Preparation of three-dimensionally oriented polycrystalline Si film

Article abstract of DOI:10.1063/1.116760

The crystal structure and morphology of polycrystalline Si films deposited using a plasma-enhanced chemical vapor deposition method, have been investigated by x-ray diffraction and atomic force microscopy. When the 〈110〉-oriented films were prepared at 690°C with a hydrogen dilution ratio H2/SiH4=3 and rf power of 20 W, it was found that the grains with almost the same size are uniformly distributed within the film, and their shapes in a top view become rectangular. The longer sides of rectangles stand in line in the direction of gas flow (the plasma also spreads in this direction), indicating that the gas-flow direction and the plasma assist the growths of essentially three-dimensionally oriented grains.

Full text of DOI:10.1063/1.116760

Preparation of threedimensionally oriented polycrystalline Si film
G. Q. Di, H. Lin, N. Uchida, Y. Kurata, K. Koumoto, and S. Hasegawa
Citation: Applied Physics Letters 68, 69 (1996); doi: 10.1063/1.116760
View online: http://dx.doi.org/10.1063/1.116760
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/68/1?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Control of the azimuthal orientation of grains in polycrystalline Si films
AIP Advances 2, 022153 (2012); 10.1063/1.4730402
Current fluctuations in three-dimensionally stacked Si nanocrystals thin films
Appl. Phys. Lett. 96, 092112 (2010); 10.1063/1.3294329
Three-dimensional crystalline Si film growth by the Ni silicide mediation
Appl. Phys. Lett. 92, 043501 (2008); 10.1063/1.2828202
Atomistic Monte Carlo simulations of three-dimensional polycrystalline thin films
J. Appl. Phys. 94, 163 (2003); 10.1063/1.1577814
Exact analysis of a threedimensional cylindrical model for a polycrystalline solar cell
J. Appl. Phys. 64, 5214 (1988); 10.1063/1.342435
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
155.33.120.209 On: Fri, 21 Nov 2014 23:42:10

