Roles of SiH4 and SiF4 in growth and structural changes of poly-Si films

Article abstract of DOI:10.1016/j.susc.2006.04.013

The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH₄/SiF₄ mixtures at 300 °C were investigated. This study indicates that the low

Full text of DOI:10.1016/j.susc.2006.04.013

Surface Science 600 (2006) 4418–4425
www.elsevier.com/locate/susc
Roles of SiH and SiF in growth and structural changes of poly-Si films
4
4
*
A. Haddad Adel , T. Inokuma, Y. Kurata, S. Hasegawa
Department of Electrical and Electronic Engineering, Faculty of Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
Received 5 September 2005; accepted for publication 18 April 2006
Available online 11 May 2006
Abstract
The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different
flow rates of SiH /SiF mixtures at 300 ꢀC were investigated. This study indicates that the low hydrogen coverage on the growing surface,
4
4
under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage
surface. Moreover, the studies of the role of SiH and SiF radicals show that the SiH radicals are important in the nucleation and growth
4
4
4
4
of grains. However, SiF radicals are effective in the structural change of grain boundaries regions and by this way, in the present system,
establish the growth of grains under the dominant h110i direction. The stress investigation indicates that addition of high flow rate of SiF
4
in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic
compressive and tensile stress in the both polycrystalline and amorphous silicon films.
ꢁ
2006 Published by Elsevier B.V.
Keywords: Plasma-enhanced chemical vapor deposition; Polycrystalline thin films; Preferential orientation; Low-angle/high-angle grain boundary;
Hydrogen coverage; Crystallization; g-Value of electron spin resonance; Stress
1
. Introduction
icon materials are typically heterogeneous and anisotropic
consisting of crystalline grains and amorphous-like silicon
tissue in grain boundaries (GBs). The GBs and intregranu-
lar defects can act as electrical potential barriers and scat-
tering sites, which decrease the carrier transport mobility
and also serve as midgap states working to increase the
leakage currents [6]. A reduction in the GBs effects is a
key to enhance the TFTs performance. Some methods for
this purpose are as follows. The low oxygen content that
avoids extrinsic GB activation [7,8] and high hydrogen
incorporation that results in passivation of the Si dangling
bonds in the GBs, which decreases the density of GBs
states in the film [9,10]. The reduction in the GBs regions
by an increase in the grain size is another method to reduce
the GBs effects and laser crystallization is a well-known
method for this purpose [11]. However, high cost and poor
film uniformity reduce some of the advantages of this tech-
nique. Beside the above-mentioned methods that cause a
reduction in the effects of GBs, the formation of low-angle
GBs that is responsible for a preferential oriented struc-
ture, also may enhance the transistor characteristics [12].
Polycrystalline silicon films (poly-Si) have attracted
intensive interest because of the potential to prepare high
performance and large area semiconductor devices such
as solar cells [1,2] and thin film transistor (TFT) in flat pa-
nel displays [3–5]. The direct deposition of poly-Si films at a
temperature below the strain point on the glass substrates
is an attractive and economical technology for producing
silicon displays. For this purpose, recent main research
subjects are low-temperature deposition of poly-Si films,
which can be widely used in the active-matrix liquid–crystal
displays that are now typically found in computer technol-
ogies and an increasing number of monitors.
In the poly-Si films, the charge carrier mobility as an
important parameter in the electrical properties is mainly
determined by polycrystalline structure. Polycrystalline sil-
*
Corresponding author. Tel.: +81 76 234 4882; fax: +81 76 234 4870.
E-mail addresses: azadadel@yahoo.com, aahadad@yahoo.com (A.H.
Adel).
0
039-6028/$ - see front matter ꢁ 2006 Published by Elsevier B.V.
