Additive-free electrodeposition of cobalt on silicon from 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

Abstract

This study describes the use of the ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMI][BF4]) as an efficient and environmental-friendly electrolyte to obtain nanocrystalline cobalt (Co) electrodeposits on a silicon substrate without any additives. Co films were successfully electrodeposited from CoCl2 dissolved in a Lewis basic [BMI]Cl-[BF4] IL system, at 25 °C and 50 °C. Electrochemical techniques were employed to evaluate electrochemical parameters and to investigate the nucleation/growth mechanism involved. Results show that the electrodeposition of Co, under both temperatures, takes place by three-dimensional instantaneous nucleation with diffusion-controlled growth. Considerably small diffusion coefficients of the dissolved Co(II) species were found (1.60 × 10−10 cm2.s−1 and 8.21 × 10−10 cm2.s−1 for 25 °C and 50 °C, respectively). Chemical composition, surface morphology and crystallinity of the electrodeposits were characterized by energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. High-quality and purity metallic cobalt electrodeposits were obtained. Co films were compact, smooth, bright and adhering to the substrate, exhibiting hcp single-phase structure with nanosized metallic particles of Co. Elevated current efficiencies (94.8% and 96.8% for 25 °C and 50 °C, respectively) were found, showing that [BMI][BF4] IL shows a large potential for the Co electrodeposition on a silicon substrate.