Electron Spin Resonance Study of Hydrogen-Free Germanium-Doped Diamond-Like Carbon Films

Hydrogen-free germanium (Ge)-doped diamond-like carbon (DLC) films deposited on SiO2 substrates are studied by electron spin resonance (ESR) technique at T = 5-295 K. The Ge-free and DLC films with 1 and 2.5 at% Ge content having low cytotoxicity are investigated. A single Lorentzian ESR line resulting from exchange interaction between localized (LE) and nonlocalized electrons (NE) is observed. The spin susceptibility temperature dependence deviates from the Curie-Weiss law at T > 75 K due to Pauli contribution caused by NE. The antiferromagnetic low-temperature spin ordering in DLC films, decreasing with Ge content, is observed. The LE contribution is significant for Ge-free and DLC:1%Ge, whereas NE makes the main contribution to DLC:2.5%Ge. The LEs are related to sp (2)/sp carbon-related centers, while the NEs are assigned with electrons in the sp (3) orbital. The correlation between the sp (3)/sp (2) ratio and the shift of g-factor for NE to lower values with Ge doping is established. The Korringa relaxation model is provided for the ESR linewidth temperature dependence. The higher density of LE states on Fermi energy level caused by Ge doping of DLC films is proposed.