Nitrogenated tetrahedral amorphous carbon films prepared by ion-beam-assisted filtered cathodic vacuum arc technique for solar cells application

Article abstract of DOI:10.1063/1.122486

Fabrication and characterization of nitrogenated tetrahedral amorphous carbon (ta-C:N) semiconductor/crystalline p-type silicon (p-Si) heterojunction structures are reported. The electron-hole pairs generated from both ta-C:N and Si depletion regions were observed from photoresponse measurements. The peaks are centered at about 540 and 1020 nm, which correspond to the optical absorption edge of ta-C:N and p-Si, respectively. The reverse current increased by three orders of magnitude when the structures were exposed to AM1 light. A photovoltaic effect was observed from ta-C:N and the values of short circuit current, open circuit voltage, and field factor obtained are 5.05mAcm^-2, 270 mV, and 0.2631, respectively.

Full text of DOI:10.1063/1.122486

Nitrogenated tetrahedral amorphous carbon films prepared by ion-beam-assisted
filtered cathodic vacuum arc technique for solar cells application
L. K. Cheah, X. Shi, E. Liu, and J. R. Shi
Citation: Applied Physics Letters 73, 2473 (1998); doi: 10.1063/1.122486
View online: http://dx.doi.org/10.1063/1.122486
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/73/17?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Enhanced passivation at amorphous/crystalline silicon interface and suppressed Schottky barrier by deposition of
microcrystalline silicon emitter layer in silicon heterojunction solar cells
Appl. Phys. Lett. 104, 113901 (2014); 10.1063/1.4868726
Hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase and usage as an intermediate
reflector in thin-film silicon solar cells
J. Appl. Phys. 109, 113109 (2011); 10.1063/1.3592208
Photovoltaic characteristics of amorphous silicon solar cells using boron doped tetrahedral amorphous carbon
films as p -type window materials
Appl. Phys. Lett. 90, 083508 (2007); 10.1063/1.2539767
Fluorinated amorphous carbon films prepared by plasma enhanced chemical vapor deposition for solar cell
applications
J. Vac. Sci. Technol. A 21, 1784 (2003); 10.1116/1.1604130
Effect of thermal annealing on the electronic properties of nitrogen doped amorphous carbon/p-type crystalline
silicon heterojunction diodes
J. Vac. Sci. Technol. A 21, 582 (2003); 10.1116/1.1562477
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
128.143.199.160 On: Mon, 15 Dec 2014 05:03:11

APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 17
26 OCTOBER 1998
Nitrogenated tetrahedral amorphous carbon films prepared
by ion-beam-assisted filtered cathodic vacuum arc technique
for solar cells application
L. K. Cheah,a) X. Shi, E. Liu, and J. R. Shi
Ion Beam Processing Lab, School of Electrical and Electronic Engineering, Nanyang Technological
University, 639798 Singapore
͑Received 9 June 1998; accepted for publication 25 August 1998͒
Fabrication and characterization of nitrogenated tetrahedral amorphous carbon (ta-C:N)
semiconductor/crystalline p-type silicon (p-Si) heterojunction structures are reported. The
electron-hole pairs generated from both ta-C:N and Si depletion regions were observed from
photoresponse measurements. The peaks are centered at about 540 and 1020 nm, which correspond
to the optical absorption edge of ta-C:N and p-Si, respectively. The reverse current increased by
three orders of magnitude when the structures were exposed to AM1 light. A photovoltaic effect was
observed from ta-C:N and the values of short circuit current, open circuit voltage, and field factor
obtained are 5.05 mA cmϪ2, 270 mV, and 0.2631, respectively. © 1998 American Institute of
Physics. ͓S0003-6951͑98͒02643-6͔
Photoconduction behavior has been observed from dia-
mond like carbon ͑DLC͒ films.1–3 Two configurations have
been introduced, i.e., the gap cell structure1,2 and the hetero-
junction structure.3 In the first configuration, the gaps are in
the range of 10–50 ␮m. Light is sighted between the gaps
and the photogenerated current is measured from the two
contacts. A photoconductivity effect in this configuration has
been reported.1,2 However, the depletion region cannot be
formed in this configuration and no photovoltaic effect has
been observed. In the second configuration, with a DLC film
deposited on a silicon substrate forming the heterojunction
structure, the electron-hole pair excitation was observed in
the Si depletion region under reverse bias as reported by
Veerasamy et al.3 The filtered cathodic vacuum arc ͑FCVA͒
technique was reported to be an efficient method of produc-
ing high quality, macroparticle free, and large uniformity
DLC at room temperature.3 The electronic properties were
reported to be adjustable by the incorporation of nitrogen
during deposition.4 More efficient doping can be achieved by
nitrogen ion bombardment during the growth of DLC films
͑ion-assisted FCVA͒.5 In this letter, the measurements on
optical absorption edge, current–voltage characteristics un-
der dark and AM1 conditions, and photoresponse were con-
ducted. An observation of electron-hole pairs generated from
both DLC and Si depletion regions from photoresponse mea-
surements under no bias condition is reported.
The DLC films were deposited by the FCVA technique.
The substrates in the vacuum chamber were negatively bi-
ased at 80 V, which corresponds to an impinging carbon ion
energy about 100 eV. The first set of ta-C films was depos-
ited directly by a Cϩ beam. The second set of samples was
deposited by a Cϩ beam together with a 100 eV Nϩ ion
beam produced by an ion beam source at a current density of
3 mA cmϪ2 and a N2 flow rate of 2 sccm. The DLC films
produced by the FCVA technique have a high sp3 content
(ϳ88%) and are termed as tetrahedral amorphous carbon
(ta-C).6 The FCVA technique has been described
elsewhere.6 The substrates used in this study were ͗100͘ p-
type silicon (p-Si) with a resistivity of 1–10 ⍀ cm. Before
deposition, all substrates were cleaned with Ar ions to re-
move the oxide layers. Gold contacts with an area about
20 mm2 and a thickness about 20 nm were deposited on the
top of ta-C layers. The gold transmittance varied from 20%
to 50% as the wavelength varied from 350 to 1100 nm.
As-deposited ta-C films exhibit a band gap of 2.7 eV7
and a Fermi level measured by activation energy is about
0.35 eV below the midgap.5 In this case, the films exhibit a
defect-controlled p-type semiconductor characteristic. When
nitrogen is incorporated in the film, the Fermi level moves up
with the doping level.5 At a nitrogen flow rate of 2 sccm, the
Fermi level of the films is above the midgap, thus the films
become doped n type.5 The Fermi level is 0.8 eV below the
conduction band and the band gap reduces to about 2.5 eV
due to the C–N alloying effects.
The absorption coefficient ͑␣͒ as a function of photon
energy for the ta-C films was determined by spectroscopic
phase modulated ellipsometry ͑UNIVEL, ISA Jobin Yvon͒.7
The realization of the efficient photovoltaic energy conver-
sion is that the film must have a sufficiently large optical
absorption coefficient to absorb a significant fraction of the
solar energy. As shown in Fig. 1, the optical absorption co-
efficient of nitrogenated ta-C (ta-C:N) is larger than that of
the as-deposited ta-C. The absorption spectra for the ta-C:N
are similar to those obtained from hydrogenated amorphous
silicon. Each absorption spectrum shows two main absorp-
tion regions: The first is located at the interband region,
which can be used to define the optical energy band gap. The
second is the shoulder region, which located at the intraband
region. In our previous spectral ellipsometry study ͑using the
Forouhi and Bloomer, F. B. formulations͒, we can only
simulate the first ͑interband͒ region.7 However, with the
Tauc and F. B. formulations, both interband and intraband
regions can be clearly observed and the optical modeling for
ellipsometry measurement is described elsewhere.8 The op-
tical band gap can be derived from the optical absorption
a͒
Corresponding author. Electronic mail: pa1870668@ntu.edu.sg
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
0003-6951/98/73(17)/2473/3/$15.00 2473 © 1998 American Institute of Physics
128.143.199.160 On: Mon, 15 Dec 2014 05:03:11

