Fabrication of silicon thin films with defects below detection limit of electron spin resonance for solar cells by high-speed zone-melting crystallization of amorphous silicon

Full text of DOI:10.1063/1.1421231

Fabrication of silicon thin films with defects below detection limit of electron spin
resonance for solar cells by high-speed zone-melting crystallization of amorphous
silicon
Manabu Ihara, Shuhei Yokoyama, Chiaki Yokoyama, Koichi Izumi, and Hiroshi Komiyama
Citation: Applied Physics Letters 79, 3809 (2001); doi: 10.1063/1.1421231
View online: http://dx.doi.org/10.1063/1.1421231
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/79/23?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 79, NUMBER 23
3 DECEMBER 2001
Fabrication of silicon thin films with defects below detection limit
of electron spin resonance for solar cells by high-speed
zone-melting crystallization of amorphous silicon
Manabu Ihara,a) Shuhei Yokoyama, and Chiaki Yokoyama
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku,
Sendai 980-8577, Japan
Koichi Izumi
Iwatani International Corporation, Nishi-Shinbashi 3-21-8, Minato-ku, Tokyo 105-8458, Japan
Hiroshi Komiyama
Department of Chemical System Engineering, University of Tokyo, Hongo 7-3-1, Bunkyo-ku,
Tokyo 113-8656, Japan
͑
Received 23 April 2001; accepted for publication 21 September 2001͒
Silicon ͑Si͒ thin films with very-low defect density for solar cells were fabricated by using
high-speed ͑0.7–4.5 mm/s͒ zone-melting crystallization ͑ZMC͒ of amorphous-silicon ͑a-Si͒ thin
films, resulting in films that had defects below the detection limit of electron spin resonance ͑ESR͒.
In this letter, poly-crystalline silicon ͑poly-Si͒ films for zone-melting recrystallization ͑ZMR͒ and
a-Si films for ZMC were each sandwiched between two SiO films. The Si films were 0.3–2.0 ␮m
2
thick, the top SiO films were 0.35–1.5 ␮m thick, and the bottom SiO films were 0.18–1.2 ␮m
2
2
thick. The a-Si ZMC films had higher crystal quality than did the poly-Si ZMR films. Over 90% of
the grains in the a-Si ZMC films had preferred ͑100͒ orientation when the films were formed at scan
speeds 0.7–4.5 mm/s. Transmission electron microscopy ͑TEM͒ revealed that neither distinct grain
boundaries nor defects were visible in the a-Si ZMC films within the 9-␮m-diam observation field.
The a-Si ZMC films fabricated from the a-Si films with the thickness smaller than 1 ␮m had no
voids. Such a low defect density indicates that silicon thin-film solar cells with high efficiency can
be fabricated by using such very-low defect density silicon thin films. © 2001 American Institute
of Physics. ͓DOI: 10.1063/1.1421231͔
ZMR of Si on SiO was extensively researched in the
Compared with SIMOX and wafer bonding, ZMR is more
suitable as an SOI technique for making solar cells because
significant reduction in the fabrication cost of Si films for
solar cells is a higher priority than reduction of crystal de-
fects. The two advantages of using ZMR to fabricate SOI
structures is that it is a simple process and that it has high
throughout ͑high-speed scanning͒, both of which reduce the
cost of fabrication. However, such high-speed ZMR makes it
difficult to fabricate Si films with low defect density. Any
increase in crystal defects causes a decrease in the energy
conversion efficiency of solar cells. Therefore, in the devel-
opment of processes to fabricate solar cells that use ZMR,
simultaneously the scan speed must be increased and the
crystal defects decreased.
2
1
͑
980’s as a technique for depositing silicon on insulators
SOI͒ for large-scale integration ͑LSI͒ applications.1 For
–3
high-quality crystal Si films, ZMR films for LSIs were usu-
ally fabricated using poly-Si and a relatively slow scan speed
of less than 1 mm/s. However, research on ZMR as a SOI
technique decreased due to improvement of other SOI tech-
4
niques, such as separation-by-implanted oxygen ͑SIMOX͒
5
and wafer bonding. Although the cost to fabricate SOI struc-
tures using either of these two methods is high, SIMOX and
wafer bonding are more suitable than ZMR for LSIs because
they can be used to fabricate films that have fewer defects
than films fabricated by using ZMR.
