Control of microstructure and composition of reactively sputtered vanadium nitride thin films based on hysteresis curves and application to microsupercapacitors

Abstract

Vanadium nitride (VN) thin films were prepared by reactive DC magnetron sputtering of a vanadium target using nitrogen as reactive gas. The structural, morphological, and compositional evolution of these films is described based on hysteresis diagrams plotting the sputtering power versus nitrogen flow rate. These diagrams, measured across various cathode voltages and discharge pressures, unveil three distinct deposition regimes: metallic, intermediate, and contaminated. The microstructure of the films was found to be closely linked to the deposition regime, ranging from dense and amorphous in the metallic regime to porous and crystalline in the contaminated regime, while the composition varies from vanadium-rich to near-stoichiometric VN. Sputtered VN thin films used as electrodes for microsupercapacitors were investigated by cyclic voltammetry. Results highlight that the intermediate deposition regime, characterized by high crystallinity and porosity, yields the highest capacitance values, above 900 F cm−3. Such high volumetric capacitance is attributed to the highly porous structure and large specific surface area. In addition, in these deposition conditions, films are composed of crystalline VN with a significant amount of amorphous VOx on the surface, which allow these thin film electrodes to behave both as current collectors and pseudocapacitive electrodes. This work gives detailed insights into VN thin film microstructure and composition in reactive sputtering based on hysteresis curves. It emphasizes how we could use these curves to target specific microstructure, composition, and eventually achieve functional properties. In particular, these findings have important implications for the design and optimization of microstructured electrodes for energy storage applications.