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JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, Vol. 11, No. 9, September 2009, p. 1079 - 1085
Correlation between the UV-reflectance spectra and the
structure of poly-Si films obtained by Aluminium
Induced Crystallization♣
D. DIMOVA-MALINOVSKA*, O. ANGELOV, M. SENDOVA-VASSILEVA, V. MIKLIa
aCentral Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko
Chaussee Blvd., 1784 Sofia, Bulgaria.
aCentre for Materials Research, Tallinn Technical University, Tallinn, Estonia
The structural properties of poly-Si films prepared by the method of Aluminium Induced Crystallization (AIC) of amorphous
Si films (a-Si:H) deposited on glass substrates covered with Al layers were studied. Raman and XRD (X-Ray Diffraction)
spectroscopy were used for characterization of their short and long range order, respectively. The UV (Ultra-Violet)
reflectance spectra of poly-Si films were measured, as well. The surface morphology was revealed by optical microscopy.
The dependence of the structural and optical properties of the obtained poly-Si films on the hydrogen pressure during the
deposition of the a-Si:H precursor was studied. A correlation between the short and long range ordering in poly-Si films,
their surface morphology and the UV optical reflectance spectra was identified. Poly-Si films with better structural
properties are obtained by AIC, using a-Si:H precursor layers with moderate concentrations of hydrogen.
(Received November 5, 2008; accepted December 15, 2008)
Keywords: Poly-Si thin films, UV reflectance, Raman spectroscopy
♣ Paper presented at the International School on Condensed Matter Physics, Varna, Bulgaria, September 2008
1. Introduction
The method of aluminium induced crystallization
(AIC) has been widely used in recent years to obtain poly-
Si films, mainly because of the lower thermal budget of
this technology and the generation of grains that are larger
than the film thickness in the obtained poly-Si films [1,2].
Additionally, it has to be pointed out that a low cost
substrate such as glass could be used for deposition of the
films. For solar cell applications, the poly-Si films are
used as seeding layers for epitaxial thickening [3]. This is
why the qualities of their surface and crystalline structure
are important.
In this work, the influence of the temperature of
deposition (Tsa-Si) of unhydrogenated (a-Si) and
hydrogenated (a-Si:H) amorphous silicon layers, prepared
by magnetron sputtering, on the structural properties of
poly-Si films obtained by AIC of glass/Al/a-Si(a-Si:H)
configurations is reported. The influence of the hydrogen
pressure during the magnetron sputtering of the a-Si:H
precursor films is also reported. XRD (X-Ray Diffraction)
and Raman spectroscopy were applied for characterization
of their structural properties. UV (Ultra Violet) reflectance
spectroscopy was used to study the surface quality of the
poly-Si films.
A correlation between the long range ordering in the
poly-Si films, their surface morphology and the UV
optical reflectance spectra was examined.
2. Experimental
Poly-Si films were obtained by AIC of the structures
glass/Al/a-Si:H(or a-Si). The precursor layers of Al were
deposited at a substrate temperature TsAl = 300 0C, and
were kept for 24 hours in air before the deposition of the a-
Si:H (or a-Si) films. Precursor Al films prepared under
these conditions result in better structural properties of the
obtained poly-Si films, as described earlier [4].
Unhydrogenated (a-Si) and hydrogenated (a-Si:H) films
were deposited on top of the Al by magnetron sputtering
with 130 W rf power at different substrate temperatures:
RT (without heating of the substrates), 250, 300, 350 and
4000C. Four different sets of samples were prepared with
a-Si:H precursors deposited without hydrogen (a-Si) and
with 0.05, 0.1Pa and 0.2 Pa of H2 partial pressure in the
Ar+H2 sputtering gas mixture. These conditions resulted in
a hydrogen concentration in the deposited precursor
amorphous Si layers between 4 and 25% (measured by
Elastic Recoil Detection Analyses (ERDA) [5]), and it
increased with increasing H2 partial pressure and reduction
of the substrate deposition temperature [6]. The
thicknesses of both precursors, Al and a-Si:H (or a-Si)
1080 D. Dimova-Malinovska, O. Angelov, M. Sendova-Vassileva, V. Mikli
were equal - about 100 nm. The structures were annealed
in forming gas (N2+ 5%H2) under atmospheric pressure at
5300C for 6 h. After annealing, Al was removed from the
surface of the obtained poly-Si films by etching with a
chemical solution based on phosphoric acid.
The UV hemispherical reflectance spectra in the range
250 - 400 nm were measured by Perkin-Elmer
UV/VIS/NIR Lambda 900 spectrometer. The surface
morphology was observed by optical light microscopy [7].
