Enhanced Thermoelectric Properties of Strongly Degenerate Polycrystalline Silicon upon Second Phase Segregation
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Enhanced Thermoelectric Properties of Strongly Degenerate Polycrystalline Silicon upon Second Phase Segregation Dario Narducci1, Ekaterina Selezneva1, Andrea Arcari1, Gianfranco Cerofolini1, Elisabetta Romano1, Rita Tonini2, and Gianpiero Ottaviani2 1 Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, via R. Cozzi 53, 20125 Milano (Italy) 2 Dipartimento di Fisica, Università di Modena e Reggio Emilia, via Campi 213, 41100 Modena (Italy) ABSTRACT We report the study of the thermoelectric properties of degenerate, boron-doped polycrystalline silicon on insulator structures. The occurrence of a regime where both the Seebeck coefficient and the conductivity increase is confirmed. This results in a power factor P of 13 mW K-2 m-1. We propose that such high values of P may be determined by adiabatic energy filtering occurring at grain boundaries decorated by segregated boron.
INTRODUCTION Silicon nanowires and other dimensionally constrained systems were demonstrated [1] to conveniently tackle the issue of increasing the thermoelectric figure of merit zT (= α2σT/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature) by decreasing κ while not lowering either α or σ. In this way zT can reach values close to 1 at T = 300 K. On the other hand, the power factor P = α2σ is intrinsically limited in diluted semiconductors by the functional dependency of α and σ upon the carrier density [2]. In such a density range, neglecting the marginal dependence of the mobility upon the ionized impurity density and referring e.g. to p-type materials, α scales with the logarithm of the hole density p while σ ∝ p. Actually, in single-crystal Si and based upon early investigations [3,4] it has been computed [5] that the optimal carrier density for zT is around 1018-1019 cm-3. However, interesting thermoelectric properties were anticipated for degenerate or nearlydegenerate semiconductors [6, 7], although only a moderate research effort has walked this alley. Recently, Ikeda and coworkers [8] disclosed an unexpected increase of the Seebeck coefficient in phosphorous-doped silicon-on-insulator (SOI) structures occurring at electron densities n in excess of 3.5×1019 cm-3, claiming for α(n) a maximum at n ≈ 7×1019 cm-3. In this paper we report the results of an investigation of the electronic transport properties of degenerate, boron-doped polycrystalline silicon structures, confirming the occurrence of a regime where both the Seebeck coefficient and the conductivity increase. We will show that, differently than in n-type single crystalline films, the high values of the figure of merit results from the energy filtering of grain boundaries (GB) decorated by segregated impurities. EXPERIMENT Films of polycrystalline silicon (450 nm thick) deposited onto oxidized Si substrates were implanted with boron through an Al sacrificial layer with a fluence of 2×1016 cm-2 at an energy
of 60 keV, and were then annealed at 1050 °C for 30 s. This led to a total
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