(Invited) Epitaxial Growth of Quasi-Low Dimensional Materials for Novel Thermoelectric Films

Abstract

Thermoelectric conversion devices or modules with high performance have been expected for the electrical power generation from wasted heat. Then, thermoelectric materials with high dimensionless figure of merit zT at temperature T , described as S 2 σT/κ have been intensively sought, where S is Seebeck coefficient, σ is electrical conductivity, and κ is thermal conductivity. Therein, intercorrelation among three thermoelectric properties has been a big barrier against high zT. In conventional approach, the heavy elements such as rare metals are used because they inherently bring small κ in the materials. Now, ecofriendly ubiquitous element materials without toxic, rare, or high cost elements, showing high zT, are expected in the application view.Introduction of nanostructures into materials is one of the promising ways to increase zT because of phonon scattering [1], quantum confinement effect in low dimensional materials and by energy filtering [2], and so on. Therein, structure design determines the properties, and then, this method can be expected to bring high zT in the ecofriendly and low-cost ubiquitous element materials. Furthermore, it is recently found that low dimensional materials and quasi-low dimensional materials such as layered materials with 2D structures in the unit cell show high zT values. Therefore, low and quasi-low dimensional materials for extraordinarily high zT values have been expected to be developed.We have been developing a lot of kinds of well-controlled nanostructure films by SiO2 film technique. One of the examples: Si-based films including ultrasmall semiconductor nanodots with controlled interfaces, strains, crystal orientations, and compositions [3]. Furthermore, we have developed the epitaxial growth method of quasi-low dimensional material films composed of ecofriendly group IV elements on Si substrates: FeGeγ including 1D Ge helical structure and Ca-intercalated multi-layered silicene films. Here, we will introduce the epitaxial growth results of quasi-low dimensional material films. Epitaxial growth of FeGeγ films on Si were achieved by using Ge epitaxial nanodots as seed crystals[4]. In the case of Ca-intercalated multi-layered silicene, silicene bucked structure was found to be deformed. The deformation enhanced Seebeck coefficient. As a result, this sub-angstrom atomic displacement brought power factor enhancement while keeping high electrical conductivity [5]. In this talk, we will present the epitaxial growth process of quasi-low-dimensional material films on Si substrates for thermoelectric film devices composed of ecofriendly and low-cost thermoelectric materials .[1] Y. Nakamura, et al., Nano Energy 12, 845 (2015).[2] T. Ishibe, et al., ACS Appl. Mater. Inter. 10, 37709 (2018).[3] Y. Nakamura, et al., Sci. Technol. Adv. Mater. 19, 31 (2018).[4] T. Terada, Acta Materialia 236, 118130 (2022).[4] T. Terada, et al., Adv. Mater. Inter. 9, 2101752 (2022).