Fabrication and thermoelectric properties of sintered group-IV nanoparticles

Energy harvesting using solid-state-based thermoelectric generators is one of the top- ical areas of materials research. High conversion efficiencies require materials with a high Seebeck coefficient, a high electrical and a low thermal conductivity. Structurally complex and heavily doped semiconductors are proposed to fulfill these needs. In this work, the fabrication of laser-sintered Si, Ge and SiGe nanoparticle (NP) thin films and their thermoelectrical performance are investigated . The resulting films are structurally complex on a variety of length scales. Using a high intensity pulsed laser a characteristic structure of the laser-sintered film in the form of well connected meanders evolves, the structural dimensions of this mesoporous network are tunable by the sinter-fluence. Elec- trical conduction is enhanced by more than 5 orders of magnitude by the laser-sintering process, caused by the increased interconnection of the former NPs. Typical values of 0.5 S/cm are found for undoped Ge, while undoped Si exhibits 10−5 S/cm. Doping of the films can further increase the electrical conductivity to up to 100 S/cm. The 2- dimensionality of the meander network effectively reduces the electrical conductivity by one order of magnitude compared to non-porous reference material or non-porous films. The Seebeck coefficient is not influenced by the film morphology but only depends on the samples’ doping concentration, which can be as high as 1 %. Both, p- and n-type doping can be realized using two different methods in this work: by already doping the precursor NPs during their synthesis or by application of dopants onto the NP film prior to its laser-sintering. In the latter method the dopant source is a liquid and the sinter laser incorporates and electrically activates the dopants in the host material. The method is called laser-assisted wet-chemical doping and is developed and extensively investigated in this work. Almost all group-III and -V elements can be used to dope Si as well as Ge films using this novel method, leading to charge carrier densities of up to 1020 cm−3, as determined by Raman spectroscopy. Beyond the electrical characterization the determination of the in-plane thermal conductivity using optical non-contact methods is a further focus of this work. Prepared in a freely suspended manner, the films are thermally excited by a laser which at the same time evokes Raman scattering. The resulting characteristic wavenumber shift is related to the sample’s temperature. Numerical simulations of the temperature distribution allow to deduce the effective thermal conductivity, which is found to be 0.5 W/m K for laser-sintered undoped Ge NPs and 0.05 W/m K for heavily doped laser-sintered SiGe NP films. The thermoelectric figure of merit calculated from these experiments indicates that laser-sintered NP thin films are competitive with other SiGe thermoelectric materials.