Silicon nanocrystals by thermal annealing of Si-rich silicon oxide prepared by the LPCVD method

Article abstract of DOI:10.1016/j.molstruc.2006.09.036

The Si-rich silicon oxide (SiO_x) thin films are prepared on silicon crystalline substrates by low pressure chemical vapor deposition (LPCVD) method. The oxygen concentration x are controlled by the ratio of the partial pressures of N₂

Full text of DOI:10.1016/j.molstruc.2006.09.036

Journal of Molecular Structure 834–836 (2007) 461–464
www.elsevier.com/locate/molstruc
Silicon nanocrystals by thermal annealing of Si-rich silicon
oxide prepared by the LPCVD method
a,
a
a
a
a
a
b
*
ˇ
´
´
´
´
M. Ivanda , H. Gebavi , D. Ristic , K. Furic , S. Music , M. Ristic , S. Zonja ,
b
c
c
d
e
f
´
P. Biljanovic , O. Gamulin , M. Balarin , M. Montagna , M. Ferarri , G.C. Righini
a
-
Ruder Bosˇkovic´ Institute, P.O. Box 180, 10002 Zagreb, Croatia
Faculty for Electrotechnic and Computing, University of Zagreb, Unska 3, 10000 Zagreb, Croatia
b
c
Medical School, Department of Physics and Biophysics, University of Zagreb, Salata 3b, 10000, Zagreb, Croatia
d
`
Dipartimento di Fisica, Universita di Trento and INFM, I-38050 Povo, Trento, Italy
e
Instituto di Fotonica e Nanotechnologie, CSMFO group, Via Sommarive 14, Trento, I-38050 Trento, Italy
Department of Optoelectronics and Photonics, Nello Carrara Institute of Applied Physics, IFAC – CNR, Via Panciatichi 64, I-50127 Firenze, Italy
f
Received 8 September 2006; received in revised form 28 September 2006; accepted 28 September 2006
Available online 14 November 2006
Abstract
The Si-rich silicon oxide (SiOx) thin films are prepared on silicon crystalline substrates by low pressure chemical vapor deposition
(LPCVD) method. The oxygen concentration x are controlled by the ratio of the partial pressures of N2O and SiH4 gases in the reaction
chamber. In order to induce the phase separation on SiO2 and Si nanostructures the samples are annealed at the temperatures 900–
1100 ꢀC. The structural and optical properties of the samples are investigated by Raman and infrared spectroscopy and scanning electron
microscopy.
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Si-rich silicon oxide; LPCVD; Raman scattering; Low frequency particle modes
1. Introduction
other amorphous oxides of silicon such as silicon monoxide
and silicon sesquioxide (Si2O3) also exists. These amor-
phous silicon suboxides have been known for decades
and are used in a variety of technical applications. The pos-
sibility of phase separation of silicon suboxides into silicon
and SiO2 was first proposed by Brady [1]. On the basis of a
theoretical approach this concept was discussed in more
detail by Temkin [2] who proposed a random-mixture
(RM) model. The RM model assumes small domains in
which either silicon is bonded only to silicon or to oxygen.
This corresponds to a two-phase mixture of Si and SiO2
The growth of thin films by low pressure chemical vapor
deposition (LPCVD) is one of the most important tech-
niques for deposition of thin films in modern technology.
The reasons of a broad application of the LPCVD method
are in possibility of deposition of different elements and
compounds at relatively low temperatures in amorphous
and crystalline phase with high degree of uniformity and
purity. A simple handling, high reliability of operations,
fast deposition, homogeneity of deposited layers and high
reproducibility are the basic characteristics the LPCVD
method.
˚
domains with thin boundary layer of ꢀ10 A between [2].
Dupree et al. [3] performed magic-angle spinning (MAS)
NMR investigations on silicon monoxide and estimated
Apart from vitreous silica as the archetypal oxide net-
work former and the basis of traditional silicate glasses
˚
phase separated regions of Si and SiO2 near to 20 A.
In this paper we show the results of the LPCVD deposi-
tion of non stoichiometric oxide SiOx thin films with SiH4
and N2O precursors. The chemical reaction we used is
*
Corresponding author.
E-mail address: ivanda@irb.hr (M. Ivanda).
0022-2860/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.09.036

