Origin of electrical contact resistance and its dominating effect on electrical conductivity in PbTe/CoSb3 composite

Abstract

The electrical contact resistance (Rcont) is a crucial parameter that influences the electrical conductivity (σ) in a composite material. Herein, we study the origin of Rcont and its effect on electrical properties of PbTe/CoSb3 composite. In particular, a series of (1-x)PbTe/(x)CoSb3 polycrystalline composite is prepared for x = 0.00, 0.25, 0.50, 0.75 and 1.00. The phase and chemical stability of the composites are investigated using X-ray diffraction and electron microscopy techniques. The Seebeck coefficient (S) and electrical conductivity (σ) measured across a wide temperature range of 25–400 °C depict a degenerate semiconductor nature for all the samples. The σ decreases from 3300 to 1000 S/cm with x and is attributed to a significant decrease in carrier mobility (µ) from 330 to 25 cm2/(s·V). For an in-depth investigation of the reduced σ in the composite, Rcont between the phases is measured on PbTe/CoSb3 layered sample using the scanning thermoelectric microprobe (STM) technique (Rcont=14.1 μΩ·cm2). Bruggeman asymmetrical model that considers Rcont further re-establishes the changes in σ vs. x. A deeper understanding of the origin of Rcont is developed by preparing the band diagram for both phases using the Kelvin probe force microscopy (KPFM) technique, which suggests the presence of a potential barrier at the junction. The current-voltage (I-V) characteristic of the heterojunction indicates that the charge carrier follows the ohmic nature in one direction and non-ohmic in another. Hence, it indicates a possible scattering of carriers at the PbTe/CoSb3 interface in the composite that reduces µ. This study paves a new direction to select the composite components with aligned band structure that can be promising to design efficient TE composite materials.