Atomic structure of the nanocrystalline Si particles appearing in nanostructured Si thin films produced in low-temperature radiofrequency plasmas

TLDR: In this article, a Si face-cubic-centered structure is demonstrated in nanocrystalline particles produced in low-pressure silane plasma at room temperature, and the existence of a well-defined crystalline structure different from the diamond-like structure of Si is discussed.

Abstract: Nanostructured Si thin films, also referred as polymorphous, were grown by plasma-enhanced chemical vapor deposition. The term “polymorphous” is used to define silicon material that consists of a two-phase mixture of amorphous and ordered Si. The plasma conditions were set to obtain Si thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful analysis by electron transmission microscopy and electron diffraction is reported with the aim to clarify the specific atomic structure of the nanocrystalline particles embedded in the films. Whatever the plasma conditions, the electron diffraction images always revealed the existence of a well-defined crystalline structure different from the diamondlike structure of Si. The formation of nanocrystallinelike films at low temperature is discussed. A Si face-cubic-centered structure is demonstrated here in nanocrystalline particles produced in low-pressure silane plasma at room temperature.

Figures

FIG. 6. HRTEM images of the samples~a! ns-Si~A!, ~b! ns-Si~B!, and ~c! ns-Si~C!. Some of the ordered domains embedded in the amorphous m are surrounded by a white line.

FIG. 7. Simulated 3D structures,~a! fcc projected on the@110# direction with a53.9 Å, ~b! diamond projected on the@110# direction with a 55.4 Å, ~c! hexagonal projected on the@0 0 1# direction witha52.46 Å, ~d! hcp projected on the@001# direction witha53.8 Å.

TABLE I. Samples of nanostructured films and free-standing nanoparticles of Si obtained using different plasma conditions and different plasmator .

FIG. 9. HRTEM image~5.5 nm width! of a hcp structure for Si~Ref. 37! and the 3D simulated structure projected along the@001# direction with a 53.8 Å.

FIG. 12. Detailed analysis of the SAED image of the ns-Si~C! sample. The diffraction rings of the fcc structure are highlighted by black circumf ences. The white arrows point out diffraction spots coming from the d mond symmetry.

FIG. 11. SAED images of the nanostructured Si samples reported in experimental section. For comparison, the SAED images of amorphous nanocrystalline diamondlike silicon are shown in the bottom.

