Mechanism of high $n$-type conduction in nitrogen-doped nanocrystalline diamond

Dramatic enhancement of the $n$-type conductivity of nanocrystalline diamond films by introducing up to $0.2\mathrm{at.}\%$ of nitrogen into the film has been reported. Previously, there were some theoretical predictions about the change of electronic structure of the films by nitrogen incorporation, but the origin of the enhanced conductivity is still not clearly understood. In this article the mechanism of high conductivity and the change of the electronic structure by nitrogen incorporation is investigated by low-temperature conductivity and other supporting measurements. It is shown that nitrogen induces percolative paths in the grain boundary regions and there is an increase in the density of states at the Fermi level that helps increase the conductivity. Low-temperature conductivity has been explained from a change over from Arrhenious behavior to Efros-Shklovokii–Pollak–Mott variable range hopping conductivity. Using a model combining band and hopping conduction electrical conductivity of highly doped samples over wide range of temperature has been explained. This approach also helps us to improve the understanding of the electronic structure and transport of conducting amorphous carbon by resolving some typical problems in the analysis of temperature dependent conductivity.

Figure 1

(a) Variation of conductivity with temperature of NCD samples prepared with different nitrogen concentration. (b) Variation of room temperature conductivity, spin density, and nitrogen atomic concentration with nitrogen gap percentage in the plasma. Inset: Typical ESR peak of NCD films. (See Refs. 1, 14 for details.)

Figure 2

Arrhenious plot of resistivity of the 0.5% ${\mathrm{N}}_2$ sample shows activated behavior of conduction.

Figure 3

For 1% ${\mathrm{N}}_2$ sample plots show (a) activated behavior (b) variable range hopping conductivity.

Figure 4

(a) Variation of ln (conductivity) with ln (frequency) and (b) Variation of ln$\left(\Delta H_{PP}\right)$ with $T^{1∕4}$ for 1% sample. The 1% ${\mathrm{N}}_2$ film shows VRH fit. Inset: Variation of $\Delta H_{PP}$ with $T$ for the 10% ${\mathrm{N}}_2$ sample (Ref. 14).

Figure 5

Change over from Mott’s VRH to Efros hopping with temperature in the 5% ${\mathrm{N}}_2$ sample can be seen from the variation of the resistivity with (a) $T^{1∕4}$ and (b) $T^{1∕2}$ at different temperature ranges.

Figure 6

Conductivity has been fitted with a combination of extended state and variable range hopping conduction in the 10% ${\mathrm{N}}_2$ sample.