Analysis of the electrical properties of Cr/n-BaSi2 Schottky junction and n-BaSi2/p-Si
heterojunction diodes for solar cell applications
Weijie Du, Masakazu Baba, Kaoru Toko, Kosuke O. Hara, Kentaro Watanabe, Takashi Sekiguchi, Noritaka
Usami, and Takashi Suemasu
Citation: Journal of Applied Physics 115, 223701 (2014); doi: 10.1063/1.4882117
View online: http://dx.doi.org/10.1063/1.4882117
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/22?ver=pdfcov
Published by the AIP Publishing
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Analysis of the electrical properties of Cr/n-BaSi
2
Schottky junction
and n-BaSi
2
/p-Si heterojunction diodes for solar cell applications
Weijie Du,
1
Masakazu Baba,
1
Kaoru Toko,
1
Kosuke O. Hara,
2
Kentaro Watanabe,
1,3
Takashi Sekiguchi,
3
Noritaka Usami,
2,4
and Takashi Suemasu
1,4
1
Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
2
Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
3
National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
4
Core Research for Evolutional Science and Technology, Japan Science and Technology Agency,
Chiyoda, Tokyo 102-0075, Japan
(Received 13 May 2014; accepted 27 May 2014; published online 9 June 2014)
Current status and future prospects towards BaSi
2
pn junction solar cells are presented. As a
preliminary step toward the formation of BaSi
2
homojunction diodes, diodes with a Cr/n-BaSi
2
Schottky junction and an n-BaSi
2
/p-Si hetero-junction have been fabricated to investigate
the electrical properties of the n-BaSi
2
. Clear rectifying properties were observed in the current
density versus voltage characteristics in both diodes. From the capacitance-voltage
measurements, the build-in potential, V
D
, was 0.53 V in the Cr/n-BaSi
2
Schottky junction diode,
and the Schottky barrier height was 0.73 eV calculated fro m the thermoionic emission t heory; the
V
D
was about 1.5 V in the n-BaSi
2
/p-Si hetero-junction diode, which was consistent with the
difference in the Fermi level between the n-BaSi
2
and the p-Si.
V
C
2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4882117]
I. INTRODUCTION
In recent years, thin-film solar cells such as Cu(In,Ga)Se
2
,
Cu
2
ZnSnS
4
, and CdTe have been attracting so much attention
due to their high efficiency and low cost.
1–5
We think that bar-
ium disilicide (BaSi
2
) could be another candidate material.
6,7
The band gap of BaSi
2
is approximately 1.3 eV that matches
the solar spectrum better than crystalline Si.
8,9
Both theoretical
and experimental researches revealed that BaSi
2
has a very
large absorption coefficient of over 3 10
4
cm
–1
for photon
energies greater than 1.5 eV.
10,11
We have already grown
high-quality BaSi
2
epitaxial layers on both Si(111) and
Si(001) substrates even though BaSi
2
has an orthorhombic
crystal structure.
12–15
We have further formed polycrystalline
BaSi
2
layers on h 111i-oriented Si films prepared on SiO
2
using an Al-induced crystallization method.
16
Besides, the
undoped n-BaSi
2
has a large minority-carrier (holes) diffusion
length (10 lm) and thereby a long minority carrier lifetime
(>10 ls).
17–19
These results have suggested that BaSi
2
is a
very promising material for thin-film solar cell applications.
To form BaSi
2
homojunction diodes, the properties of BaSi
2
thin-films doped with impurities such as Cu, Ag, Sb, P, As, In,
Al, and B have also been investigated.
20–26
Among these ele-
ments, Sb and B were proved to be the suitable candidates for
n
þ
-BaSi
2
and p
þ
-BaSi
2
, respectively. Their diffusion coeffi-
cients are another important parameter that should be taken
into account. Most impurities, such as Al, Sb, and As, except
B have large diffusion coefficients in BaSi
2
layers.
