Electrical and Electrochemical Properties of a-C:N:H Films
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Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
20 Hz to 200 kHz using an ac bridge; elements of the equivalent circuit were calculated as described in [3]. Potentiodynamic curves were recorded at a potential scan rate v from 5 to 100 mV s-1. We used ultra-high purity H2S0 4 and analytical grade K 3Fe(CN) 6 and K4Fe(CN) 6 chemicals. Table 1. Growth, optical and electrical parameters of the films. , io10 N 3 ,
104 AC-H/
10-4AN-H/
CM-
cm-2
cm"2
6 10.8
9.6 9.2
2.4 5.7
1.34 0.96
4.5 11.1
9.2 7.3
0 1.9
0.85
27
6.1
4.1
CH4 flow/
N2 flow/
Ub /
V
Eop,/
sccm
2 3
40 40
5 10
100 100
eV 1.2 1.0
5 6
60 60
0 5
250 250
7
60
15
250
No. of
film
sccm
For the electrical measurements, Ni contacts (1 mm in diameter) were evaporated on the film surface in vacuum. Parameters of the electrically active point defects were determined by the Charge-based Deep Level Transient Spectroscopy (Q-DLTS), using a computerized system ASEC-3 [4]. Transmittance and absorption spectra were recorded with spectrophotometers Specord-M80 (in the 2.5-5.0 itm range) and Specord-M400 (in the 0.185--0.9 ýtrm range). We note that both optical and electrical properties of the films studied did not depend on the type of substrate. RESULTS AND DISCUSSION The Electrochemical Impedance Measurement The electrochemical impedance was measured in a well conducting background electrolyte solution (5 N H 2 SO 4 ) at the electrodes' steady-state potential. The impedance spectra were analyzed using an equivalent circuit shown on Fig. 1. It comprises two RC-chains of which the first one relates to the a-C:N:H film bulk; the second, to the film/electrolyte interface. Here C'g is the film geometrical capacitance, R, is the film bulk resistance. The second chain includes, as the frequency analysis of the measured impedance showed, a Constant Phase Element (CPE). The CPE impedance is ZCPF = (Y I(i (0) -a, where i = -1)1/2, (o = 27f is the ac angular frequency, and
Rs
RF
Fig. ! Equivalent circuit of electrode. the factor a is measured in the [pFa na-i cm_2] units. RF is the leakage (faradaic) resistance at the interface. Values of aYand a calculated from the impedance spectra are given in Table 2. The experimental capacitance Cm values related to the first chain turned to somewhat exceed the geometrical capacitance calculated by the films' thickness (d) and the DLC permittivity. Obviously, the experimental capacitance is contributed to, in addition to the geometrical capacitance proper, by the capacitance of electrically active point defects (see below). We calculated the a-C:N:H resistivity Pac (related to the ac measurements) from the films' «series)) resistance R, and thickness d. When comparing it with the resistivity pd, calculated from
440
the slope of dc current--voltage curves taken at the film/metal contact, we see that the difference in their values, if any, is no more than 1.5 to 2 times (Table 2)'. Thus, the ac- and dc-determined p values agree quite well. Table 2. Impedance and kinetic characteristics
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