Nanostructure enhanced thermoelectric properties in SnSe and SnTe thin films

Abstract

The consumption of energy increases continuously because of the growth of economy, industry and global population; meanwhile there is large amount of heat dissipation lost as waste energy. This problem presents from large-scale, such as the hot fluids including exhaust gases in power plants, to small-scale such as the hot spots with higher power density in the microprocessors. There is a need for developing appropriate ways to collect or reuse the dissipated heat, among them thermoelectric devices can address the problem with advantages of compact, reliable and produce no pollutants. Throughout the development of thermoelectricity, different approaches for enhancing thermoelectric figure of merit were proposed and investigated, and different materials were discovered to have extraordinary thermoelectric properties. Among these materials, SnSe single crystal exhibits ultralow thermal conductivity and very high figure of merit. SnTe is another candidate who has similar crystal and electronic band structure as PbTe which is a well-known good thermoelectric material but contain toxic lead. Both SnSe and SnTe show potential to be prominent thermoelectric materials. In this work, two approaches enhancing the thin film thermoelectric figure of merit are investigated, namely grain size reduction and multilayered structure. SnSe thin films with different grain sizes are deposited, and their structural and thermoelectric characteristics are studied. SnTe and multilayered thin films composed of SnSe and SnTe are also deposited to investigate the difference of structural and thermoelectric properties between them. SnSe thin films are fabricated by pulsed-laser deposition at normal angle and glancing angle on different substrates. With glancing angle deposition, nanopillar structure is achieved in the SnSe film, presenting smaller grain size than that of the normal angle grown film. The Seebeck coefficient of the nanopillar structured film is enhanced with the power factor attains 18.5 uWcm-1K-2 due to the energy barrier scattering. SnTe film and a multilayered thin film with six alternating layers of SnSe and SnTe are deposited, and a layer-by-layer growth is demonstrated in the multilayered film. The studies in thermoelectric properties for the multilayered film also show an improvement in Seebeck coefficient compared to the SnTe film, leading to an increased power factor. The investigations done in this work revealed the effect of grain size and multilayered structure to the thermoelectric properties of SnSe and SnTe thin films. These results provide strategies for enhancing figure of merit of thermoelectric thin films.