Local impedance imaging of boron-doped polycrystalline diamond thin films

Abstract

Local impedance imaging (LII) was used to visualise surficial deviations of AC impedances in polycrystalline boron-doped diamond (BDD). The BDD thin film electrodes were deposited onto the highly doped silicon substrates via MW PE CVD. The studied boron dopant concentrations, controlled by the [B]/[C] ratio in plasma, ranged from 1 10^16 to 2 10^21 atoms cm-3. The BDD films displayed microcrystalline structure, while the average size of crystallites decreased from 1 to 0.7 µm with increasing [B]/[C] ratios. The application of LII enabled a direct and high-resolution investigation of local distribution of impedance characteristics within the individual grains of BDD. Such an approach resulted in greater understanding of the microstructural control of properties at the grain level. We propose that the obtained surficial variation of impedance is correlated to the areas of high conductance which have been observed at the grain boundaries by using LII. We also postulate that the origin of high conductivity is due to either preferential boron accumulation, the presence of defects, or sp2 regions in the intragrain regions. The impedance modulus recorded by LII was in full agreement with the bulk impedance measurements. Both variables showed a decreasing trend with increasing [B]/[C] ratios, which is consistent with higher boron incorporation into BDD film.

Full text

Local impedance imaging of boron-doped polycrystalline diamond thin films
A. Zieli
nski,
1
R. Bogdanowicz,
2,a)
J. Ryl,
1
L. Burczyk,
1
and K. Darowicki
1
1
Department of Electrochemistry, Corrosion and Material Engineering, Gdansk University of Technology,
11/12 Narutowicza St., 80-233 Gdansk, Poland
2
Department of Metrology and Optoelectronics, Faculty of Electronics, Telecommunications and Informatics,
Gdansk University of Technology, 11/12 G. Narutowicza St., 80-233 Gdansk, Poland
(Received 30 August 2014; accepted 25 September 2014; published online 2 October 2014)
Local impedance imaging (LII) was used to visualise surficial deviations of AC impedances in
polycrystalline boron-doped diamond (BDD). The BDD thin film electrodes were deposited onto the
highly doped silicon substrates via microwave plasma-enhanced CVD. The studied boron dopant
concentrations, controlled by the [B]/[C] ratio in plasma, ranged from 1 10
16
to 2 10
21
atoms
cm
ÿ3
. The BDD films displayed microcrystalline structure, while the average size of crystallites
decreased from 1 to 0.7 lm with increasing [B]/[C] ratios. The application of LII enabled a direct and
high-resolution investigation of local distribution of impedance characteristics within the individual
grains of BDD. Such an approach resulted in greater understanding of the microstructural control of
properties at the grain level. We propose that the obtained surficial variation of impedance is corre-
lated to the areas of high conductance which have been observed at the grain boundaries by using
LII. We also postulate that the origin of high conductivity is due to either preferential boron accumu-
lation, the presence of defects, or sp
2
regions in the intragrain regions. The impedance modulus
recorded by LII was in full agreement with the bulk impedance measurements. Both variables
showed a decreasing trend with increasing [B]/[C] ratios, which is consistent with higher boron incor-
poration into BDD film. V
C2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4897346]
In recent years, boron-doped diamonds (BDD) have
been extensively investigated because of their remarkable
properties. The unique physical and chemical properties ena-
ble BDD to be an ideal anode material in electrochemical ox-
idation,
1,2
biosensing,
3
or electroanalysis.
4
The properties of
BDD electrodes, including their lifetime and stability, are
significantly determined by their manufacturing process.
5
The structural and electronic properties of boron-doped
electrodes have been extensively studied.
6
Moreover, the
influence of boron acceptor on surface conductivity,
7,8
mor-
phology,
9,10
and phase structure defined as sp
3
/sp
2
ratio,
11–14
was investigated in polycrystalline diamond films.
Liao et al.