Full text

Atomic structure of the nanocrystalline Si particles appearing
in nanostructured Si thin films produced in low-temperature
radiofrequency plasmas
G. Viera and M. Mikikian
Groupe de Recherche sur l’Energe
´
tique des Milieux Ionise
´
s (GREMI), BP 6744, Universite
´
d’Orle
´
ans,
45067 Orle
´
ans, Cedex 02, France
E. Bertran
FEMAN, Departament Fı
´
sica Aplicada i O
`
ptica, Universitat de Barcelona. Avgda. Diagonal, 647, E08028
Barcelona, Spain
P. Roca i Cabarrocas
Laboratoire de Physique des Interfaces et des Couches Minces. (CNRS, UMR7647), Ecole Polytechnique,
91128 Palaiseau, Cedex France
L. Boufendi
a)
Groupe de Recherche sur l’Energe
´
tique des Milieux Ionise
´
s (GREMI), BP 6744, Universite
´
d’Orle
´
ans,
45067 Orle
´
ans, Cedex 02, France
共Received 15 July 2002; accepted for publication 17 July 2002兲
Nanostructured Si thin films, also referred as polymorphous, were grown by plasma-enhanced
chemical vapor deposition. The term ‘‘polymorphous’’is used to define silicon material that consists
of a two-phase mixture of amorphous and ordered Si. The plasma conditions were set to obtain Si
thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful
analysis by electron transmission microscopy and electron diffraction is reported with the aim to
clarify the specific atomic structure of the nanocrystalline particles embedded in the films. Whatever
the plasma conditions, the electron diffraction images always revealed the existence of a
well-defined crystalline structure different from the diamondlike structure of Si. The formation of
nanocrystallinelike films at low temperature is discussed. A Si face-cubic-centered structure is
demonstrated here in nanocrystalline particles produced in low-pressure silane plasma at room
temperature. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1506382兴
I. INTRODUCTION
The generation of powder in silane-based radiofrequency
共rf兲 low-pressure plasmas has attracted a great interest in the
last decade.
1–4
The initial studies were driven by the con-
tamination effects of powder during the preparation of amor-
phous silicon 共a-Si:H兲 thin films by plasma enhanced chemi-
cal vapor deposition 共PECVD兲. It is widely recognized that
the formation of powder in PECVD is originated by gas-
phase polymerization reactions.
5–7
The long residence time
of the powder particles in the plasma, electrically confined
by the plasma sheaths, favors their further growth, first by
agglomeration of small particles and then by deposition of
neutral and ion radicals on their surface.
8–10
The formation
of powder is known to be reduced by decreasing either the
reactive gas pressure or the electrical power supplied to the
discharge, or by increasing the substrate temperature. How-
ever, this often leads to a limitation of the film deposition
rate, which is contrary to the industrial requirements of mak-
ing silicon-based devices with high deposition rate and film
qualities.
The square wave modulation 共SQWM兲 of the rf power
amplitude applied to the plasma has been revealed as a suit-
able method for inhibiting powder formation.
5,11–13
Plasma
modulation consists of alternating periods of plasma-on time
(T
on
) with afterglow periods (T
off
). For any given set of
plasma conditions, it is possible to find a plasma modulation
that inhibits the appearance of powder particles: T
on
is set
short enough so that no particles are formed, and T
off
is
chosen long enough to completely extinct the anion popula-
tion in order to avoid their further polymerization in the next
T
on
. Therefore, modulated discharges allow to use high rf
powers and moderate substrate temperatures while avoiding
the formation of powder particles.
Recently, however, silicon thin films grown, with a sig-
nificant contribution of nanoparticles coming from the
plasma, have been found to exhibit improved properties of
transport and stability and high optical gap as compared to
a-Si:H.
14–17
The knowledge of the powder formation path-
way and the use of modulated rf plasma have permitted anew
the selective incorporation of nanocrystallites of few nanom-
eters into the growing thin film.
18,19
These films have been
described as a mixture of amorphous and ordered material
and called as polymorphous Si 共pm-Si:H兲
14–16
or as nano-
structured Si 共ns-Si:H兲.
20–22
It has been claimed than the un-
usual structure of these films, dominated by the ordered
a兲
Author to whom correspondence should be addressed; electronic address:
laifa.boufendi@univ-orleans.fr
JOURNAL OF APPLIED PHYSICS VOLUME 92, NUMBER 8 15 OCTOBER 2002
