Pulsed Supermagnetron Plasma CVD of a-CN<sub>x</sub>:H Electron-Transport Films for Au/a-CN<sub>x</sub>:H/p-Si Photovoltaic Cells

doi:10.4236/jmp.2011.25049

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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

E-mail: [email protected]

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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