doi:10.1016/j.susc.2006.04.013

A.H. Adel et al. / Surface Science 600 (2006) 4418–4425
4419
The low-angle GBs are those GBs that the angle between
the two grains is small (<10ꢀ). The low-angle boundaries
can be represented as an array of dislocations that the re-
gions of the two grains placing in between the dislocations
are in good match with each other. In such grain bound-
aries, cores of dislocation are regions of poor fit with highly
distorted crystal. However, when the angle between two
grains is above 10–15ꢀ, a high-angle boundary is formed
and has amorphous-like structure. In high-angle bound-
aries, the dislocation spacing is so small that cores of indi-
vidual dislocations start overlapping; therefore, individual
dislocations can no longer be physically identified. The dif-
ferences between low-angle and high-angle grain bound-
aries can be summarized as follows:
power and gas pressure during film deposition were main-
tained at 20 W and 33.3 Pa, respectively. The unique fea-
ture in the preparation process of the samples is that the
substrate before deposition of the poly-Si films was sequen-
tially exposed to the hydrogen and nitrogen plasma for
20 min to make its surface clean. The plasma exposure also
causes the change in the surface roughness [15], besides the
above-mentioned surface cleaning effects. In a previous pa-
per [15], we have reported that the presence of a proper de-
gree of the surface roughness improves the crystallinity and
increases the grain size [15].
Polycrystalline Si films were deposited on glass (corning
7059) substrates for measurements of X-ray diffraction
(XRD), Raman scattering and stress. The single h111i
3
crystal Si substrates with a resistivity higher than 10
–
–
–
High-angle boundaries contain large areas of poor fit
while low-angle boundaries contain large areas of good
fit separated by misfit dislocations.
High-angle boundaries have an open structure with lots
of free volume while low-angle boundaries have little
free-volume.
In case of high-angle boundaries, the interatomic bonds
are either broken or highly distorted at the boundary
while in case of low-angle boundaries the interatomic
bonds are only slightly distorted.
Xcm were used for infrared (IR) absorption measurement
and electron spin resonance (ESR). For the ESR measure-
ments, the fused quartz substrates were also used to inves-
tigate effects for the use of the different substrates in
addition to the above crystal Si substrates with a proper
oxidized layer. As a result, we did not find significant differ-
ence between the ESR spectra for the samples on both sub-
strates. The morphology was investigated by XRD
measurements. The degree of preferential orientation in
hhkli texture was represented by X-ray relative intensity,
I
(hhkli), for a given texture, which was normalized
XRD
Moreover, in the previous article [13], we reported that
by the corresponding X-ray intensity for Si powder with
a roughly completely random texture. The different film
thickness in each sample was also corrected using the
absorption coefficient of X-ray for Si. The details for the re-
sults of the IXRD were presented in a previous paper [13].
The grain size, d, in a given depth direction was estimated
from the half-width value of the corresponding X-ray spec-
tra by means of the Scherrer formula.
the high degree preferentially oriented poly-Si films, pre-
vents the oxygen contamination owing to establishment
of the low-angle GBs regions in such films. These proper-
ties for a low-angle GBs indicate that the growth of poly-
Si films with a preferentially orientation seems to be a key
to enhance the TFTs performance by reducing of defects
and oxidation rate in the GBs regions. In that work [13],
the poly-Si films with h110i texture were deposited by
plasma enhanced chemical vapor deposition (PECVD)
method using a SiH /SiF system. In the present work,
in order to enhance the degree of the h110i preferential
orientation and to reduce the defects, such as stress in
these films, we investigate mechanism of addition of
SiH4 and SiF4 on the enhancement of crystallization
and preferentially orientation of poly-Si films by using
different flow rate of SiH , [SiH ], and flow rate of SiF ,
The crystallization ratio (volume fraction of crystalline
phase), q, was estimated from the intensity of the Raman
spectra using the procedure proposed by Tsu et al. [16].
The Raman spectra was decomposed into two components
with narrow and broad line shapes corresponding to a crys-
4
4
ꢀ
1
talline Si phase (c-phase) at around 520 cm and an amor-
ꢀ1
phous Si phase (a-phase) at around 480 cm . The third
ꢀ
1
component between 480 and 520 cm , due to very small
crystallites, was relatively weak; therefore, we ignored the
contribution of it to the values of q. Then the q values were
estimated from the intensity ratio of both the a- and c-
phase components [16]. Therefore, the real q values may
be somewhat different from the values shown in the dia-
gram. The g factor was measured using an X-band ESR
spectrometer with magnetic-field modulation frequency of
100 kHz, and the measurements were carried out at room
temperature. The infrared (IR) vibrational absorptions
were measured using a Fourier-transform spectroscope at
a normal light incidence.
4
4
4
[
SiF4].
