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

Article abstract of DOI:10.1016/j.susc.2006.04.050

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.2006.04.050

Surface Science 601 (2007) 1429–1436
www.elsevier.com/locate/susc
Roles of SiH4 and SiF4 in growth and structural changes
of poly-Si films
*
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 19 December 2006
Abstract
The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with dif-
ferent flow rates of SiH4/SiF4 mixtures at 300 ꢀC were investigated. This study indicates that the low hydrogen coverage on the growing
surface, 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 SiH4 and SiF4 radicals show that the SiH4 radicals are important in the nucleation
and growth of grains. However, SiF4 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 SiF4 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
silicon 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
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
panel displays [3–5]. The direct deposition of poly-Si films
at a temperature below the strain point on the glass sub-
strates is an attractive and economical technology for pro-
ducing 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 com-
puter technologies 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
*
Corresponding author. Tel.: +81 76 234 4882; fax: +81 76 234 4870.
E-mail address: azadadel@yahoo.com (A. Haddad-Adel).
0039-6028/$ - see front matter ꢁ 2006 Published by Elsevier B.V.
doi:10.1016/j.susc.2006.04.050

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A. Haddad-Adel et al. / Surface Science 601 (2007) 1429–1436
structure, also may enhance the transistor characteristics
[12]. 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 regions of the two grains placing in between the
dislocations are in good match with each other. In such
GBs, 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
differences between low-angle and high-angle GBs can be
summarized as follows:
tion time. The rf power and gas pressure during film
deposition were maintained at 20 W and 33.3 Pa, respec-
tively. The unique feature in the preparation process of
the samples before deposition of the poly-Si films is that
the substrate were sequentially exposed to the hydrogen
and nitrogen plasma for 20 min to make their surface
clean. The plasma exposure also causes the change in
the surface roughness [15], beside the above-mentioned
surface cleaning effects. In a previous paper [15], we have
reported that the presence of a proper degree of the sur-
face roughness improves the crystallinity and increases
the grain size [15].
Polycrystalline Si films were deposited on glass (corning
7059) substrate for measurements of X-ray diffraction
(XRD), Raman scattering and stress. The single h111i
crystal Si substrate with a resistivity higher than 103 X cm
was used for infrared (IR) absorption measurement and
electron spin resonance (ESR). For the ESR measurement
the fused quartz substrate was also used to investigate
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 difference between
the ESR spectra for the samples on both substrates. The
morphology was investigated by XRD measurement. The
degree of preferential orientation in hhkli texture was rep-
resented by X-ray relative intensity, IXRD(hhkli), for a
given texture, which was normalized 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 results 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 spectra by means of the Scherrer
formula.
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 crystalline
Si phase (c-phase) at around 520 cmꢀ1 and an amorphous Si
phase (a-phase) at around 480 cmꢀ1. The third component
between 480 and 520 cmꢀ1, 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
theintensity ratio ofboth the a- andc-phase components [16].
Therefore, the real q values may be somewhat different from
the values shown in the diagram. 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 vibra-
tional absorptions were measured using a Fourier-transform
spectroscope at a normal light incidence.
– 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.
Moreover, in the previous article [13], we reported that
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 the 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 de-
fects 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 SiH4/SiF4 system. In the present work, in
order to enhance the degree of the h110i preferential orien-
tation 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 preferen-
tially orientation of poly-Si films by using different flow
rate of SiH4, [SiH4], and flow rate of SiF4, [SiF4].
2. Experimental details
Polycrystalline Si films were deposited by PECVD sys-
tem at a temperature of 300 ꢀC using SiH4/SiF4 mixtures
in a hot wall type fused quartz reactor, employing the
inductive coupling of rf power at 13.56 MHz. This depo-
sition system has been presented elsewhere [14]. In this
work, a diluted gas of SiF4 with He gas (SiF4: 5%, He:
95%) was used. Therefore, the net flow rate of SiF4 was
changed between 0 and 0.5 sccm under five different con-
ditions of [SiH4] = 0.2, 0.5, 1, 2, and 5 sccm. The film
thickness was kept at 0.5–0.6 lm by varying the deposi-
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:

