Biomedical Engineering Reference
In-Depth Information
crystallites are highly defected with many twins and other planar defects. Khomich et al.
[76] deposited 100- to 200-nm crystallites with the appearance of polycrystalline conglom-
erates. Lee et al. [75] prepared nanocrystalline films from CH
4
/H
2
mixtures and concluded
real-time spectroscopic ellipsometry to measure activation energies. Microhardness, elec-
trical conductivity, and the effect of methane pressure on film growth rate were studied by
Fedoseev et al. [77]. Films produced by Zarrabian et al. [79] from an electron cyclotron res-
onance (ECR) plasma were studied by TEM and electron energy loss spectroscopy (EELS)
and found to consist of 4- to 30-nm crystallites embedded in diamond-like carbon (DLC).
Magnetron sputtering of vitreous carbon produced films in which the nanocrystallites
were embedded in an amorphous carbon matrix [78]. By adjusting the ratio of noble gas to
hydrogen in the gas mixture, Gruen's group [82] has achieved a continuous transition from
microcrystalline to nanocrystalline.
However, in essence, the technique allows an equivalent circuit representation of the
material system under study to be proposed, and the experimental data determined to be
compared with simulated data based on manipulation of the circuit parameters within the
equivalent circuit. This chapter shows evidence for both grain boundaries and grain inte-
rior conduction within the silicon-supported nanocrystalline diamond films used here.
All films studied here were produced using a commercial supplied 2.45-GHz resonant
standing wave cavity microwave plasma-enhanced CVD system. It has been already dem-
onstrated that good quality, microcrystalline diamond films can be really produced on
suitably treated Si substrates using CH
4
/H
2
/O
2
gas mixtures [83-86]. The introduction
of Ar in place of the oxygen has been shown to lead to nanocrystalline film production
[73,87]; this approach has been used here.
The films synthesized comprise randomly oriented fine grains 50-100 nm in size. The
temperature dependence of the characteristic Cole-Cole plots measured for these dia-
mond films is shown in Figure 4.18a-e. Figure 4.18a presents the data measured at 25°C
and 100°C, respectively, showing the presence of a single semicircular response, with some
scatter in the data in the low-frequency impedance range. Similar data are shown in Figure
4.18b for the temperatures of 150°C and 200°C. The diameter of the semicircular response
is reduced dramatically with the increase in temperature. Beyond 250°C, shown in Figure
4.18c-e, the semicircular response is accompanied by an additional semicircle (or an arc)
that extends to low frequencies.
In Figure 4.19, the calculated resistance for the higher-frequency semicircular response
within the Cole-Cole plot, which persists over the whole temperature range investigated
here, is presented in a logarithmic plot against reciprocal temperature. A straight line is
apparent, enabling a single electrical activation energy to be estimated from the slope of the
curves. Attention should be paid to the steep transition of the activation energy from 0.13
to 0.67 eV at 250°C, coincident with the emergence of the second semicircular response.
It has been found that each Cole-Cole plot below 250°C shows only one depressed semi-
circle, as indicated in Figure 4.18a and b. The single semicircle indicates that one primary
mechanism exists for the polarization within the diamond film at these temperatures.
The presence of the single semicircle in this frequency range corresponds to the electrical
conduction from bulk grain interior. The diameter of each semicircle corresponds to the
resistance for the particular contribution from the diamond grain interior at each tempera-
ture. As the temperature increases, the diameter of the semicircle decreases, indicating
a reduction of the grain interior resistance. There appears to be no secondary semicircu-
lar response over the frequency range measured here for temperatures below 250°C; it
can therefore be summarized that the electrical conduction within these diamond films is
being dominated by the diamond grain interiors.
