street, 95064 Santa Cruz, U.S.A 3 Department of Physics, University of Modena and Reggio Emilia, via Campi 213, 41100 Modena, Italy
ABSTRACT Nanostructuring has opened new ways to increase the thermoelectric performance of a host of materials, mainly by decreasing their thermal conductivity κ while preserving the Seebeck coefficient S and electrical conductivity σ. The thermoelectric properties of degenerated polycrystalline silicon films with nanocavities (NCs) have been studied as a function of annealing temperature upon isochronous annealings in argon carried out every 50°C in the range 500 – 1000°C which were used to modify the shape of the NCs. We found that presence of the NCs had no negative effect on the electronic properties of the system. The measured values of S and σ were close to those previously reported for the blank polycrystalline silicon films with the same doping level. The thermal conductivity was also found to be close to the value measured on the blank sample, about half of the reported value in polycrystals. This led to a power factor of 15.2 mWm-1K-2 and a figure of merit of 0.18 at 300 K.
INTRODUCTION Thermoelectric efficiency of a material is described by the so-called thermoelectric figure of merit ZT = S2σκ-1T , where S is the Seebeck coefficient, σ and κ are correspondingly the electrical and the total thermal conductivities, and T is the absolute temperature. Experiments on silicon nanowires [1, 2] demonstrated an enhancement of thermoelectric performance arising mainly from phonon scattering on dimensionally constrained structures, which lead to a remarkable decrease of κ. However, killing thermal conductivity is of little use when nanostructuring does not preserve electrical conductivity. In porous silicon, for instance, thermal conductivity was found to be up to three orders of magnitude smaller than in single-crystal silicon. However, a severe degradation of the electrical conductivity was also observed [3]. This has motivated the quest for phonon scatterers able to preserve the charge carrier mean free path. Among them, nanocavities (NCs) may be considered of special interest because of the possibility of continually modulating their size and spacing by annealing; and of decorating their internal surfaces with oxygen, hydrogen, and hydroxyl groups, thereby enabling the fine tuning of the inner surface transparency for phonons and electrons [4,5]. Recent report of Tang et al.[6] demonstrated that holey silicon (HS), a new type of nanostructure where high density nanoscopic
holes are created in thin single-crystalline silicon membrane, exh
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