  We investigate transport properties of ultrananocrystalline diamond films for
a broad range of temperatures. Addition of nitrogen during plasma-assisted
growth increases the conductivity of ultrananocrystalline diamond films by
several orders of magnitude. We show that films produced at low concentration
of nitrogen in the plasma are very resistive and electron transport occurs via
a variable range hopping mechanism while in films produced at high nitrogen
concentration the electron states become delocalized and the transport
properties of ultrananocrystalline diamond films can be described using the
Boltzmann formalism. We discuss the critical concentration of carriers at which
the metal to insulator transition in ultrananocrystalline diamond films occurs
and compare our results with available experimental data.
