The roughness of tetrahedral amorphous carbon (ta-C) films grown at room temperature is measured as a function of film thickness by atomic force microscopy, to extract roughness and growth exponents of $\alpha \sim 0.39$ and $\beta \sim 0–0.1$, respectively. This extremely small growth exponent shows that some form of surface diffusion and relaxation operates at a homologous temperature of 0.07, much lower than in any other material. This is accounted for by a Monte Carlo simulation, which assumes a smoothening during a thermal spike, following energetic ion deposition. The low roughness allows ta-C to be used as an ultrathin protective coating on magnetic disk storage systems with $\sim 1\mathrm{T}\mathrm{b}\mathrm{i}\mathrm{t}/{\mathrm{i}\mathrm{n}\mathrm{.}}^2$ storage density.

• Received 25 July 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.226104