27–29
As a next step toward forming a BaSi
2
homojunction
diode on a Si substrate, we need to investigate the electrical
properties of an undoped n-type BaSi
2
film, which is to be an
active layer in a solar cell. Due to the small electron affinity
of BaSi
2
(3.2 eV), band offsets exist at the BaSi
2
/Si inter-
face,
30,31
that is, 0.8 eV for the conduction band, and 0.6 eV
for the valence band. These values are predicted from the
electron affinities of BaSi
2
andSi.Thebandoffsetsblockthe
photocurrent flowing across the BaSi
2
/Si interface. An
Sb-doped n
þ
-BaSi
2
/p
þ
-Si tunnel junction (TJ) solved this
problem.
32–34
Recently, we have achieved large photocurrent
corresponding to the internal quantum efficiency exceeding
70% for the 400 nm-thick undoped n-BaSi
2
layer grown on
the Sb-doped n
þ
-BaSi
2
/p
þ
-Si TJ.
35
The remaining process is
the formation of p-BaSi
2
layer on the undoped n-BaSi
2
layer
to complete the BaSi
2
homojunction diode. However, there
has been no report even on the built-in potential, V
D
,ina
BaSi
2
/Si heterojunction diode or in a metal/BaSi
2
Schottky
junction diode, which is one of the most fundamental electri-
cal properties in semiconductors. In an n-BaSi
2
/p-Si hetero-
junction diode, for example, the V
D
is given ideally by the
difference in work function between the n-BaSi
2
and p-Si.
Assuming that the effective density of states of conduction
band, N
C
, is approximately 2.6 10
19
cm
3
from the effective
mass tensors of electron in BaSi
2
,
11
the separation of the bot-
tom of the conduction band, E
C
, from the Fermi level, E
F
,that
is E
C
E
F
, can be estimated using the value of electron con-
centration. Thus, the work function of the n-BaSi
2
can be cal-
culated. The same is true for the p-Si. In this article, we grew
an undoped n-BaSi
2
layer on a p-Si(111) for that purpose. An
undoped n-BaSi
2
layer was also formed on the TJ, and a
Schottky junction was formed on top of the n-BaSi
2
with Cr
for comparison. Then, we measured the current-density versus
voltage (J-V) and capacitance-voltage (C-V) characteristics of
the above two diodes and compare their V
D
’s with those pre-
dicted from their work functions.
II. EXPERIMENTAL
An ion-pumped molecular beam epitaxy (MBE) system
was used for the growth of samples. For the Schottky junc-
tion diode, the p
þ
-Si (q 0.01 Xcm) substrate was adopted.
0021-8979/2014/115(22)/223701/4/$30.00
V
C
2014 AIP Publishing LLC115, 223701-1
JOURNAL OF APPLIED PHYSICS 115, 223701 (2014)
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First, the substrate was heated at 900
C for 30 min for clean-
ing the surface. A thin (5 nm) BaSi
2
template layer was
then grown by Ba deposit ion on the Si substrate at 500
C for
5 min (reactive deposition epitaxy; RDE). The template was
used to control the crystal orientation of the BaSi
2
over-
layers. An approximately 30-nm-thick Sb-doped n
þ
-BaSi
2
layer was grown at 550
C by MBE for 20 min, to form a TJ
with the p
þ
-Si substrate. After that, a 1150-nm-thick
undoped n-BaSi
2
layer was grown by MBE for 15 h at
600
C. After the growth, 1-mm-diamter front-surface Au/Cr
electrodes were formed by vacuum evaporation and the
back-surface Al electrodes by sputtering. For the heterojunc-
tion diode, the p-Si substrate (q ¼ 0.1 Xcm) was used. First,
a thin BaSi
2
template layer was grown by RDE at 500
C.
After that, an approximately 650-nm-thick undoped n-BaSi
2
layer was grown by MBE at 600
C for 590 min. In order to
facilitate to make ohmic contacts on the front-surface,
another 10-nm-thick Sb-doped n
þ
-BaSi
2
layer was grown.
To avoid the Sb diffusion into the undoped n-BaSi
2
layer,
the growth temperature was decreased to 500
C for the
n
þ
-BaSi
2
layer. Finally, 0.5-mm-d iameter front-surface Al
electrodes were formed by sputtering. Backside electrodes
were also formed with Al by sputtering. The electrical prop-
erties were measured at room temperature (RT).
