The effect of surface nucleation on the evolution of crystalline microstructure during solid phase crystallization of amorphous Si films on SiO2

Article abstract of DOI:10.1063/1.119437

The effect of surface nucleation on the evolution of crystalline microstructure during the solid phase crystallization (SPC) of an amorphous Si (a-Si) film, deposited by low pressure chemical vapor deposition (LPCVD) on SiO₂, has been investigated. The surface nucleation phenomenon was observed by suppressing the interface (a-Si/SiO₂) nucleation by the incorporation of oxygen atoms during the initial deposition period of a-Si. It was found that the surface-nucleated polycrystalline Si (poly-Si) had equiaxial grains with the size of about 3-5 μm, while interface-nucleated one had elliptical grains with the size of about 0.3-1 μm.

Full text of DOI:10.1063/1.119437

The effect of surface nucleation on the evolution of crystalline microstructure during
solid phase crystallization of amorphous Si films on SiO 2
Myung-Kwan Ryu, Seok-Min Hwang, Tae-Hoon Kim, Ki-Bum Kim, and Seok-Hong Min
Citation: Applied Physics Letters 71, 3063 (1997); doi: 10.1063/1.119437
View online: http://dx.doi.org/10.1063/1.119437
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/71/21?ver=pdfcov
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The effect of surface nucleation on the evolution of crystalline
microstructure during solid phase crystallization of amorphous
Si films on SiO2
Myung-Kwan Ryu, Seok-Min Hwang, Tae-Hoon Kim, and Ki-Bum Kim
Division of Materials Science and Engineering, Seoul National University, san 56-1, Shillim-Dong,
Kwanak-Gu, Seoul, Korea, 151-742
Seok-Hong Min
Center for Advanced Materials Research, Seoul National University, san 56-1, Shillim-Dong, Kwanak-Gu,
Seoul, Korea, 151-742
͑Received 20 June 1997; accepted for publication 23 September 1997͒
The effect of surface nucleation on the evolution of crystalline microstructure during the solid phase
crystallization ͑SPC͒ of an amorphous Si ͑a-Si͒ film, deposited by low pressure chemical vapor
deposition ͑LPCVD͒ on SiO2, has been investigated. The surface nucleation phenomenon was
observed by suppressing the interface (a-Si/SiO2) nucleation by the incorporation of oxygen atoms
during the initial deposition period of a-Si. It was found that the surface-nucleated polycrystalline
Si ͑poly-Si͒ had equiaxial grains with the size of about 3–5 ␮m, while interface-nucleated one had
elliptical grains with the size of about 0.3–1 ␮m. © 1997 American Institute of Physics.
͓S0003-6951͑97͒01747-6͔
Solid phase crystallization ͑SPC͒ of amorphous Si ͑a-Si͒
films deposited on SiO2 by low pressure chemical vapor
deposition ͑LPCVD͒ has been intensively investigated to uti-
lize the crystallized film as an active layer of the polycrys-
talline Si ͑poly-Si͒ thin film transistor ͑TFT͒.1–3 In these ap-
plications, the electrical properties of poly-Si TFT are
strongly dependent on the microstructure of the poly-Si film.
In particular, grain boundaries and crystalline defects inside
grains are known to affect the device properties since both of
these act as scattering centers of the charge carriers. Thus,
many efforts have been attempted to increase the grain size
and to reduce the density of crystalline defects in the grains.
Recently, we have demonstrated a possibility to signifi-
cantly improve the microstructure of the polycrystalline film
by promoting the surface nucleation during solid phase crys-
tallization ͑SPC͒ process. By using an a-(Si0.7Ge0.3 /Si) bi-
layer structure deposited on SiO2 instead of an a-Si single
layer, we have observed that the film forms equiaxial grains
of about 7 ␮m in size with much reduced density of crystal-
line defects.4 We have explained these results based on the
site of nucleation, namely, surface vs interface. In that work,
the nucleation starts at the surface since the a-Si0.7Ge0.3 layer
deposited on top of the a-Si has a lower thermal budget
compared to a-Si. The question now is firstly whether we can
obtain a similar phenomenon of surface nucleation in an a-Si
single layer film and secondly how the microstructure of the
surface-nucleated poly-Si is different from that of the inter-
face nucleated.
Two kinds of a-Si films with a thickness of 80
ϳ100 nm were deposited on 100 nm-thick thermal oxide
substrates. One is a conventional a-Si a-Si͑I͒ film depos-