15
investigated the influence of boron concen-
tration on the structure of diamond thin films for the carrier
density ranging from 3.4 10
17
to 1.8 10
21
holes/cm
ÿ3
. It
should be noted that the boron-dopant density not only
affects the electrical properties of electrode but also its mor-
phological and structural characteristics (sp
3
/sp
2
ratio).
16,17
Information about the electrochemical response of BDD
electrodes with different boron doping levels can be found in
literature.
18
Furthermore, the position and coordination of B
dopants in this BDD electrode strongly modify the electronic
transport as well as impedance properties.
22
The uptake of
boron was found to be non-uniform across the surface of
BDD.
19–21
Wilson et al.
23
evidenced two different conduc-
tivity domains. These local heterogeneities influenced elec-
troactivity of the BDD surface and various electron transfer
in Ru(NH
3
)
6
3þ
. The characterization of these structural
defects and boron position is fundamental in understanding
the physical and electrochemical properties of BDD electro-
des and, in particular, of microelectrode arrays.
24
Lu et al.
22
experimentally measured the variation of
local bond length in boron-doped nanocrystalline diamond
(B:NCD) films by using spatially resolved STEM-EELS in
an aberration-corrected electron microscope. The bond elon-
gation and a significant difference in boron energy-loss near-
edge structure have been reported at defective regions in
B:NCD grains. In another study, Lu et al.
25
demonstrated the
presence of B dopant in the diamond lattice as well as the
enrichment of B dopant within twin boundaries and defect
centres. Turner et al.
26
claimed that boron concentrations of
1 to 3 at. % were found tetrahedrally embedded into the core
of diamond grains. The results of Muramatsu and Yamamoto
27
indicate that B atoms in heavily B-doped diamonds form caged
B-clusters in the defect space of the diamond lattice.
The majority of earlier investigations focused on the use
of advanced surface techniques to elucidate the boron uptake
heterogeneity. Nevertheless, the electrochemical perform-
ance and AC impedance are typically represented as an aver-
age calculated over the whole electrode area. To address the
issue of surface heterogeneity, we propose to apply high-
resolution local impedance imaging (LII) to visualise spatial
deviations of AC impedances on polycrystalline BDD over a
wide range of boron dopant concentrations (1 10
16
–2
10
21
at. cm
ÿ3
). To our best knowledge, the direct investi-
gation of local distribution of the impedance properties
within the individual grains of BDD films has not yet been
reported.
Thus, in this study, we directly investigate AC imped-
ance of hydrogen-terminated thin BDD films synthesized in
an microwave plasma-enhanced CVD system (AX5400S,
Japan, Seki Technotron) on p-type Si wafers with (111)
a)
Author to whom correspondence should be addressed. Electronic mail:
rbogdan@eti.pg.gda.pl. Tel.: þ48 58 347 1503. Fax: þ48 58 347 18 48.
0003-6951/2014/105(13)/131908/5/$30.00 V
C2014 AIP Publishing LLC105, 131908-1
APPLIED PHYSICS LETTERS 105, 131908 (2014)
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orientation. Si substrates were seeded by sonication in nano-
diamond suspension (crystallite size of 5–10 nm) for
2 h.
6,7,28
The substrate temperature was kept at 1000 C dur-
ing the deposition. The plasma microwave power
(@2.45 GHz),
29–31
optimized for diamond synthesis, was
kept at 1300 W. The gas mixture ratio was 1% of the molar
ratio of CH
4
-H
2
at gas volume 300 sccm of the total flow
rate. The base pressure was about 10
ÿ6
Torr, and the process
pressure was kept at 50 Torr. The boron level expressed as
[B]/[C] ratio in the gas phase was 200, 2000, or 10 000 ppm.
Diborane (B
2
H
6
) was used as dopant precursor. The growth
time was 6 h, which resulted in a film of approx. 2 lm thick-
ness. According to the [B]/[C] ratio, the prepared samples
were ascribed as AX-BC02k, AX-BC2k, and AX-BC10k.
