Field emission properties of porous diamond-like films produced by chemical vapor deposition

Article abstract of DOI:10.1063/1.1517724

The field emission properties of porous diamond-like carbon structures have been characterized. A hot filament chemical vapor deposition system fed with ethyl alcohol vapor diluted in helium was used to deposit the samples. Morphological analysis by field emission scanning electron microscopy revealed that they had a highly porous structure, which was attributed to the modification of the kinetics of the carbon deposition process due to the presence of helium as a buffer gas. Micro-Raman spectroscopy showed two peaks in the graphene and microcrystalline graphite frequencies and a new peak at 1620cm^-1. Low threshold fields (E_t) and hysteresis in the current versus voltage characteristic have been observed, and a model to explain the hysteresis is proposed.

Full text of DOI:10.1063/1.1517724

Field emission properties of porous diamond-like films produced by chemical vapor
deposition
V. P. Mammana, T. E. A. Santos, A. P. Mammana, V. Baranauskas, Helder J. Ceragioli, and A. C. Peterlevitz
Citation: Applied Physics Letters 81, 3470 (2002); doi: 10.1063/1.1517724
View online: http://dx.doi.org/10.1063/1.1517724
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/81/18?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 18
28 OCTOBER 2002
Field emission properties of porous diamond-like films produced
by chemical vapor deposition
V. P. Mammana, T. E. A. Santos, and A. P. Mammana
Instituto Centro de Pesquisas Renato Archer, Minist e´ rio da Ci eˆ ncia e Tecnologia,
Rodovia SP-65, km 143,6, 13089-500, Campinas, SP, Brazil
a)
V. Baranauskas, Helder J. Ceragioli, and A. C. Peterlevitz
Faculdade de Engenharia El e´ trica e Computa c¸ ˜
ao, Universidade Estadual de Campinas,
Avenida Albert Einstein N. 400, 13083-970, Campinas, SP, Brazil
͑
Received 24 June 2002; accepted 5 September 2002͒
The field emission properties of ‘‘porous diamond-like’’ carbon structures have been characterized.
A hot filament chemical vapor deposition system fed with ethyl alcohol vapor diluted in helium was
used to deposit the samples. Morphological analysis by field emission scanning electron microscopy
revealed that they had a highly porous structure, which was attributed to the modification of the
kinetics of the carbon deposition process due to the presence of helium as a buffer gas.
Micro-Raman spectroscopy showed two peaks in the graphene and microcrystalline graphite
Ϫ1
frequencies and a new peak at 1620 cm . Low threshold fields (E ) and hysteresis in the current
t
versus voltage characteristic have been observed, and a model to explain the hysteresis is
proposed. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1517724͔
Considerable attention has been given in recent years to
the electronic properties of different forms of nanostructured
materials, since these properties are different from those in
the bulk state. For example, nanostructured carbon has the
exceptional property of easy emission of free electrons from
its surface to the vacuum when low external electric fields
are applied, even at room temperature.1 This process,
named cold field emission, has potential applications in
spectroscopy and field emission scanning electron Micros-
copy ͑FESEM͒ are discussed.
The samples were prepared by a hot-filament CVD reac-
9
tor originally optimized for diamond synthesis. However,
instead of the conventional H gas chemistry, the system was
2
fed with a vapor mixture of ethyl alcohol (C H OH) diluted
2
5
–7
only in helium. Total gas flow rate ͑regulated by precision
mass flow meters͒ and pressure were maintained at about 110
sccm and 20 mbar, respectively. Substrates were polished Si
wafers of low electrical resistivity, with temperature moni-
tored by a thermocouple, embedded in the substrate holder.
Temperature was kept constant at 1128 K by the control of
the power applied to the filament ͑about 200 W͒. A total
processing time ͑nucleation and growth͒ of 7 h was used.
The samples were characterized by micro-Raman spectros-
copy ͑at ambient temperature͒ employing a Jobin–Yvon
T64000 Raman spectrometer, with the argon line at ␭
ϭ514.5 nm used for excitation at a laser power of about 6
mW. Morphological analyses were made by FESEM using a
JEOL JSM6330F. The characterization of field emission
properties was performed by the measurement of current ver-
sus voltage (I–V) curves for different anode–cathode
8
vacuum microelectronics, microwave amplifiers, and lumi-
7
nescent flat panel displays, among others.
Field emission from different samples of diamond-like
3
1,2,4–7
carbon and carbon nanotubes ͑CNTs͒
have been stud-
ied. From these reports, the best field emission characteris-
tics have been achieved using CNT samples, and this is often
attributed to their high aspect ratio geometry and small tip
radii which enhance the local electrical field at the tips for
the electron emission process.2 Therefore, it is also of inter-
est to investigate the field-emission properties of other forms
of high roughness nanostructured carbon, such as the variety
of ‘‘porous diamond-like’’ materials that may be produced
using new preparation techniques.
,7
In this letter we investigate the electron emission from
porous diamond-like structures synthesized by chemical va-
por deposition ͑CVD͒ in a hot-filament reactor fed with ethyl
alcohol vapor and helium. The use of inert helium instead of
reactive hydrogen was to modify the kinetics of the carbon
crystallization process at the atomic level, and to contribute
to the increase of the final surface roughness of the samples.
We show that the samples prepared by this process have very
6
͑sample͒ spacing in parallel plate configuration. Spacers of
known thickness (d) and anodes and cathodes of the same
area have been used. The threshold field (E ) for a given
t
threshold current (I ) was determined by the slope of the V
t
t
versus d curves fitted to straight lines. To plot the I–V
curves in the FN form the emission current was measured
either with increasing or decreasing bias voltage, indicated in
the plot as ‘‘rise’’ or ‘‘fall,’’ respectively.
Ϫ2
high porosity and electron emission of 10 ␮A cm with a
Ϫ1
threshold field (E ) of 1.3 V ␮m . A clear hysteresis in the
Figure 1 shows the typical micro-Raman spectrum of the
samples. Three scattering peaks may be seen: two prominent
t
Fowler–Nordheim ͑FN͒ current–voltage characteristic is ob-
served. Analysis of the structures made by micro-Raman
Ϫ1
Ϫ1
peaks centered at 1350 cm and 1580 cm , together with
Ϫ1
a small broad peak centered at 1620 cm . The peak at
Ϫ1
a͒Author to whom correspondence should be addressed; electronic mail:
vitor@dsif.fee.unicamp.br
1350 cm
corresponds to disorders on the graphene layers,
Ϫ1
and the peak at 1580 cm corresponds to graphene crystal-
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003-6951/2002/81(18)/3470/3/$19.00 3470 © 2002 American Institute of Physics
34.129.164.186 On: Mon, 22 Dec 2014 06:59:13
0
1

