Electron states and microstructure of thina-C:H layers

Abstract

Thin layers of amorphous hydrogenated carbon (a-C:H) deposited on (100)Si/${\mathrm{SiO}}_2$ 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 Å. 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 Å 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 the 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. © 1996 The American Physical Society.