Transport properties of n-type ultrananocrystalline diamond films

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.

Figure 1

Calculated DOS for diamond with $\Sigma 13$ grain boundary and without (left) or with (right) nitrogen (relative to the Fermi level of undoped system). Occupied states are shaded. Sketch of electron levels in the presence of nitrogen is shown on the right. The data are taken from Ref. 7.

Figure 2

(Color online) Sketch of ultrananocrystalline diamond film with a schematic electron path. The grey regions are insulating grains with $a$ and $d$ being the characteristic grain size and distance between the grains respectively.

Figure 3

Dependence of the conductivity of UNCD films on temperature. Different curves correspond to different nitrogen concentrations in the feed gas during film growth. Reproduced from Ref. 2.