Appl. Phys. Lett., Vol. 81, No. 18, 28 October 2002
Mammana et al.
3471
FIG. 1. Typical room-temperature micro-Raman spectrum of the porous
diamond-like samples.
FIG. 3. Typical threshold voltage V versus distance d measurements nec-
t
Ϫ2
essary to produce a current density of about 10 ␮A cm
.
line layer. Both peaks are observed in the Raman spectra of
1
0,11
12
several types of CNT,
fullerenes. The small broad peak at 1620 cm
microcrystalline graphite, and
1
3
Ϫ1
was not
surements, carried out for a fixed threshold current, is shown
in Fig. 3. The error bars are associated with the emission
fluctuation due to flickering and spacing uncertainty. A linear
identified and was not observed in other carbon materials. It
may, therefore, represent a signature of the present porous
diamond-like samples.
Ϫ1
fit to this data provides a threshold field (Et) of 1.3 V ␮m
.
The typical morphology of the sample shows the coales-
cence of round-shaped features of diameter in the range of 1
to 2 ␮m, forming three-dimensional mesostructures of en-
tangled wires, deposited on the Si surface, with no apparent
preferential orientation. Figure 2 shows a FESEM image of a
typical entangled wire surface. There is clear evidence of a
highly porous structure, since the pores and flaws which ap-
pear dark are much larger than the thickness of the pore
walls (ϳ10 nm). A rough estimate indicates that the porosity
is higher than 80%. The elevated inert gas concentration in-
creases the gas-phase diffusion barriers and also decreases
the adatom mobilities at the growing surface. Starting from
the first nucleation step these mechanisms inhibit the lateral
deposition rate of carbon atoms on the substrate to the end of
the growth process, and may be the main factors responsible
for production of the highly porous morphologies observed
in these samples.
The threshold current used (Itϭ8.4 ␮A) averaged over the
2
cathode area (84 mm ) corresponds to a current density of
Ϫ2
10 ␮A cm , which is a value normally used for comparing
emission data in the literature.7
The typical electron emission results using a fixed dis-
tance (dϭ1.08 ␮m) are shown in Fig. 4 by plotting the
(I–V) data in FN diagram for increasing or decreasing bias
voltage, as indicated. For high voltages the data points may
be fitted to a single statistical slope, that is, a usual FN plot,
either for increasing or decreasing bias. However, for me-
dium voltages there is a remarkable hysteresis; that is, the
slope for the sweep rising bias is much higher than the slope
for the sweep decreasing bias. Although the data points of
1
4
The typical threshold voltage V versus distance d mea-
t
FIG. 4. Typical current–voltage data plotted in FN diagram for increasing
͑
open squares͒ or decreasing bias voltage ͑black squares͒, as indicated. A
FIG. 2. Typical FESEM image of the samples grown with ethyl alcohol and
parallel plate configuration was used with anode and cathode having the
same area of about 84 mm , separated by a distance d of 1.08 ␮m.
34.129.164.186 On: Mon, 22 Dec 2014 06:59:13
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Ϫ2
He in the chamber feed.
1

