Proceedings of 7th Thin Film Materials & Devices Meeting
November 5-6, 2010, Nara, Japan
http://www.tfmd.jp/

Lateral Crystallization of Amorphous Silicon films
Induced by Flash Lamp Annealing
Keisuke Ohdaira1,2, Shohei Ishii1, Naohito Tomura1, and Hideki Matsumura1
1

School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST),
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
2PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama
332-0012, Japan
Keywords: Flash lamp annealing, Polycrystalline silicon, Explosive crystallization, Periodicstructure, Thin-film solar cells
Abstract. Flash lamp annealing (FLA) induces lateral crystallization of a few µm-thickprecursor amorphous silicon (a-Si) films from film edges toward center. This lateralcrystallization can be understood as explosive crystallization (EC), driven by the release oflatent heat, and leaves behind periodic microstructures in poly-Si films with an interval of
approximately 1 µm. Based on the results of crystallization induced by multi-pulse FLA, weestimate the velocity of lateral crystallization to be 2.5-5.0 m/s. This value is significantly
smaller than that of liquid-phase EC of ~15 m/s, suggesting that observed EC includes
solid-phase process.

Introduction
Thin-film silicon (Si) is an attractive photovoltaic material because of low Si consumption andresulting high adaptivity for future enormous increase in photovoltaic production. There are,however, some disadvantages to be overcome for chemical vapor deposited (CVD) Si thin films
such as light-induced degradation for amorphous Si (a-Si) and difficulty of realizing highdeposition rate for microcrystalline Si (µc-Si).
Hence, other approaches to form
polycrystalline Si (poly-Si) films on low-cost substrates have been proposed [1,2,3]. In
particular, poly-Si films formed through solid-phase crystallization of a-Si films by
conventional furnace annealing has demonstrated 10%-efficiency solar cells [1]. This
crystallization process, however, requires hour-order, time-consuming annealing, resulting in
low throughput, and we requires rapid crystallization technique. Another disadvantage oflong-duration furnace annealing is thermal-equilibrium treatment, which prevents us to usesubstrates with poor thermal tolerance.
To overcome this issue, we have investigated flash lamp annealing (FLA), withmillisecond-order duration, for a technique to crystallize micrometer-order-thick a-Si films.
The millisecond-order duration of FLA leads to the sufficient heating of a few µm thick a-Sifilms with avoiding thermal damage to entire glass substrates. We have actually confirmed
the formation of poly-Si films with a thickness up to 4.5 µm on quartz and conventional soda
lime glass substrates [4,5]. We have also observed that hydrogen (H) atoms in Si films are
hardly desorbed off during FLA, and formed poly-Si films contains H atoms on the order of1021 /cm3, which can be effectively utilized to terminate dangling bonds inflash-lamp-crystallized (FLC) poly-Si films by post-furnace annealing [6]. Diode and solar
cell operations have been confirmed in devices fabricated using FLC poly-Si films [7].
Interestingly, the crystallization induced by FLA takes place laterally, and in this paper, wediscuss the mechanism of this particular crystallization induced by FLA.

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Experimental details
First, we deposited Cr films on 20×20×0.7 mm3-sized
quartz glass substrates by RF sputtering in order toprevent the peeling of Si films during FLA [4,5]. We
then deposited 2.0-4.5 µm-thick a-Si films by catalytic
CVD (Cat-CVD). The detailed deposition conditions
have been summarized elsewhere [8].
We alsoattempted sputtered a-Si films as precursor films. We
have confirmed the formation of FLC poly-Si films byusing sputtered a-Si films and similar microstructure to
those formed from Cat-CVD a-Si films.
Figure 1 shows the schematic diagram of an FLAsystem. Flash lamp light has a broad spectrum mainly
in visible range, which can be seen elsewhere [9].
FLA was performed under a fixed duration of 5 ms,
while its irradiance was systematically changed around
20 J/cm2. Flash lamp light was irradiated onto an
entire Si film surface with sufficient areal uniformity.
Besides a single-pulse FLA system we have used so far,
we also newly utilized a multi-pulse FLA system,
whose pulse light is schematically illustrated in Fig. 2.
The frequency of individual pulse emission can besystematically changed in the multi-pulse FLA system.
The total irradiance of a set of a multi-pulse was also
approximately 20 J/cm2.
The microstructure of FLC poly-Si films was observedwith scanning electron microscope (SEM), atomicforce microscope (AFM), and transmission electron
microscope (TEM). Crystallization and crystalline
fraction of FLA Si films were evaluated by means ofRaman spectroscopy.

Fig. 1 Schematic diagram of an FLA
system used in this study.

Fig. 2Schematics of conventional(single-pulse) and multi-pulse flash-lamplight.

Results and discussion
Figure 3 shows surface images of Si films after FLA with various lamp irradiances. One cansee partial crystallization in the vicinity of Si film edges after low-irradiance FLA, andcrystallized area enlarges towards center with increasing lamp irradiance. These facts indicatethat crystallization starts from edges and propagates laterally. We can also observe
rainbow-colored surfaces on FLC poly-Si films.

