WDS'10 Proceedings of Contributed Papers, Part III, 86–90, 2010.
ISBN 978-80-7378-141-5 © MATFYZPRESS
Electrostatic Charging Differences in Ultrathin Nanocrystalline Diamond
E. Verveniotis, J. Čermák , A. Kromka, M. Ledinský, and B. Rezek
Institute of Physics ASCR, Cukrovarnicka 10, Prague 6, Czech Republic.
Abstract. Nanocrystalline diamond (NCD) thin films are deposited on p-type Si
substrates in thicknesses bellow 100 nm using different deposition temperatures
(600–820°C). The films are then terminated by oxygen using r.f. oxygen plasma. Atomic
force microscopy (AFM) is employed to induce electrostatically charged micrometersized patterns on the diamond films by applying a bias voltage on the AFM tip during
contact mode scan. Retained charge is detected by Kelvin force microscopy showing
electrical potential shifts (100–400 mV) that differ in geometry and amplitude on each
film for the same bias voltages. The films have similar morphology and grain size as
measured by AFM and scanning electron microscope (SEM). Material differences are
resolved via micro-Raman spectroscopy using UV illumination (λ=325 nm). Different
charging properties are thus attributed to the differences in relative amount of diamond
and sp2 phase (39–61% vs. 63–37% samples A, B, respectively).
Introduction
Electrostatic charging of surfaces is widely used in a variety of technological processes. It improves wetting
of plastics for painting or it is used in printers and copiers for toner positioning on paper. Electrostatic charging
is also an effective method for guiding self-assembly of micro- and nanosized particles on insulating materials
[1, 2, 3]. It is also employed in electronics, e.g. in memory devices.
A large variety of materials have been used for electrostatic charge storage (semiconductors [4] including
amorphous silicon [5] as well as polytetrafluoroethylene and poly(methyl methacrylate) [6]) by various methods
(laser, ion, or electron beam illumination, using electrodes etc). Charged patterns of submicrometer resolution
can be created by using nanometer-sized probes, such as those employed in atomic force microscopy (AFM)
[4, 5].
For such local and intentional charging, diamond has been only little investigated [7, 8] even though it
exhibits a unique set of properties for applications. It can, for example, be used as a semiconductor for device
fabrication [9], or as a biointerface [10, 11] and can be deposited on diverse substrates [12]. From the electronic
point of view, diamond is a wide band gap semiconductor (5.5 eV). Intrinsic diamond is thus generally
electrically insulating and transparent for visible light. It is transformed into p- or n-type semiconductor by boron
[13] or phosphorus [14] doping respectively. Only when the intrinsic diamond is hydrogen-terminated (Hdiamond), a thin (<10 nm) conductive layer is formed close to the diamond surface (surface conductivity) under
ambient conditions [15]. While this feature attracted a lot interest and research effort in past, research on
electronic properties of highly resistive oxygen-terminated intrinsic diamond (O-diamond) has been focused to
only a few applications (e.g. radiation or UV detectors [16]). Detecting and understanding of electrostatic
charging of diamond is crucial for many diamond-based electronic applications from detectors to field-effect
transistors, batteries, and silicon on diamond (SOD) systems.
In this paper we report on local electrostatic charging of oxygen-terminated nanocrystalline diamond (NCD)
thin films deposited on silicon. The films were deposited in thicknesses below 100 nm using different
temperatures. Diverse behavior and properties of the films according to the specific deposition conditions are
discussed for resolving charge contributions both qualitatively and quantitatively.
Materials and Methods
NCD films were prepared by microwave plasma chemical vapor deposition using the following parameters:
substrate temperature 820°C or 600°C, deposition time 16 or 80 minutes, microwave plasma power 900 W,
CH4:H2 dilution 3:300. Resulting thickness was 74 nm (sample A) and 45 nm (Sample B) respectively. The
substrates were conductive p-doped silicon wafers nucleated by water-dispersed detonation diamond powder of
5 nm nominal particle size (NanoAmando, New Metals and Chemicals Corp. Ltd., Kyobashi) using an ultrasonic
treatment for 40 min. After the deposition, the diamond films were oxidized in r.f. oxygen plasma (300 W,
3 min).
Localized charging was performed by scanning in contact mode with an atomic force microscope
(N-TEGRA system by NT-MDT). Conductive, diamond coated silicon probes were used (DCP11 by NT-MDT).
The bias voltage was applied to the tip while the silicon substrates were grounded. An external voltage amplifier
86
VERVENIOTIS ET AL.: CHARGING DIFFERENCES OF DIAMOND THIN FILMS
(HP 6826A) was connected to the cantilever and controlled by the AFM software via a signal access module, to
apply voltages bigger than 10 V. Applied voltages were in the range of -20 V to 20 V and the scan speed was
always 10 μm/s. KFM was then used to detect potential differences across the sample [17]. The KFM potential
values and differences are given as measured, not with respect to the vacuum level.
