Electronic properties of the emission sites of low-field emitting diamond films

Using the functional capabilities of a special scanning tunneling field emission microscope (STFEM), a new measuring procedure for studying the electronic properties of the emission sites of low-held emitting materials has been developed. The position of intensive, separately situated emission sites ('individual' emission centres) was determined over a large surface area, whereupon both an electron escape and a surface electron potential at the 'individual' centre were investigated in detail. We report on the experimental results of the STFEM study of electronic properties of low-field emitting diamond films which have a composite structure consisting of diamond and nanocrystalline graphite phases, and show stable electron emission at fields 3-8 V/mu,. We observed a non-monotonous dependence of the electron escape from the emission site on the bias voltage - i.e, in a certain voltage range, the electron escape decreased with increasing voltage. Also. in the same voltage range, the effective surface potential barrier at the emission site shows a peak value. It is supposed that such anomalous electronic properties are due to resonant tunneling through the grain boundary region near the interface of the diamond and graphite phases. In addition, the results of energy resolved electron emission measurements are analyzed. The emission spectra show a cut-off at high energies and a tail towards low energies. The best approximation of the spectrum tail is reached if electron tunneling through a triangular potential barrier of 0.1-0.2 eV relative to the Fermi level of the diamond film substrate, and the emission field of 50-100 V/mu m are taken into account. It thus can be supposed that the electrons are emitted from the valence band, and the electric field at the emission site is enhanced. A possible mechanism for the low-held electron emission from chemical vapor deposited (CVD) diamond films, including both the geometric field enhancement and the quantum well effect at the grain boundary, is discussed. (C) 2000 Elsevier Science S.A. All rights reserved.