The 5th International Symposium on Advanced Science and Technology of Silicon Materials (JSPS Si
Symposium), Nov. 10-14, 2008, Kona, Hawaii, USA
Electrical Properties of Grain Boundaries in Cast-Grown Polycrystalline Silicon
Yoshio Ohshita1*, Koji Fukuda1, Koji Arafune2, Masafumi Yamaguchi1
1
Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511 Japan
2
University of Hyogo, 2167 Shosha, Himeji 214-8571, Japan
e-mail: y_ohshita@toyota-ti.ac.jp, sd03067@toyota-ti.ac.jp, arafune@eng.u-hyogo.ac.jp,
masafumi@toyota-ti.ac.jp
Abstract
Recombination velocities of grain boundaries in cast-grown polycrystalline silicon are evaluated as a function of
the annealing temperature, based on the line profile of electron beam induced current. The distribution of metals
and these chemical structures are also determined by ȝ-x-ray fluorescence and x-ray absorption near edge spectra
methods. In the as-grown crystal, most of grain boundaries including Ȉ3 act as recombination centers. The
recombination velocity at Ȉ3 drastically decreases by the thermal annealing. The velocities at Ȉ9 and Ȉ27
boundaries also decrease due to the annealing, but still relatively high. The recombination activities at some
random grain boundaries remain high and a large amount of nickel exists as nickel silicide along the boundary
even after the annealing. In some grains, the electrical properties are deteriorated by the high temperature
annealing, and the nickel and cupper silicide are formed there.
Introduction
Crystalline silicon (Si) technology has dominated the production of solar cells, which contributed about 90%
of the world’s PV production. In the crystalline Si solar cell market, the poly- crystalline Si solar cells are
dominant because of the lower cost and relatively better conversion efficiencies. The recent rapid growth of Si
solar cell market causes the lack of Si feed stock. Then, to increase the production of Si feed stock and to reduce
the cell cost, much effort has been focused on the development of the solar-grade Si (SOG-Si). However, the
impurity concentration in the Si wafer grown by using SOG-Si is commonly high. The small amount of metal
impurities, such as iron (Fe) and nickel (Ni), cause the minority carrier lifetime severe degradation, resulting in
the low conversion efficiency of solar cells. For example, low concentration less than 1 ppb Fe drastically
deteriorate Si based solar cells [1]. In order to establish new processes for fabricating the solar cells with the
satisfactory high conversion efficiency even from the SOG-Si, it is important to understand the behaviors of these
impurities and to reduce them after the cell fabrication processes. The most of metal impurities can be removed
from the polycrystalline Si by a phosphorous gettering. During the gettering process, many impurity atoms diffuse
in the crystal due to the high temperature and they are trapped at the gettering sites. Then, the average minority
carrier lifetime is improved and the relatively high conversion efficiency is realized. But, the concentrations of
metals decreased with the increasing of grain boundaries density and the grain boundaries acted as gettering
centers [2]. Then, some impurity atoms may still remain at some particular grain boundaries and at the defects in
the crystal. They deteriorate the solar cell performance. Therefore, it is strongly required to understand the
electrical properties of grain boundaries and metal impurity distributions in the crystal, which are strongly affected
by the thermal annealing. In this study, the recombination velocities at the grain boundaries, the distributions of
impurity metals, such as, Ni, Fe, and cupper (Cu), and these chemical states were studied as a function of
annealing temperature by electron beam induced current (EBIC), x-ray microprobe fluorescence (XRF) and x-ray
absorption in the near edge structure (XANSE) analysis.
Experiment
The gallium (Ga) doped polycrystalline Si was
used in this study. It was grown by our original
traveling
heater
furnace [3]. The schematical
illustration of the furnace is shown in Fig. 1. The
grown ingots were cylindrical shape with a diameter
of 10cm and a height of 10cm. The quartz crucible
was coated with Si3N4. The silicon feedstock used
here was off-specification of electronics grade. The
heater transfer rate during growth was 15 - 30 mm/hr.
The ingot was horizontally sliced to a thickness of
300ȝm wafer. The neighboring three substrates,
Fig.1.