LII was implemented on the commercial atomic force
microscopy (AFM)
32
device (NTegra Prima, Russia,
NT-MDT). The combined LII technique is based on a mea-
surement of the current flowing between the sample and the
conductive AFM probe. In the case of DC measurement,
known as Scanning Spreading Resistance Microscopy
(SSRM),
33
an AFM probe is used to perform the local con-
ductivity measurements on the sample surface. Self-designed
connection to the conductive probe was installed and
coupled with the external impedance system. The clip fixed
on the top surface of the sample served as the second electri-
cal terminal (Figure 2(d)). The independent system for elec-
trical measurements consisted of a current-voltage converter
(SRS 570, USA, Stanford Research System) and A/D con-
version card (NI USB-6356, USA, National Instruments).
The additional measuring channel in the card was used to
synchronize the acquisition of impedance data with topogra-
phy. Control and acquisition have been created in LabVIEW
(v. 2012, USA, National Instruments). The impedance acqui-
sition was performed in multi-frequency mode; thus, it was
possible to extract the modulus and phase images for any
investigated frequency. The details of the approach used are
given in Ref. 34. The frequency span of perturbation signal
equalled 1.6 kHz with 100 mV amplitude. The applied DC
bias applied via a conductive probe was equal to 3 V
(CDTP-NCHR-10, Switzerland, Nanosensors). The initial
contact resistance measured for the described probe type was
2.2 k Xon gold surface. An analogous technique was used
by Hasegawa et al.
35
to investigate the transport properties
of nanocontacts based on GaAs and InP. O’Hayre et al.
40
discussed the details of the contact impedance measurement
such as the frequency limits or impedance components.
Furthermore, the bulk impedance amplitude and phase
angle spectra were characterized using a custom-built
setup.
36–39
The Au needle probes with programmable auto-
matic RCL meter (Fluke PM6306) acted as an impedance
analyser in the frequency range from 50 Hz to 1 MHz. The
shielded cables were used to reduce a phase error in the high
frequency range. The Ohmic contact was made to the BDD
surface by using Ti/Pt/Au to average the impedance over the
sample area.
The surface morphology and the size of BDD crystalli-
tes were investigated with scanning electron microscopy
(SEM, S-3400N, Japan, Hitachi). As shown in Figure 1, the
dimensions of crystallites decrease with increasing [B]/[C]
ratio. The average grain size for the least doped electrode
(AX-BC02k) was approx. 2 lm, while for AX-BC10k elec-
trode, it was 4 times smaller. A similar topographical effect
was previously reported.
16
It is related to the fact that the
addition of boron influences the nucleation process and
degenerates the growth of diamond. Lu et al. reported that
boron mainly accumulates along the grain boundaries of
polycrystalline diamond.
22
In the case of AX-BC10k elec-
trode, this phenomenon produces large amounts of a small
crystalline agglomerates positioned in-between large crys-
tals (see Fig. 1). In this specific case, SEM gives a relatively
low lateral resolution of the electrode morphology com-
pared to the proposed LII technique. Nevertheless, SEM
microimages prove that the investigated BDD films
fully encapsulate Si substrates without cracks or layer
discontinuities, which could cause potential conductivity
disturbances.
Figure 2(a) illustrates an example of the spatial distribu-
tion of impedance modulus for the highly boron-doped
diamond sample (AX-BC10k). For the selected vertical
(Fig. 2(b)) and horizontal (Fig. 2(c)) profiles, there are visi-
ble conduction heterogeneities between the grains and their
boundaries, manifested by changes of 5 orders of magnitude.
The LII spectrum in Figure 2(a) shows the presence of
both types of areas, i.e., the conductive areas and fully insu-
lating areas. The “dark” regions corresponding to high im-
pedance regions with approximately 400 kXare mostly
FIG. 1. SEM micrographs presenting
the topography of BDD electrodes
with different [B]/[C] ratios (200, 2 k,
10 k). Magnification 10 000.