2
. Experimental details
Polycrystalline Si films were deposited by PECVD sys-
tem at a temperature of 300 ꢀC using SiH /SiF mixtures
in a hot wall type fused quartz reactor, employing the
inductive coupling of rf power at 13.56 MHz. This deposi-
tion system has been presented elsewhere [14]. In this work,
a diluted gas of SiF with He gas (SiF : 5%, He: 95%) was
4
4
4
4
used. Therefore, the net flow rate of SiF was changed be-
tween 0 and 0.5 sccm under five different conditions of
The film stress was estimated from changes in the curva-
ture of the substrate/film system with a rectangular shape
(the length L being sufficiently longer than the width),
using Stoney’s formula [17] as:
4
[
SiH ] = 0.2, 0.5, 1, 2, and 5 sccm. The film thickness was
4
kept at 0.5–0.6 lm by varying the deposition time. The rf

4420
A.H. Adel et al. / Surface Science 600 (2006) 4418–4425
ꢀ
ꢁ
2
1
b d
r ¼ ðY =ð1 ꢀ m ÞÞ
S
S
2
3
dL
here, Y is Young’s modulus and m the Poisson ratio for
S
S
the substrate. The parameters b and d are the thicknesses
of the substrate and film, respectively. The displacement
is represented by d from before and after the film deposi-
tion of one edge of the substrate in the longitudinal direc-
tion, which respect to another edge. Values of Y =
S
1
0
2
7
.7 · 10 N/m and m = 0.28 were used for the corning
S
glass substrate.
Fig. 2. XRD spectra for poly-Si films with a fixed value of [SiF
4
] =
0
.5 sccm with different [SiH ] values of 0.2, 0.5, 1, 2 and 5 sccm and a fixed
4
3
. Results
temperature of 300 ꢀC.
Fig. 1 shows (a) deposition rate, and (b) q values of
poly-Si films as a function of [SiF ] for films prepared with
4
different [SiH ]. As seen in Fig. 1, changes in the [SiH ] are
(a)
(b)
4
4
more effective than the [SiF ] in the changes of both depo-
4
sition rate and q. This result means that the main precur-
sors for film growth in this system arise from SiH . As
4
shown in Fig. 1(b), a completely amorphous phase is ob-
tained in the films having [SiH ] = 5 sccm, while the q val-
4
ues in the samples having [SiH ] < 5 sccm can be increased
4
with decreasing [SiH ] to 0.5 sccm and reaches to 90% un-
4
der the high [SiF4].
Fig. 2 shows the XRD spectra for the poly-Si films
deposited at [SiF ] = 0.5 sccm under different [SiH ] values
4
4
as a parameter. In these diagrams an amorphous structure
has been obtained for the film with [SiH ] = 5 sccm, which
is consistent with the Raman results seen in Fig. 1(b). A
4
Fig. 3. (a) XRD intensity and (b) grain size, d, of the h110i texture as a
function of [SiH ], with open symbols, for films with different [SiF ] values
of 0, 0.3, 0.4 and 0.5 sccm at a fixed temperature of 300 ꢀC.
4
4
reduction in [SiH ] causes crystallized films with the random
4
structure at [SiH ] = 2 sccm and with a further decrease in
4
[
SiH ] in the range of [SiH ] 6 1 sccm, a dominant h110i
4
4
direction should start at [SiH ] = 1 sccm in films with
4
texture can be obtained. However, all the other crystal-
lized films with [SiF ] = 0 sccm exhibit random textures.
Thus, formation of a preferred oriented structure in h110i
[
SiF ] > 0 sccm. These results mean that SiF plays an
4 4
4
important role in creation of dominant h110i texture,
though the degree of these preferred textures is likely to
be controlled by the ratio of SiH to SiF .
4
4
In Fig. 3(a) IXRD(h110i) and (b) d(h110i) values are
plotted as a function of [SiH ] with different [SiF ] values
4
4
(
a)
b)
as a parameter. Both the IXRD(h110i) and d(h110i) values
for films with [SiF ] = 0 sccm monotonically decrease with
4
an increase in the [SiH ], different from the behaviors of
4
films with other [SiF ] values. According to Fig. 3, the
4
IXRD(h110i) values for films with [SiF ] P 0.3 sccm have
4
maximum at [SiH ] = 0.5 sccm, while d(h110i) for these
4
films have maximum at [SiH ] = 1 sccm, which means that
4
(
in the films with [SiH ] < 1, with a reduction in the d(h110i)
4
the I
(h110i) increases. An increase in the intensity with
XRD
a reduction in the grain size can take place when the num-
ber of grains increases. In other words, the h110i nucle-
ation rate increases during the growth of poly-Si films
with [SiH ] < 1 sccm. Furthermore, Fig. 3 shows that the
4
IXRD and d values for [SiH ] 6 1 sccm monotonically in-
4
crease with increasing [SiF ] that acts as etchant.
