Calculation of electronic properties of defects in diamond: Application to electron emission

Electron field emission from diamond or diamond coated surfaces has been known experimentally to yield large currents at low fields. It has been speculated by the authors [Z.-H. Huang, P. H. Cutler, N. M. Miskovsky, and T. E. Sullivan, Appl. Phys. Lett. 65, 2562 (1994); J. Vac. Sci. Technol. B 13, 526 (1995)] that electron transport through band-gap states can be responsible for sustaining such currents. These states may be generated by point and/or extended defects such as vacancies and grain boundaries in chemical-vapor-deposition diamond films. Electronic and atomic properties of these defects are calculated using a tight binding approach. The model consists of a single vacancy or H substitution in a repeated extended cell consisting of 27 carbon atoms. The presence of vacancies or H substitution produces states that are spatially localized and whose energies lie in the gap. These states span an energy spectrum of about 1-2 eV just above the top of the valence band. The calculations for a concentration of about 10(19) cm(-3) suggest that these defect states are too low in energy to couple to those states which can produce the tunneling current in field emission. Additional work is needed to clarify the transport mechanism and how it depends on vacancy concentration including the possibility of vacancy-vacancy interaction which can lead to broadening of the discrete states into bands. (C) 1996 American Vacuum Society.