3472
Appl. Phys. Lett., Vol. 81, No. 18, 28 October 2002
Mammana et al.
the sweep decreasing bias may be statistically fitted to a
single slope for high, medium, and low voltages, the current
fluctuation is comparatively high in the mid and low voltages
range. This kind of hysteresis has been observed in vertically
the hysteresis is related to the electrostatic alignment of the
terminating species with the externally applied electric field
vector which creates tips with extremely high localized elec-
trical fields. The result that porous diamond-like structures
are good electron emitters may be of great importance for
future electronic devices.
1
5
aligned CNTs also.
As a nanoscopic phenomenon, the properties of the field
emission depend strongly on: ͑i͒ the atomic and radical spe-
cies that terminate the emitting surface ͑carbon atoms, me-
tallic impurities, adsorbed radicals, adsorbed gases͒, and ͑ii͒
on the local field at these terminating species ͑which depends
on the nanoscopic geometry of the emitting tips͒. During the
rising bias sweep, besides the polarization of the terminating
species, long molecules like the CNTs themselves, other car-
bon chains, or carbon-whiskers, that are lying on the surface,
are electrostatically induced to be ‘‘piled-up’’ to align their
main longitudinal axis with the externally applied electric
field vector. This process creates tips with extremely high
localized electrical fields, which further enhances the current
emission. In sequence, when the falling sweep bias starts, the
aligned tips have an inherent inertia to remain piled-up, since
there is no change in the direction of the externally applied
electric field, but only its intensity decreases. Therefore, the
current emission during the falling sweep bias remains high
compared to that during the rising sweep bias current, which
explains the hysteresis.
´
We thank ‘‘Grupo de Propriedades Opticas do IFGW/
UNICAMP’’ for use of their Raman spectrometer. The elec-
tron microscopy work was performed at LME/LNLS-
Campinas. We also gratefully acknowledge Dr. Kanad Mallik
for his technical assistance and ‘‘CNPq,’’ ‘‘FAPESP,’’ and
Minist e´ rio da Ci eˆ ncia e Tecnologia from Brazil for financial
support.
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We demonstrated that it was possible to obtain porous
diamond-like structures of carbon with good field emission
characteristics by the direct CVD of ethyl alcohol vapor, us-
ing He as a noble gas buffer. Although the complex surface
structure of this material is not yet know, micro-Raman
analysis indicated scattering peaks corresponding to
1
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graphene layers, microcrystalline graphite, and a new peak at
Ϫ1
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