Electronic structure of sulfur-modified nanocrystalline carbon films

Abstract

Thin films of nanocrystalline diamond were grown by filament-assisted chemical-vapor deposition using methane as carbon precursor with high hydrogen dilution and hydrogen sulfide concentration ranging from 0to500ppm in the gas phase. The surface topography and electronic structure of these films (n-C:S) were investigated using ultrahigh-vacuum scanning tunneling microscopy and scanning tunneling spectroscopy (STS), respectively. Topographic image analyses depict that the root-mean-square roughness of the film surface and average grain size decreases with increasing sulfur incorporation either in gas phase or solid films. High-resolution scanning tunneling microscopy images reveal the localized regions of high conductivity (white) surrounded by less conductive regions (black) pointing at the existence of inhomogeneous mixture of sp2- and sp3-bonded carbon in aggregate or clustered and dispersed state. The surface density of states was determined using scanning tunneling spectroscopy where normalized differential conductivity, i.e., (dI∕dV)∕(I∕V) mimics local density of states (DOS). These methods were employed to understand the role of sulfur in the modification of both the surface microstructure and electronic structure near the Fermi level. The band edges were derived by taking tangents to the differential conductivity (dI∕dV) within a certain potential window of ±2eV of the Fermi level. The resulting band gap is found to be similar to that measured optically (Tauc gap). The Fermi level for undoped nanocrystalline carbon (n-C) was found just below the midgap indicating that n-C is a weakly p-type semiconductor. The STS DOS shows oscillatory behavior or peaks which we ascribe to states of the surface layer having relatively more graphitic or sp2-bonded carbon bonds. With higher sulfur addition, the Fermi level is found to move above the midgap. These results seem to agree quite well with our early work on electrical conductivity exhibiting n-type doping taking place and declining band gap at higher sulfur contents measured with spectroscopic ellipsometry technique on the same samples.