Journal of Modern Physics, 2011, 2, 398-403
doi:10.4236/jmp.2011.25049 Published Online May 2011 (http://www.SciRP.org/journal/jmp)
Pulsed Supermagnetron Plasma CVD of a-CNx:H
Electron-Transport Films for Au/a-CNx:H/p-Si
Photovoltaic Cells
Haruhisa Kinoshita, Hiroyuki Suzuki
Research Institute of Electronics, Shizuoka University, Shizuoka, Japan
Received February 28, 2011; revised April 16, 2011; accepted April 27, 2011
Abstract
Hydrogenated amorphous carbon nitride (a-CNx:H) films were formed on p-Si wafers set on a lower elec-
trode by pulsed supermagnetron plasma CVD using i-C4H10 and N2 gases. Lower electrode RF power (LORF)
of 13.56 MHz (50 - 800 W) was modulated by a 2.5-kHz pulse at a duty ratio of 12.5%, and upper electrode
RF power (UPRF) of 50 - 400 W was supplied continuously. The optical band gap decreased with an in-
crease in LORF at each UPRF. The open circuit voltage of Au/a-CNx:H/p-Si photovoltaic cells (a-CNx:H
film thickness: 25 nm) was about 200 mV for each cell, and the short circuit current density and energy con-
version efficiency increased with LORF for each UPRF. The highest energy conversion efficiency of 0.81%
was obtained at UPRF/LORF of 200/800 W.
Keywords: a-CNx:H film, Supermagnetron Plasma, Pulsed Plasma, CVD, Photovoltaic Cell
1. Introduction
Until now, silicon and compound-semiconductor-based
materials have dominated the research and market for
photovoltaic cells (PVCs). These PVCs utilize p-n junc-
tions for the separation of photo-generated carriers. Hy-
drogenated amorphous carbon (a-C:H) has been applied
in PVCs, as a semiconductor, though the use of these
PVCs is still rather limited [1-4]. a-C:H and its nitride
(a-CNx:H) showed more than ten orders of magnitude
difference in room temperature conductivity, depending
on the deposition method used [5-7]. The proportion of
sp2 hybridized carbon atoms and their clustering in
smaller or larger size determine the optical band gap and
produce a significant influence on the electrical transport
as well [7-9].
Pulsed discharge of RF plasma is useful to suppress
wafer temperature, because the mean RF power supplied
to the wafer becomes small because of the pulsing. In the
case of pulsed discharge of RF plasma, high pulse-fre-
quency is important because the oscillation of wafer
temperature can be reduced by its enhancement [10]. In
the case of supermagnetron plasma chemical vapor de-
osition (CVD) by continuous plasma discharge, the sub-
strate set on a room-temperature stage was easily heated
to over 100˚C with an increase in RF power [11]. The
increase in substrate temperature degraded the character
of the a-CNx:H films. By using RF powers modulated by
a 2.5-kHz pulse frequency and a duty ratio (defined as
the ratio of pulse on-time to total cycle time) of 12.5%,
in the case of supermagnetron plasma CVD, the substrate
was cooled stably and relatively hard and homogeneous
a-CNx:H films were obtained [12].
In this study, we formed a-CNx:H films on p-Si sub-
strates and measured the diode I-V characteristics. We
report herein the optical properties and PVC properties
of a-CNx:H films, including the short circuit current den-
sity (ISC), open circuit voltage (VOC), and energy conver-
sion efficiency, for fabricated Au/a-CNx:H/p-Si hetero-
junction PVCs.
2. Experimental Procedure
A pulsed RF supermagnetron plasma CVD apparatus
was used for the deposition of a-CNx:H films [12], as
shown in Figure 1. The amplitude of one of the two RF
power sources with the same RF frequency of 13.56
MHz was modulated using a pulse generator. Continuous
and pulsed RF powers were supplied to the upper and
lower electrodes with respect to the grounded metal

H. KINOSHITA ET AL. 399
Figure 1. Schematic of the pulsed supermagnetron plasma
CVD apparatus.
chamber, respectively. The upper electrode was covered
with a graphite plate. The phase difference between the
two RF voltages was controlled to be approximately 180°.
The magnetic field applied in parallel with the two elec-
trode surfaces was approximately 80 G. The deposition
substrates were placed on the lower electrode, which was
heated to 100˚C during film deposition. i-C4H10 (50 sccm)
and N2 (120 sccm) were introduced into the CVD cham-
ber, and the gas pressure was controlled at 30 mTorr.