131908-2 Zieli
nski et al. Appl. Phys. Lett. 105, 131908 (2014)
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present on the grain edges or in the intergrain areas. The
“white” areas, originating from the intragrain areas or flat
surfaces of crystals, are characterized by lower values of im-
pedance, i.e., below 1 kX.
In addition, the distribution of increased conductivity
areas has been consistently confirmed by pertinent topo-
graphic images of the samples tested. This suggests that the
contribution of local spreading impedance, assumed to be
constant under experimental conditions, predominates over
the contact impedance component of the material.
These observations are in agreement not only with the
presence of boron between the grains (mostly in the sp
2
phase form) but also inside the grains themselves.
41
Furthermore, it should be mentioned that impedance is not
only a function of the sample properties but also of the tip ra-
dius (30 nm in this study). The intragrain impedance regions
can be burdened with error because the radius of AFM tip
does not enable the achievement of surface details.
In a recent study of H-terminated polycrystalline BDDs
electrode, Dealouis et al. hypothesized that a significant
decrease in the local surface conductivity could be explained
by a partial passivation of BDD film due to the formation of
B–H pairs leading to a decrease in the acceptor levels in the
film.
19
Thus, the local impedance measurement allowed the
identification of regions with different electrical character,
causing the complex electrochemical response of BDDs.
The LII results are presented as the acquired collection
of images that have been subjected to decomposition to the
form of impedance maps corresponding to one selected mea-
surement frequency. Figure 3illustrates the surficial distribu-
tion of impedance modulus for a frequency of 1.6 kHz. A
similar impedance scale was kept for all images presented in
Figure 3. The impedance images were recorded for the same
value of the bias voltage 3 V, the impedance measurement
parameters, area, and topographical scanning speed.
The local impedance images show gradual decrease in
the average surface impedance versus boron doping level.
The low doped sample AX-BC02k exhibits smooth variation
of impedance along the crystal boundaries. The intragrain
zones of low impedance become rounded and fuzzy in con-
tradiction to sharp crystal shapes observed in SEM microi-
mages (Figure 1). Furthermore, the inhomogeneous
distribution of impedance in the intergrain regions was also
observed. This allows for the conclusion that B dopant is not
FIG. 2. Image of impedance modulus
for the BDD film (AX-BC10k) with
10 k boron content (a). Frequency of
impedance spectrum: 1600 Hz. AFM
scan in contact mode, velocity
16.19 lm/s, contact force 1.24 lN.
Selected vertical (b) and horizontal (c)
profiles. Scheme of the electrodes posi-
tioned on the sample surface (d).
FIG. 3. Impedance modulus maps for
three BDD samples arranged according
to increasing boron content: 200 (a),
2000 (b), and 10 000 (c). Analysed fre-
quency of impedance spectrum:
1600 Hz. AFM scan in contact mode,
velocity 16.19lm/s, contact force
1.24 lN.
131908-3 Zieli
nski et al. Appl. Phys. Lett. 105, 131908 (2014)
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heterogeneously distributed over the grain. For the highest
boron levels (sample AX-BC10k) approaching the metallic
transition, the wide conductive zones preferentially formed
at the intragrain boundaries. The image of average boron-
doped sample (AX-BC2k) demonstrates the combined com-
position of “white” intragrain zones, characterized by the
metallic transition, and “grey” intergrain regions represent-
ing semiconducting diamond.
Based on the LII maps shown in Figure 3, the average
values of modulus for each BDD sample have been calcu-
lated (see Table I). The impedance modulus recorded by LII
was in full agreement with the bulk impedance measure-
ments. Both variables showed a decreasing trend with
increasing [B]/[C] ratios, which is consistent with higher bo-
ron incorporation into BDD film. Moreover, the bulk imped-
ance measurements confirm the tendencies determined by
using LII (see Figure 4). These phenomena are most likely
linked together since an increase in boron doping level also
decreases the percentage of sp
3
hybridized carbon.