4
ꢀ
1
Fig. 4 shows the absorption intensity at 2100 cm as a
function of [SiF ] with different [SiH ] values as a parame-
Fig. 1. (a) Deposition rate and (b) the crystalline volume fraction, q, for
poly-Si films with varying [SiF ] from 0 to 0.5 sccm with [SiH ] = 0.2, 0.5,
, 2, and 5 sccm at a fixed temperature of 300 ꢀC.
4
4
4
4
ꢀ
1
1
ter. It is well known that the 2000 and 2100 cm peaks

A.H. Adel et al. / Surface Science 600 (2006) 4418–4425
4421
(
a)
(b)
ꢀ
1
Fig. 4. Absorption intensity of Si–H
FT–IR as a function of [SiF ], for films with different [SiH
.5, 1, 2 and 5 sccm and at a fixed temperature of 300 ꢀC.
2
bonds at 2100 cm measured by
4
4
] values of 0.2,
0
Fig. 5. Stress in the poly-Si films as a function of (a) [SiF
symbols) and (b) [SiH
4
] (closed
4
] (open symbols). In these diagram, Positive and
negative values of stress denote tensile and compressive stress,
respectively.
arise from the structure involving Si–H and Si–H bonds,
2
respectively. The Si–H bonds usually exist in the amor-
phous region [18] and the Si–H bonds depend on the struc-
2
ture of Si films can appear on GBs regions of poly-Si films
or on internal surfaces of amorphous silicon films [19]. If
means a stretch in film in reference to substrate. The mea-
sured stress, r, is composed of two different types of stres-
the oscillator strength is fixed, the Si–H absorption inten-
ses: one is the intrinsic stress, r , depending on the growth
2
i
sity in Fig. 4 would be proportional to the density of incor-
porated H-related bonds. In the present work, all films
process of the film, and another is due to thermal expan-
sion mismatch between the film and the substrate, r . The
t
consist of Si–H bonds while the Si–H bonds are observed
rt values can be given by
2
only in the films with higher [SiH ] values of 1, 2 and
4
Y F
r
t
¼ ða
F
ꢀ a
S
ÞDT =ð1 ꢀ m Þ
F
5
sccm in which more than 30% of the films area are amor-
phous, as shown in Fig. 1(b). The absorption intensity of
Si–H bonds in those films (not presented here) increases
with increasing [SiH ] values independently of the [SiF ]
where a and a are the thermal expansion coefficient for
F S
the deposited film and the substrate, respectively. We used
ꢀ
6
the value of a = 5 · 10 /K for the corning 7059 glass sub-
4
4
S
values.
strates, which was cited from a catalog. The thermal expan-
sion coefficient for c-Si, (a ), is 3 · 10 /K [20], but that for
ꢀ
6
The presented results in Fig. 4 shows that in the depos-
ited poly-Si films, addition of SiF to SiH in the feed gases
F
a-Si will be considerably smaller than this value. The DT va-
lue is assumed as the difference between the deposition tem-
perature and room temperature at which the stress was
measured. The YF and mF are Young’s modulus and the
Poisson ratio for the deposited films, respectively. The total
values of Y /(1 ꢀ m ) for c-Si and a-Si films were assumed to
4
4
results in the both reduction and increase in the H density
in the films with [SiH ] 6 1 sccm and [SiH ] = 2 sccm,
4
4
respectively. This different behavior may be interpreted
by the presented results in Figs. 2 and 4, which indicate
that the spectral profiles for hydrogen in the GB regions
may change between the films with random structure and
preferentially oriented films. As mentioned before the pref-
erentially oriented poly-Si film has a low-angle GBs, while
a high-angle GBs is formed in a randomly oriented poly-Si
film. According to the changes in the absorption intensity
F
F
1
1
11
2
be 1.8 · 10 and 4.2 · 10 N/m [21], respectively. Using
these values, we expect occurrence of a compressive stress,
especially, the absolute magnitude of this value will become
large in films with poor crystallization ratio. However, the
presented results show an opposite behavior, which means
that the observed stress was due to some intrinsic stress, dif-
ferent from thermal expansion mismatch.
of Si–H bonds in the GBs regions, two assumptions are
2
possible for these films; (1) interaction between silicon
and hydrogen in a low-angle and high-angle GBs is differ-
ent, and/or (2) extraction of hydrogen by fluorine in low-
angle GBs is more practicable than in a high-angle GBs,
which means that the behavior of F radicals in the high-an-
gle GBs and low angle GBs is different.