The potential differences were studied as a function of the charging voltage, effective field and specific
sample properties. Relative humidity and temperature during all AFM experiments were in the ranges of
20–32% and 22–26°C.
For resolving typical grain size, shape and film homogeneity we used SEM. Film thickness was measured by
ellipsometry. Micro-Raman Spectroscopy and FTIR in reflection regime were employed to determine the thin
films’ material properties and nano-structure respectively.
Results
Figures 1 (a) and 2 (a) show 15 μm2 KFM images after a typical charging experiment (Fig 1 (a) sample A,
Fig 2 (a) sample B). The charging voltage of 10 V, 20 V, -10 V, -20 V was applied in an 8 μm2 area during
contact mode AFM scan, while scanning horizontally and with slow scan direction from the bottom up. We can
see that the charged features on sample A are homogeneous and are observed for every value of bias voltage
applied. On the contrary, the features on sample B are relatively inhomogeneous, and present only for the higher
voltage settings (±20 V).
The surface potential values are also significantly different. Figures 1 (b) and 2 (b) show the graphs of the
cross sections indicated by the arrows on the KFM images. We can see four distinct surface potential peaks on
the sample A graph, while on the one corresponding to sample B there are only two. The overall contrast is
600 mV (sample A) and 270 mV (sample B). The absolute potential values with respect to the background for
the highest voltage settings on both polarities were 210 mV, -390 mV and 70 mV, -200 mV respectively. The
above described trend concerning the relative electric potential variations with respect to the charge voltage is
typical. The absolute potential values vary depending to the specific place on the sample [8].
Figure 1. (a) Surface potential map of charged features on sample A. Voltages used for charging are shown next
to the exposed areas. (b) Potential cross section as indicated by the arrow.
Figure 2. (a) Surface potential map of charged features on sample B. Voltages used for charging are shown next
to the exposed areas. (b) Potential cross section as indicated by the arrow.
87
VERVENIOTIS ET AL.: CHARGING DIFFERENCES OF DIAMOND THIN FILMS
The I/V characteristics measured by AFM (Figure 3 (a)), show that for the same values of bias voltage the
generated charging currents are lower on sample B. This is more pronounced for the negative polarity where a
higher voltage is needed for current generation on the same sample (-15 V vs. -6 V). This effect is even more
noticeable if we plot the current as a function of electric field (it is larger on the sample B due to its smaller
thickness). Figure 3 (b) shows that the thinner sample B needs larger electric fields to facilitate electronic
conduction.
Figure 4 shows SEM images of sample A (a) and sample B (b). As observed also on AFM, surface
morphology of the films is very similar and the grain size is comparable. Indicated within the images we can see
measured some typical grain sizes: 31 nm, 64 nm, 85 nm for sample A and 29 nm, 42 nm, 70 nm for sample B.
Note that the similar morphology was corroborated by AFM. Surface roughness of 5 nm root-mean-square
(RMS) obtained also by AFM is identical on both samples.
Micro-Raman spectra (UV laser, λ = 325 nm) shown in Figure 5 reveals that despite the similar surface
structure, the material is different in the volume. Both films exhibit clear sp3 peak at 1332 cm-1 indicating
diamond character. Yet, in relation to the diamond peak, sp2 (graphitic) phase detected on the sample A is higher
compared to the sample B. Calculating relative percentage of the diamond phase from the Raman spectra
(ID/(ID+Isp2)) we find 63 % in sample B and 39 % in sample A. Higher sp3 content in sample B deposited at
600°C is in agreement with previous report that the optimum temperature for depositing NCD is around 600°C
[18]. Note that both 600°C and 820°C films exhibit surface conductivity when hydrogen-terminated, which
indicates generally good electronic quality.
Discussion
As seen on the KFM images, both positive and negative potential changes are observed. This is different
from what was reported on Si thin films [5] and can be attributed to the electret-like behavior of the NCD films
[7]. The electric potential shifts achieved on each sample were different though: three times more positive and
two times more negative potential for the high charge voltage setting.
Figure 3. Charge current plotted against the (a) applied voltage and (b) applied electric field.
Figure 4. SEM of (a) sample A and (b) sample B. In the images we can see measured some typical grain sizes.
88
VERVENIOTIS ET AL.: CHARGING DIFFERENCES OF DIAMOND THIN FILMS
Figure 5. Micro-Raman spectra on NCD thin films, normalized to the diamond peak.
The different electric potential shifts can be partially explained by I/V curves. Typically, we observe higher
currents on sample A for given voltage, except for the barrier region between about +-7V. This is even more
pronounced when plotted as a function of electric field. As the sample is thicker than sample B, one would
expect smaller currents. Hence it must be the material difference indicated by Raman spectra that is behind this
effect.