Preparation of three-dimensionally oriented polycrystalline Si film
G. Q. Di, H. Lin,a) N. Uchida, Y. Kurata, K. Koumoto,a) and S. Hasegawa
Department of Electronics, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan
͑Received 17 April 1995; accepted for publication 24 October 1995͒
The crystal structure and morphology of polycrystalline Si films deposited using a plasma-enhanced
chemical vapor deposition method, have been investigated by x-ray diffraction and atomic force
microscopy. When the ͗110͘-oriented films were prepared at 690 °C with a hydrogen dilution ratio
H2 /SiH4ϭ3 and rf power of 20 W, it was found that the grains with almost the same size are
uniformly distributed within the film, and their shapes in a top view become rectangular. The longer
sides of rectangles stand in line in the direction of gas flow ͑the plasma also spreads in this
direction͒, indicating that the gas-flow direction and the plasma assist the growths of essentially
three-dimensionally oriented grains. © 1996 American Institute of Physics.
͓S0003-6951͑96͒02601-6͔
Polycrystalline silicon ͑poly-Si͒ has attracted extensive
attention as an electronic material for integrated circuits and
thin-film transistors ͑TFTs͒. However, the presence of grain
boundaries in the active region of poly-Si TFTs degrades the
carrier mobility because of Si dangling bonds which intro-
duce deep traps for carriers within the band gap, therefore,
limiting the device performance. In order to obtain high-
quality TFTs, the grain boundary effects can be generally
reduced by growing the enhanced fiber-textured poly-Si
films with large grains.1 It has been shown that the best TFTs
performance can be obtained in the film with only interme-
diate average grain size but with a strong ͕110͖ fiber texture
͑not in the film with the largest average grain size2͒, and that
the randomly oriented boundaries in the fiber-textured film
cause a singular problem when the TFTs channel dimensions
are scaled down to submicron size.3 This might suggest that
the oriented grain boundaries are a more important factor in
determining the quality of devices so it is necessary to de-
velop effective and economical techniques for obtaining
poly-Si films composed of grains having controlled orienta-
tions and configurations. It has been known that poly-Si
films with a strong ͗110͘ texture can be grown on amorphous
substrates using a plasma-enhanced chemical vapor deposi-
tion ͑PECVD͒ technique.4,5 Through observing the morphol-
ogy of those films by atomic force microscopy ͑AFM͒, we
recently found a new type of ͗110͘-oriented poly-Si PECVD
film in which grains have a regular geometry and orient three
to 20 W, and the substrate temperature was set at 690 or
720 °C. The deposition rate was between 12 and 20 nm/min
under the above conditions. Crystal structures of the films
were characterized by x-ray diffraction ͑XRD͒ using Cu K␣
radiation. And, the morphologies of films were observed by
AFM ͑SFA-300, Seiko Instruments Ltd.͒ with the operating
force on an order of 10Ϫ9 N in a clean room of the class
1000. The observations were carried out at more than two
different points over an area of about 5 mm.
Figure 1 presents the typical XRD spectra for low-
pressure CVD ͑LPCVD͒ and PECVD films. The LPCVD
film ͑denoted as ‘‘film A’’ below͒ were deposited at 720 °C
with H2 /SiH4ϭ3 and rf power of 0 W, and the PECVD films
͑film B͒ at 690 °C with H2 /SiH4ϭ3, and rf power of 20 W.
The XRD spectrum from film A exhibits ͑111͒, ͑220͒, ͑311͒,
and ͑400͒ peaks, and therefore the LPCVD film is a near-
random polycrystalline film. In contrast, only stronger ͑220͒
and very weak ͑111͒ peaks appear in that from film B, indi-
cating that film B is ͗110͘ textured. The XRD spectra do not
appreciably change as varying the ratio H2 /SiH4 from 1 to 5
or the rf power from 8 to 20 W, and injecting SiF4 below
SiF4 /SiH4ϭ50%.
dimensionally. We refer to such
a film as ‘‘three-
dimensionally oriented poly-Si film’’ in this letter.
In this work, 0.73–0.84-␮m thick PECVD films were
deposited on fused quartz substrates in a horizontal fused
quartz reactor inserted into a rf inductive coil that generates
plasma. This system is heated by an electric furnace.6 Gases
used for depositions were hydrogen diluted SiH4 with dilu-
tion ratios H2 /SiH4, of 1–6. For some films, SiF4 gas was
further injected into the feed gases at a ratio SiF4 /SiH4 of
15%. The total flow rates and gas pressure were 2–7 sccm
and 0.15 Torr, respectively. The rf power was varied from 0
FIG. 1. X-ray diffraction spectra of poly-Si film. Film A was prepared at
720 °C with H2 /SiH4ϭ3 and rf power of 0 W; film B at 690 °C with
H2 /SiH4ϭ3 and rf power of 20 W.
a͒
Also with Department of Applied Chemistry, Faculty of Engineering,
Nagoya University, Nagoya 464-01, Japan.
Appl. Phys. Lett. 68 (1), 1 January 1996
0003-6951/96/68(1)/69/3/$6.00
© 1996 American Institute of Physics
69
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
155.33.120.209 On: Fri, 21 Nov 2014 23:42:10