A. Haddad-Adel et al. / Surface Science 601 (2007) 1429–1436
1431
ꢀ
ꢁ
1
b2d
dL2
r ¼ ðY S=ð1 ꢀ mSÞÞ
3
here, YS is Young’s modulus and mS the Poisson ratio for
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
direction, which respect to another edge. Values of YS =
7.7 · 1010 N/m2 and mS = 0.28 were used for the corning
glass substrate.
Fig. 2. XRD spectra for poly-Si films with
a
fixed value of
[SiF4] = 0.5 sccm with different [SiH4] values of 0.2, 0.5, 1, 2 and 5 sccm
at a fixed temperature of 300 ꢀC.
3. Results
Fig. 1 shows (a) deposition rate, and (b) q values of
poly-Si films as a function of [SiF4] for films prepared with
different [SiH4]. As seen in Fig. 1, changes in the [SiH4] are
more effective than the [SiF4] in the changes of both depo-
sition rate and q. This result means that the main precur-
sors for film growth in this system arise from SiH4. As
shown in Fig. 1(b), a completely amorphous phase is
obtained in the films having [SiH4] = 5 sccm, while the q
values in the samples having [SiH4] < 5 sccm can be
increased with decreasing [SiH4] to 0.5 sccm and reaches
to 90% under the high [SiF4].
Fig. 2 shows the XRD spectra for the poly-Si films
deposited at [SiF4] = 0.5 sccm under different [SiH4] values
as a parameter. In these diagrams an amorphous structure
has been obtained for the film with [SiH4] = 5 sccm, which
is consistent with the Raman results seen in Fig. 1(b).
A reduction in [SiH4] causes crystallized films with the
random structure at [SiH4] = 2 sccm and with a further
decrease in [SiH4] in the range of [SiH4] 6 1 sccm, a
dominant h110i texture can be obtained. However, all
the other crystallized films with [SiF4] = 0 sccm exhibit ran-
dom textures. Thus, formation of a preferred oriented
structure in h110i direction should start at [SiH4] = 1 sccm
in films with [SiF4] > 0 sccm. These results mean that SiF4
plays an important role in creation of dominant h110i tex-
ture, though the degree of these preferred textures is likely
to be controlled by the ratio of SiH4 to SiF4.
In Fig. 3(a) IXRD(h110i) and (b) d(h110i) values are
plotted as a function of [SiH4] with different [SiF4] values
as a parameter. Both the IXRD(h110i) and d(h110i) values
for films with [SiF4] = 0 sccm monotonically decrease with
an increase in the [SiH4], different from the behaviors of
films with other [SiF4] values. According to Fig. 3, the
IXRD(h110i) values for films with [SiF4] P 0.3 sccm have
maximum at [SiH4] = 0.5 sccm, while d(h110i) for these
films have maximum at [SiH4] = 1 sccm, which means that
in the films with [SiH4] < 1, with a reduction in the d(h110i)
the IXRD(h110i) increases. An increase in the intensity with
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 [SiH4] < 1 sccm. Furthermore, Fig. 3 shows that the
IXRD and d values for [SiH4] 6 1 sccm monotonically in-
crease with increasing [SiF4] that acts as etchant.
(a)
(b)
Fig. 1. (a) Deposition rate and (b) the crystalline volume fraction, q, for
poly-Si films with varying [SiF4] from 0 to 0.5 sccm with [SiH4] = 0.2, 0.5,
1, 2, and 5 sccm at a fixed temperature of 300 ꢀC.
Fig. 3. (a) XRD intensity and (b) grain size, d, of the h110i texture as a
function of [SiH4], with open symbols, for films with different [SiF4] values
of 0, 0.3, 0.4 and 0.5 sccm at a fixed temperature of 300 ꢀC.

