Role of substrate effects on the morphological, structural, electrical and thermoelectrical properties of V2O5 thin films

The present work focuses on the influence of different substrates on the morphological, compositional, phase purity, structural and transport properties of vanadium pentaoxide (V2O5) thin films. Thin films of V2O5 were fabricated on different substrates: glass, quartz, Si, and alumina (hereafter these films are referred to as V2O5:G, V2O5:Q, V2O5:Si and V2O5:A, respectively) using inorganic sol-gel with V2O5 powder and hydrogen peroxide (H2O2) as precursors by spin coating. Films deposited on glass substrates were found to be amorphous in nature with smooth surfaces, whereas films deposited on quartz, silicon, and alumina substrates exhibited a polycrystalline nature, having an orthorhombic structure with space group Pmmn. The crystallinity improves from quartz to silicon and the best crystalline films were fabricated on alumina. Electrical measurements as a function of temperature and substrate are investigated and characterized by measuring the resistivity, Hall and Seebeck coefficients. Negative values of Hall and Seebeck coefficients reveal that all the films are of n-type semiconductors. Electrical resistivity as well as charge carrier density decreases from the films on glass to quartz to silicon and to alumina. Carrier mobility decreases in the following order V2O5:G > V2O5:A > V2O5:Si > V2O5:Q, whereas the Seebeck coefficient varies in the reverse order. Variation of these transport parameters has been understood on the basis of scattering and trapping of charge carriers along the grain boundaries. Furthermore, a model based on thermodynamics is proposed to explain the effect of substrates on the crystallinity of thin films. Interactions between sol and substrate (adhesive forces) determine the thickness, phase purity, structural and morphological properties of thin films. As the magnitude of adhesive forces increases, both film thickness and crystallinity increase.