III. RESULTS AND DISCUSSION
A. Cr/n-BaSi
2
Schottky junction diode
Figure 1 shows the J-V characteristics of the sample
measured at RT. The current increased rapidly as the positive
bias was applied to the Au/Cr electrode with respect to the p-
Si substrate. Clear rectifying properties were observed in this
sample, indicating that the Schottky junction was surely
formed on the surface of the thick undoped BaSi
2
layers
grown on the TJ. The series resistance, R
s
, can be deduced
from the slope of the I(dV/dI) versus I plot in a large I region,
and the shunt resistance, R
sh
, from the slope of dV/dI at
around V ¼ 0.
36,37
They were 180 X and 1 MX, respectively.
The logarithmic plot of J with respect to V is inserted in Fig.
1, in which the series resistance and shunt resistance were
subtracted. The reverse saturation current density J
S
can be
deduced from the intercept of the straight line of the
Log(J)-V plot at V ¼ 0 and was found to be 6.1 10
6
A/cm
2
. For a Schottky junction diode, the reverse saturation
current density J
S
can be expressed by the thermoionic emis-
sion theory:
38
J
S
¼ A
T
2
exp
q/
S
k
B
T
; (1)
where A* is the effective Richardson constant, k
B
the
Boltzmann constant, q/
S
the barrier height for electrons in
the metal. The q/
S
was calculated to be 0.73 eV for this
Schottky junction. This value is smaller than that predicted
from the electron affinity of BaSi
2
(3.2 eV) and the work
function of Cr (4.5 eV). Thus, a thin interfacial layer is likely
to exist between the Cr and the n-BaSi
2
due to the oxidation
of the BaSi
2
surface, and voltages also drop at the interfacial
layer.
39,40
Figure 2(a) shows the 1/C
2
versus V plot of the Schottky
junction diode. The V
D
can be deduced by extending the
straight line to the voltage axis and it was found to be 0.53 V.
The positive ionized donor density, N
þ
D
, near the surface
region of the n-BaSi
2
layer, was calculated to be
3 10
16
cm
3
from the C-V characteristics, assuming that the
permittivity of BaSi
2
approaches 15 for long wavelengths.
11,41
Figure 2(b) shows a simple band diagram of the Cr/n-BaSi
2
Schottky junction according to the Schottky-Mott model.
42
FIG. 1. J-V characteristics of the Schottky junction diode measured at RT.
The logarithmic plot is inserted. The bias voltage is applied to the Au/Cr
electrode with respect to the p-Si substrate.
FIG. 2. (a) 1/C
2
versus V plot of the Schottky junction diode and a line for
fitting. (b) Band diagram of the Cr/n-BaSi
2
Schottky junction.
223701-2 Du et al. J. Appl. Phys. 115, 223701 (2014)
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The E
C
E
F
was determined to be 0.2 eV in the n-BaSi
2
assuming that it is equal to q(/
S
V
D
). The electron concen-
tration, n, is given by
n ¼ N
C
exp
E
C
E
F
k
B
T
: (2)
Assuming complete ionization of impurity atoms in the n-
BaSi
2
, and thereby n ¼ N
þ
D
¼ 3 10
16
cm
3
,theE
C
E
F
was
thus calculated to be 0.18 eV at RT. This value is consistent
with q(/
S
V
D
), thereby the band diagram shown in Fig. 2(b).
B. n-BaSi
2
/p-Si hetero-junction diode
Now let us discuss the electric properties of the
n-BaSi
2
/p-Si heterojunction diode. Figure 3(a) shows the J-V
characteristics of the diode measured at RT. The bias voltage
was applied to the p-Si substrate with respect to the
n
þ
-BaSi
2
top layer. Clear rectifying properties were also
confirmed in this heterojunction diode. The R
s
and R
sh
were
calculated to be 0.8 kX and 40 kX, respectively. The inserted
figure shows the logarithmic plot of J-V characteristics,
where the R
s
and R
sh
were subtracted. The ideal factor of the
diode, which should be between 1 and 2, was 1.7 under the
bias voltages smaller than 0.3 V. However, this value
increased to 14.6 when the bias voltage was higher than
0.5 V. This might be due to the carrier trapping at the defect
states at the interface and a nonlinear series resistance in the
diode structure. The reverse saturation current density J
0
was
deduced to be 1.2 10
4
A/cm
2
from the Log(J) versus V
plot in the inserted figure.
