[Bi]:[Te] Control, Structural and Thermoelectric Properties of Flexible Bi _x Te _y Thin Films Prepared by RF Magnetron Sputtering at Different Sputtering Pressures

Abstract

In this work, flexible Bi _x Te _y thin films were prepared by radio frequency (RF) magnetron sputtering using a Bi₂Te₃ target on polyimide substrate. The effects of sputtering pressures, which ranged between 0.6 Pa and 1.6 Pa on the [Bi]:[Te] ratio, and structural and thermoelectric properties were investigated. The [Bi]:[Te] ratio of thin film was determined by energy-dispersive spectrometry (EDS). The EDS spectra show the variation of the [Bi]:[Te] ratio as the sputtering pressure is varied. The film deposited at 1.4 Pa almost has a stoichiometric composition. The selective films with different [Bi]:[Te] ratios and sputtering pressures were characterized by their surface morphologies, crystal and chemical structures by field emission scanning electron microscopy (FE-SEM), x-ray diffraction (XRD) and Raman spectroscopy, respectively. Electrical transport properties, including carrier concentration and mobility, were measured by Hall effect measurements. Seebeck coefficients and electrical conductivities were simultaneously measured by a direct current four-terminal method (ZEM-3). The XRD and Raman spectroscopy results show a difference in microstructure between BiTe and Bi₂Te₃ depending on the [Bi]:[Te] ratio. Electrical conductivity and Seebeck coefficient are related to the crystal and chemical structures. The maximum power factor of the Bi₂Te₃ thin film is 9.5 × 10⁻⁴ W/K² m at room temperature, and it increases to 12.0 × 10⁻⁴ W/K² m at 195°C.