46840021-8979/2002/92(8)/4684/11/$19.00 © 2002 American Institute of Physics
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structure of the Si nanoparticles embedded therein, is respon-
sible for the unusual properties of pm-Si:H. However, there
is no clear picture on the atomic structure of these Si nano-
particles or clusters of ⬃ 2 nm. This question will be dis-
cussed in this article from results of high-resolution trans-
mission electron microscopy 共HRTEM兲 and selected area
electron diffraction 共SAED兲.
In the following, an overview of the atomic structure of
Si, both in the amorphous and crystalline phase, is presented.
The atomic structure of microcrystalline Si 共
␮
c-Si兲 con-
sists of Si ordered domains with the diamond crystal struc-
ture. The diamond lattice is formed by two interpenetrating
face-centered-cubic 共fcc兲 lattices, displaced along the body
diagonal of the cubic cell by
1
4
the diagonal length. Each Si
atom is surrounded by four near neighbors forming a tetra-
hedron 共the coordination number is 4兲. The unit cell contains
8 atoms and the lattice constant a is 5.4282 Å. The diamond
structure is the less dense of the different phases that Si can
attain when subjected to compression.
For crystalline domains of a fraction of a micron, the
films are referred to as polycrystalline Si 共pc-Si兲. In previous
works, Veprek et al.
23
reported on diamond-structured nano-
crystals formed in different chemistries and claimed that 3
nm represents the lower limit size for their stability. By tak-
ing into account that the number density of Si atoms in the
diamond lattice is ⬃5⫻ 10
28
at m
⫺ 3
共calculated from the
quotient between the number of atoms of the unit cell and its
volume兲 and by considering Si spheres of 30 Å, the total
number of atoms is around 700. This is, therefore, the mini-
mum number of atoms necessary to form a Si diamondlike
crystal thermodynamically stable. However, as we will dis-
cuss in this article, Si crystallites smaller that 3 nm, but with
atomic structures different from the diamond lattice, have
been experimentally observed.
20
The amorphous structure of Si is described as a disor-
dered lattice of atoms, bonded with tetrahedral coordination,
and with a small distortion both in bond length and angle
when compared to diamond crystalline Si. The short-range
order reaches only the first and second neighbors. The dis-
tance to first neighbors is the same as in crystalline Si and
equal to 2.35 Å 共with a distortion of 1%–3%兲, and the dis-
tance to second neighbors is 3.5 Å 共with a distortion larger
than 10%兲.
Ordered domains of Si of a few nanometers are normally
referred to as Si clusters, rather than small crystallites. Al-
though it is recognized that the most stable structure of small
Si clusters of some ten of atoms is not the diamond
structure,
24
there is no general agreement on the structure of
the Si nanocrystallites, which are only a few nanometers in
size. Different kinds of structures, such as cage-core or clath-
rate structures can be found in the literature.
25–29
This article will highlight that the Si ordered structures
of 1–5 nm formed in SiH
4
-based rf plasmas present an
atomic structure with well-defined crystalline geometry dif-
ferent from those known for Si clusters and for stable bulk
Si.
II. EXPERIMENT
A. Sample preparation
For the synthesis of nanoparticles containing Si thin
films by PECVD, the range of plasma conditions should be
different from those usually adopted for a-Si:H thin film
deposition. These films can be obtained using a wide range
of plasma conditions 关temperature, rf 共13.56 MHz兲 power
and modulation, gas pressure and SiH
4
gas dilution兴 but al-
ways close to the formation of powder in order to allow the
formation of Si nanoparticles in the discharge. In this study,
samples obtained in different plasma reactors and using dif-
ferent plasma conditions have been considered. This allows
to avoid any spurious effect coming from a particular plasma
reactor setup. In addition, the structure of nanoparticles em-
bedded in the matrix of thin films or deposited directly on a
suitable TEM grid will be analyzed. Therefore, the
nanoparticle/matrix interface will be taken into account. In
the following, the different plasma conditions are presented
and summarized in Table I. From here, we will use the term
nanostructured Si 共ns-Si兲 to group all nanoparticle-
containing thin films.
ns-Si„A…: pm-Si:H thin films from continuous-wave
共cw兲 rf plasmas under conditions of very low particle devel-
