Electronic properties of tetrahedral amorphous carbon investigated by scanning tunneling microscopy

Scanning tunneling spectroscopy (STS) has been used to investigate the elec tronic structure of tetrahedral amorphous carbon (ta-C) films. The density of states (DOS) was derived from the normalized conductivity spectra at fix ed tip heights. Band edges were defined by extrapolating the generalized co nductivity (dI/dV) within +/-2 eV of the Fermi level. The resulting band ga p was found to be similar to that measured optically for ta-C films of diff erent sp(3) 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 b and tail is found to be steeper than the conduction band tail, the opposite of the situation in aSi:H. With nitrogen addition, the Fermi level is foun d to move above midgap, confirming that n-type doping is occurring. The ban d gap is found to decline at higher N contents. The DOS at larger tip heigh ts and as a function of surface etching shows evidence of a surface layer w ith a narrower band gap, which we identify as the sp(2)-rich surface layer seen previously by cross-sectional electron energy loss spectroscopy. The S TS DOS shows peaks which we attribute to states of the surface layer having more graphitic bonds. The scanning tunneling microscopy images show that t a-C deposited on Si has a root mean square surface roughness of 1.7 Angstro m and this increases to about 10 Angstrom for films deposited on metals suc h as Ti. (C) 1999 American Institute of Physics. [S0021-8979(99)05203- 2].