Relations between the deposition conditions, the microstructure and the defects in PECVD hydrogenated amorphous carbon films; influence on the electronic density of states

The relationships between the deposition conditions, the growth mechanisms, the microstructure and the electronic density of states of hydrogenated am orphous carbon (a-C:H) films prepared by PECVD from hydrocarbons are not ye t fully understood. We therefore performed a systematic study using several complementary techniques to determine the changes in the microstructure an d in the optical properties of a-C:H samples deposited from a dual micro-wa ve/radio-frequency discharge in methane as a function of the negative r.f. bias voltage applied to the substrate - V-b. The results reveal the existen ce of two successive film growth regimes when varying - V-b from - 30 to - 600 V, i.e. when increasing the ion bombardment during deposition, which le ad to different types of sp(2) C atoms clustering and H incorporation, but to the same density of paramagnetic defects. Models of the a-C:H microstruc ture are proposed in each case, and their influence on the electronic state s density is analysed in detail. It is shown that the H content and the pro portion of sp(2) C atoms play a minor role on the electronic properties, as compared to the nature, the size, the number and probably the distortions of the pi -bonded clusters. (C) 2001 Elsevier Science B.V, All rights reser ved.