opment 共low deposition temperature, high H
2
dilution, low
pressure and low rf power兲.
20,21
The plasma reactor is a
grounded cylindrical box, with two parallel electrodes of 15
cm diameter, 2.8 cm apart.
30
The gas mixture is injected
from the back of the rf electrode 共cathode兲, confined by the
plasma box, and is flowed out through the edges of the
sample plate located on the grounded electrode 共anode兲. The
process temperature is related to the substrate temperature.
TABLE I. Samples of nanostructured films and free-standing nanoparticles of Si obtained using different plasma conditions and different plasma reactors.
Sample SiH
4
共sccm兲 inert gas 共sccm兲 T 共°C兲 p 共Pa兲
P
INC
(mW/cm
2
) T
on
(s) T
off
(s) n° cycles
ns-Si 共A兲
a
1 30 sccm H
2
100 6 10 cW
ns-Si 共B兲
a
2% SiH
4
in H
2
200 160 110 cW
ns-Si 共C兲
b
1.2 30 sccm Ar 25–150 12 60 0.1–5 19
ns-Si 共D兲
a
1 30 sccm Ar RT 20 175 1 19 10
ns-Si 共E兲
c
7 133 sccm Ar RT 30 500 5 15 10
a
Thin films and nanoparticles obtained in the laboratory—Laboratoire de Physique des Interfaces et des Couches Minces, Ecole Polytechnique, Palaiseau
共France兲.
b
Thin films deposited in the laboratory—Groupe de Recherche sur l’Energe
´
tique des Milieux Ionise
´
s 共GREMI兲, Universite
´
d’Orle
´
ans, Orle
´
ans 共France兲.
c
Nanoparticles obtained in the laboratory—Grup de Fisica i Enginyeria de materials amorf i nanostructurats 共FEMAN兲, Dep. Fisica Aplicada i Optica,
Universitat de Barcelona 共Spain兲.
4685J. Appl. Phys., Vol. 92, No. 8, 15 October 2002 Viera
et al.
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The process parameters were optimized to be just before
the onset of the formation of powder, thus allowing the for-
mation of Si nanoparticles of 1–3 nm in the plasma, but not
larger powder particles.
18
These nanoparticles are not elec-
trostatically confined by the plasma sheaths; they can leave
the plasma and thus contribute to film growth in a cw dis-
charges.
ns-Si„B…: pm-Si:H thin films from cw rf plasmas, high
deposition temperature, high H
2
dilution, high pressure, and
high rf power.
31
The plasma setup was the same than for the
sample ns-Si共A兲.
ns-Si„C…: pm-Si:H thin films from square-wave modu-
lated 共SQWM兲 rf plasmas using different plasma-on time,
different gas temperature, high Ar dilution, low pressure and
moderate rf power.
19
The plasma reactor is a grounded cy-
lindrical box, with two parallel electrodes of 13 cm diameter,
3.5 cm apart.
32
The gas mixture is shower-like injected, using
a showerhead cathode and it is flowed out through the bot-
tom of the box that is closed with a 20% transparency grid.
Samples are located on the anode. Plasma box is surrounded
by a cylindrical oven that allows the gas temperature to be
varied from room temperature to 200 °C. The gas tempera-
ture is measured in the gas flow below the bottom grid by
means of a J-type thermocouple.
The Si films were deposited in dusty plasma conditions,
i.e., in the presence of powder particles in the plasma gas
phase. Plasma modulation and gas temperature were changed
to control powder development pathway. These experimental
conditions were adapted from previous particle generation
studies done by laser light scattering and laser induced par-
ticle explosive evaporation.
5,32
ns-Si„D…: Free-standing nanocrystalline Si particles from
SQWM rf plasmas, room temperature, high Ar dilution,
moderate pressure, and high rf power .
18,22
The plasma reac-
tor setup is similar to that described for ns-Si共A兲, but the
parallel electrodes were 12 cm diameter, 3.7 cm apart.
The plasma conditions were adapted from the studies
referenced for the ns-Si共C兲 sample and from ex situ TEM
studies.
18
For TEM analysis, particles were directly collected
using suitable grids placed on the base of plasma box, onto
which powder particles fell down during the plasma-off pe-
riods. The process was maintained for a few number of
modulation cycles. This limits both the formation of an ex-
cessive amount of particles and the deposition of a thin film
that would make the characterization of the particles diffi-
cult.
ns-Si„E…: Free-standing nanocrystalline Si particles us-
ing similar plasma conditions than for ns-Si 共D兲, but with
different reactor geometry.
33
The plasma reactor is a
grounded square box, with two parallel electrodes of 20 cm
diameter, 9 cm apart. The gas mixture is injected through an
edge of the anode, flows parallel to the electrodes, and is
evacuated through an outlet seam located on the opposite
edge of the anode. This configuration allows a laminar flow