Figure 4(a) shows the 1/C
2
versus V plot of the sample.
In our previous works, the electron density in the undoped
n-BaSi
2
was usually of the order of 10
16
cm
3
from the Hall
measurement.
43
This value is smaller by one order of magni-
tude than that in the p-Si substrate (p 2 10
17
cm
3
) used
in this work. Thus, it is reasonable to think that the depletion
region extended mostly toward the n-BaSi
2
layer. Figure
4(b) shows the n distribution in the n-BaSi
2
layer assuming
complete ionization of impurity atoms. The average electron
density was about 2 10
16
cm
3
. Homogeneous carrier con-
centration profile indicated the high quality of the BaSi
2
epi-
taxial thin film. As a result, the 1/C
2
versus V plot was well
fitted by a linear broken line as shown in Fig. 4(a). The V
D
was deduced to be about 1.5 V in this heterojunction diode.
The E
C
E
F
is calculated to be 0.19 eV for n ¼ 2 10
16
cm
3
in BaSi
2
, while the separation of E
F
from the top of the va-
lence band, E
V
, that is E
F
E
V
, is estimated to be 0.10 eV
using p ¼ 2 10
17
cm
3
in the p-Si substrate. Thereby, the
difference in work function between the n-BaSi
2
and p-Si is
approximately 4:0 þð1:1 0:1Þð3:2 þ 0:19Þffi1:6eV:
This value is consistent with the experimen tally obtained V
D
of 1.5 V, as shown in Fig. 4(a).
On the basis of these results on the two kinds of diodes,
we conclude that the band diagrams of the undoped
n-BaSi
2
/p-Si and the metal/BaSi
2
heterojunctions are well
explained by a conventional way widely used in semicon-
ductor physics although BaSi
2
is really an unfamiliar
FIG. 3. (a) J-V characteristics of the hetero-junction diode measured at RT.
The insertion is the logarithmic plot of J–V characteristics. (b) Band diagram
of n-BaSi
2
/p-Si hetero-junction.
FIG. 4. (a) 1/C
2
versus V plot and a broken line for fitting (blue). The bias
voltage is applied to the p-Si substrate with respect to the n
þ
-BaSi
2
. (b)
Electron density distribution in the BaSi
2
layer.
223701-3 Du et al. J. Appl. Phys. 115, 223701 (2014)
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130.158.56.140 On: Tue, 10 Jun 2014 14:20:52
semiconductor material. These results facilitate us to design
the device structure of BaSi
2
homojunction and heterojunc-
tions solar cells.
IV. CONCLUSIONS
We have formed the Cr/n-BaSi
2
Schottky junction and
n-BaSi
2
/p-Si heterojunction diodes. In both samples, clear
rectifying properties were observed in the J-V characteristics
at RT. In the Schottky junction diode, the J
S
was 6.1 10
6
A/cm
2
, the Schottky barrier height was calculated to be
0.73 eV and the V
D
deduced from the 1/C
2
versus V plot, was
found to be 0.53 V. These results were in good agreement
with the Schottky-Mott model. In the heterojunction diode,
the V
D
was found to be 1.5 V, which was well explained by
the difference in E
F
between the n-BaSi
2
and p-Si. The
reverse saturation current density was 1.2 10
4
A/cm
2
.
The electron concentrations of the undoped n-BaSi
2
layers
were of the order of 10
16
cm
3
in both samples from the C-V
measurement.
ACKNOWLEDGMENTS
This work was financially supported by Core Research
for Evolutional Science and Technology, Japan Science and
Technology Agency (JST-CREST).
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