Conductivity of boron-doped polycrystalline diamond films: influence of specific boron defects

    Diamond is known to be a remarkable material due to its particularly attractive properties combining chemical resistance, optical transparency, thermal conductivity, high bio-inertness and electrochemical properties. Diamond is generally recognized as an insulating material. Once successfully doped it is a wide bandgap semiconductor material with an excellent application potential due to the unique combination of its physical and electronic properties. The small boron (B) atom seems to be the only efficient dopant atom in diamond, which can be incorporated with high reproducibility and in high enough concentrations to be useful for electronic devices. Here we studied the conductivity of BDD polycrystalline films which were synthesized by microwave plasma enhanced chemical vapour deposition on quartz substrates. We focused on the role of B atoms inside diamond grain in terms of B contributionto the continuum of diamond electronic states. We studied B concentration close or above themetal-insulatortransition.Usingacombinationof theoretical and experimental techniques (plane-wave Density Functional Theory, NDP, resistivity and Hall effect measurements, AFM, Raman spectroscopy) we studied B defects location, vibration states, conductivity, free hole concentration inside the grains, holes mobility. When the concentration of B is low, single substitution defects are predominant. Hopping conduction using the excited level of B plays a very important role. With the impurity concentration increase, the wave functions of the B excited states overlap and form an impurity band – with the increasing concentration of single B atoms the defects states smoothly pass to the top of the valence band, which enables metal-like conductivity behaviour  of the material. With an increase in B concentration more complex defects – mainly B dimers – accompanied by a larger deformation of the lattice are set. B dimers have such a high acceptor energy that ionization from valence band to impurity level is practically excluded. The impurity states do not merge with the valence band and free carrier concentration increase in the whole system is reduced (as indicated also by our electrical trans- port measurement/Hall effect, resistivity). Moreover, due to original lattice deformation and compensated spins of 2s and2p electronsin B dimers the probability of charge hopping is also reduced.