Electronic properties of tetrahedral amorphous carbon investigated by scanning tunneling microscopy

Abstract

Scanning tunneling spectroscopy (STS) has been used to investigate the electronic structure of tetrahedral amorphous carbon (ta-C) films. The density of states (DOS) was derived from the normalized conductivity spectra at fixed tip heights. Band edges were defined by extrapolating the generalized conductivity (dI/dV) within ±2 eV of the Fermi level. The resulting band gap was found to be similar to that measured optically for ta-C films of different sp3 content. The Fermi level of undoped ta-C was found to lie below midgap, confirming ta-C to be a weakly p-type semiconductor. The valence band tail is found to be steeper than the conduction band tail, the opposite of the situation in a-Si:H. With nitrogen addition, the Fermi level is found to move above midgap, confirming that n-type doping is occurring. The band gap is found to decline at higher N contents. The DOS at larger tip heights and as a function of surface etching shows evidence of a surface layer with a narrower band gap, which we identify as the sp2-rich surface layer seen previously by cross-sectional electron energy loss spectroscopy. The STS DOS shows peaks which we attribute to states of the surface layer having more graphitic bonds. The scanning tunneling microscopy images show that ta-C deposited on Si has a root mean square surface roughness of 1.7 Å and this increases to about 10 Å for films deposited on metals such as Ti.