ited at 475 °C and 1 Torr by using Si2H6 source gas ͑5
SCCM͒ and H2 carrier gas ͑200 SCCM͒ and the other
͓
͔
a-Si a-Si͑II͒ was deposited at the same temperature and
͓
͔
pressure except that oxygen was blown into the LPCVD
chamber at the initial deposition period. The flow rate of
oxygen was gradually decreased from 10 SCCM to zero
within 5 min. Both of these films were annealed at 600 °C in
a nitrogen ambient for SPC. In order to record the depth
profiles of oxygen in the as-deposited a-Si films, we utilized
secondary ion mass spectrometry ͑SIMS͒. The microstruc-
ture of the films was investigated by using transmission elec-
tron microscopy ͑TEM͒.
The SIMS depth profiles of chemical species, including
oxygen, in the two a-Si films are given in Fig. 1. It is clearly
shown that the oxygen profile changes steeply at the
a-Si/SiO2 interface for the a-Si͑I͒ film. On the contrary, as
for a-Si͑II͒, the oxygen profile changes gradually through the
a-Si/SiO2 interface. It appears that the flow of oxygen is
large enough to deposit CVD-SiO2 at the beginning of the
a-Si͑II͒ deposition. However, it is insufficient to form
CVD-SiO2 at the end of the oxygen blowing process and
thus forms oxygen-rich a-Si at the a-Si/CVD-SiO2 interface.
The CVD-SiO2 is distinguishable from the thermal SiO2
since the former has higher hydrogen concentration com-
pared to the latter, due to the incorporated hydrogen during
the CVD-SiO2 deposition.
In this letter, we report the successful results of obtain-
ing the surface nucleation phenomenon during the SPC of an
a-Si film deposited on SiO2. The interface (a-Si/SiO2)
nucleation commonly observed in a conventional a-Si
film5–8 was suppressed by the incorporation of oxygen atoms
during the initial deposition period of an a-Si film. The evo-
lution of crystalline microstructure by the surface nucleation
has been investigated and compared with that by the inter-
face nucleation.
Figures 2͑a͒ and 2͑b͒ show the plan-view TEM images
of a two a-Si films annealed for 7 hours. It is clearly ob-
served that many elliptical grains of about 0.3ϳ1 ␮m size
are randomly distributed in an a-Si͑I͒ ͓Fig. 2͑a͔͒. The forma-
tion of elliptical grains is commonly observed in a solid
phase crystallized a-Si film and is known to occur by the
preferential growth along the direction of microtwins exist-
ing in each grain.9 Unlike the case of a-Si͑I͒, the TEM mi-
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Appl. Phys. Lett. 71 (21), 24 November 1997 0003-6951/97/71(21)/3063/3/$10.00 © 1997 American Institute of Physics 3063
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FIG. 3. Cross-sectional TEM micrograph of partially crystallized ͑a͒ a-Si͑I͒
and ͑b͒ a-Si͑II͒ film at 600 °C. The dashed line in ͑b͒ indicates the interface
between CVD-SiO2 formed by oxygen blowing and thermal SiO2.
face in a-Si͑II͒ is believed due to the oxygen-rich a-Si region
formed by the incorporation of oxygen at the initial deposi-
tion period.
Comparing both the results of plan-view and cross-
sectional TEM, it is clear that the grains nucleated at the
surface form an equiaxial shape unlike the interface nucle-
ated ones which grew as an elliptical shape. It is also noted
that the density of equiaxial grains in Fig. 2͑b͒ is much
smaller than that of the elliptical grains in Fig. 2͑a͒. This
result implies that the surface nucleation rate is much lower
than that of the interface.
FIG. 1. SIMS depth profile of the as-deposited ͑a͒ a-Si͑I͒ and ͑b͒ a-Si͑II͒.
crograph of the a-Si͑II͒ ͓Fig. 2͑b͔͒ shows the formation of
equiaxial grains of about 2 ␮m in an amorphous matrix.
While the elliptical shape grains also appeared in the micro-
graph, the density of the elliptical grain was remarkably re-
duced compared to that of a-Si͑I͒. The selected area diffrac-
tion pattern ͑SADP͒ of an equiaxial grain shows ͗111͘-zone
axis pattern, indicating that the ͕111͖-planes are parallel to
the film surface. In addition, the diffraction pattern shows
It has been known that oxygen retards the crystallization
process of an a-Si during the solid phase epitaxial regrowth
of an oxygen-implanted a-Si.11 Our results also demonstrate
that the incorporated oxygen at the interface effectively sup-
presses the interface nucleation during the solid phase crys-
tallization. When the interface nucleation is suppressed, it is