Recently, several Si film-transfer methods have been de-
veloped for fabricating low-cost, high-efficiency solar cells,
such as a process using porous Si,6 for producing monoc-
rystalline Si films, and a process using ZMR on SiO2.9 In
the fabrication of solar cells using ZMR, a Si film for ZMR
has a three-layer structure on Si wafers, in which the Si film
is ‘‘sandwiched’’ between two SiO films. The top SiO film
In current fabrication processes of solar cells using ZMR
–8
9–11
on SiO2,
the Si film for the ZMR is polycrystalline. How-
–11
ever, no studies have been done on the effect of the type of Si
film on ZMR. For Si films fabricated by using high-speed
ZMR and ZMC, in this study we investigated the crystallin-
ity of ZMC films fabricated from a-Si compared with that of
ZMR films fabricated from poly-Si. We called films that we
fabricated from a-Si, a-Si ZMC films, because the word ‘‘re-
crystallization’’ means the second crystallization of a crystal-
line film. Our results showed that compared with the grains
of poly-Si ZMR films, more of the grains of a-Si ZMC films
preferred the ͑100͒ orientation and had fewer crystal defects.
This indicates that Si films having a very-low defect density,
2
2
prevents Si aggregation during ZMR, and the bottom SiO2
film is etched by using hydrofluoric acid to separate the Si
film from the Si wafer after ZMR. After the separation of the
Si film, the relatively expensive Si wafer can be reused.
a͒Author to whom all correspondence should be addressed; electronic mail:
ihara@tagen.tohoku.ac.jp
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003-6951/2001/79(23)/3809/3/$18.00 3809 © 2001 American Institute of Physics
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0
1

3810
Appl. Phys. Lett., Vol. 79, No. 23, 3 December 2001
Ihara et al.
i.e., below the detection limit of ESR can be fabricated by
using high-speed ZMC of a-Si.
In the fabrication of ZMR and ZMC films in our current
study, the first of two steps was fabrication of the sandwich
structure, SiO /Si/SiO , on a Si wafer. We evaluated two
2
2
types of Si films, amorphous, and poly-crystal. The sandwich
structure for the ZMR or the ZMC was fabricated by first
depositing SiO , then depositing Si ͑either polycrystalline or
2
amorphous͒, and finally depositing SiO on either a 4° off
2
͑100͒ Si wafer or a 4° off ͑111͒ Si wafer with a mirror-finish
surface ͑Shin-Etsu Co.͒. The SiO films were deposited by
2
using chemical vapor deposition ͑CVD͒ with either a mix-
ture of tetraethoxysilane ͑TEOS͒ and nitrogen (N ) at 710 °C
2
at a partial pressure of TEOS of 3.0 Torr or a mixture of
TEOS and helium ͑He͒ at 725 °C at a partial pressure of
TEOS of 1 Torr. The total pressure for all conditions was 76
Torr. The a-Si films were deposited by using rf magnetron
FIG. 1. Orientation of poly-Si ZMR films normalized by the XRD powder
pattern ͑film thickness of 1 ␮m, lower-heater temperature of 1550 °C͒.
higher for the a-Si ZMC films than for the poly-Si ZMR
films. Even at the high scan speed of 4.5 mm/s, less than
about 10% of grains in the ZMC films of a-Si had orienta-
tions other than ͑100͒.
Ϫ3
sputtering ͑400 W͒ in argon ͑Ar͒ of 5.0ϫ10 Torr. The
poly-Si films were deposited by using CVD with a mixture
of dichlorosilane (SiH Cl ) and hydrogen (H ), 0.25%,
2
2
2
0
.5%, and 1% of SiH Cl , at 1000–1100 °C. The Si films
Cross sections of the a-Si ZMC film fabricated from the
a-Si film with the thickness of 2 ␮m at a scan speed of 0.7
mm/s ͑TEM micrographs in Fig. 3͒ show that the voids were
formed in the film. However, neither a clear grain boundary
nor any clear defects were detected within the 9 ␮m-
diameter observation field, although two samples in the film
were studied. The electron diffraction reveals a spot pattern,
indicating a single crystal. These results show that the grain
size for the a-Si ZMC films was not less than 9 ␮m ͑the
diameter of the observation field͒ and that the grain had an
extremely low number of defects. Furthermore, the a-Si
ZMC films fabricated from the a-Si films with the thickness
smaller than 1 ␮m had no voids ͑data not shown͒.
2
2
were 0.3–2.0 ␮m thick, the top SiO films were 0.35–1.5
␮
thick.
In the second step, the Si films were recrystallized or
2
m thick, and the bottom SiO films were 0.18–1.2 ␮m
2
crystallized by using ZMR or ZMC, respectively, to improve
the crystal quality. Each sample was preheated by a lower
heater and then melted by linear irradiation of an upper lamp
heater with an elliptical mirror. Both the a-Si films and
poly-Si films were crystallized or recrystallized at different
scan speeds of the upper heater to investigate the effect of
scan speed on the ZMR. The temperature of the lower heater
was set between 1470 °C and 1570 °C, and the scan speed of
the upper lamp heater between 0.3 and 4.5 mm/s. The linear
melting zone moved as the scanning upper lamp heater was
moved.
