Reduction of thermal conductivity in semiconducting composite films consisting of silicon and transition-metal silicide
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Reduction of thermal conductivity in semiconducting composite films consisting of silicon and transition-metal silicide nanocrystals N. Uchida 1, Y. Ohishi2, K. Kurosaki2, S. Yamanaka2,3, T. Tada1, and T. Kanayama4 1
Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Japan 2 Graduate School of Engineering, Osaka University, Japan 3 Research Institute of Nuclear Engineering, University of Fukui, Japan 4 National Institute of Advanced Industrial Science and Technology, Japan ABSTRACT We observed significant reduction of thermal conductivity in semiconducting composite films of Si and molybdenum (Mo)-silicide nanocrystals (NCs). These films were synthesized by phase separation due to annealing at 700 -1000°C from sputtered amorphous Mo–Si alloy. Transmission electron microscope images showed that the NCs were grown to diameters of~10 nm in the films by annealing at 800°C. Raman scattering spectra showed lower shift of peak positions of Si transverse optical (TO) phonon due to the confinement effect and the tensile stress. The electrical resistivity of the films was 0.17- 9 Ωm at room temperature and showed a semiconducting temperature dependence at 20-400 K. Thermal conductivity of the film was reduced to 4.4 W/mK by enhancement of phonon scattering at NC interfaces, suggesting that the composite film is promising as a high-efficiency Si-based thermoelectric material. INTRODUCTION Silicon is a useful material for wide range of applications, such as electronic devices, solar cells and microelectromechanical systems etc., but is not suited to thermoelectric power generation because the thermal conductivity к is high [1]. However, it is possible to reduce the к value by introducing nanostructures into Si and thereby enhancing phonon scattering. Many groups have reported the reduction of thermal conductivity in Si nanostructures such as nanocrystals (NCs) [2-4], nanowires [5, 6], and nanomeshes [7, 8]. We can develop highperformance Si-based thermoelectric devices assembled from Si nanostructures. While the Si nanowires and nanomeshes have very low к of 0.76-1.6 W/mK and 1.5-1.9 W/mK respectively, it is difficult to mass-produce Si nanowires and nanomeshes, because they are fabricated by lithographic techniques. Therefore, as a first step, Si NCs were selected for use as unit nanostructures. The SiNCs composite materials have к of 6 -70 W/mK, which is much lower than the value of 87.3 W/mK for heavily doped Si at room temperature, resulting from phonon scattering at the Si NC interfaces [2-4]. This к reduction is very sensitive to the size of Si NCs. To obtain a larger reduction in к, we must optimize the size distribution of Si NCs in nanostructured Si materials. Dense Si NCs are obtained by phase separation of amorphous transition metal (M=Ni, Mo, Nb and W) and Si alloys into Si and M-silicide phases by thermal annealing at 550–900°C in Ar gas [10, 11]. For Mo-Si alloy films with a composition of Si/Mo= 10 to 12, the formation of Si NCs with diameters of ~ 10 nm was obs
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