Schematic
illustration
of
directional
solidification furnace.
which have the relatively same grain structures,
around the fraction solidified, X=0.85, were used, because the concentration of impurities were relatively high due
to the segregation phenomena. The averaged metal impurities concentrations were as follows; iron (Fe): 3.5 x1013,
cupper (Cu): 1.0x1014, and nickel (Ni): 3.3 x 1014 atoms/cm3. They were determined by the inductively coupled
plasma mass spectrometer and the atomic absorption spectrometry methods. The resistivity of crystal was 0.3
ȝȍcm, and the average lifetimes of the as-grown substrate were 0.3 - 0.4 ȝsec. Before the measurements, the
surface damage layers were removed by chemical etching with a mixture of nitric and hydrofluoric acids. One
substrate was unprocessed, the others were annealed at 650͠ for 120 min and 1000͠ for 90 min in a N2
ambient, respectively. The electrical activities were characterized by the EBIC measurements. Here, aluminum
(Al) - Schottly structures were adopted. The crystallographic orientation was analyzed by the scanning electron
microscope (SEM) equipped with an electron backscatter diffraction (EBSD) pattern collection system, and the
sigma number of each boundary was determined. The distribution of nickel and its chemical state was evaluated
by using the synchrotron beam line 37 XU at the Spring-8. The distribution was determined by the ȝ-XRF
measurement with energy of 10keV. The beam size was 0.7ȝm x 1.5ȝm and the sampling pitch was 5ȝm. The
chemical states were determined by XANES measurements.
Results and Discussion
The EBIC and crystallographic images were shown in Fig. 2. The darker line in the EBIC images indicates
the higher recombination velocity. There are some grain boundaries defined by coincidence site lattice (CSL),
such as, ƴ3, ƴ9, and ƴ27, and random boundaries. In the as-grown crystal, most of grain boundaries including Ȉ3
act as recombination centers. The contrast differences indicate that the recombination velocities vary depending
on the structures of the grain boundaries and these values change by the low temperature (LTA) and high
temperature (HTA) thermal annealing. For example, Ȉ3 becomes not recombination site after the annealing. To
discuss the effects of annealing condition on the recombination velocity at each grain boundary quantitatively, the
Fig. 2 EBIC images and crystallographic orientation maps determined by EBSD.
recombination velocities at the grain boundaries were evaluated from the line profile of EBIC current (Fig. 3) by
using the following Zook’s equation [4].
Here, I(S, X, Z) is the EBIC current at the
position x and I’ is the one at the position far
from the boundary. X=x/L and L is the diffusion
length of minority carrier. The experimental
results were fitted by using eq. (1) as functions
of recombination velocity S and diffusion length
L. The experimental line profile of EBIC current
Fig. 3. EBIC image and line profile of EBIC current.
can be well explained by the theoretical curve
obtained by eq. 1 (Fig. 4), and the estimated
㩷
recombination velocities at grain boundaries are summarized
in Fig. 5, as a function of annealing temperature. The existed
Ȉ3 is a twin boundary, which has a high symmetry and has no
0.8
㩷
I/I0
1.0
0.6
0.4
0
dangling bond along the boundary. However, in the as-grown
crystal, this boundary acts as a recombination center. On the
experimental data
theoretical data
S=17000[cm/sec], L=11.2[Pm]
10
20
30
40
other
hand,
recombination
50
x[Pm]
the
annealing
velocity,
drastically
resulting
in
decreases
not
being
the
the
recombination site. The Ȉ9 and Ȉ27 boundaries also act as the
recombination centers (Fig. 4). The recombination velocities
increasing as the CSL number increases. These velocities also
Fig. 4 Experimental line profile of EBIC
decreased by the thermal annealing. But, the recombination
current and the theoretical curve.
velocity remains still high, and the value at Ȉ27 is larger than
that at Ȉ9. As increasing the annealing temperature, the
㩷
4.0
3.0
impurities near the coincide boundaries diffused and the
electrical properties were improved. Ȉ9 and Ȉ27 boundaries
2.5
have the larger strain energy than Ȉ3, and they might attract
2.0
㩷
Sg [u104cm/s]
3.5
driving force of dissolution increased. Then, the metal
as-grown
650qC annealing
1000qC annealing
1.5
the metal impurities even after the annealing. However, it is
1.0
not clear yet which the residual impurity at Ȉ9 and Ȉ27
0.5
boundaries,
0.0
6
6
㩷
6
R
Fig. 5. Recombination velocity at grain
boundaries.