to be piped on the samples. The process temperature is re-
lated to the substrate temperature.
The plasma parameters have been adapted, for particle
formation in Ar-diluted SiH
4
plasmas, on the basis of previ-
ous ex situ TEM studies on particle growth in pure SiH
4
rf
discharges in order to attain similar particle population in
both cases.
B. Sample characterization
HRTEM and TEM images as well as SAED patterns
were obtained with a Philips CM30 microscope working at
300 kV. When nanoparticles were analyzed, the TEM grids
used to collect them inside the plasma reactor had a holey
membrane 共allowing HRTEM and SAED images to be done兲
which was covered by a thin carbon layer to avoid particles
charging during electron beam irradiation. When thin films
were analyzed, an ex situ sample preparation was required.
The samples were prepared for cross-section observation us-
ing the conventional thinning method: first they are mechani-
cally polished using abrasive materials and finally thinned
with ion milling. The magnification of the HRTEM images,
used to calculate structural characteristics of the films, was
verified from measurements on the c-Si substrate oriented
along
具
110
典
by knowing that the interplanar distance of the
兵111其 faces is 3.14 Å.
III. RESULTS AND DISCUSSION
A. Conventional TEM analysis
Figure 1 shows TEM and SAED micrographs of the ns-
Si共A兲 sample. The interface with the substrate corresponds to
an a-Si:H layer of 0.8
␮
m thick 共layer A兲. The following
layers 共layers B and C兲 result from the incorporation of small
Si nanoparticles of few nanometers, which can contribute to
film deposition during cw rf discharges. These nanoparticles
represent the first population of particles appearing in the
plasma gas phase before the onset of coagulation.
8–10
Due to
their very small size 共1–2 nm兲 they experience charge fluc-
tuations and consequently when a neutral 共or positive兲 state
occurs they are not electrostatically confined by the plasma
sheaths.
34
At this stage they can leave the plasma and con-
tribute to film deposition. Dark field and SAED images were
taken for each individual layer 共central and right images in
Fig. 1兲. The SAED of the first layer contains the diffuse rings
FIG. 1. Cross-sectional bright field TEM images 共left兲, SAED patterns 共cen-
ter兲, and the corresponding dark-field images 共right兲 of a ns-Si共A兲. The first
layer over the substrate 共A兲 corresponds to an amorphous Si thin film. The
layers 共B兲 and 共C兲 were grown by the simultaneous deposition of Si radicals
and nanoparticles 共conditions reported in Sec. A of the experimental part兲.In
the SAED image of layers 共B兲 and 共C兲, a sharp diffraction ring is pointed out
by an arrow.
4686 J. Appl. Phys., Vol. 92, No. 8, 15 October 2002 Viera
et al.
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characteristics of amorphous Si. The corresponding dark
field image appears uniformly lighted, thus confirming the
amorphous character of the layer. However, for the subse-
quent layers, a careful inspection of the corresponding SAED
images revealed the existence of sharp rings 共the most in-
tense ring is pointed out by an arrow in the figure兲 superim-
posed on diffused rings, thus indicating the presence of or-
dered structures in an amorphous matrix. As it will be
discussed in Sec. III C, such a SAED pattern is different
from that of the diamondlike structure of crystalline Si. The
dark-field images of the layers 共B兲 and 共C兲 clearly reveal the
presence of nanocrystalline regions, corresponding to the
specular reflections in dark field. Direct measurements on
high-resolution TEM micrographs, not shown here, indicate
a crystallite size of about 1–2 nm. In order to determine the
size of the crystallites from the dark-field images, we have
used image processing and analysis software to identify the
bright points and to extract their features 共quantity, area, pe-
rimeter, roundness, etc.兲. By means of this software, the size
histograms were determined.
19
Figure 2 shows another example of nanostructured Si
thin film but with a higher concentration of ordered domains,
which is evident from its dark-field and SAED images. This
film corresponds to a ns-Si共C兲 sample using T
ON
⫽ 5 s and
T
G
⫽ 100 °C. The plasma conditions used here were chosen
to attain powder formation contrary to the previous case
关ns:Si共A兲 film shown in Fig. 1兴. The selective incorporation
of nanoparticles into the growing film was controlled
through the square-wave modulation of the rf plasma.
19
Dur-
ing the plasma-on time (T
on
