Pulsed laser deposition of Bi2Te3-based thermoelectric thin films

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Abstract

Thin films of p-type Bi0.5Sb1.5Te3, n-type Bi2Te2.7Se0.3, and n-type (Bi2Te3)90(Sb2Te3)5(Sb2Se3)5 (with 0.13 wt % SbI3) were deposited on substrates of mica and aluminum nitride (on silicon) using pulsed laser ablation at substrate temperatures between 300 °C to 500 °C. The films were characterized using x-ray diffraction and transmission electron microscopy for crystalline quality and epitaxial growth on the substrates. The surface morphology and microstructure were examined using scanning electron microscopy. X-ray mapping and energy-dispersive spectroscopy were performed to determine nonstoichiometry in the composition and homogeneity. The quality of the films, in terms of stoichiometric composition and crystal perfection, was studied as a function of growth temperature and laser fluence. The values of the Seebeck coefficient, electrical resistivity, and Hall mobility in the thin films were measured and compared with those in the bulk. Thermoelectric figure of merit of the films was evaluated from the measured parameters. Correlation of the thermoelectric properties, with the crystalline quality and stoichiometric composition of the films, showed the advantages of pulsed laser deposition of the multicomponent thermoelectric thin films. The results illustrate that laser physical vapor deposition is a suitable choice for deposition of multicomponent thermoelectric films. However, optimization of target composition, substrate temperature, and annealing of the films after deposition were found necessary to maintain the desired stoichiometry and low defect density. AlN/Si substrates provided better quality films compared to substrates of mica. Poor adhesion and cracking of the films on mica were found to be detrimental factors. Films deposited on AlN/Si substrates were found to show higher carrier mobility and higher values of Seebeck coefficient.

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