Tuning Deposition Conditions for VN Thin Films Electrodes for Microsupercapacitors: Influence of the Substrate Bias Voltage

This study delves into the impact of substrate bias voltage on vanadium nitride thin films deposited via DC magnetron sputtering. By increasing the substrate bias voltage from 0 to −200 V, the microstructure of the films changes from crystalline and porous to amorphous and dense. This obvious change in microstructure is due to atomic peening effect, which is the sputtering of the film by energetic cations at high bias voltage during thin film growth. For capacitive storage devices, it is known that the microstructure of the electrode is key to achieve high capacitance, with the aim to maximize the electrode surface area and concomitantly the areal capacitance, while keeping a low characteristic time to achieve fast charge/discharge rates. In this study, we reveal that a trade-off must be found between areal capacitance and characteristic time. Samples sputtered with low substrate bias voltages present higher areal capacitance but also higher characteristic time compared to thin films sputtered with high substrate bias voltages. The evolution of the characteristic time associated with fast charge/discharge aligns with the electrical conductivity of VN films as determined by four-point probes measurement and indicates that the cycling rate is limited by electrical properties of the VN film.