FIG. 3. Schematic representations: ͑a͒ for grains appearing in Fig. 2͑e͒; ͑b͒
for that in Fig. 2͑d͒. a0 is the lattice constant.
gram although the geometric arrangement of grains along the
gas-flow direction does not appreciably change. The XRD
spectrum for this film was similar to that for film B as shown
in Fig. 1, indicating that the grain surfaces parallel to the film
surface are composed of ͑110͒ crystal planes. On the other
hand, the angles between the neighboring edge lines in the
parallelograms are ϳ55° and 125°. In a Si lattice we know
that a ͑110͒ plane is at a right angle with two ͑100͒ planes
and produces an angle of 35.26° with two ͕111͖ planes, and a
͑100͒ plane makes an angle of 54.74° with the four ͕111͖
planes. So, the grain with the top view shape of a parallelo-
gram can be deduced to be parallelepiped surrounded by
FIG. 2. AFM micrographs of poly-Si films. ͑a͒ Solid-phase recrystallized
film; ͑b͒ film A shown in Fig. 1; ͑c͒ film B in Fig. 1; ͑d͒ five-magnified
micrographs of ͑c͒; ͑e͒
a film prepared with an injection of
SiF4 /SiH4ϭ15%, the other deposition conditions are the same as ͑d͒; ͑f͒ a
film prepared with rf power of 8 W, the other deposition conditions the same
as ͑d͒. Scale mark represents 1 ␮m for the left-hand side column and 0.2 ␮m
for the right-hand side column, respectively.
Figure 2 shows some AFM micrographs. We note that
the horizontal line in Fig. 2 agrees with the direction of the
gas flow ͑the plasma also spreads in this direction͒. For com-
parison, a randomly oriented poly-Si film, prepared by an-
nealing amorphous Si ͑a-Si͒ films at 600 °C,7 was also ob-
served as shown in Fig. 2͑a͒. The size of grains in this
recrystallized film are larger than 1 ␮m, but their shapes are
random. Grains in film A shown in Fig. 2͑b͒, however, ap-
pear in various geometries such as triangles, rectangles,
squares, and other irregular polygons in a top view. Their
sizes range from about 0.08 to 0.45 ␮m. Such complex
shapes and widely different sizes would be connected with
the random texture as seen in the XRD spectrum of this film
͑Fig. 1͒. When we look at the distribution of grains in Fig.
2͑b͒ carefully, it is found that the direction of the longitudi-
nal edges of grains with rectangular shapes orients well with
the direction of the gas flow, while the edges of the polygo-
nal grains appear to be randomly arranged. In the micro-
graphs for film B shown in Figs. 2͑c͒ and 2͑d͒ which is the
magnified diagram of Fig. 2͑c͒ with five diameters, all grains
in film B exhibit rectangular shapes and align well in the
gas-flow direction. The grain length in the longitudinal direc-
tion is found to be about 0.16 ␮m being ϳ1.4 times as large
as that in the lateral direction, and the average grain size
determined by XRD is about 0.1 ␮m. This result indicates
that the applied plasma assists grains to grow into rectangles.
When SiF4 is added to the feed gas with the ratio
SiF4 /SiH4ϭ15% under the same deposition conditions as
those for film B, it is found in Fig. 2͑e͒ that the shapes of
some grains change from that of a rectangle to a parallelo-
¯
͑110͒, ͑100͒, and ͑111͒ planes as shown in Fig. 3͑a͒. Conse-
quently, the grain with top view shape of a rectangle in Fig.
2͑d͒ is near a tetragonal prism surrounded by ͑110͒ planes
¯
for top and bottom, ͑100͒ for two sides, and ͑110͒ for two
other sides as shown in Fig. 3͑b͒. Thus, the grains are ar-
ranged three dimensionally in the film B. The gas-flow di-
rection ͑i.e., the horizontal line in Fig. 2͒ is parallel to a
¯
͑100͒ plane along the ͓110͔ axis of grain. Such shape regu-
larity and alignment of grains along the gas-flow direction
was not perfect when the rf power was only 8 W as shown in
Fig. 2͑f͒, even if other deposition conditions for this film
were the same as that for film B. Figures 2͑b͒, 2͑d͒, and 2͑f͒
suggest that the higher rf power plasma is favorable for
growing poly-Si films with regular and uniform grains. The
regularity and uniformity of grains are also strongly depen-
dent on the H dilution ratio. As a result, it was found that the
ratio around H2 /SiH4ϭ3, which was the condition used for
film B, is optimal for obtaining uniform and three-
dimensionally oriented poly-Si films.
The growth of poly-Si films composed of three-
dimensionally oriented grains on amorphous substrates, as
far as we know, is a novel result. The mechanism is yet
unclear. For the appearance of ͑110͒ planes, the specific
surface-free energy ␴ is a possible explanation. It is known
that the growth rate of a crystal plane depends on its ␴. The
70
Appl. Phys. Lett., Vol. 68, No. 1, 1 January 1996
Di et al.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
155.33.120.209 On: Fri, 21 Nov 2014 23:42:10

