Iodine doping in amorphous carbon thin-films for optoelectronic devices

Article abstract of DOI:10.1016/j.physb.2005.12.081

We report the effects of iodine doping on the optical and structural properties of amorphous carbon thin-films grown on silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (CVD) at low temperature (<100 °C). For film d

Full text of DOI:10.1016/j.physb.2005.12.081

ARTICLE IN PRESS
Physica B 376–377 (2006) 316–319
www.elsevier.com/locate/physb
Iodine doping in amorphous carbon thin-films for
optoelectronic devices
a,
Ashraf M.M. Omer , Sudip Adhikari , Sunil Adhikary , Mohamad Rusop , Hideo Uchida ,
Ã
a
b
c
b
b
Masayoshi Umeno , Tetsuo Soga
c
a
Department of Electrical and Electronic Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan.
b
Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan.
Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 464-8501, Japan.
c
Abstract
We report the effects of iodine doping on the optical and structural properties of amorphous carbon thin-films grown on silicon and
quartz substrates by microwave surface wave plasma chemical vapor deposition (CVD) at low temperature (o100 1C). For film
deposition, we used Ar and CH as plasma source gases. The films were characterized by UV/Vis/NIR spectroscopy, X-ray photoelectron
4
spectroscopy (XPS) and Raman spectroscopy measurements. The optical band gap of the films decreased from 3 to 0.8 eV corresponding
to non-doping to iodine doping conditions. The XPS results confirm the successful doping of iodine in the films. The Raman results show
that iodine doping induced more graphitization in the films.
r 2005 Elsevier B.V. All rights reserved.
Keywords: Amorphous carbon; Iodine doping; Microwave surface wave plasma CVD; Optoelectronic properties
1
. Introduction
several researchers [3–7]. However, the doping prospect of
a-C remains unclear. A new method of doping by iodine (I)
called oxidation has been introduced by Shirakawa et al. [8]
Carbon (C) is a remarkable element exists in different
forms ranging from insulator diamond to metallic graphite
to conducting/semiconducting nanotubes [1]. In recent
research, amorphous carbon (a-C) has been shown to
behave as a semiconducting material, which is able to
accept dopants, shows photoconductivity and suitable for
optoelectronic devices [2]. However, there are some
problems such as high density of defects due to the sp /
3
sp bonding structure and the difficulties in controlling the
conduction type, carrier concentration and optical band
gap [3].
When we attempt to utilize a-C as an alternative material
in optoelectronic devices, control of the conduction type of
a-C film is indispensable [4]. Effective doping can modify
optoelectronic properties of semiconductor materials, in
particular optical band gap and photoconductivity. Dop-
ing of a-C with n-type (e.g. phosphorus (P) and nitrogen
for hydrocarbon polyacetylene (CH) [9–11]. This method
x
has also been applied for a-C and diamond-like carbon
(DLC) films, and the preliminary results showed improve-
ment on optoelectronic properties [12]. However, the
mechanism of doping and the doping induced structural
changes were not clear in the earlier study.
In this paper, we report the effects of I-doping on optical
and structural properties of a-C thin-films grown on quartz
and silicon (Si) substrates by microwave (MW) surface
wave plasma (SWP) chemical vapor deposition (CVD) at
low temperature (o100 1C).
2
2. Experimental details
a-C thin-films were deposited on quartz and Si substrates
by a 2.45 GHz MW SWP CVD, a newly developed
deposition method [12–14]. The substrates were cleaned
beforehand by acetone and methanol in an ultrasonic bath
and then rinsed by ultra-pure water. The Si substrates were
(
N )) and p-type (e.g. Boron (B)) has been attempted by
2
Ã
Corresponding author. Tel.: +81 568 51 9348; fax: +81 568 51 1478.
E-mail address: e03801@isc.chubu.ac.jp (A.M.M. Omer).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.12.081