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A. Haddad-Adel et al. / Surface Science 601 (2007) 1429–1436
Fig. 4. Absorption intensity of Si–H2 bonds at 2100 cmꢀ1 measured by
FT–IR as a function of [SiF4], for films with different [SiH4] values of 0.2,
0.5, 1, 2 and 5 sccm and at a fixed temperature of 300 ꢀC.
Fig. 5. Stress in the poly-Si films as a function of (a) [SiF4] (closed
symbols) and (b) [SiH4] (open symbols). In these diagrams, positive and
negative values of stress denote tensile and compressive stress,
respectively.
Fig. 4 shows the absorption intensity at 2100 cmꢀ1 as a
function of [SiF4] with different [SiH4] values as a parame-
ter. It is well known that the 2000 and 2100 cmꢀ1 peaks
arise from the structure involving Si–H and Si–H2 bonds,
respectively. The Si–H bonds usually exist in the amor-
phous region [18] and the Si–H2 bonds depend on the struc-
ture of Si films can appear on GBs regions of poly-Si films
or on internal surfaces of amorphous silicon films [19]. If
the oscillator strength is fixed, the Si–H2 absorption inten-
sity in Fig. 4 would be proportional to the density of incor-
porated H-related bonds. In the present work, all films
consist of Si–H2 bonds while the Si–H bonds are observed
only in the films with higher [SiH4] values of 1, 2 and
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 [SiH4] values independently of the [SiF4]
values.
The presented results in Fig. 4 shows that in the depos-
ited poly-Si films, addition of SiF4 to SiH4 in the feed gases
results in the both reduction and increase in the H density
in the films with [SiH4] 6 1 sccm and [SiH4] = 2 sccm,
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
of Si–H2 bonds in the GBs regions, two assumptions are
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.
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
means a stretch in film in reference to substrate. The mea-
sured stress, r, is composed of two different types of stres-
ses: one is the intrinsic stress, ri, depending on the growth
process of the film, and another is due to thermal expan-
sion mismatch between the film and the substrate, rt. The
rt values can be given by
rt ¼ ðaF ꢀ aSÞDTY F=ð1 ꢀ mFÞ
where aF and aS are the thermal expansion coefficient for
the deposited film and the substrate, respectively. We used
the value of aS = 5 · 10ꢀ6/K for the corning 7059 glass sub-
strates, which was cited from a catalog. The thermal expan-
sion coefficient for c-Si, (aF), is 3 · 10ꢀ6/K [20], but that for
a-Si will be considerably smaller than this value. The DT
value is assumed as the difference between the deposition
temperature 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 YF/(1 ꢀ mF) for c-Si and a-Si films were
assumed to be 1.8 · 1011 and 4.2 · 1011 N/m2 [21], respec-
tively. Using these values, we expect occurrence of a com-
pressive 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, different from thermal expansion
mismatch.
As shown in Fig. 5(a), changes in the stress are rather
complex. In films with [SiF4] P 0.3 sccm, with an increase
in [SiH4] the stress values are likely to change from com-
pressive to tensile stress and again from tensile to compres-
sive stress. Thus, the tensile stress is likely to take a
maximal at around [SiH4] = 1 sccm. In other words, the
minimal compressive stress is obtained in [SiH4] = 1 sccm.
Changes in the stress results can be summarized as follows.
Fig. 5(a) and (b) shows stress, respectively, as a function
of [SiF4] and [SiH4] with different [SiH4] and [SiF4] values
as their respective parameters. In these diagrams, the stress
as a function of [SiF4] are shown by closed symbols, and as
a function of [SiH4] are represented by open symbols. The