sequence of appearance of the crystal plane in diamond-type
structures is ͕111͖, ͕100͖, ͕311͖, and ͕110͖ for solid-phase
crystal growth,8 but the ͗110͘-textured poly-Si film can also
be obtained in gas-phase growth.9,10 This difference might
originate from changes in ␴ due to the different growth en-
vironments in the two cases. Although ␴ of ͑100͒ and ͑110͒
planes has been reported to be ␴100ϭ2.130 and ␴110ϭ1.510
(10Ϫ4 J/cm3͒ at low temperature, respectively,11 the entropy
term becomes very significant and ͑110͒ would be the plane
of highest ␴ at high temperature.9 In addition, it is possible
that the adsorption and collision of foreign particles on the
growth surface during the gas-phase growth and the applied
external fields would also influence ␴ of planes. Thus, the
observed order of crystal planes is likely to change. In our
case, the random texture characteristic of a LPCVD film
changes into a ͗110͘ texture and becomes a three-
dimensional arrangement with increasing rf power as shown
in Figs. 1 and 2. This result implies that the applied plasma
may bring about changes in ␴ of planes through particle
collisions, and the effects of the rf electromagnetic field
and/or light excitation. In fact, for grains with the shape of a
tetragonal prism hexahedron shown in Fig. 3͑b͒, we can
grains can all be observed in LPCVD, PECVD, and
SiF4-doped PECVD films as stated above. The details are
under further investigation.
In the three-dimensionally oriented film, very low-angle
even 0°-angle boundaries can be expected to form in the
grain boundary regions because the neighbor crystal planes
of two grains are the same. As evidence, we recently found
that the g value in ESR for such films exhibits anisotropy
when the direction of the magnetic field is varied with re-
spect to the film surface,12 implying that the dangling bonds
are oriented in an orderly manner at grain boundaries. The
results above are believed to be significant scientifically and
practically.
The authors thank S. Watanabe for the sample of a
poly-Si film prepared by annealing an a-Si film. This work
was supported in part by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Science, Sports, and
Culture.
1 For example, see, R. Reif and J. E. Knott, Electron Lett. 17, 586 ͑1981͒;
K. T.-Y. Kung and R. Reif, J. Appl. Phys. 59, 2422 ͑1986͒.
2 K. T.-Y. Kung and R. Reif, J. Appl. Phys. 62, 1503 ͑1987͒.
3 N. Yamaguchi, J. Appl. Phys. 75, 3235 ͑1994͒.
4 T. I. Kamins and K. L. Chiang, J. Electrochem. Soc. 129, 2326 ͑1982͒.
5 S. Hasegawa, M. Morita, and Y. Kurata, J. Appl. Phys. 64, 4154 ͑1988͒; S.
Hasegawa, S. Yamamoto, and Y. Kurata, J. Electrochem. Soc. 137, 3663
͑1990͒.
roughly estimate ␴110ϭ&␴ by utilization of Wulff’s law.
100
Indeed, ͑110͒ became the plane of higher ␴ under the depo-
sition conditions of film B. So, ͑110͒ planes once nucleated
will grow faster along their axes than other planes. On the
other hand, there is a question of what influences the nucle-
ation of Si so that the rectangular grains grow with their
6 S. Hasegawa, S. Narikawa, and Y. Kurata, Philos. Mag. B 48, 431 ͑1983͒.
7 S. Hasegawa, S. Watanabe, T. Inokuma, and Y. Kurata, J. Appl. Phys. 77,
1938 ͑1995͒.
8 G. A. Wolff, Am. Mineral 41, 60 ͑1956͒.
¯
9 P. Rai-Choudhury and D. K. Schroder, J. Electrochem. Soc. 120, 664
͑1973͒.
͓110͔-axis parallel to the gas-flow direction. A possibility is
that the gas flow, with an average velocity of about 30 cm/s
parallel to the substrate surface, would greatly dominate the
dynamics of the nucleation process and the stability of nuclei
under proper growth conditions, since this arrangement of
10 R. Bisaro, J. Magarino, N. Proust, and K. Zellama, J. Appl. Phys. 59, 1167
͑1986͒.
11 R. J. Jaccodine, J. Electrochem. Soc. 110, 524 ͑1963͒.
12 G. Q. Di, N. Uchida, Y. Kurata, and S. Hasegawa ͑unpublished͒.
Appl. Phys. Lett., Vol. 68, No. 1, 1 January 1996
Di et al.
71
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
155.33.120.209 On: Fri, 21 Nov 2014 23:42:10