) of the modulation cycle, an
amorphous Si film is deposited onto the substrate and, at the
same time, nanoparticles nucleate and grow in the plasma
gas phase. During the afterglow periods (T
off
), the particles
leave the plasma and are deposited onto the amorphous Si
film. Consequently, after a great number of cycles, the final
structure will consist of Si nanoparticles embedded in an
amorphous matrix. In particular, the T
on
used to grow the
ns-Si 共C兲 sample shown here is slightly larger than the char-
acteristic time for particle coagulation, thus explaining the
presence of larger crystallites in the film as observed in Fig.
2.
8,10,19
Of great relevance is that the ns-Si共C兲 sample shown in
Fig. 2 presents the same distribution of sharp rings in the
SAED pattern as the ns-Si共A兲 film analyzed in Fig. 1, in spite
of the very different plasma conditions and reactor geometry.
As it will be carefully discussed in Sec. III C, the indexation
of these electron diffraction patterns highlights a fcc cell. A
similar crystalline structure was found in free-standing Si
nanoparticles formed in modulated SiH
4
–Ar rf plasmas.
22
Figure 3 shows a bright field image and a SAED pattern
of nanocrystalline Si particles 关corresponding to the ns-Si共E兲
sample兴 deposited directly on a TEM grid 共black points in
the TEM figure兲. These particles are spherical, appear iso-
lated and monodispersed in the TEM grid, and have a radius
of about 5 nm. They have a medium-range ordered structure,
as revealed by the SAED image. The indexation of this pat-
tern anew reveals the existence of a fcc structure. This result
is very important, because it proves that the Si nanoparticles
maintain the same atomic arrangement once incorporated in
the film.
The questions which arise now are 共i兲 why the Si nano-
particles formed under such plasma conditions appear as
crystallites?; 共ii兲 how to explain the formation of a fcc crys-
talline structure; and 共iii兲 why this structure has not been
previously detected in these kinds of Si nanoparticles?
The reason why such particles are crystallites is not
straightforward. The formation of powder particles in rf plas-
mas of Ar-diluted SiH
4
is known to be governed by the dis-
charge conditions and the plasma-on duration.
8–10
Particle
development can be divided into three phases: nucleation,
coagulation, and powder growth by molecular sticking. Dur-
ing the nucleation phase 共particles 1–2 nm in size兲, particles
with an ordered atomic structure can be formed. The coagu-
lation of such nanocrystallites 共second stage兲 gives rise to
larger particles. Indeed, the presence of small ordered do-
mains of a few nanometers embedded in bigger powder par-
ticles has been reported since the studies on particle forma-
tion on Ar-diluted SiH
4
discharges.
9
The appearance and development of the first particles
occurring in rf plasmas have been studied ‘‘in situ’’ by mass
spectrometry. These studies emphasized an evolution of the
particle structure during its initial growth stage.
35,36
The re-
sults indicated that these particles could not be understood as
silicon cores covered by hydrogen, but as cross-linked struc-
tures. This cross linking was found to increase with the resi-
dence time of the particles inside the plasma. Two different
FIG. 2. Bright field 共left兲 and dark-field 共right兲 TEM images of a nanostruc-
tured Si thin film deposited from modulated rf plasmas of Ar-diluted SiH
4
in
dust-forming conditions 关ns-Si共C兲, with T
ON
⫽ 5 s and T
G
⫽ 100°C]. The
insert in the dark-field image shows the corresponding SAED pattern.
FIG. 3. Bright field TEM image of Si nanoparticles of 10 nm 共black points
in the image兲 obtained from modulated rf plasmas of Ar-diluted SiH
4
关ns-
Si共E兲兴. The insert shows the corresponding SAED. The background image
corresponds to a thin carbon layer covering the membrane of the TEM grid.
4687J. Appl. Phys., Vol. 92, No. 8, 15 October 2002 Viera
et al.
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physical mechanisms could explain this evolution: 共a兲 the
energy supplied by ion and electron bombardment, which is
enhanced in Ar-diluted silane plasmas and 共b兲 the tempera-
ture spike resulting from the collision of two smaller par-
ticles. This is due to the need to accommodate the excess of
surface energy into the bulk. The temperature reached during
coagulation is known to decrease as the particle size in-
creases, and this agrees with the fact that only small particles
appear as crystallites.
The effect of the inert-gas dilution on particle structure
