Biomedical Engineering Reference
In-Depth Information
As previously mentioned, the diameter of each semicircle indicates the resistance (
R
) con-
tributed either from grain interiors or from grain boundaries. To understand the different
semicircular response quantitatively, the resistance for the contribution from both grain
interiors and grain boundaries was estimated and extracted as shown in Table 4.3.
R
gi
,
R
gb
,
and
R
gi
/
R
gb
are the resistance from the grain interior, the resistance from grain boundaries,
and the ratio between them. The first observation is that the resistance either from grain
interiors or from grain boundaries decreases with increasing temperature. Particular atten-
tion should be paid to the marked decrease in diamond grain interior resistance (from 300
to 0.02 k
Ω
) with increasing temperature (from 25°C to 500°C). The resistance contributed
from the grain boundaries is not measurable below 250°C, implying that it is relatively
small. Comparison of the relative variation of both resistances shows that the resistance
from grain boundaries made more dominant contribution with increasing temperature.
Recent studies of the temperature-dependent resistance of polycrystalline CVD diamond
films have shown that the thermal activation energy can cover a wide range between 0.09
and 1.5 eV [90]. However, the grain interior and grain boundary resistances have not pre-
viously been identified and separated. In the field of CVD diamond, it is often difficult
to make direct comparisons with a body of published work, since films grown in differ-
ent laboratories can display significantly different properties. Jin et al. [90] have reported
activation energy changes from 0.4 to 0.9 eV with increasing temperature. They suggested
that the weakly temperature-dependent resistivity at lower temperature was due to leak-
age along the grain boundaries; however, no directly evidence was presented. The resis-
tance they reported became nearly independent of frequency at the high temperature end,
which is consistent with similar observations by Nath and Wilson [91].
A few reports exist that discuss different mechanisms to explain the electrical conduction
properties of microcrystalline diamond films, rather than the nanocrystalline diamond
films studied here [92-96]. Landstrass and Ravi [97] proposed that the conduction transi-
proposed that the conduction transi-
tion they observed with temperature was caused by the movement of hydrogen and other
defects from electrically active deep levels to nonactive sites during annealing. Mori et al.
Landstrass and Ravi [97] proposed that the conduction transi-
TABLE 4.3
Temperature Dependence of Grain Boundary Resistance (
R
gb
),
Grain Interior Resistance (
R
gi
), and the Relative Resistance
(
R
gb
/
R
gi
) between Them
T
(°C)
R
gi
(k
Ω
)
R
gb
(k
Ω
)
R
gb
/R
gi
25
300.00
-
-
100
133.00
-
-
150
71.00
-
-
200
50.00
-
-
250
28.00
-
-
275
14.00
5.20
0.37
300
5.60
3.68
0.66
350
1.00
1.32
1.32
375
0.50
0.80
1.60
400
0.22
0.54
2.45
425
0.11
0.35
3.18
450
0.05
0.24
4.80
500
0.02
0.12
6.00
Source:
Ye, H.
J. Appl. Phys.
, 94, 7878, 2003. With permission.