As shown in Fig. 5(a), changes in the stress are rather
complex. In films with [SiF ] P 0.3 sccm, with an increase
4
in [SiH ] the stress values are likely to change from compres-
4
sive to tensile stress and again from tensile to compressive
stress. Thus, the tensile stress is likely to take a maximal at
Fig. 5(a) and (b) shows stress, respectively, as a function
of [SiF ] and [SiH ] with different [SiH ] and [SiF ] values
around [SiH ] = 1 sccm. In other words, the minimal com-
4
pressive stress is obtained in [SiH ] = 1 sccm. These results
4
4
4
4
4
as their respective parameters. In these diagrams, the stress
can be summarized as follows. For those crystallized films
as a function of [SiF ] are shown by closed symbols, and as
which exhibit compressive stress ([SiH ] = 0.2 and 0.5 sccm),
4
4
a function of [SiH ] are represented by open symbols. The
an increase in [SiF ] causes an increase in the absolute value
4
4
tensile stress, which shows shrinkage of the film in reference
to substrate, is shown by positive value. On the contrary,
negative values of stress denote compressive stress, which
of compressive stress. However, in amorphous films with
[SiH ] = 5 sccm, addition of SiF results in a reduction of
4
4
compressive stress and finally under high [SiF ], nearly stress
4

4422
A.H. Adel et al. / Surface Science 600 (2006) 4418–4425
free amorphous film can be obtained. Also in those films that
tent, which causes a decrease in the volume of GBs regions,
the volume of the lower portions of the films decrease and
finally a small compressive stress occurs in these films,
which results in a reduction in the total tensile stress.
were randomly oriented at [SiH ] = 2 sccm and exhibiting
4
tensile stress, the film becomes a nearly stress free film under
high [SiF ]. In films with [SiH ] = 1 sccm, an increase in the
4
4
[
SiF ] from 0 to 0.3 sccm acts to convert their stress from
More reduction in the [SiH ] causes the development of
4
4
compressive to tensile. However, for [SiF ] higher than
crystallization (Fig. 1(b)) and a reduction in the grain size
with an increase in the number of grains (Fig. 3). In these
films, which are more than 80% crystallized, addition of
4
0
.3 sccm, the tensile stress slightly decreases with an increase
in [SiF4].
As shown in Fig. 5(b), for [SiH ] 6 0.5 sccm and
SiF to the feed gases causes the preferentially orientation
4
4
[
SiH ] = 5 sccm, all films have a compressive stress, while
of h110i grains and hence fabrication of low angle GBs.
In these films may be due to low interaction of hydrogen
with silicon or high etching rate of hydrogen by F radicals
(as discussed under Fig. 4), the hydrogen content in the
GBs regions is low. In such GBs regions that have a low
angle and low hydrogen content, the Si atoms should bond
to each other instead of additional hydrogen that is usually
available in the high-angle GBs regions. These Si–Si bonds
may have longer bond length or larger bond angle in com-
parison with normal Si–Si bonds. Formation of such bonds
in the GBs regions around the grains should cause the
stretch of the crystallized Si–Si bonds inside the grains
(Fig. 5(b)). Presence of such stretched Si–Si bonds in the
GBs regions and grains area cause a compressive force
from the substrate to relax these stretched bonds. With a
reduction in the number of grains in the films with
4
under the intermediate values of [SiH ] = 1 to 2 sccm, it
4
changes to a tensile stress. The maximum tensile stress for
films with [SiF ] 5 0 sccm occurs at around [SiH ] =
4
4
1
sccm, which is similar to the d behavior in these films as
seen in Fig. 3(b). Kitahara et al. [22] has reported that in
large grains (d > 500 nm), the tensile stress is accumulated
in the grains and will be relaxed at GBs. The similarity be-
tween the behaviors of the stress and d in the grains with
d > 50 nm, indicates that the model proposed for the stress
of poly-Si by Kitahara et al. [22] can be also applied for
the poly-Si films with d > 50 nm. Furthermore, the accumu-
lated stress in individual grains may change from a tensile
stress to a compressive stress, with a reduction in d (d <
5
0 nm).