can be analyzed by comparing the particles formed in pure
SiH
4
and in Ar-diluted SiH
4
rf plasmas. The plasma condi-
tions were adjusted to have particles coming from the same
development stage.
33
As compared to the case of Ar-diluted
discharges, particles growing in pure SiH
4
discharges have
higher nucleation rate and faster kinetics of particle develop-
ment. Therefore, to obtain similar particle size and particle
distribution in both discharges, the processing time and the
plasma-on time must be reduced in the pure SiH
4
discharges.
Figure 4 compares TEM micrographs of Si nanoparticles
generated in pure SiH
4
共a兲 and in Ar-diluted SiH
4
共b兲 using
the same plasma conditions but different processing time and
T
on
: 共a兲 pure SiH
4
共12 sccm兲 during 2 cycles with T
on
⫽ 0.05 s, and 共b兲 5% SiH
4
diluted in Ar 共total flow of 140
sccm兲 during 10 cycles with T
on
⫽ 5 s. Both kinds of particles
are spherical, appear isolated and monodispersed in the TEM
grid, and have a radius of about 5 nm. However, the SAED
patterns 关insets in Figs. 4共a兲 and 4共b兲兴 reveal important struc-
tural differences between both samples. The SAED pattern of
the Si nanoparticles grown in pure SiH
4
shows diffuse rings
that must be assigned to an amorphous structure. However,
the nanoparticles obtained in Ar-diluted plasmas exhibit the
fcc ordered structure reported before. This result is very im-
portant because it emphasizes the role of the Ar dilution on
the particle structure as a result of collisions of high ener-
getic plasma species with the particles, which is clearly ab-
sent when nanoparticles are grown from pure SiH
4
plasmas.
Moreover, it is important to notice that the growth rate of
nanoparticles is about 100 times faster in the case of pure
SiH
4
. Thus, the growth kinetic is an important factor to be
considered when obtaining silicon crystallites. As a matter of
fact, no crystalline particles have been observed in pure
SiH
4
.
4
The question which remains now is: what atomic struc-
ture or structures do the particles have?
B. High-resolution TEM analysis
HRTEM images are widely used to characterize the
atomic structure of small crystalline domains. Nanocrystal-
lite size, shape, surface 共crystal/matrix interface兲 and crystal-
line lattice can be determined. However, the analysis and
identification of nanocrystals of few nanometers by this
method is highly arduous, first because crystallites in the
range of 1–2 nm are not easily detected on the images, and
second because the crystallites analyzed must be well ori-
ented along the optical axis. In addition, the number of crys-
talline planes observed in HRTEM images is too small 共just 5
planes for a silicon crystallite of 1.5 nm兲 and may be irregu-
larly distributed due to boundary effects. This, therefore, lim-
its the quantitative information that one can take out by nu-
merical processing of the HRTEM images. Moreover, the
ns-Si samples analyzed here present a low density of ordered
domains 关⬍5% 共Ref. 19兲兴. This makes the Fourier transform
of the images very noisy, since the amorphous matrix hides
the information coming from the nanocrystalline regions.
Therefore, we preferred to compare directly the simulated
patterns to those observed on the HRTEM images.
Figure 5 shows a HRTEM image of an amorphous Si
thin film deposited on a crystalline silicon substrate. The
periodic structure being observed on the micrograph corre-
sponds to the 兵111其 planes of the Si diamond structure, cross-
ing each other at 70°. The image is seen along the 具110典
direction. For the amorphous Si film, the short-range order
reaches only the first and second neighbors and, therefore,
nonperiodicity appears on HRTEM image as observed in Fig.
5.
However, for ns-Si films, a medium-range order is ex-
pected, associated to Si crystallites of 1–5 nm embedded in
the amorphous matrix. Figure 6 shows HRTEM images of
FIG. 4. TEM images of Si nanoparticles obtained 共a兲 from pure SiH
4
dis-
charges, during 2 modulation cycles of T
on
⫽ 0.05 s and 共b兲 from high Ar-
diluted SiH
4
discharges, during 10 cycles of T
on
⫽ 5 s. Inserts are the corre-
sponding SAED patterns.
FIG. 5. Cross-section HRTEM image of an amorphous Si thin film depos-
ited on a substrate of crystalline Si.
4688 J. Appl. Phys., Vol. 92, No. 8, 15 October 2002 Viera
et al.
Downloaded 15 Jun 2010 to 161.116.168.169. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Atomic structure of the nanocrystalline si particles appearing in nanostructured si thin films produced in low-temperature radiofrequency plasmas" ?

