It was recalled that electron field emission from diamond surfaces yielded large currents at low fields, and that it had been proposed that electron transport through band-gap states was responsible for sustaining such currents. The states could be generated by point defects and extended defects such as vacancies and grain boundaries in chemical vapor deposited films. The electronic and atomic properties of these defects were calculated by using a tight-binding approach. The model consisted of a single vacancy or H atom which was located substitutionally in a repeated extended cell that consisted of 27 carbon atoms. The presence of vacancies or substitutional H produced states that were spatially localized and whose energies lay in the band gap. The states spanned an energy spectrum of some 1 to 2eV, just above the top of the valence band. Calculations that were performed for a concentration of about 1019/cm3 suggested that these defect states were too low in energy to couple to those states that could produce the tunnelling currents during field emission. It was concluded that further work was required in order to clarify the transport mechanism, and its dependence upon the vacancy concentration; including the possibility of vacancy-vacancy interactions which could lead to a broadening of the discrete states into bands.

N.M.Miskovsky, P.H.Cutler, Z.H.Huang: Journal of Vacuum Science and Technology B, 1996, 14[3], 2037-40