ICT2014 Abstracts

 To use Si for thermoelectric applications, reduction of high thermal conductivity k of 142 W/mK is a key. Recently, we developed semiconducting composite films of Si and Ni silicide NCs (Ni-Si nanocomposite film) have much lower k of 3-7 W/mK at RT than that of bulk Si. The Ni-Si nanocomposite films have much higher dimensionless figures of merit (ZT , 0.13 for p-type and 0.06 for n-type Ni-Si nanocomposite films) than those of bulk Si, the amorphous Si film, and the polycrystalline Si film (ZT<0.01) at RT.However, the ZT of the n-type Ni-Si nanocomposite films is 54% lower than that of the p-type films. In this paper, for further improvement of the thermoelectric properties of the n-type Ni-Si nanocomposite films, we perform alloying of Si nanocrystals with Ge atoms (Ni-SiGe nanocomposite films), because n-type SiGe bulk nanocrystal composites have higher ZT values than those of Si bulk nanocomposites.
  The Ni-SiGe nanocomposite films were synthesized by phase separation from amorphous P doped (2mol%) Ni-SiGe alloy films with composition of NiSi₁₆Ge₄, followed by thermal annealing at 1000-1200℃. The electron mobility and carrier concentration were estimated to be 23.1 cm²/Vs and 7.4×10¹⁹ cm^-3, respectively, for the n-type Ni-SiGe film annealed at 1200℃. The Seebeck coefficient was estimated to be −98 µV/K at RT using a scanning thermal probe micro-image (STPM) method. The power factor was 0.6 mW/mK2. The k was 2.0 W/mK, which was lower than that of the n-type Ni-Si nanocomposite films (3.4-7.7 W/mK). The thermal conductivity was reduced by SiGe alloying for n-type Ni-Si nanocomposite films, resulting in the 30% increase of the ZT (=0.08) at RT. We believe that n-type Ni-SiGe and the p-type Ni-Si films are promising combination in developing film type thermoelectric applications.