Properties and structure of amorphous hydrogenated carbon films

Abstract

Results are presented of a study of physical and optical properties of amorphous hydrogenated carbon films (a-CH _x ) prepared by d.c. magnetron glow-discharge decomposition of acetylene in argon, both as prepared and after heat treatment in vacuum. The dependences of density, deposition rate and hardness on the partial pressure of acetylene used in film preparation are examined. A description of the investigations of material produced by distillation from the films is given and the optical constants of both the films and the distilled material in the energy range 0·5 to 6·5 eV are determined; these constants are used both to deduce optical gaps and ascertain the number of π-bonded electrons per carbon atom. It is shown that the average carbon—carbon coordination number of the annealed films is very close to three. Measurements of the temperature dependence of electrical conductivity indicate variable-range hopping. The results of infrared spectroscopy show that heat treatment increases the aromatic content of the films. All experimental observations suggest a close structural similarity between the annealed films and glassy carbon, both having a mixture of threefold and four-fold coordination.