Si thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful analysis by electron transmission microscopy and electron diffraction is reported with the aim to clarify the specific atomic structure of the nanocrystalline particles embedded in the films. 

Q2. What is the stable structure of ns-Si?

In addition, ns-Si films are highly hydrogenated21,31 and the saturation of the crystallite surface with hydrogen can also influence the most stable structure. 

Q3. What is the lattice constant for the fcc structure?

For the dense nanostructured thin films with the fcc structure, the lattice constant deduced from the indexation is found to be around 3.65 Å, while for both samples of Si nanoparticles, the lattice constant is around 4.05 Å. 

Q4. How many planes are there in the tHRTEM images?

In addition, the number of crystalline planes observed in HRTEM images is too small ~just 5 planes for a silicon crystallite of 1.5 nm! and may be irregularly distributed due to boundary effects. 

Q5. What was the TEM grid used to collect the nanoparticles inside the plasma reactor?

When nanoparticles were analyzed, the TEM grids used to collect them inside the plasma reactor had a holey membrane ~allowing HRTEM and SAED images to be done! 

Q6. What is the order of the ns-Si crystallites?

for ns-Si films, a medium-range order is expected, associated to Si crystallites of 1–5 nm embedded in the amorphous matrix. 

Q7. What is the structure of the ns-Si C! sample?

after a great number of cycles, the final structure will consist of Si nanoparticles embedded in an amorphous matrix. 

Q8. How was the magnification of the TEM images used to calculate structural characteristics of the films?

The magnification of the HRTEM images, used to calculate structural characteristics of the films, was verified from measurements on the c-Si substrate oriented along ^110& by knowing that the interplanar distance of the $111% faces is 3.14 Å. 

Q9. What is the kinetics of development of the particles in the plasma?

The occurrence of such phases in low-temperature plasma processes has been explained in terms of the particular kinetics of development of the particles in the plasma, and of the stability changes involving size effects. 

Q10. How many ns-Si particles are formed in a plasma?

The plasma parameters have been adapted, for particle formation in Ar-diluted SiH4 plasmas, on the basis of previous ex situ TEM studies on particle growth in pure SiH4 rfDownloaded 15 Jun 2010 to 161.116.168.169. 

Q11. What is the important information about the structure of the diamond?

Although it is accepted that the diamond structure is not the most stable structure for small Si crystallites of a few number of atoms ~, 2 nm!,38 very little information is found in the literature on the particular atomic structure of the nanoparticles created in rf plasmas. 

Q12. How did the authors find the diffraction rings of the nc-Si sample?

In order to identify their atomic structure without ambiguity, the authors have indexed the diffraction rings on the basis of cubic and hexagonal cells.