Simultaneous Measurements of Thermal Conductivity and Seebeck Coefficients of Roughened Nanowire Arrays
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Simultaneous Measurements of Thermal Conductivity and Seebeck Coefficients of Roughened Nanowire Arrays J. S. Sadhu1, T. Hongxiang1, J. Ma1, J. Kim1 and S. Sinha1 1
Department of Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801, U.S.A.
ABSTRACT We report simultaneous measurements of thermal conductivity and Seebeck coefficient on arrayscale silicon nanowires fabricated by metal assisted chemical etching. The measurements are conducted on the solid and the mesoporous nanowire arrays (NWAs) obtained from etching 1 ohm-cm and 0.002 ohm-cm Si substrates respectively. We demonstrate control on sidewall morphology and doping of the arrays that have an aspect ratio up to 20 and 30 % areal coverage. We employ differential frequency-domain measurements, separately on the array and the corresponding substrate to obtain the temperature drop and Seebeck voltage contribution of the nanowire array. The technique is validated by measurements on bulk silicon across the resistivity 0.002-1 ohm-cm. The Seebeck measurements reveal quenching of the phonon drag in the nanowires in comparison to the bulk in the measured temperature range of 300 K- 500 K. The Seebeck coefficient shows a ~18 % decrease in the solid NWAs and ~22 % increase in the mesoporous NWAs at room temperature. The thermal conductivity is close to Casimir limit for the solid wires while it drops to ~2.5 W/mK in the mesoporous nanowires. INTRODUCTION Silicon-based thermoelectrics is receiving wide attention in the recent years with the possibility of enhancing thermoelectric figure of merit (ZT) by patterning silicon into nanostructures [1-4]. The enhancement is mainly achieved by the dramatic reduction of the thermal conductivity (k) below 5 W/mK in nanostructured silicon with careful control of surface morphology [1,5,6], porosity [3,4] and doping. In particular, studies on the electrolessly etched nanowires (EE NWs) show a remarkable reduction in the thermal conductivity [1,6] well below the diffuse phonon transport limit (the Casimir limit[7]) of these wires. This reduction is attributed to the surface roughness of these nanowires, inherent to the chemistry of the etching process [6]. While the focus remains on understanding the implications of surface disorder on the thermal conductivity [8,9], the phonon transport effects on the Seebeck coefficient of EE wires remains largely unexplored. The phonon drag [10] arising from electron-phonon interaction has a significant contribution to Seebeck coefficient (S) of bulk silicon even at room temperatures [11]. Hence, change in phonon transport in EE wires should also affect the phonon drag which forms the key focus of our study. In this work, we study the thermal conductivity and the Seebeck coefficient of silicon nanowire arrays fabricated by metal-assisted electroless etching [12]. We conduct the measurement on bulk-scale nanowire arrays (NWAs) on Si substrate which has several advantages over singlenanowire measurements [13] reported thus far. Array scale characterization is more pertinent
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