A. Haddad-Adel et al. / Surface Science 601 (2007) 1429–1436
1433
For those crystallized films which exhibit compressive
stress ([SiH4] = 0.2 and 0.5 sccm), an increase in [SiF4]
causes an increase in the absolute value of compressive
stress. However, in amorphous films with [SiH4] = 5 sccm,
addition of SiF4 results in a reduction of compressive stress
and finally under high [SiF4], nearly stress free amorphous
film can be obtained. Also in those films that were ran-
domly oriented at [SiH4] = 2 sccm and exhibiting tensile
stress, the film becomes a nearly stress free film under high
[SiF4]. In films with [SiH4] = 1 sccm, an increase in the
[SiF4] from 0 to 0.3 sccm acts to convert their stress from
compressive to tensile. However, for [SiF4] higher than
0.3 sccm, the tensile stress slightly decreases with an in-
crease in [SiF4].
As shown in Fig. 5(b), for [SiH4] 6 0.5 sccm and
[SiH4] = 5 sccm, all films have a compressive stress, while
under the intermediate values of [SiH4] = 1 to 2 sccm, it
changes to a tensile stress. The maximum tensile stress
for films with [SiF4] 5 0 sccm occurs at around [SiH4] =
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 accumu-
lated in the grains and will be relaxed at GBs. The similar-
ity between the behavior 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
accumulated stress in individual grains may change from
a tensile stress to a compressive stress, with a reduction
in d (d < 50 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
as observed in films with [SiH4] = 2 sccm in the presented
results. Reduction in the stress of films with [SiH4] = 2
and 5 sccm with increasing [SiF4] may be explained in
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
[SiH4], changes in the stress cannot be explained by this
mechanism. Therefore, the changes in their stress may be
caused by the changes in the grain size and structure of
GBs regions in these films.
plained as follow. In the poly-Si films, the h110i texture
is associated with columnar grains [5]. According to the
proposed model of h110i grain growth by Kamiya 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 content, 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 value of their tensile stress.
More reduction in the [SiH4] causes the development of
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
SiF4 to the feed gases causes the preferentially orientation
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 atoms that are
usually available in the high-angle GBs regions. These Si–
Si bonds may have longer bond length or larger bond angle
in comparison 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
[SiH4] = 0.2 sccm when compared with the films with
[SiH4] = 0.5 sccm (Fig. 3), we expect that the length or an-
gle of Si–Si bonds in these 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 [SiH4] as a parameter. As revealed in Figs.
5(a) and 6, in the films with [SiH4] = 5 sccm, which are
completely amorphous, with increasing [SiF4] both the
absolute values of compressive stress and g values mono-
tonically decrease. However, those for other films show a
complex behavior. At [SiF4] P 0.3 sccm, with an increase
in [SiH4] the g-values at first decrease and then increase
again. Thus, the g-values is likely to take a minimal at
around [SiH4] = 1 sccm, which resembles the minimal com-
pressive stress in these films, as shown in Fig. 5(a). More-
over, changes in the g-value as a function of [SiF4] also
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
In films with [SiH4] = 1 sccm, observation of a reduction
in the tensile stress at [SiF4] P 3 sccm with a reduction in
the hydrogen content in these films (Fig. 4) may be ex-

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A. Haddad-Adel et al. / Surface Science 601 (2007) 1429–1436
we should at first have a short over view on the growth
mechanism of poly-Si films. The plasma diagnosis investi-
gations show that the major growth precursors in a SiH4/
SiF4 system are SiHnFm (n + m 6 3) [27,28]. The precursor
formation may be enhanced as the deposition rate increases
with increasing [SiH4] values, (Fig. 1(a)). After arrival of
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. Un-
der 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-
der excess [SiH4], the proportion of fluorine-related deposi-
tion precursors are reduced and the major deposition
precursors change from SiHnFm to SiHn, which indicates
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 SiH4/SiF4 system, in which
hydrogen atoms are obtained from the decomposition of
SiH4 radicals, a further decrease in [SiH4] may result in
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
Fig. 6. The value of g factor, obtained by ESR measurement, for films
with varying [SiF4] from 0 to 0.5 sccm with different [SiH4] values of 0.2,
0.5, 1, 2 and 5 sccm at a fixed temperature of 300 ꢀC.
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 calcu-
lation using an extended Huckel theory, they found that
¨
when a trivalent Si atom bearing the dangling bond moves
vertically toward the plane formed by three backbonded 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 amor-
phous Si films in Figs. 5(a) and 6, the resultant stress and
the g-values show same behavior with that reported by Ishii
et al. [26] with an increase in [SiF4]. The obtained results in
stress and ESR measurements demonstrate that the resul-
tant stresses should be due to the changes in the local struc-
ture 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 intrinsic compressive and
tensile stress in the both poly-Si and amorphous films.
In these films, independent on the dominant stress, the
changes in the g-values reflect, respectively, same behavior
and inverse behavior with changes of intrinsic compressive
stress and tensile stress. According to these results in amor-
phous films, with a reduction in the compressive stress, the
g-value also decreases from 2.0055 to 2.0047 in the nearly
stress free films.
4. Discussion
In the presented work, the physics of PECVD poly-Si
films using the SiH4/SiF4 system was investigated. With a
decrease in [SiH4], the deposition rate reduces, while the
q values, which were obtained from a change in the Raman
spectral intensity, increase. A reduction in [SiH4] from
5 sccm to 1 sccm, causes an increase in both the d and
XRD intensity in h110i direction. A further decrease in
[SiH4] below 1 sccm leads to a reduction in d, however
the decrease in the XRD intensity starts at [SiH4] =
0.5 sccm (Fig. 3).
1
pffiffiffiffiffiffi
d ꢁ
Nn
To explain the above-mentioned discrepancy in the peak
position of the XRD intensity and d in the h110i direction,
the grain size, d, is related to the initial density of nuclei, Nn
[5]. This equation shows that with an increase in the nucle-