2474
Appl. Phys. Lett., Vol. 73, No. 17, 26 October 1998
Cheah et al.
FIG. 3. Photoresponse measurements for the as-deposited ta-C, ta-C:N of
12 at % N and the Si standard.
FIG. 1. Absorption spectra as a function of photon energy of the as-
deposited ta-C film and the ta-C:N film with 12 at % N measured by
spectral ellipsometry. ͑Inset shows the absorption spectra as a function of
wavelength for ta-C and ta-C:N films.͒
ϫ10Ϫ3 A cmϪ2, respectively. The JSC of the ta-C:N hetero-
junction is about an order higher than the ta-C heterojunc-
tion. The FF is 0.2472 and 0.2631 for ta-C and ta-C:N
heterojunction structures, respectively.
edge.7 The optical band gap for the as-deposited ta-C and
ta-C:N are 2.7 and 2.5 eV, respectively.
The photoresponse spectra of the ta-C and ta-C:N het-
erojunction diodes were examined by using 300 W mercury
xenon arc lamp manufactured by Oriel Instruments. The
wavelength of the arc lamp ranged from 350 to 1100 nm.
The incident radiation was modulated in 25 Hz to reduce the
signal-to-noise ratio. The spectra resolution of the system is
determined by a monochromator ͑model 77200, 1/4 m grat-
ing, Oriel Instruments͒ and was fixed at 0.1 nm. The respon-
sivity of the ta-C and ta-C:N was obtained by a comparison
with a standard silicon detector head of known responsivity
͑Oriel’s model 70331͒. The ta-C and ta-C:N samples were
measured under no bias condition. The measurements show
that the photoresponse for the ta-C:N materials is larger than
the ta-C films and the standard Si detector. The noteworthy
feature of the photoresponse spectra as a function of wave-
length is that the both ta-C and ta-C:N spectra show two
peaks centered at about 520–540 nm ͑band I͒ and 1020 nm
͑band II͒. Band I is found to coincide with the ta-C and
ta-C:N optical absorption edges, as shown in the inset of
Fig. 1. Therefore, it can be inferred that the photogeneration
of carriers occurs in the depletion region on the ta-C or
ta-C:N side of the heterojunction. The generation consists of
exciting electrons and holes across the forbidden gap from a
high-density valence band tail states to high-density conduc-
tion band tail states. The photoconduction takes place pre-
dominantly in this mobility edge but not the delocalized elec-
trons or by hopping motion in the shoulder region. This
agrees with our results on thermal activation measurement,
which the band tail states transport mechanism is observed at
room temperature.5 Band II corresponds to the optical ab-
sorption edge of silicon. This band has a very similar trend in
responsivity to the standard silicon head, as shown in Fig. 3.
In summary, the ta-C and ta-C:N heterojunction solar
cells were successful fabricated. The optical absorption co-
efficient of these structures is sufficiently large for them to
absorb a significant fraction of the solar energy. The
electron-hole pair generation in the ta-C and the ta-C:N
depletion regions were illustrated in the photoresponse mea-
surements. The photovoltaic effect was observed from the
The current–voltage (I–V) characteristics were mea-
sured with a precision semiconductor parameter analyzer
͑HP4156A, HP͒. The I–V curves of the ta-C and ta-C:N
under the dark and AM1 light conditions are shown in Fig. 2.
The reverse bias current increases by about three orders of
magnitude when the ta-C and the ta-C:N heterojunction
structures are exposed to the light, i.e., the reverse bias cur-
rent of ta-C:N heterojunction increases from 6.10ϫ10Ϫ5 to
5.45ϫ10Ϫ2 A cmϪ2 with a reverse bias of 2.0 V. The for-
ward bias current was found to increase by one to two orders
of magnitude. Thus, these heterojunctions are very sensitive
to the light under the reverse bias conditions and are poten-
tially useful for photodetector applications. The photovoltaic
behavior was observed in the ta-C and ta-C:N heterojunc-
tion structures. Several parameters for the photovoltaic be-
havior, i.e., the open circuit voltage (VOC), short circuit cur-
rent density (JSC), and field factor (FF) can be extracted
from the I–V measurement. The VOC and JSC of the ta-C
heterojunction structures are about 0.22 V and 1.37
ϫ10Ϫ4 A cmϪ2, and the VOC and JSC of the ta-C:N hetero-
junction structures are about 0.27
V
and 5.05
FIG. 2. I–V curves for the as-deposited ta-C and the ta-C:N of 12 at % N
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
in dark and under AM1 condition.
ta-C and ta-C:N heterojunction structures. The analysis on
128.143.199.160 On: Mon, 15 Dec 2014 05:03:11

Appl. Phys. Lett., Vol. 73, No. 17, 26 October 1998
Cheah et al.
2475
3 V. S. Veerasamy, G. A. J. Amaratunga, J. S. Park, H. S. MacKenzie, and
W. I. Milne, IEEE Trans. Electron Devices 42, 577 ͑1995͒.
4 G. A. J. Amaratunga, V. S. Veerasamy, C. A. Davis, W. I. Milne, D. R.
McKenzie, J. Yuan, and M. Weiler, J. Non-Cryst. Solids 164–166, 1119
͑1993͒.
the optical absorption spectra, I–V characteristics under dark
and AM1 conditions, photovoltaic behavior, and photore-
sponse spectra shows that the ta-C:N heterojunction struc-
tures are more promising than the ta-C heterojunction struc-
tures for solar cell applications.
5 L. K. Cheah, X. Shi, H. Fu, B. K. Tay, Electron. Lett. 33, 1339 ͑1997͒.
6 B. K. Tay, X. Shi, L. K. Cheah, and D. I. Flynn, Thin Solid Films 308–
309, 199 ͑1997͒.
1 G. A. J. Amaratunga, V. S. Veerasamy, W. I. Milne, C. A. Davis, S. R. P.
Silva, and H. S. MacKenzie, Appl. Phys. Lett. 63, 370 ͑1993͒.
2 L. Klibanov, N. Croitoru, A. Seidman, L. Scheffer, and E. Ben-Jacob,
Diamond Relat. Mater. 6, 1868 ͑1997͒.
7 X. Shi, L. K. Cheah, and B. K. Tay, Thin Solid Films 312, 166 ͑1998͒.
8 L. K. Cheah, X. Shi, J. R. Shi, E. Liu, and B. K. Tay, J. Non-Cryst. Solid
͑to be published͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
128.143.199.160 On: Mon, 15 Dec 2014 05:03:11