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PII: S0040-6090(00)01349-3
.
Thin Solid Films 381 2001 52 56
Single semicircular response of dielectric properties of diamond
lms
Haitao Ye
a
, Chang Q, Sun
b
, Haitao Huang
a
, Peter Hing
a,
U
a
Ad
¨
anced Materials Research Center, School of Materials Engineering, Nanyang Technological Uni
¨
ersity, Singapore 639798, Singapore
b
School of Electrical and Electronic Engineering, Nanyang Technological Uni
¨
ersity, Singapore 639798, Singapore
Received 1 March 2000; received in revised form 24 June 2000; accepted 24 June 2000
Abstract
Diamond lms were synthesized by a microwave plasma-enhanced chemical vapor deposition method using H
rCH gas
2
4
mixtures. A Fluke PM6306 RCL Meter was used to study the dielectric properties of the diamond lms deposited. The dielectric
dispersion measurement yielded the real and imaginary parts of impedance of diamond lms in the form of a depressed
semicircle in a complex plane. A Cole Cole plot was observed at frequencies from 50 Hz to 1 MHz. The result was found to t
the theoretical resistor capacitor parallel circuit model. The structure and quality of diamond lms were analyzed by scanning
electron microscopy, X-ray diffraction and Raman spectroscopy.
2001 Elsevier Science B.V. All rights reserved. .
Keywords: Diamond; Dielectric properties; Chemical vapor deposition CVD
1. Introduction
Diamond lms have recently become the subject of
great interest because of their excellent optical, me-
w
x
chanical, and thermal properties 1 7 . Since the re-
markable success in synthesizing diamond lms by .
chemical vapor deposition CVD , recent attention has
been paid to the application of diamond lms in vari-
w x
ous electronic devices and packaging materials 8,9 .
Therefore, understanding the dielectric properties is
very important. The ideal diamond is a semiconductor
with an electrical resistivity of 10
9
10
12
cm and an
w x
indirect bandgap of 5.5 eV 10 . The band gap is so
wide that the diamond lm is often considered to
behave effectively as an insulator. However, only type
IIa diamond lms are pure enough to exhibit the
semiconducting properties of the undoped crystal.
Other diamond lms grown by metal-organic CVD,
laser ablation, etc., exhibit resistivities many orders of
U
Corresponding author. Tel.:
q65-7906090; fax: 65-7935297. .
E-mail address: asphing@ntu.edu.sg P. Hing .
magnitude lower than that for type IIa, usually due to
the presence of defect, non-diamond phases and the
w x
scattering of carriers at grain boundaries 11 . Never-
theless, the tendency of the sp
3
bonds of diamond to
change to sp
2
during the microwave plasma-enhanced
CVD process and, hence, to form non-diamond phases,
including graphite, nanocrystalline diamond, and amor-
phous carbon, may inuence the electrical or dielectric
properties.
The relevance of the application of man-made CVD
diamond in electric and electronic devices has in-
creased the number of projects on the dielectric
properties in the past decades. A few researchers have
reported the dielectric properties of CVD diamond
w x
lms over a wide frequency domain. Y. Muto et al. 12
emphasized that the high conductivity in as-deposited
polycrystalline diamond lms was caused by the con-
duction through a disordered graphite region between
w x
grains and at the surface of the lms. Y. Tzeng 13
reported that exposure of diamond lms with electrical
resistivity as high as 10
14
W cm to a hydrogen plasma
led to a decrease of electrical resistivity by several
0040-6090
r01r$ - see front matter
2001 Elsevier Science B.V. All rights reserved. .
PII: S 0 0 4 0 - 6 0 9 0 0 0 0 1 3 4 9 - 3 (
)
H. Ye et al.
rThin Solid Films 381 2001 52 56
53
w x
orders of magnitude. M. Yaowu et al. 14 discussed the
dielectric properties using a capacitance-in-series
model. However, most of the dielectric characterization
of diamond lms have concentrated on its capacitance
voltage characterization, isothermal capacitance tran-
sient spectroscopy, current voltage characterization,
and temperature dependence of dielectric parameters,
while not much attention has been paid to what hap-
pens to the dielectric properties when the elds are
w
x
frequency-dependent 9,13 .
This study presents the contribution of the complex
impedance RCL Meter to the investigation of the di-
electric properties of diamond lms for the rst time.
The FLUKE RCL Meter is a powerful tool for the
study of the electric and dielectric properties of ionic,
electronic, or mixed conductor ceramics. It has been
already applied successfully in the investigation of fer-
w
x
roelectric materials 15 17 . In this paper, the study of
the dielectric properties of diamond lms at frequen-
cies from 50 Hz to 1 MHz in an air atmosphere has
been reported. The dielectric dispersion measurement
yielded the real and imaginary parts of impedance of
diamond lms, consisting of a depressed semicircle in a
complex plane, the so-called Nyquist diagram or
Cole Cole plot. The result tted the theoretical resis- .
tor capacitor
RC parallel circuit model very well.