It was proposed that the obtained surficial variation of
impedance in hydrogen-terminated polycrystalline BDD is
correlated with the areas of low impedance that had been
observed at the grain boundaries by using LII. The samples
with smaller grain size were richer in the grain boundaries
containing disordered sp
3
and sp
2
phases. Consequently, the
incorporation of defects, including boron dopants was more
efficient in these samples.
42
The low impedance is a result of
preferential boron incorporation and clustering in the inter-
grain areas. Moreover, the intergrain regions introduce impu-
rity centres that cause carrier transfer perturbations via
various intragrain defects and grain boundaries.
Similar conclusions have been reached by Wilson
et al.,
23
who employed conducting atomic force micros-
copy (C-AFM) with conventional metal-coated AFM
probes to provide spatially resolved electrical information
on polycrystalline BDD. They reported two different con-
ductivity domains with roughly estimated resistances of ca.
100 kXand ca. 50 MXlinked to the boron dopant levels in
individual microcrystalline grains. However, these results
had a spatial resolution of about tens of microns.
Furthermore, an analogous effect was presented by Bennet
et al.
43
who investigated how boron dopant accumulates
into diamond surface. Based on Raman mapping, they con-
firmed that boron mainly aggregates at the grain bounda-
ries; the presence of sp
2
carbon increased with increasing
boron level.
Figure 4presents Bode plots obtained for hydrogen-
terminated polycrystalline diamond. The bulk impedance
modulus and phase angle were recorded at BDD electrodes
with Ohmic contacts. For all samples, a gradual decrease in
phase angle and modulus plotted against frequency was
observed. Moreover, no peaks were registered over a wide
frequency range (50–100k Hz), which indicates that only
simple capacitive element exists in this model. The grey line
in Figure 4marks the frequency utilized in local impedance
images. The authors decided to record images at 1600 Hz
due to relatively small differences in phase element in BDDs
with different levels of boron incorporation.
It appears that the bulk impedance modulus of highly
doped BDD samples is lower than that of low doped sample
(Figure 4(b)). The phase angle spectra (Figure 4(a)) showed
a continuous decrease down to ca. 40˚ over 5 kHz only for
low doped AXBC02k sample, which results from the impact
of resistive behaviour.
In summary, the LII provides considerable insight into
the complex nature of BDD as an electrode surface. It was
proposed that the obtained surficial variation of impedance
correlates with the areas of high conductance observed at
the grain boundaries. It was postulated that the origin of
high conductivity is due to boron accumulation in the
intragrain region. For the selected vertical and horizontal
profiles of impedance, the visible conduction heterogene-
ities between the grains and their boundaries were present
as manifested by changes of 5 orders of magnitude. The
method can be used on global scale as a technique supple-
mentary to conventional impedance measurements.
This work was supported by the Polish National Science
Centre (NCN) under grant No. 2011/03/D/ST7/03541. The
DS funds of the Faculty of Electronics, Telecommunications
and Informatics and the Faculty of Chemistry at the Gdansk
University of Technology are also acknowledged.
TABLE I. The estimated surface parameters for the set of BDD samples.
Sample
[B]/[C] ratio
(ppm)
Boron concentration
(at. cm
ÿ3
)
Mean grain size
(lm)
Mean surface
roughness (nm)
Mean jZjfrom
LII (kX)
jZj@ 1 kHz
(X)
AX-BC02k 200 1 10
16
1.8 106.23 899 153.6
AX-BC2k 2000 7 10
19
1.3 91.85 248 6.06
AX-BC10k 10000 2 10
21
0.7 87.35 168 1.15
FIG. 4. Bode plots of BDD electrodes with different [B]/[C] ratios in
plasma: phase angle (a) and jZjamplitude (b) vs. frequency. The Ohmic
contact and Au needles were applied to BDD electrode.
131908-4 Zieli
nski et al. Appl. Phys. Lett. 105, 131908 (2014)
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