Chemical structure and electrical characteristics of diamondlike carbon films

The relationship between the chemical structure and electrical characteristics of diamondlike carbon (DLC) films has been clarified. The DLC films were formed in atmospheres with different ratios of methane to argon under the photoemission-assisted Townsend discharge. The dependence of the dielectric constant of the films on methane concentration in the synthesis was arch-like with a maximum. The breakdown strength was constant irrespective of the synthesis atmosphere and was approximately the same for both electrical polarizations. Raman spectra of the films were deconvoluted into five active bands. Raman analysis in conjunction with the sp² cluster model elucidated the DLC structure. The sp² cluster model comprises sp² clusters floating in a dielectric matrix sea. The sp² clusters were rather aliphatic for films formed in low methane concentration. The clusters grew to become aromatic with increasing methane concentration. Defects or dangling bonds increased similarly, but they were terminated with hydrogen for films formed in high methane concentration. The hydrogen-terminated bonds occupied large amounts of space in the DLC films, causing internal strain. The dielectric constant of the whole DLC film was determined by the size of sp² clusters, dielectric constant of the matrix sea material, and space volume induced by the hydrogen–carbon bonds. The breakdown strength was determined by the balance between the size of sp² clusters and density of dangling bonds in the matrix sea. However, because the dependences of these factors on methane concentration were opposing, the breakdown strength was approximately constant irrespective of methane concentration.