The film thicknesses were measured by profilometry
(ULVAC; DECTAK-3) to be 300 - 600 nm. The absorp-
tion coefficient was measured using a UV/Vis/NIR spec-
trometer (Shimadzu; UV-3100PC) to calculate the opti-
cal band gap with the Tauc equation [13]. The bonding
configurations of a-CNx:H were measured using a Fou-
rier transform infrared (FTIR) spectrometer (JEOL;
JIR-WINSPEC 50). The film thickness of a-CNx:H used
in FTIR analysis was controlled to be about 500 nm for
all the samples. The refractive index of a-CNx:H film
deposited on the Si wafer was measured by ellipsometry
(Mizojiri Optical; DHA-OLXS with a wavelength of
632.8 nm).
3. Results and Discussion
In the deposition of a-CNx:H films, RF power conditions
were upper electrode RF powers (UPRFs) of 50, 100,
200, and 400 W at lower electrode RF powers (LORFs)
of 50 - 800 W, and the N2 gas concentration was 70%.
Figure 2 shows the LORF dependence of the deposition
rate (calculated as total cycle time). The deposition rate
increased significantly with the UPRF for each LORF.
At UPRFs of 50 and 100 W, the deposition rate increased
incrementally with an increase in LORF. At a UPRF of
200 W, the deposition rate was almost independent of
LORF. At a UPRF of 400 W, on the other hand, the
deposition rate decreased with an increase in LORF. This
deposition rate behavior was caused by sputter deposits
Figure 2. Deposition rate of a-CNx films measured as a func-
tion of LORF.
on a counter electrode, i.e., sputtering of reactive species
from the upper electrode surface to the lower electrode
surface. Pulse plasma deposition was performed by re-
peated deposition of soft polymer-like and hard dia-
mond-like films. In this experiment, both sputter deposits
and radicals present during the discharge off-time formed
soft polymer-like films. Soft films were bombarded with
high-energy ions and modified to hard films.
3.1. Physical and Chemical Characteristics of the
Films
The optical band gap of a-CNx:H films was measured as
a function of LORF at UPRFs of 50, 100, 200, and 400
W, as shown in Figure 3. The optical band gap de-
creased from around 1.6 eV to around 0.6 eV with an
increase in LORF from 50 to 800 W. At LORF of 200 -
800 W, optical band gap decreased a little with the in-
crease of UPRF from 50 to 400 W. This behavior was
caused by the sputter deposits from the upper electrode
during the discharge off-time. The hardening modification
(with a lower optical band gap) of sputter deposits by
high-energy ion bombardment dominates the film quality.
The refractive index of a-CNx:H films was measured
at UPRFs of 50, 100, 200, and 400 W as a function of
LORF (50 - 800 W), as shown in Figure 4. Between
LORFs of 200 and 800 W, the refractive index assumed a
value of around 2.0 within the condition of trivial experi-
mental errors and was independent of UPRF. These films
were diamond-like. In contrast, at an LORF of 50 W, the
refractive index decreased to 1.8 - 1.9, indicating that
these films included some polymer-like components [14].

H. KINOSHITA ET AL. 400
Figure 3. Optical band gap of a-CNx films measured as a
function of LORF.
Figure 4. Refractive index of a-CNx films measured as a
function of LORF.
The refractive index observed at UPRF of 400 W was a
little smaller than those observed at UPRFs of 50 - 200
W. This was caused by the sputter deposits from the upper
electrode during the discharge off-time. The hardness val-
ues of the a-CNx:H films deposited at 50 W/400 - 800 W
were about 21 GPa, which were similar to that of our
vitreous silica (SiO2) of 22 GPa.
FTIR absorption spectra were measured for the
a-CNx:H films deposited at 200 W/50 - 800 W, as shown
in Figure 5. The film thickness of a-CNx:H used in the
Figure 5. FT-IR absorption spectra of a-CNx films measured
as a function of LORF.
FTIR analysis was controlled to be around 500 nm for all
the samples, and the film thickness was almost uniform.
The intensities of the absorption bands at 2930 cm–1
(CH3, CH2, and CH bonds) decreased and those at 1100
to 1700 cm–1 (C = C, C = N, and C = N-H bonds) in-
creased with an increase in LORF [15]. The intensities of
the absorption bands at 3300 cm–1 (NH bonds) changed
little with LORF, but the absorption band at 3200 - 3700
cm–1 (OH bond) was observed at LORF of 50 W [16].
H2O molecules adsorbed on the plasma chamber wall
were estimated to be taken into the films by the H2O
sputtering at 200/50 W.
