The origin of optical absorption and the electron spin resonance signal in amorphous carbon and amorphous hydrogenated carbon: a proposed density-of-states model

Abstract

The optical absorption spectral behaviour and the amount of electron spin resonance (ESR) signal in amorphous carbon and hydrogenated amorphous carbon are discussed. It is shown that the value of the Tauc optical gap obtained by applying the standard procedure has a different meaning from that in hydrogenated amorphous silicon and related materials. From its value, information on the shape of the gaussian-like π and π ∗ peaks of the density of states (DOS) due to the sixfold rings forming the graphite-like islands can be obtained. An empirical relationship between the Tauc gap and gaussian parameters is given. It is shown that the measured ESR signal cannot arise from the tail of the gaussian bands near the Fermi level. A different contribution exists, most probably arising from the same “defects” sitting near the graphite-like islands borders which are responsible for conductivity near room temperature. A model DOS taking into account the two contributions is presented.