The physical origins of intrinsic stress can vary with dif-
ferent factors such as deposition rate, amount of bonded
hydrogen, and the structure of the grains and GBs regions,
which cause the compressive or tensile stress and conse-
quently the relative proportion of these two types of stress
determines the total film stress. It has been reported that a
reduction in bonded hydrogen content causes shrinkage in
the film and consequently tensile stress develops in these
films [23], which means that the compressive stress in these
films reduces [23]. In amorphous films, the inherent nature
of the amorphous silicon network causes a compressive
stress. Moreover, Miura et al. [24] reports that in the initial
stage of crystallization of an amorphous silicon film, the
sign of stress changes from compressive to tensile stress
[SiH ] = 0.2 sccm when compared with the films with
4
[SiH ] = 0.5 sccm (Fig. 3), we expect that the length or an-
4
gle of Si–Si bonds in those GBs regions increase more and
subsequently the compressive stress in these films improves.
In the poly-Si films, the ESR signal mainly arises from
the Si dangling bonds within GBs and amorphous regions
and the value of g reflects the local structure around the Si
dangling bonds through spin–orbit interaction. In this
work, the obtained ESR signal for poly-Si films has a
rather symmetric shape, which indicates that the most of
dangling bonds in these films will arise from GBs regions.
Fig. 6 shows the changes in g values with varying [SiF4]
with different [SiH ] as a parameter. As revealed in Figs.
4
as observed in films with [SiH ] = 2 sccm in the presented
5(a) and 6, in the films with [SiH ] = 5 sccm, which are
4
4
results. Reduction in the stress of films with [SiH ] = 2
completely amorphous, with increasing [SiF ] both the
4
4
and 5 sccm with increasing [SiF ] may be explained in
absolute values of compressive stress and g values mono-
4
terms of changes in their hydrogen contents (Fig. 4)
according to the above mentioned mechanism for the
changes in the tensile stress. However, for films with lower
[
SiH ], changes in the stress cannot be explained by this
4
mechanism. Therefore, the changes in their stress may be
caused by the changes in the grain size and structure of
GBs regions in those films.
In films with [SiH ] = 1 sccm, observation of a reduction
4
in the tensile stress at [SiF ] P 3 sccm with a reduction in
4
the hydrogen content in these films (Fig. 4) may be ex-
plained as follow. In poly-Si films, the h110i texture is
associated with columnar grains [5]. In these films, accord-
ing to the proposed model of h110i grain growth by Ka-
miya et al. [25], growth of large columnar grains results
in a greater distribution of GBs in the lower portion of
the film. As a result, with a reduction in the hydrogen con-
Fig. 6. The value of g factor, obtained by ESR measurement, for films
4 4
with varying [SiF ] from 0 to 0.5 sccm with different [SiH ] values of 0.2,
0.5, 1, 2 and 5 sccm and a fixed temperature of 300 ꢀC.

A.H. Adel et al. / Surface Science 600 (2006) 4418–4425
4423
tonically decrease. However, those for other films show a
the precursors on to the growth surface, they must diffuse
on the surface (surface migration). Since the adsorbed
atoms diffuse, they can desorb or re-evaporate, as they
are only weakly bonded to the surface, or they can encoun-
ter another diffused atom to form an adsorbed atom pair.
This atom pair because of its larger mass, is less likely to
desorb than an individual adsorbed atom. Due to the diffu-
sion on surface and consequently encountering with other
atoms, a larger and more stable cluster can be formed.
The cluster may finally reach a final critical size, beyond
the condition that it is unlikely to desorb, forming a stable
nucleus and further creating a stable Si–Si network. During
these processes, the effective etching of weakly adsorbed
bonds by hydrogen and fluorine atoms can establish the
crystalline structure. In other words, the crystalline silicon
network can be constructed by the processes of adsorption,
surface diffusion and desorption of volatile products.
