Electron states and microstructure of thin a-C:H layers

Thin layers of amorphous hydrogenated carbon (a-C:H) deposited on (100)Si/SiO2 substrates were studied using atomic force microscopy, electron spin resonance (ESR), and internal photoemission spectroscopy. Layers with optical band gap in the range 3-0.7 eV were found to be homogeneous with respect to the interaction with oxygen under ultraviolet irradiation down to a lateral size of 5 Angstrom. However, topographic features are developed under oxygen plasma exposure, where the decrease in the optical band gap is found to be correlated with the appearance of 300-500 Angstrom lateral size features, ascribed to dense regions in a-C:H. Electron states near the Fermi level of a-C:H are related to electrically neutral diamagnetic network fragments. In the a-C:H of smallest band gap delocalized unpaired electrons are revealed, the observed ESR anisotropy suggesting a unidirectional ordering along the normal to thr surface plain. Chainlike structures of carbon atoms are proposed as the network fragments responsible for the variation in the a-C:H optical gap width.