462
M. Ivanda et al. / Journal of Molecular Structure 834–836 (2007) 461–464
Table 1
oxidation of silane with N2O: SiH4 + cN2O fi p-
SiOx + (1 ꢁ p)SiH4 + 2pH2 + (c ꢁ px)N2O + px N2. The
silicon nanocrystals were formed by phase separation of
SiOx (x < 2) structure induced by thermal annealing:
SiOxfi (1 ꢁ x/2) Si + x/2 SiO2, where N2O/SiH4 ratio con-
trols the Si amount in the layer.
Deposition parameters of SiOx thin films
Sample
U(SiH4)/sccm
U(N2O)/U(SiH4)
S1
S2
S3
6.2
37.7
80.5
4.04
1.14
1.03
absorption spectroscopy and scanning electron microscopy
(SEM).
2. Experimental
The LPCVD method is most successfully applied in
deposition of polysilicon thin films from SiH4 in the tem-
perature range 580–660 ꢀC and SiO2 layers from SiH2Cl2
at 900 ꢀC. The scheme of the conventional hot-wall hori-
zontal LPCVD reactor is shown in Fig. 1. The base of
device is a quartz tube placed in a spiral heater. The tube
is evacuated on the pressure of 0.1 Pa and heated on to
the wanted temperature to 1000 ꢀC. The temperature sta-
bility is 1 ꢀC. The deposition starts with entering of the
working gas in the tube. The working (dynamical) pressure
is 10–200 Pa.
In this experiment the non-stoichiometric oxide SiOx
(x < 2) were deposited on a (111) oriented silicon substrate
with a diameter of 50 mm set at 7 mm from the 1st
(dummy) wafer. The depositions were carried out by
thermal decomposition of 2% (S1) and 26% silane (S2
and S3) diluted in argon. The deposition temperatures
was 748 ꢀC. The flow rate ratios of nitrous oxide and silane
U(N2O)/U(SiH4) are presented in Table 1. The SiOx films
were further thermally annealed at 900, 1000 and 1100 ꢀC
in air. Upon annealing the decomposition of SiOx into
SiO2 and elemental Si takes place. After the decomposition,
the excess Si atoms form Si clusters embedded in a SiO2
matrix. The size of Si clusters is expected to become larger
for the higher annealing temperatures. The deposited layers
were characterized by Raman spectroscopy using Dilor
Z-24 Raman triple monochromator spectrometer, IR
3. Results and discussion
The main difference between CVD depositions at low
pressure and atmospheric pressure is in ratio of the mass
transport velocity and the velocity of reaction on the sur-
face. At atmospheric pressure these quantities are of the
same order of magnitude. While the velocity of the mass
transport depends mainly on the reactant concentration,
diffusion, and thickness of the border layer, the velocity
of the surface reaction depends mainly on the concentra-
tion of reactants and temperature. As diffusion of gas is
reciprocal to pressure, it will decrease 1000 times if the
pressure reduces from atmospheric value to 100 Pa. Now
the carrier gas is not more necessary, the substrates could
approach more closely, and deposited films shows better
uniformity and homogeneity. The working gas, that regu-
larly consist of the gas for dilution and of the reactive
gas, after entering spreads inside the tube and flows above
the hot substrates (thin wafers of silicon, quartz or some
other material) placed in the quartz holders. The wafers
in the tube reactor are radiantly heated by resistive hearing
coils surrounding the tube. The critical factors that influ-
ence on thickness uniformity and film content are positions
of the substrates, temperature profile in the zone of deposi-
tion, reactor geometry, deposition time, working pressure,
as well as the quantity and content of all gases or vapors
that enter in the reactor.
Fig. 2a shows the FTIR spectrum of the sample S1. The
band above 1000 cmꢁ1 is assigned to the asymmetric
stretching of the Si–O–Si bridge. This peak position can
be used for reasonable stoichiometry estimation in case
of a homogeneous SiOx alloy [4]. The observed peak posi-
tion at 1072 cmꢁ1, which differs from the position of ther-
mally grown oxide at ꢂ1080 cmꢁ1, gives the composition
x = 1.9. The SEM image in Fig. 2b shows that the layer
is porous and inhomogeneous. These results show that
the structure of deposited layer is more close to the silica
structure than to SiO structure.
In order to decrease the composition x we have
decreased the flow rate ratio to: U(N2O)/U(SiH4) = 1.14
(sample S2). Fig. 3 shows the Raman spectra of as depos-
ited and annealed samples. The Raman spectra shows the
characteristic bands of SiOx structure that consists of the
broad peaks at 160 and 460 cmꢁ1 which corresponds to
the TO and TA phonon-like bands. Upon thermal anneal-
ing the decomposition of SiOx into SiO2 and elemental Si
Fig. 1. Schematic description of the LPCVD device.