Figure 4 shows that the total defect density of the a-Si
ZMC films was less than that of the poly-Si ZMR films at a
scan speed, except for the sample ͑f ͒ in Fig. 4. Figure 4 also
The cross section of the films before and after ZMR or
ZMC was studied by using scanning electron microscopy
͑SEM͒ to confirm that the sandwich structure was retained
after the ZMR or ZMC. All samples in our study retained
their sandwich structure ͑data not shown͒. The grain bound-
aries and defects of the ZMR- and ZMC-fabricated films
were evaluated by using TEM. The crystal orientation of the
films was determined by using x-ray diffraction ͑XRD͒. The
defect densities were then measured by using X-band ESR
͑BRUKER ESP300E, 9.78 GHz͒ for both the sandwich-
structured a-Si ZMC films and poly-Si ZMR films. The de-
fect densities of both ZMR and ZMC films whose top SiO2
films were etched by hydrofluoric acid were also measured to
investigate the distribution of defects. In the ZMR fabrica-
tion process of solar cells, the top SiO film is etched by
2
hydrofluoric acid and then the thick epitaxial film of Si is
deposited on the ZMR film as the layer of power generation.
Therefore, low defect density after etching of the top SiO2
film is critical.
FIG. 2. Orientation of a-Si ZMC films normalized by the XRD powder
pattern ͑film thickness of 0.3–2 ␮m, lower-heater temperature of 1550 °C͒.
Distance from the start of the ZMC scan: ͑a͒ 1, 8, 15, and 25 mm from the
left-hand side, ͑b͒ 1, 13, 23, and 33 mm from the left-hand side, ͑c͒ 3, 9, 15,
2
0, 27, and 31 mm from the left-hand side, ͑d͒ 1, 12, 23, and 34 mm from
the left-hand side, ͑e͒ 2, 12, 23, and 32 mm from the left-hand side, ͑f͒ 1, 7,
2, 24, 30, and 34 mm from the left-hand side, ͑g͒ 2, 12, 23, and 32 mm
from the left-hand side, ͑h͒ 8, 14, 20, 26, and 32 mm from the left-hand side,
i͒ 1, 12, 23, and 34 mm from the left-hand side, ͑j͒ 2, 5, 12, 24, 30, and 34
mm from the left-hand side, ͑k͒ 7, 13, 20, and 26 mm from the left-hand
Figures 1 and 2 show the ratio of crystal orientations
obtained by normalizing the measured intensity of each peak
in the XRD with each sensitivity calculated from the well-
known Si powder pattern of XRD. For all scan speeds, the
1
͑
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percentage of grains preferring the ͑100͒ orientation was
side, ͑l͒ 7, 13, 19, 26, and 33 mm from the left-hand side.
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Appl. Phys. Lett., Vol. 79, No. 23, 3 December 2001
Ihara et al.
3811
located at the interface between the SiO top film and the Si
2
film and that the defects in the bulk films were below the
detection limit of ESR. Figure 4 also shows that the a-Si
ZMC films were very-low-defect films when fabricated even
at the high scan speed of 4.5 mm/s. This suggests that the
scan speed can be increased further, while keeping the defect
density low.
In conclusion, high-speed ZMC of a-Si produced Si
films that had very-low defect density, high crystal quality,
and grains whose preferred orientation was ͑100͒. The ZMR
and ZMC films seemed to be polycrystalline because each
grain possibly has each crystal direction in the ͑100͒ plane.
However, the grain seemed to be very large because no clear
grain boundary and no clear defect were detected within the
9
-␮m-diam observation field by TEM. Furthermore, the a-Si
ZMC films fabricated from the a-Si films with the thickness
smaller than 1 ␮m had no voids. Therefore, a-Si ZMC films
can be used to fabricate higher efficiency solar cells than
cells fabricated by currently used ZMR processes that in-
volve poly-Si ZMR films.
FIG. 3. Cross section and electron diffraction of a-Si ZMC film with voids
revealed by TEM. ͑Fabricated from the a-Si film with the thickness of 2␮m,
scan speed of 0.7 mm/s.͒
shows that the defect density of the poly-Si ZMR films was
not affected by the etching, and that the defect density of the
a-Si ZMC films was drastically decreased by the etching and
was below the detection limit of ESR. This drastic decrease
indicates that most of the defects in the a-Si ZMC films were
The authors thank Dr. Ikoma and Professor Tero ͑To-
hoku University͒ for useful discussion about the ESR mea-
surements and also thank Yoshifumi Ogawa and Toshio
Osawa ͑University of Tokyo͒ for their assistance in the TEM
observation.
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FIG. 4. Defect density of a-Si ZMC films ͑a͒–͑e͒ and of poly-Si ZMR films
͑
f͒–͑j͒ before and after etching of the SiO top layer.
2
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