Under such an assumption for growth of crystallized Si,
an amorphous structure will result when the reaction time
for producing volatile products by etchants is longer than
the effective adsorption time of precursors; i.e., by increas-
ing the arrival rate of silicon atoms under an excess [SiH4]
flux, the structure will tend to be amorphous structure
(Fig. 1). Furthermore, Nakahata et al. [27] shows that un-
complex behavior. At [SiF ] P 0.3 sccm, with an increase
4
in [SiH ] the g-values at first decrease and then increase
4
again. Thus, the g-values is likely to take a minimal at
around [SiH ] = 1 sccm, which resembles the minimal com-
4
pressive stress in these films, as shown in Fig. 5(a). More-
over, changes in the g-value as a function of [SiF ] also
4
show same behavior with the changes in the compressive
stress in these films. This result suggests that there is a close
relationship between occurrence of stress and the type of
defects. Ishii et al. [26] have reported that the changes in
the bond length and specially in the bond angle are effective
on the changes of g-values. In that paper [26], through a
calculation using an extended H u¨ ckel theory, they found
that when a trivalent Si atom bearing the dangling bond
moves vertically toward the plane formed by three backb-
onded Si atoms, the g-value decreases. This result states a
reduction in the g-value with an increase in the intrinsic
tensile stress or the same behavior between the changes in
the g-value and compressive stress. As shown in the poly-
Si and amorphous Si films in Figs. 5(a) and 6, the resultant
stress and the g-values show same behavior with that re-
ported by Ishii et al. [26] with an increase in [SiF ]. The ob-
4
tained results in stress and ESR measurements demonstrate
that the resultant stresses should be due to the changes in
the local structure within the GBs regions. These results
also indicate that the investigation of the changes in g is
a suitable substitute for studying the changes in the intrin-
sic compressive and tensile stress in the both poly-Si and
amorphous films. In those films, independent on the dom-
inant stress, the changes in the g-values reflect, respectively,
same behavior and inverse behavior with changes of intrin-
sic compressive stress and tensile stress. According to these
results in amorphous films, with a reduction in the com-
pressive stress, the g-value also decreases from 2.0055 to
der excess [SiH ], the proportion of fluorine-related deposi-
4
tion precursors are reduced and the major deposition
precursors change from SiH F to SiH , which indicates
n
m
n
that the fluorine-related species are effective for growing
crystalline silicon.
Those deposition conditions, such as low deposition
rate, which allow adsorbed silicon atoms to diffuse farther
on the surface before being immobilized by subsequently
arriving silicon atoms, would lead to larger grains. There-
fore, an increase in d values due to a reduction in [SiH4]
from 5 sccm to 1 sccm (Fig. 3(b)) should be caused by a de-
crease in the deposition rates with a decrease in the [SiH4]
in these films (Fig. 1(a)). In the SiH /SiF system, in which
2
.0047 in the nearly stress free films.
4
4
4
. Discussion
hydrogen atoms are obtained from the decomposition of
SiH radicals, a further decrease in [SiH ] may result in
4
4
In the presented work, the physics of PECVD poly-Si
lower hydrogen content in the plasma region. Conse-
quently, we expect this phenomenon to lead a reduction
in the H coverage over the growing surface. Thus, the dif-
fusion length reduces and adsorbed silicon will easily find a
suitable position to form atomic layers, leading to an in-
crease in the nucleation rate in these films [29]. According
to the following equation
films using the SiH /SiF system was investigated. With a
4
4
decrease in [SiH ], the deposition rate reduces, while the q
4
values, which were obtained from a change in the Raman
spectral intensity, increases. A reduction in [SiH ] from
4
5
sccm to 1 sccm, causes an increase in both the d and
XRD intensity in h110i direction. A further decrease in
SiH ] below 1 sccm leads to a reduction in d, however the
[
4
1
decrease in the XRD intensity starts at [SiH ] = 0.5 sccm
d ꢁ pffiffiffiffiffiffi
4
N
n
(
Fig. 3).
To explain the above-mentioned discrepancy in the peak
the grain size, d, is related to the initial density of nuclei, N
[5]. This equation shows that with an increase in the nucle-
n
position of the XRD intensity and d in the h110i direction,
we should at first have a short over view on the mechanism
of growth of poly-Si films. The plasma diagnosis investiga-
tions show that the major growth precursors in a SiH /SiF
ation rate, d decreases [30], as was observed in films with
[SiH ] < 1 sccm.