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A.M.M. Omer et al. / Physica B 376–377 (2006) 316–319
317
etched with diluted hydrofluoric acid (10%) in order to
remove the resistive native oxide layer over the surface. The
CVD chamber was evacuated to a base pressure at
05
1
Before doping
After doping
ꢁ
4
approximately 5 ꢀ 10 Pa using a turbomolecular pump.
For film deposition, we used Ar (280 sccm) as carrier gas
and CH4 (10 sccm) as carbon precursor. The launched
microwave power was typically 500 W at a gas composition
pressure of 55 Pa in the CVD chamber. The duration of
film deposition was 60 min. For I-doping, we employed
pyrolysis of the CVD-deposited a-C films to I-vapor for
about 10 min. Pyrolysis was carried out using a double
furnace set-up, one for heating the source material (I)
placed in a quartz boat and the other for controlling the
sample temperature at 80 and 200 1C, respectively, using Ar
as a carrier gas [12].
1
04
1
03
The optical properties of the films were investigated by
JASCO V-570 UV/VIS/NIR spectrophotometer. The
X-rays photoelectron spectroscopy (XPS) was measured
by ESCA-3300 KM Electron Spectrometer utilizing an Al
K (hn ¼ 1486.6 eV) radiation as an X-ray source, under
a
ꢁ
7
high vacuum conditions of about 10 Pa. The Raman
spectra (RS) was obtained at room ambient conditions in
the quasi-backscattering geometry using 488 nm line of
02
1
+
0
1
2
3
4
5
Ar laser operating at a power of 200 mW while at the
sample it was about 5 mW.
Photon Energy (eV)
Fig. 1. Optical absorption edge of the a-C films before and after I-doping.
3
. Results and discussion
The film (thickness: 460 nm) deposited in this experiment
is diamond-like a-C, which promises a good-quality
semiconducting thin-film for optoelectronic devices [12].
To study the optical characteristics of a-C films, we
carried out the reflectance and transmittance measurements
by UV/Vis/NIR spectroscopy in the wavelength range of
C 1s
2
00-2000 nm. The optical band gaps were obtained by Tauc
equation [15]. As shown in Fig. 1, we found that the
absorption coefficient (a) of a-C films increased signifi-
cantly after I-doping. Interestingly, the optical band gap of
the films decreased from 3 to 0.8 eV after I-doping. The
results suggest that the I-doping in a-C films induced
graphitization and, consequently, decreasing of the optical
band gap.
O 1s
I 3d
After doping
The XPS was used to investigate chemical bonding and
structural properties of the films [13]. Although there are
still some controversies about the assignments of the
individual components of the C 1s and I 3d core level
spectra, the XPS analysis is one of the most likely used
techniques in the literatures to characterize the chemical
bonding structures, and to acquire useful information on
the chemical environment around O, C and I of a-C phases
as shown in Fig. 2. In order to confirm the homogenous
distribution of I in the film, the XPS data has been taken as
an average from different positions of the sample. As
shown in Fig. 3, the XPS result reveals a clear difference
between undoped and I-doped a-C films. The I-doped a-C
film contains I at atomic concentration of 0.26% and mass
concentration of 2.66%.
C 1s
O 1s
Before doping
2
00
300
400
500
600
700
Binding Energy (eV)
Fig. 2. XPS (hn ¼ 1486.6 eV) wide scans of a-C films before and after I-
doping.

