Nano-structured thermoelectric (Ag, Sb) PbTe thin films deposited by fs Ti:Sapphire pulsed laser ablation

The spontaneous tendency to deposit nanostructured material by ablation operated by an ultra-short laser beam has been used to grow thin films of PbTe material, for thermoelectric applications. It is in fact well known that the process of nano-structuring with dimensions smaller than the mean free path of phonons while larger than those of charge carriers is an essential element for obtaining a material with good thermoelectric characteristics. In these conditions, in fact, the phonons are scattered and the lattice thermal conductivity reduced, while the mobility of charge carriers and electron conduction remains unchanged.The deposition process has been performed in vacuum, at room temperature. The substrate was mirror polished technical alumina. Different atomic percentage of Ag and Sb has been added by a multi-target operating system in vacuum. A thermal post annealing, in vacuum at temperature around 500°C, close to the operating conditions of thermoelectric material, has been performed to test the structural and morphological stability of the deposited layers. The surface morphology and chemical composition have been analysed by FE-SEM and EDS, while the structure of the film was analysed by grazing incidence X ray diffraction (GI-XRD) . The Scherrer analysis for dimensions of crystallites confirmed the presence of nano-structures, of the order of 30-40 nm. Electrical resistivity of the samples has been studied by the four point probe method. From conductivity values and Seebeck parameter determination, the power factor of deposited films was calculated. From the first data of thermal conductivity, obtained by the method "3 ?", the films appear to have good thermal insulation properties. Data obtained are comparable with the values obtained on the corresponding bulk material.