4
These findings indicate that in poly-Si films, high H cov-
erage over the growing surface would enhance the surface
migration of adsorbates and leads to an increase in d, while
low H coverage cause an increase in nucleation rate and a
4
4
system are SiH F (n + m 6 3) [27,28]. The precursor for-
n
m
mation may be enhanced as the deposition rate increases
with increasing [SiH ] values, (Fig. 1(a)). After arrival of
4

4424
A.H. Adel et al. / Surface Science 600 (2006) 4418–4425
decrease in d. However, the Raman results demonstrate that
in as deposited poly-Si films prepared with the PECVD
system which have a limitation in the magnitude of grains,
films consist of higher number of small grains are more crys-
tallized than films with low number of large grains. In other
words, an increase in the nucleation rate under optimum
condition should be more important than an increase in
the grain size in the enhancement of the crystallization ratio
as shown in Figs. 1(b) and 3 [31].
gions, demonstrates that the structure of GBs regions
should be effective in the growth direction of grains.
5. Conclusion
The structural properties of the PECVD poly-Si films
deposited with different [SiH ] and [SiF ] were investigated.
4
4
The high degree orientation of the grains in the h110i direc-
tion were observed in film with [SiH ]/[SiF ] = 0.5 sccm/
4
4
As discussed before, it is well known that in a such sys-
tem the concentration of F radicals that reach the growing
surface of the film and etch the undesirable atoms and weak
Si–Si bonds is an important factor in the crystal growth.
The binding energies related to film growth with A–B bonds
can be obtained as E(A–B) as follows: E(H–F) = 5.8 eV,
E(Si–F) = 5.6 eV, E(Si–H) = 3.1 eV and E(Si–Si) = 1.8 eV
0.5 sccm, while the largest grain size (120 nm) was obtained
for film under [SiH ]/[SiF ] = 1 sccm/0.5 sccm. The high
4
4
flow rate of [SiH ] = 5 sccm, resulted in an amorphous Si
4
film and no films with the desired thickness were obtained
at [SiH ]/[SiF ] = 0.1 sccm/0.5 sccm. According to these
4
4
structural changes in the deposited poly-Si films using a
SiH /SiF mixture, the following results were obtained.
4
4
[
32]. Formation of H–F bonds as the more favorable bond
The H coverage, which depends on the flux of both H
and F radicals in the plasma region, can affect the grain size
and number of grains. This study demonstrates that
although a high hydrogen coverage over the growing sur-
face is necessary to obtain a large grain, but a low H cov-
erage on the growing surface and under optimum condition
could be more effective in increasing the crystallized region
and the degree of preferential orientation of poly-Si films.
Moreover, further investigation shows that the change in
in the plasma regions is an effective parameter for the con-
trol of the density of F atoms arriving on the growing sur-
face. Therefore, in the films with the same [SiH ], an
4
increase in the [SiF ] should cause an enhancement in the
4
etching effect by F and consequently both the IXRD(h110i)
and d(h110i) values can be increased as seen in Fig. 3.
According to the presented binding energies, after the H–
F bond, formation of Si–F bond has the highest possibility.
In some conditions such as very low density of H radicals,
formation of H–F bonds as a volatile product decreases
and consequently in the absence of H–F bonds, formation
[SiH ] is effective in the nucleation, crystallization and size
4
of grains, while [SiF ] is effective in the changes of GBs re-
4
gions. The studies on the intrinsic stress indicate that in
amorphous and high-angle grain boundary regions, addi-
of Si–F bonds increases. Thus, in films with [SiH ] =
4
0
.2 sccm with the high probability of Si–F bond formation,
tion of high [SiF ] results in the nearly stress free films.
4
desorption of Si–F bonds as a volatile gas results in a
reduction in the desirable Si atoms for the nucleation. In
turn, in these films with a small d and lower number of
grains, the XRD intensities will also decrease. Our unsuc-
cessful experience in the achievement of poly-Si films with
our desired thickness under the condition of low
Acknowledgements
The authors wish to thank Professors M. Kumeda and
A. Morimoto for use of the X-ray diffractometer and
Raman spectrometer. This work was partially supported
by a Grant-in-Aid for Scientific Research (B) from the
Ministry of Education, Sports, Culture, Science and Tech-
nology in the Japanese Government.
[
SiH ] = 0.1 sccm and high [SiF ] = 0.5 sccm, may confirm
4 4
this assumption.
The above-mentioned results indicate that although the
F radicals are an etchant with higher ability when com-
pared with the H etchant, the amount of H in the plasma
region and its reaction on the growing surface is much
more important factor in the crystallization processes in
the SiH /SiF system. This investigation show that the
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