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A.M.M. Omer et al. / Physica B 376–377 (2006) 316–319
318
After Doping
Before Doping
620.5 eV
After doping
D peak
G peak
Before doping
615
620
625
1
000
1200
1400
Raman Shift (cm )
1600
1800
Binding Energy (eV)
-1
Fig. 3. XPS (hn ¼ 1486.6 eV) I 3d core-level spectrum of I-doped a-C film
Fig. 4. RS of the a-C films before and after I-doping.
compared with the spectrum of undoped film.
Raman spectroscopy is a very popular, non-destructive
tool for investigating the structural characteristics of a-C
and DLC films [16–18]. RS provides a wide range of
structural and phase disorder information. RS of two
crystalline forms of C; graphite and diamond are well
known [19–22]. The first order RS consist of a single line at
ꢁ
1
ꢁ1
1
crystal graphite [20,24]. The latter, known as graphite peak
332 cm
for diamond [23], and 1580 cm
for single
(
while another line appears also at 1355 cm
G peak), results from the Raman allowed E2g mode [25],
ꢁ
1
for poly-
crystalline graphite, known as disorder peak (D peak)
results from the breakdown of the k-vector conservation
rule of the disordered lattice [26] and grain boundary effect
[20]. Generally, the G and D peaks are the most used lines
in RS to characterize the C films. The D peak represents
2
3
disordered sp -hybridized C with an amount of sp -
hybridized C, while the G peak represents graphite-like
1000
1200
1400
Raman Shift (cm )
1600
1800
-1
2
sp -hybridized C in the films [27,28]. Fig. 4 shows the RS of
Fig. 5. Gaussian fitting for RS of I-doped a-C film.
the films before and after I-doping in the range of
ꢁ
1
000–1800 cm . The RS of the undoped film revealed its
1
amorphous nature. It is clear that the board band of the
film splitted into D and G peaks after I-doping due to the
graphitization, as can also be seen from the optical
properties of the films (Fig. 1). In order to evaluate the
structural features, the experimental data were best fitted
(FWHM) and the integrated intensity ratio of the two
peaks (I /I ) are obtained from the Gaussian fittings. For
D
G
D-peak, the position and FWHM were 1357.62 and
ꢁ
1
233.11 cm , respectively. While for G-peak, the position
ꢁ
1
and FWHM were 1588.96 and 90.07 cm , respectively.
(
accuracy factor ꢂ0.95) by two peaks, considering Gaus-
The I /IG was calculated as 2.35. The Raman results
D
sian line shapes and linear background, and a typical plot
of this kind is shown in Fig. 5 for the I-doped film. The
peak position, the peak full-width at half-maximum
suggest that the film contains a mixture of tetrahedral
2
3
bonded C (sp -C bond) and trihedral bonded C (sp -C
bond) bonds, and the latter acts as a localized conduction

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A.M.M. Omer et al. / Physica B 376–377 (2006) 316–319
319
state [29]. Therefore, the optical band gap is determined by
[3] Y. Hayashi, S. Ishikawa, T. Soga, M. Umeno, T. Jimbo, Diamond
Relat. Mater. 12 (2003) 687.
3
the sp /sp -hybridized C ratio.
2
[4] M. Rusop, S.M. Mominuzzaman, T. Soga, T. Jimbo, M. Umeno,
Finally, the present results suggest that I-doping in a-C
films induces graphitization as a result of structural
Jpn. J. Appl. Phys. 42 (2003) 2339.
5] C.W. Chen, J. Robertson, Carbon 37 (1999) 839.
[
[
3
2
changes due to sp /sp carbon bonding network. Conse-
quently, the I-induced graphitization helps to improve
optoelectronic properties of a-C films applicable for
photovoltaic devices.
6] X.M. Tian, M. Rusop, Y. Hayashi, T. Soga, T. Jimbo, M. Umeno,
Solar Energy Mater. Solar Cells 77 (2003) 105.
[
[
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Gilkes, M. Weiler, L.M. Brown, Phys. Rev. B 48 (1993) 17954.
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4
. Conclusions
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We investigated the properties of the a-C film before and
[
[
10] C.K. Chiang, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis,
A.G. MacDiarmid, J. Chem. Phys. 69 (11) (1978) 5098.
after I-doping. The optical band gap of the film decreased
from 3 to 0.8 eV after I-doping. The XPS analysis
confirmed the presence of I-bonded a-C that indicates the
successful doping of I in the film. The Raman results
showed that the graphitization of film intensified after I-
doping. Further research is in progress to optimize the
dopants, and to improve quality of the films by reducing
the bonding defects.
11] C.K. Chiang, M.A. Druy, S.C. Gau, A.J. Heeger, E.J. Louis, A.G.
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Acknowledgements
This work was supported by New Energy and Industrial
Technology Development Organization (NEDO) under
Ministry of Economy, Trade and Industry (METI),
Government of Japan. We gratefully acknowledge Mr.
A. Marui for Raman spectroscopy measurements and Mr.
M. Kawamura for XPS measurements. A.M.M. Omer is
grateful to ASJA International for its financial support.
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