A. Haddad-Adel et al. / Surface Science 601 (2007) 1429–1436
1435
ation rate, d decreases [30], as was observed in films with
[SiH4] < 1 sccm.
in the low GBs regions with low H content. According to
the presented results by Raman, XRD, FT–IR, stress and
ESR measurements, in the poly-Si films addition of SiH4
is more effective in the growth and crystallization of Si
films, while addition of SiF4 may cause changes in the
GBs regions. Enhancement in the preferentially orientation
of grains by increasing in [SiF4], which changes GBs re-
gions, demonstrates that the structure of GBs regions
should be effective in the growth direction of grains.
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
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 crystallized 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 crys-
tallization ratio of the deposited poly-Si films in PECVD
system, as shown in Figs. 1(b) and 3 [31].
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 [32]. Formation of H–F bonds as the
more favorable bond in the plasma regions is an effective
parameter for the control of the density of F atoms arriving
on the growing surface. Therefore, in the films with the
same [SiH4], an increase in the [SiF4] should cause an
enhancement in the 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 ener-
gies, 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 of Si–F bonds increases. Thus, in films
with [SiH4] = 0.2 sccm with the high probability of Si–F
bond formation, 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 unsuccessful experience in the achievement of poly-Si
film with our desired thickness under the condition of
low [SiH4] = 0.1 sccm and high [SiF4] = 0.5 sccm, may con-
firm this assumption.
5. Conclusion
The structural properties of the PECVD poly-Si films
deposited with different [SiH4] and [SiF4] were investigated.
The high degree orientation of the grains in the h110i
direction were observed in film with [SiH4]/[SiF4] =
0.5 sccm/0.5 sccm, while the largest grain size (120 nm)
was obtained for film under [SiH4]/[SiF4] = 1 sccm/
0.5 sccm. The high flow rate of [SiH4] = 5 sccm, resulted
in an amorphous Si film and no film with the desired thick-
ness was obtained at [SiH4]/[SiF4] = 0.1 sccm/0.5 sccm.
According to these structural changes in the deposited
poly-Si films using a SiH4/SiF4 mixture, the following
results were obtained.
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
[SiH4] is effective in the nucleation, crystallization and size
of grains, while [SiF4] is effective in the changes of GBs re-
gions. The studies on the intrinsic stress indicate that in
amorphous and high-angle grain boundary regions, addi-
tion of high [SiF4] results in the nearly stress free films.
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.
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 SiH4/SiF4 system. This investigation show that the
amount of [SiH4], which determine the arrival rate of main
precursors on the growing surface, can be considered as a
main factor in the change of structure from amorphous
to highly crystallized film. However, addition of SiF4 is also
necessary for changing structure from random to preferen-
tially growth of grains in the h110i direction, which results
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