3.2. Electrical and Optical Characteristics of the
Photovoltaic Cells
Gold ohmic contacts with a thickness of 30 nm were
deposited for the solar cell by a magnetron sputter depo-
sition system on the a-CNx:H surfaces. The 30-nm-thick
gold film was semitransparent in visible light. Back oh-
mic contacts between the p-Si (0.02 Ωcm) and Al
film-like wire were made using conductive silver pastes.
I-V characteristics of Au/a-CNx:H/p-Si PVCs were
measured at 25˚C in the dark, as shown in Figure 6.
They showed a rectifying curve in the dark, indicating
the formation of a heterojunction between the a-CNx:H
film and the p-Si substrate [4]. a-CNx:H films with a
thickness of 25 nm and a 70% N2 concentration were
deposited at a UPRF of 200 W and LORF of 200 - 800
W. At a forward current density of 1 mA/cm2, the for-
ward voltage (VF) of the 200/800 W solar cell reached a
minimum (0.18 V). With a decrease of LORF from 800

H. KINOSHITA ET AL. 401
Figure 6. I-V characteristics of the Au/a-CNx:H/p-Si photo-
voltaic cells measured under darkness. a-CNx:H films were
deposited at 200 W/200 - 800 W.
to 200 W, the values of VF were increased. The increase
of VF with the decrease in LORF from 800 to 200 W is
ascribed to an increase in resistivity, which usually varies
proportionally to the optical band gap [17,18].
The open circuit voltage (VOC) and short circuit current
density (ISC) of the PVCs were measured at 25˚C under
illumination by a Xenon lamp (close to AM 1. 5), as
shown in Figures 7 and 8, respectively. With increases
of LORF from 200 to 400 W and from 400 to
Figure 7. VOC of the photovoltaic cells measured under the
illumination of a Xenon lamp. a-CNx:H films were depos-
it ed at 5 0 - 400 W/200 - 800 W.
Figure 8. ISC of the photovoltaic cells measured under the
illumination of a Xenon lamp. a-CNx:H films were depos-
it ed at 5 0 - 400 W/200 - 800 W.
800 W, VOC increased slightly and a little for each UPRF,
respectively. With increases of LORF from 200 to 800
W, on the other hand, ISC increased significantly for each
UPRF. A drastic increase of ISC was observed at the
UPRF of 200 W. Maxima of VOC (226 mV) and ISC (9.3
mA/cm2) were obtained at 200/800 W. These tendencies
at UPRF of 200 W are somewhat similar to the case of
VF with regard to the I-V characteristics shown in Figure
6. At the minimum VF, the maxima of VOC and ISC were
achieved.
The energy conversion efficiencies of Au/a-CNx:/p-Si
PVCs were evaluated using the I-V characteristics ob-
tained under the illumination of a Xenon lamp (Figure 9).
With the increase of LORF from 200 to 800 W, the en-
ergy conversion efficiency increased for each UPRF,
whose tendencies are somewhat similar to the case of ISC
(Figure 8). At UPRF of 200 W, the energy conversion
efficiency increased significantly from 0.05% (LORF:
200 W) to 0.81% (800 W), which is the maximum en-
ergy conversion efficiency obtained in this experiment.
4. Conclusions
a-CNx:H films were deposited by pulsed supermagnetron
plasma CVD. RF power (13.56 MHz) supplied to the
lower electrode was modulated by a 2.5-kHz pulse fre-
quency and a pulse duty ratio of 12.5% (50 - 800 W). On
the other hand, the upper electrode was connected to a
continuous RF power source (50 - 400 W). The optical
band gap decreased with an increase in LORF. The refrac-
tive index was around 2.0 at LORF of 200 - 800 W, and
decreased to 1.8 - 1.9 at LORF of 50 W for each UPRF.

H. KINOSHITA ET AL. 402
Figure 9. Energy conversion efficiencies of photovoltaic
cells obtained under the illumination of a Xenon lamp.
a-CN x: H film s were d eposited at 50 - 400 W/ 200 - 800 W.
Au/a-CNx:H/p-Si PVCs deposited a-CNx:H film (25
nm thick) at UPRF/LORF of 200/200 - 800 W showed
rectifying I-V characteristics in the dark. The energy
conversion efficiency increased with increase in LORF
for each UPRF. At 200/800 W, VOC, ISC, and the energy
conversion efficiency took maxima of 226 mV, 9.3
mA/cm2, and 0.81%, respectively.
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