or
these
boundaries
themselves
without
impurities was the cause of high recombination velocity even
after the high temperature annealing. The recombination
velocity at the random grain boundary R2 decreases by the
low temperature annealing, but becomes high by the high
temperature annealing (Fig. 5). By the high temperature
annealing, many dark spots appeared in the grains. In some grains, there were many defects, which appeared as a
etch pit by the etching. The high temperature annealing caused the defect formation which acted as strong
recombination centers.
The distributions of metals, such as, Ni, Fe,
and Cu, of as-grown, low temperature annealed,
and high temperature annealed substrates were
shown in Fig. 6. In the present crystals, Fe could
not be detected. In the as-grown crystal, the Ni
atoms exist at the particular grain boundaries,
such as random boundary, R1, R2, R7, R8, and
R9. On the other hand, Ni was not detected
along R5, R6, and coincide sites. In the grains,
Ni can not be detected from the as-grown and
low temperature annealed crystals. Due to the
low temperature annealing, the concentration of
Ni at several areas decrease. By the high
temperature annealing, Ni exist at the random
boundaries R2, R3, R5, R7, and at some
coincide boundaries. The strong Ni signals were
also obtained at the intra-grains. The signal
Fig. 6 Metal distributions obtained by XRF.
intensity of Cu is not strong in the as-grown and
low-temperature annealed crystals. But, many
cupper atoms appeared at the dark spot obtained
by the EBIC measurement in the grains. The defects in the grain accumulated these metal impurities and became
the strong recombination centers.
The XANES spectrums of Ni and Cu are shown in Fig. 7. The Ni signals are slightly different from each
other depending on the annealing conditions. But, they are basically similar to that of Ni silicide. This indicates
that many Ni atoms existed as Ni silicide and that they were precipitated along the grain boundaries and at the
defects in the grain. The spectrums of Cu are the same independent of the annealing conditions. It is suggested
that the cupper atoms existed at the defect regions as the Cu silicide.
(a) Ni
(b) Cu
Fig. 7 XANES spectrums of nickel and cupper.
Summary
Effects of annealing on the recombination velocities at grain boundaries and on the distributions of Ni, Fe
and Cu impurities were studied by EBIC, ȝ-XRF, and XANES measurements. In the as-grown crystal, most of
grain boundaries including Ȉ3 acted as recombination centers. The recombination velocity at Ȉ3 drastically
decreases by the thermal annealing. These velocities of Ȉ9 and Ȉ27 boundaries also decreased, but still remained
high value. The recombination activity at some random grain boundaries decreased by the low temperature
annealing, but became high due the high temperature annealing. The large amount of Ni existed as Ni silicide at
the random boundaries before and even after the annealing. The high temperature annealing also caused the Ni
and Cu silicide precipitations in the grains. They also deteriorated the electrical properties of polycrystalline Si.
Acknowledgement
The authors would like to thank to Yasuko Terada (JASRI) for XRF and XANES measurements. We also
would like to thank Ken Kajiwara and Kenji Kondo for their technical cooperation. Part of this work was
supported by the Incorporated Administration Agency New Energy and Industrial Technology Development
Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI) 2006-2007.
References
[1] O. F. Vyvenko, T. Buonassisi, A. A. Istratov, H. Hieslmair, A. C. Thompson, and W. R. Weber, J. Appl. Phys.,
91(6) (2002) 3614.
[2] A. A. Istratov, T. Buonassisi, R. J. McDonald, A. R. Smith, R. Schindler, J. A. Rand, J. P. Kalejs, and E. R.
Weber, J. Appl. Phys., 94(10) (2003) 6552.
[3] K. Arafune, E. Ohishi, H. Sai, Y. Ohshita, and M. Yamaguchi, J. Crystal Growth, 308(1) (2007) 5.
[4] J. D. Zook, Appl. Phys. Lett., 42(7) (1983) 602.