Enhanced room temperature thermoelectric power factor of β-Zn4Sb3 thin films via surface roughness optimization

Abstract

Roughness induced resistivity and surface scattering components play vital role in optimization of charge carrier transport properties in thin film surfaces. In present work, effects of surface roughness on the thermoelectric (TE) parameters Seebeck coefficient (S), electrical conductivity (σ) and the power factor (PF) were investigated. Melt-quenching method was employed for the synthesis purpose of β-Zn4Sb3. Thin films of various thickness ranging from 69 nm to 363 nm were deposited using thermal evaporation process which resulted in to variation of surface roughness from 6.94 nm to 37.51 nm. Maximum S and PF values of 227 μVK-1 and 814 μWm-1K-2 at room temperature (RT) were obtained for thin film with roughness of 28.94 nm which were an enhancement of 3.05 and 41.84 times respectively compared to film with roughness of 6.94 nm. Enhancement of S values with increasing roughness was attributed to the origination of surface energy filtering effects. Maximum σ value of 2.25×104 Sm-1 was obtained for film with roughness magnitude of 21.75 nm. Uptrend in σ values was attributed to the longer mean free path for carrier caused by increased crystallite sizes and then downtrend was attributed to the increased resistivity, surface scattering and trapping of carriers caused by dominance of roughness effects. X-ray Diffraction (XRD) and Raman Spectroscopy were employed to investigate structural characteristics of surfaces which revealed enhancement in crystallite sizes with increasing film thickness. Thin films thickness values of 69 nm, 146 nm, 239 nm, 286 nm and 363 nm for prepared surfaces were determined by employing cross-sectional Field Emission Scanning Electron Microscope. Atomic Force Microscope (AFM) was employed to investigate topographical characteristics and height irregularities of surfaces. 3D micrographs of surfaces were constructed and parameters including roughness, skewness and kurtosis were determined. Surface roughness consistently enhanced with increasing thickness attributed to the vertical accumulation and growth of larger crystallites. Ultraviolet Visible Spectroscopy (UV-Vis) was employed to investigate optical properties and estimation of band gap. Reduction is band gap was attributed to the lowered confinement effects and enhanced light absorbing tendency of rougher surfaces.