The structure and quality of diamond lms were ana-
lyzed by scanning electron microscopy, X-ray diffrac-
tion, and Raman spectroscopy.
2. Experimental procedure
Diamond lms were deposited on silicon substrates,
with an area of 15
=30 mm, by the microwave plasma-
enhanced CVD method. The apparatus and depositing
w x
conditions have been discussed in detail previously 18 . .
Commercial single crystal silicon
001
wafers were
mechanically roughened with a 1- m diamond paste
and then cleaned with acetone and deionized water in
an ultrasonic cleaner for half an hour in order to
increase the nucleation sites for higher nuclei density.
The microwave plasma-enhanced CVD system was sup-
plied by the Coaxial Power system Pte. Ltd., UK. The
output frequency was 2.45 GHz. The detailed experi-
mental parameters used in the deposition process are
listed in Table 1. The surface temperature of the
substrate during the deposition of diamond lm was
monitored by a thermocouple attached to the backside
of the substrate holder and an optical pyrometer.
The coating quality or diamond purity was evaluated
using a micro-Raman spectroscopy with the 514.5-nm
line of an argon ion laser. The residual stresses in the
diamond coatings could be estimated from the quantity
w
x
of the diamond Raman line shift 19,20 . The crystal
structure of the coatings was obtained by X-ray
diffraction with CuK
40 kV
r30 mA with incident
Table 1
Deposition conditions for diamond lms on Si substrate .
Microwave power kW
2.0 .
Gas mixture: Ar
rH rCH sccm
20
r174r4
2
4 .
Gas pressure torr
50
.
Deposition time h
12 .
Substrate temperature
C
700
angle of 1 . The precise lattice parameters were de-
termined by the least-squares method using more than
10 reection peaks that appeared in the scan range of
10 120 , with a step of 0.08 and a scan speed of
4
rmin. The surface morphology of the deposited coat-
ing was investigated using a scanning electron micros- .
cope SEM, JSM-5410LV, JEOL, Japan attached to an
energy dispersive X-ray spectrometer. The samples were
coated with gold to reduce the charging effects before
observations were conducted. The dielectric properties
of diamond lms were determined by using a FLUKE
PM6306 RCL Meter in the frequencies from 50 Hz to 1
MHz. Silver paste was used to form an ohmic contact
to both the diamond lm and silicon substrate. The
effect of current conduction through the silicon subs-
trate on the measured resistance was estimated to be
less than 0.1% over the frequency range used, and thus
could be ignored.
3. Results and discussion
3.1. Characterization of lm quality
The diamond deposited in the H
rCH gas mixture
2
4
was uniform and fully covered the silicon substrate

.
Fig. 1a . The coating surface showed a dense, well .
faceted, and polycrystalline morphology Fig. 1b , pre- .
dominantly in the 220 orientation. The average crystal
size was approximately 2 3
m. The roughness of the
diamond lm was approximately 0.376
"0.12 m. The .
Raman spectrum of 220 textured diamond lm de-
posited using H
rCH gas mixture is shown in Fig. 1c,
2
4
which clearly demonstrates the presence of the charac-
teristic diamond and diamond-like phases in the de-
posited lms. The sharp Raman peak at approximately
1345 cm
y1
corresponded to the diamond peak, which
was shifted to the standard peak located at 1331 cm
y1
w x
24 , probably due to the residual compressive stress
caused by the difference in the coefcient of thermal
expansion between the diamond lms and the silicon
w x
substrate 2,3 . The Raman band at approximately 1515
cm
y1
was attributed to sp
2
-bonded carbon, which ex-
isted at the grain boundaries of the diamond crystals.
Grazing incidence X-ray diffraction analysis of the .
220 textured diamond lm using Rigaku diffractome-
ter are shown in Fig. 1d. The XRD patterns conrmed
the formation of a polycrystalline diamond lm on the (
)
H. Ye et al.
rThin Solid Films 381 2001 52 56
54
.
.
.
.
Fig. 1. Characterization of diamond lm. a and b : Surface morphology of diamond lm; c Raman spectroscopy of diamond lm; and d XRD
prole of diamond lm.
surface layer. The index of the peaks are denoted in
Fig. 1d, which are consistent with the Powder Diffrac- . tion Standard PDS card of natural diamond No.
.
6-0675 . The lattice parameter calculated from the
peaks in the XRD spectrum was slightly smaller than
the data from the standard PDF card, which was in the
order of m