Effect of nitrogen on the electronic properties of ultrananocrystalline diamond thin films grown on quartz and diamond substrates

The electronic transport properties of ultrananocrystalline diamond thin films grown from an argon-rich $\mathrm{Ar}∕\mathrm{C}{\mathrm{H}}_4$ microwave plasma have been investigated in the temperature range from 300 up to $700\mathrm{K}$ and as a function of nitrogen added to the gas phase (from 0 to 20%). The influence of nitrogen incorporation on the electronic transport properties of the ultrananocrystalline diamond films was examined by conductivity and Hall effect experiments. Electron spin resonance and electrically detected magnetic-resonance measurements complement the electronic transport study. In the case of films grown with a high nitrogen content in the gas phase, it was possible to perform Hall effect experiments, which showed n-type conductivity, with carrier concentrations up to ${10}^{20}{\mathrm{cm}}^{-3}$ and mobilities above $1{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ at room temperature. From the temperature dependence of the conductivity, we propose that electron transport via grain boundaries can explain the high conductivity (up to $150{\Omega }^{-1}{\mathrm{cm}}^{-1}$) of nitrogen containing ultrananocrystalline diamond films. The conduction mechanism in these films is explained by a transition from variable range-hopping transport in localized states near the Fermi level (in the case of low-conductivity films) to defect band conduction (in the case of high-conductivity films). The results have been discussed using a hopping model which assumes an exponential distribution of the density of states near the Fermi level, in order to explain the temperature dependence of the conductivity in the temperature range from 300 up to $700\mathrm{K}$. Electrically detected magnetic resonance confirms that the transport of the low-conductivity samples can be explained by hopping via carbon dangling bonds.

Figure 1

Room-temperature conductivity versus nitrogen content in the gas phase for samples from the $R$ series, grown from an argon-rich gas phase. The conductivity increases strongly with increasing nitrogen content.

Figure 2

Arrhenius plots of the conductivity $\sigma$ for samples of the $R$ series, in the range 300 up to $700\mathrm{K}$. Also shown is the linear fit in the high-temperature range, suggesting a change from variable-range hopping to simply activated conductivity.

Figure 3

(a) A comparison of the carrier concentration determined from Hall measurements with the concentration of nitrogen obtained from ERDA. (b) The concentration of nitrogen obtained from ERDA as a function of nitrogen in the gas phase.

Figure 4

Variation of the ESR spin density ${\mathrm{N}}_{\text{spin}}$ with increasing nitrogen content obtained from ERDA measurements. A clear trend with nitrogen content cannot be observed.

Figure 5

Variation of $\sigma \sqrt{T}$ with $T^{-0.25}$ for the same set of samples of the $R$ series as in Fig. 2, in the temperature range $300–700\mathrm{K}$. Also shown are the linear fits at low temperatures:

Figure 6

(a) The correlation between computed prefactor ${\sigma }_{\text{Godet}}^0$ $\left[{\Omega }^{-1}{\mathrm{cm}}^{-1}\right]$ and the activation energy $E_{\mathrm{act}}$ [eV] derived from the hopping model in an exponential bandtail, using the disorder parameter $E_0$ and the density of states $\mathrm{N}\left(E_F\right)$ as parameters. Data taken from Ref. 8. The localization length ${\alpha }^{-1}$ is kept fixed at $5\mathrm{\AA }$ (Ref. 8). Black data points correspond to experimental values (seeTable ). (b) The relationship between the parameters ${\sigma }_{\text{Godet}}^{00}$ and $T_0$ derived from modeling of hopping conductivity in an exponential density of states. Data taken from Ref. 7. The disorder parameter $E_0$ varies in the range $0.05–0.20\mathrm{eV}$ and the localization parameter $\mathrm{N}\left(E_F\right)$ ${\alpha }^{-3}$ in the range ${10}^{-4}–{10}^{-1}{\mathrm{eV}}^{-17}$. Black data points correspond to experimental values (see Table ).