Biomedical Engineering Reference
In-Depth Information
(a)
MPECVD
(b)
HFCVD
400
800
1200
1600
2000
Wavenumber (cm
-1
)
FIGURE 4.14
Raman spectroscopy of MPECVD (a) and HFCVD (b) diamond films. (From Ye, H.T., PhD thesis, University
College London, 2005. With permission.)
at 1580 cm
-1
that indicates that the HFCVD diamond has more graphite and non-diamond
phases formed during the CVD process.
The Cole-Cole plots for MPECVD diamond films from 25°C to 400°C are shown in
Figure 4.15. The Cole-Cole plot at room temperature shows the presence of a single semi-
circular response, with some scatters in the data of the low frequency range. Similar data
are shown for the other temperature ranges. The diameters of the semicircle response
reduce dramatically with the temperature increase. At 400°C, the semicircular response
is accompanied by a linear tail extended to low frequency. It is found that each Cole-Cole
plot shows only one depressed semicircle. The single semicircle indicates that only one
primary mechanism exists for the electrical conduction within the diamond film at tem-
peratures below 300°C. The depressed semicircles make their centers on a line below the
real axis, which indicates the departure from the ideal Debye behavior [1]. The diameter of
each semicircle indicates the electrical resistance of diamond films. With increasing tem-
perature, the diameter decreases indicating the reduction of the resistance from 62 M
Ω
at
room temperature to 4 k
Ω
at 300°C. The Cole-Cole plot at 400°C shows a small semicircle
and a long linear tail at low frequency. Such phenomena have been observed in other sys-
tems and have been attributed to the AC polarization of diamond/electrode interface at
this high temperature [48].
The Cole-Cole plots for HFCVD diamond films from 25°C to 300°C are shown in Figure
4.16. The Cole-Cole plot at room temperature shows the presence of an arc, instead of a
complete semicircle, with a linear trend at the low-frequency impedance range. Similar
data are shown at other temperatures. The diameters of these arcs reduce dramatically with
the temperature increase. It is difficult to derive the thermal activation energy for HFCVD
diamond because there are no clear semicircles in their Cole-Cole plots. However, through
the simulation of the each arc, it was found that the capacitance value for each arc is main-
tained at 0.7 pF, which indicates that the impedance spectrum is from the grain interior.
A central issue to be addressed is which portion of the equivalent circuit in Figure 4.2
corresponds to the observed single semicircular response. This interpretation is centered
to determine the dominance of conduction from grain interiors or grain boundaries or oth-
erwise injection from the electrodes. For ionic conductors, electrode porosity and polar-
ization must be considered for the general impedance analysis [2], whereas for diamond,
different electrode configuration effects have been investigated [48], and the results show
