Positioning of cobalt atoms in amorphous carbon films by pre-selecting the hydrogen concentration

Amorphous carbon and hydrogenated amorphous carbon layers were implanted at room temperature with Co ions to investigate the role of hydrogen on the Co distribution. Amorphous carbon (a:C) and hydrogenated amorphous carbon (a:C-H) films were prepared by mass selective ion beam deposition with a 5 kV acceleration voltage using C⁺ and C₃H₆ ⁺ ions, respectively. The typically 100 nm thin films were implanted with Co using a 30 kV acceleration voltage to a fluence of 4×10¹⁶ cm^-2. Raman measurements showed that Co implantation in hydrogenated amorphous carbon causes increased sp² clustering while in amorphous carbon there is significant rehybridisation of carbon from sp³ to sp² bonding. High resolution Rutherford backscattering measurements indicated that in the absence of hydrogen in the base matrix, the implantation profile assumes a unimodal distribution as predicted by simulations. However, in the presence of hydrogen the effects of collision cascade enhanced diffusion are significant in altering the implantation profile resulting in a bimodal distribution. The difference in the Co depth distribution between a:C and a:C-H films is explained by the change in thermal conductivity of the carbon matrix in the presence of hydrogen. The ability to position Co (magnetic atoms) in the surface region of diamond-like carbon films offers great advantages for applications in novel magnetic devices.