Fig. 3 Surface images of Si films after FLA with various lamp irradiances.

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Figure 4 shows SEM images of an FLC poly-Sifilm. One can see a periodic microstructure along
a crystallization direction.
This structure isformed spontaneously during crystallizationinduced by FLA.
Surface root-mean-squareroughness (Rrms) of the FLC poly-Si surface,evaluated by AFM measurement, is typically 120
nm, which is significantly larger than Rrms ofprecursor a-Si films of less than 10 nm. Theseperiodic structures act as an optical grating,resulting in the emergence of rainbow-coloredsurface on FLC poly-Si films.
We have Fig. 4 Surface SEM image of an FLA
poly-Si film.
confirmed that this surface roughness effectively
acts to reduce optical reflection on the FLA poly-Si
surface [10].
Figure 5 shows the cross-sectional TEM image ofan FLC poly-Si film. The cross section wasformed along a crystallization direction. Periodic
surface roughness, shown in the SEM image, isalso shown in the TEM image. We can also see
periodic structures inside poly-Si, which consist of
two kinds of regions with different grain features.
One region, large-grain region, contains surface
protrusions on the surface and relatively large, afew hundred nm-sized grains inside Si film, someof which are stretched along the lateralcrystallization direction.
The other region,
fine-grain region, connects to flat surface andincludes no hundred-nm-sized large grains.
The onset of crystallization at Si film edges can be Fig. 5 Cross-sectional TEM image of an
FLC poly-Si film. Solid and dashed arrows
understood as additional heating by angled flash indicate large-grain and fine-grain regions,lamp light into film edges. One may worry about respectively.
thermal loss at Si film edges through convection orradiation. These effects are, however, negligible during ms-order short-duration process.
More quantitative consideration about heat loss at Si film edges has been summarizedelsewhere [11].
Since the lateral crystallization propagates ~1 cm during 5-ms flash lamp pulse, thecrystallization velocity must be more than 2 m/s. Such a rapid lateral crystallization can beexplained only by explosive crystallization (EC), autocatalytic crystallization driven by therelease of latent heat [12,13,14,15], and both liquid-phase [16,17,18] and solid-phase EC[18,19] have been investigated experimentally and theoretically. FLC poly-Si films containtwo regions with different grain features existing periodically, as shown above, and we canconsider that these two regions are formed through different crystallization mechanisms. As
we have discussed in a previous paper [11], the large-grain region is formed thorough a mixed
phase of solid-phase nucleation and liquid-phase epitaxy, while the fine-grain region is formed
only through solid-phase nucleation. This means that two types of EC occurs alternatively
during one flash irradiation. The reason of this has not been fully clarified, and we will
perform an analysis of lateral thermal diffusion including the effect of heat generation bysolid-phase and liquid-phase crystallization.
Figure 6 shows the surfaces of FLC poly-Si films formed from sputtered a-Si films using the110107015-3

multi-pulse FLA system. We have confirmedthat poly-Si films crystallized by using
multi-pulse
flash
lamplighthave
microstructures and crystalline fraction similar
to FLC poly-Si films formed using asingle-pulse FLA system.
One can see
macroscopic stripes on the FLC poly-Si films.
Since we have observed such stripes only on
FLC poly-Si films formed using multi-pulse Fig. 6 Surface images of FLC poly-Si films
FLA and the width of stripes systematically formed using a multi-pulse FLA system with pulse
varies depending on pulse emission frequency, frequency of (a) 5 kHz and (b) 7 kHz, respectively.
we can understand that these stripes are formedby discrete pulses. By using known pulse emission frequency and the width of stripes, we caneasily estimate the velocity of EC induced by FLA to be 2.5-5.0 m/s. In the case of simple
liquid-phase EC, its velocity is governed by the speed of liquid-phase epitaxy that depends on
the temperature of supercooled Si melt, and an EC speed of ~15 m/s has been experimentallyconfirmed [16]. On the other hand, an EC velocity of 0.5-3.0 m/s has been theoretically
predicted in the case of complete solid-phase EC [19]. The EC velocity estimated for FLC
poly-Si films is between the two typical values, indicating the validity of the proposedmechanism of mixed-phase EC. The estimated EC velocity is also consistent with the fact that
2×2 cm2-sized a-Si films are not wholly crystallized when we use 1-ms pulse light forcrystallization [20].

Summary
FLA induces EC of a-Si films, leaving behind periodic microstructures on the surface of andinside FLC poly-Si films. This periodic structure consists of two kinds of characteristic
regions formed through different mechanisms: one is partially related to liquid-phase growth
and the other is governed only by solid-phase process. This fact is consistent with an EC
velocity of 2.5-5.0 m/s, estimated from the results of multi-pulse FLA, larger than the velocityof solid-phase EC and smaller than that of liquid-phase EC.

Acknowledgments
The authors would like to thank T. Yokomori and T. Owada of Ushio Inc. for their expert
operation of FLA, and T. Yoshida of JAIST for his assistance for experiments. This work was
supported by JST PRESTO program and Grant-in-Aid for Young Scientists (B) No. 40396510
from MEXT Japan.

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