Effective correlation energies for defects in a-C:H from a comparison of photelectron yield and electron spin resonance experiments

Abstract Amorphous hydrogenated carbon films (a-C:H) were deposited by r.f. plasma CVD from methane, varying the self bias potential of the substrate electrode by means of the r.f. power coupled into the discharge. Films were characterized by IR and optical spectroscopy, confirming a transition from polymer-like to diamond-like (DLC) material with increasing self bias. One set of samples was investigated in situ by photoelectron and photoelectron yield spectroscopy, from which the density of gap states and their spectral distribution was derived. An identical set of samples was then examined by electron spin resonance to determine the density of paramagnetic defects. From a comparison of the results of both experiments, a lower limit for the effective correlation energy of the defect states was extracted which gave surprisingly large values for the correlation energy of the DLC material. In addition to the interpretation of the results within a spatially uniform model, the influence of a possible surface band bending on the evaluation of the correlation energies is also discussed.