M. Ivanda et al. / Journal of Molecular Structure 834–836 (2007) 461–464
463
a
b
1.2
1.0
0.8
0.6
0.4
T = 866 ˚C
400
600
800
1000 1200 1400
-1
Wavenumber/cm
Fig. 2. FTIR spectrum (a) and SEM image (b) of the sample S1.
a
b
*
as prepared
*
54
as prepared
Tann.=900 oC
Tann.=1000 oC
Tann.=1100 oC
22
Tann.=900 o C
13
Tann. = 1000 o C
Tann. =110 0 o C
0
50
100
150
200
250
0
200
400
600
Wavenumbers/cm-1
Wave numbers/cm
-1
Fig. 3. Raman spectra of SiOx structure in the range of TA and TO phonon-like bands of the sample deposited at 748ꢀC with the gas flow rate ratio
U(N2O)/U (SiH4) = 1.14 (a); the low frequency reduced Raman spectra of the same samples (b) the arrow indicate the symmetric vibrational mode of
silicon nanoparticles. The asterisk indicates the plasma line at 39 cmꢁ1
.
S0vL
cD
takes place. After decomposition, the excess Si atoms form
Si clusters embedded in a SiO2 matrix. The size of Si clus-
ters is expected to become larger for higher annealing tem-
peratures. The Raman scattering on nanosized silicon
particle manifests in broadening and red shift of the
TO(C) phonon band at 521 cmꢁ1 and blue shift of the
low frequency spherical mode with decrease of particle size.
The model of phonon confinement of optical modes [5] and
the calculation of spherical acoustical modes are applied in
order to deduce the mean size and distribution width of
silicon nanocrystals [6].
Low frequency modes indicated by an arrow in Fig. 3b
corresponds to the spherical acoustical modes of silicon
nanocrystals. Vibrations of elastic spheres have been stud-
ied for a long time by Lamb [7]. The frequency (in wave-
numbers) of the symmetric spherical mode is given by [8]:
m0 ¼
;
ð1Þ
where m0 is the frequency of the symmetric mode, D is the
diameter of the spherical particle and c is the velocity of
light. The constant S0 = 0.76 [6]. The mean value of the
longitudinal sound velocities calculated across three crys-
talline directions is vL = 8790 m/s. These parameters when
inserted in Eq. (1) give the mean size of silicon nanoparti-
cles from the know frequency of the symmetric spherical
mode, i.e. D = 2.29 · 10ꢁ7/mo. Experimentally, by compar-
ison of the low frequency Raman results with the particles
size distributions obtained by TEM, the size of silicon par-
ticles deduced by Eq. (1) are by the factor of 0.5 smaller [6].
The reason for such factor is still unclear and could be con-
nected with the resonance phenomena of incident laser

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M. Ivanda et al. / Journal of Molecular Structure 834–836 (2007) 461–464
Fig. 4. SEM images of the SiOx film prepared at 748 ꢀC by silane reaction with N2O at flow rate: U(SiH4) = 80.5 sccm, and gas flow rate ratio: U(N2O)/
U(SiH4) = 1.03.
confirmed by the Raman scattering on the confined TO(C)
509
phonon mode that appear as a shoulder and sharp peak at
509 cmꢁ1 of the as prepared and annealed sample. By
applying the phonon confinement model, the mean particle
size is possible to obtain by the relation [5]: D = 0.337/
(521/m-1), where m is observed frequency of TO(C) phonon
mode. The particles of mean size of 1.4 nm were found by
this formula which is similar to those found from the LFR
modes Fig. 5.
509
50
43
As a conclusion here we have shown that by using
LPCVD technique a number of different silicon SiOx nano-
structures important for microelectronic and photonic
application is possible to prepare with rather simple
approach. The content of oxygen atoms is possible to con-
trol by of nitrous oxide and silane partial pressure gas ratio
and the particle size by the temperature of annealing pro-
cess. The different size of silicon nanocrystals in silica
matrix are obtained by subsequent annealing of SiOx struc-
ture. The Raman scattering techniques showed to be simple
and reliable technique in determination of size of prepared
silicon nanoparticles.
0
100
200
300
400
-1
500
600
Wavenumber/cm
Fig. 5. Raman spectra reduced for the Bose–Einstein factor of SiOx
prepared with silane flow rate U(SiH4) = 80.5 sccm, and gas flow rate
ratio: U(N2O)/U(SiH4) = 1.03 (diamonds) and of the same sample
annealed at 900 ꢀC for 1 h in air (circles). The asterisk indicates the
plasma line at 39 cmꢁ1
.
energy with the exciton transition in silicon [6]. Taking this
into the consideration, the silicon nanocrystals with mean
sizes of 3.2, 5.2 and 8.8 nm are formed in the films annealed
at 900, 1000 and 1100 ꢀC, respectively. Some broad bands
with maximum at 54 cmꢁ1 also exist for the as deposited
samples. This could be also ascribed to spherical vibrations
that correspond to the broad distribution of silicon nano-
particles. The mean particles size of 2.2 nm estimated from
the peak maximum agree with the value obtained by NMR
measurements of Dupree et al. [3].
With further decrease of the flow rate ratio to U(N2O)/
U(SiH4) = 1.03 (sample S3), the black porous structure
with porous surface morphology shown in Fig. 4 is
obtained. The sample is than further annealed at 900 ꢀC
for 1 h in air. The low frequency Raman spectra of the as
deposited and annealed sample shows the broad peaks of
spherical modes with maximums at 43 and 50 cmꢁ1 which
corresponds to the mean particle size of 2.7 and 2.3 nm,
respectively. The existence of such small crystallites are also
Acknowledgement
This work was supported by the Ministry of Science and
Technology of the Republic of Croatia.
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