expected that the nucleation will start at another preferable
nucleation site, namely, the surface of the film. It is expected
that the Si atoms at the surface are laid in different surround-
ings from those at the a-Si/SiO2 interface since surface at-
oms are loosely bounded to a very thin (ϳ20 Å) native ox-
ide layer. Based on this, Morimoto et al.12 claimed that the
atomic rearrangement for crystallization is more favorable at
the vicinity of a-Si surface than at the a-Si/SiO2 interface.
They also claimed that crystalline defects were developed
mainly at the a-Si/SiO2 interface during the lateral solid
phase epitaxial ͑LSPE͒ growth in order to relieve the tensile
stress resulted by the phase transformation from an amor-
phous to a crystalline. They had demonstrated the formation
of a nearly defect-free crystalline Si layer by removing the
SiO2 underlayer before the SPE process.
From the above discussion, it is considered that the crys-
talline defects such as microtwins and stacking faults in the
interface-nucleated grains were introduced in order to relieve
the large magnitude of tensile stress at the vicinity of the
a-Si/SiO2 interface. After the nucleation, the grains grow to
elliptical shape by a preferential growth along the microt-
wins and stacking faults. For the case of the surface nucle-
ation, however, the defects are not formed at the nucleation
step since the stress is relieved by the surface. However,
even for this case, the stress gradually builds up at the
growth front as the grain grows and, when the stress in-
creases above a critical level, the defects are generated at the
growth front. Since the crystalline phase has a ͕111͖-
orientation, the twinned region is parallel to the film surface.
After the in-plane twin is formed, the lateral growth is fur-
ther promoted to make the lens-like shape in the cross-
section view and the equiaxial form in the plan view.
1/3 422 spots resulted from the diffraction from the twinned
͕
͖
regions.10
Figures 3͑a͒ and 3͑b͒ are the cross-sectional TEM im-
ages of partially crystallized a-Si͑I͒ and a-Si͑II͒, respec-
tively. These micrographs show the initial crystallization
process in these two a-Si films. The micrograph of Fig. 3͑a͒
clearly shows that the nucleation starts from the interface
between a-Si and SiO2, as is commonly observed for the
conventional SPC process, and the nucleated grains grow
toward the surface of the film. Compared to this result, the
TEM image of Fig. 3͑b͒ shows the formation of a lens-like
shape grain at the surface of the film with a few microtwins
parallel to the film surface. It is also noted that the amor-
phous phase still remains below the grain which clearly sug-
gests that the grain nucleates at the film surface and subse-
quently grows down to the interface. The interface between
a-Si and SiO2 in Fig. 3͑b͒ appears to be graded unlike the
sharp interface in Fig. 3͑a͒. The appearance of graded inter-
FIG. 2. Plan-view TEM micrograph of ͑a͒ a-Si͑I͒ and ͑b͒ a-Si͑II͒ film after
a furnace annealing at 600 °C for 7 hours. The SADP was obtained from an
equiaxial grain in ͑b͒.
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3064 Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997 Ryu et al.
129.49.170.188 On: Fri, 19 Dec 2014 11:44:01

In conclusion, we have investigated the microstructure
during the SPC of an a-Si film in which the surface nucle-
ation of crystalline phases is dominantly observed. The sur-
face nucleation phenomenon was achieved by suppressing
the interface nucleation by incorporating oxygen atoms at the
initial deposition period of a-Si. TEM observation revealed
that the surface-nucleated grains had equiaxial shape with
͕111͖-orientation. The surface nucleation rate was much
lower than the interface nucleation rate. After the full crys-
tallization at 600 °C, the surface-nucleated poly-Si film con-
sisted of many equiaxial grains with the size of about 3–5
␮m while the conventional SPC poly-Si had plenty of ellip-
tical grains with the size of about 0.3–1 ␮m.
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The authors appreciate the financial support offered by
Samsung Electron Company for this work. The Research
Center for Thin Film Fabrication and Crystal Growing of
Advanced Materials ͑RETCAM͒ assisted in meeting the pub-
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