When we applied 10 V, which means that we attempted to charge with similar current amplitudes (< 1 nA)
on both samples, a potential shift was observed only on sample A. Furthermore, we can also see that -3 nA on
sample A (-10 V bias) are inducing practically the same potential shift as -12 nA (-20 V bias) do on sample B
(190 mV vs. 200 mV). From the above we can conclude that sample B needs higher currents in order to charge
and charges with less efficiency. This current is generated by generally higher bias voltages.
The above is despite the fact that the applied electric field for any given voltage is 60% stronger on sample
B as compared to sample A due to the thickness difference between the two (E = Voltage/Thickness).
Furthermore, considering the generally higher voltages needed to generate current on the sample B, the effective
electric field for any given non-zero charge current can have up to double the intensity on this sample. We also
assume that the geometry of the field is always the same as we use the same type of AFM tips which have the
same apex radius (70 nm) meaning that the contact area is not changing taking into consideration the low surface
roughness of both our films (5 nm).
Contributions to the observed potential shifts may come from charge being trapped in the grain
surface/boundary states and from polarization of the material [7]. As capacitance of the thinner layer is expected
to be higher compared to the thicker one (assuming same material properties) more charge (Q = C*V) should be
stored in the thinner film. This is not the case. Hence the charging is most likely dominated by the trap states in
our case. This explains higher charging of sample A which has relatively higher sp2 content and larger volume.
The states should be of graphitic nature so that they exhibit continuous density of states (DOS) that is able to
keep charges irrespective of the polarity [19].
Conclusions
It was shown that NCD diamond thin films deposited for 16 minutes in 820°C have similar morphology and
grain size with films deposited for 80 minutes in 600°C. In spite of similar surface morphology, the amount of
stored charge differs significantly per both applied voltage and absolute current values. Material differences of
the films were resolved by Raman spectroscopy that detected relatively more sp2 phase in the thicker sample.
Still both samples exhibit good electronic quality (surface conductivity when H-terminated).
The charging properties of the ultra-thin NCD films can be thus tailored by the deposition conditions
according to application requirements in SOD, self-assembly, batteries etc. More sp2 phase in the films leads to
more intensive and spatially better defined charged features. Minimizing sp2 phase reduces induced charge as
well as conduction.
Acknowledgements. We would like to acknowledge the kind assistance of Z. Poláčková with surface oxidation, Dr.
J. Potměšil with NCD deposition, Dr. K. Hruska with SEM imaging and Dr. Z. Vyborny with ellipsometry. This research
was financially supported by AV0Z10100521, research projects KAN400100701 (GAAV), LC06040 (MŠMT), LC510
(MŠMT), 202/09/H0041, SVV-2010-261307 and the Fellowship J.E. Purkyně (ASCR).
89
VERVENIOTIS ET AL.: CHARGING DIFFERENCES OF DIAMOND THIN FILMS
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
H. Fudouzi, M. Kobayashi, and N. Shinya, Adv. Mater. 14, 1649 (2002).
W. Wright and D. Chetwynd, Nanotechnology 9, 133 (1998).
P. Mesquida and A. Stemmer, Microelectron. Eng. 61, 671 (2002).
H. Jacobs and A. Stemmer, Surf. Interface An. 27, 361 (1999).
B. Rezek, T. Mates, J. Stuchlík, J. Kočka, and A. Stemmer, Appl. Phys. Lett. 83, 1764 (2003).
N. Naujoks and A. Stemmer, Microelectron. Eng. 78/79, 331 (2005).
J. Čermák, A. Kromka and B. Rezek, Phys. Stat. Sol. (a) 205 (2008) 2136–2140.
E. Verveniotis, J. Čermák, A. Kromka and B. Rezek., Phys. Status Solidi (b) 246, (2009), 2798
B. Rezek, D. Shin, H. Watanabe, C.E. Nebel Sens. Act B 122 (2007) 596–599.
B. Rezek, D. Shin, H. Uetsuka, C. E. Nebel, Phys. Stat. Sol. (a) 204 (2007) 2888–2897.
B. Rezek, D. Shin, C. E. Nebel, Langmuir 23 (2007) 7626–7633.
A. Kromka, B. Rezek, Z. Remeš, M. Michalka, M. Ledinský, J. Zemek, J. Potměšil, M. Vaněček, Chem. Vap. Dep. 14
(2008) 181–186.
C. E. Nebel, Nature Mater. 2, 431 (2003).
S. Koizumi, M. Kamo, Y. Sato, H. Ozaki, and T. Inuzuka, Appl. Phys. Lett. 71, 1065 (1997).
F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, Phys. Rev. Lett. 85, 3472 (2000).
Y. Koide, M. Liao, and J. Alvarez, Diam. Relat. Mater. 15, 1962 (2006).
B. Rezek and C. E. Nebel, Diam. Relat. Mater. 14, 466 (2005).
H. Kozak, A. Kromka, M. Ledinsky and B. Rezek , Phys. Stat. Sol. (a) 206, (2009), 276.
J. -C. Charlier and J. -P. Michenaud, Phys. Rev. B 46, (1992), 4540.
90