The degree of crystallization of the poly-Si films was
studied by XRD and Raman spectroscopy. The XRD
spectra of the samples were obtained using a Brucker D8
Advance spectrophotometer with CuKα radiation: λ
CuKα1= 1.540560 Å and λ CuKα2 = 1.544426 Å (intensity
half that of λ CuKα1). The instrumental broadening was
0.040 in a 2Θ geometry. Raman spectra were excited by
the 488 nm line of an Ar+ laser, and all of them were
measured under the same conditions. The peak positions
and the Full Width at Half Maximum (FWHM) of the
Raman bands were measured with a mean error of 0.5 cm-1.
3. Results and discussion
Raman spectroscopy was used to study the influence
of the deposition temperature and the hydrogen partial
pressure during the sputtering of the a-Si (a-Si:H)
precursor layers on the quality of the structure of the
resulting poly-Si films. The Raman spectra of films
obtained from a-Si or a-Si:H precursors deposited at
different Tsa-Si and three different hydrogen partial
pressures are shown in Fig. 1. All of the samples display
Raman spectra typical of the crystalline Si structure - a Si-
Si TO band, centered between 518.5 and 520.5 cm-1. The
Si-Si TO peak for crystalline silicon, measured under the
same conditions, is at 521 cm-1 and has a Full Width at
Half Maximum (FWHM) of 4.5 cm-1. The dependence of
the Si-Si TO peak position and its FWHM (estimated from
a Lorentzian fit) on the a-Si (a-Si:H) substrate temperature
and the H2 partial pressure are presented in Fig. 2a and
Fig 2b, respectively. The accuracy of the values is
indicated by the error bars.
An estimate of the grain size can be deduced from
both the downshift and the FWHM of the
450 500 550
Ts=400oC
Ts=350oC
Ts=300oC
Ts=250oC
Ts=RT
PH2=0 Pa
Normalized Raman Intensity
Raman Shift [cm-1]450 500 550
Raman Shift [cm-1]
Ts=400oC
Ts=350oC
Ts=300oC
Ts=250oC
Ts=RT
PH2=0.05 Pa
Normalized Raman Intensity
(a) (b)
450 500 550
Raman Shift [cm-1]
Ts=400oC
Ts=350oC
Ts=300oC
Ts=250oC
Ts=RT
PH2=0.1 Pa
Normalized Raman Intensity
(c)
Fig.1. Raman spectra of poly-Si films obtained from a-Si and a-Si:H precursors deposited at different Tsa-Si and
different H2 pressures: 0 Pa (a), 0.05 Pa (b) and 0.1 Pa (c).
Correlation between the UV-reflectance spectra and the structure of poly-Si films obtained by Aluminium ... 1081
100 200 300 400
518
519
520
521 0 Pa
0.05 Pa
0.1 Pa
TO Peak Position [cm-1]
Substrate Temperature [0C] 100 200 300 400
6
7
8
9
10 0 Pa
0.05 Pa
0.1 Pa
FWHM [cm-1]
Substrate Temperature [0C]
(a) (b)
Fig. 2. Si-Si TO-like peak position (a) and FWHM (b) taken from Raman spectra of the samples presented in Fig 1.
The lines are guides to the eye.
Raman peak [8]. Although accurate values cannot be
determined from the relationships described previously in
the literature, comparisons between the spectra obtained
from similar materials are valid [8, 9]. The grain size is
inversely proportional to the FWHM of the peak [9]. On
the other hand, a shift in the Si-Si TO peak position, ωTO,
to a lower wave number could be related to an increase in
the value of the tensile stress [10]. The following
tendencies can be seen. The Si-Si TO peak position shifts
slightly to a higher wave number with increasing Tsa-Si.
This is an indication of a reduction in the tensile stress in
the poly-Si samples with increasing Tsa-Si [11]. A weak
tendency for increased stress in the poly-Si films, when a-
Si:H precursors were used, could be noticed, and could be
explained by the effusion of H during the AIC. This could
leave pinholes and microvoids in the poly-Si films,
resulting in higher tensile stresses.
The FWHM of the Si-Si TO peak passed through a
minimum for poly-Si films obtained from hydrogenated
precursor layers deposited at Tsa-Si between 250 and 350o C
- in this case, larger grains were obtained in the poly-Si
films. It should be noted that we observed similar results
for the AIC of glass/a-Si(a-Si:H)/Al structures [12].
During the annealing, effusion of H from the a-Si:H films
takes place. It is possible to suppose that this will enhance
the disorder in the a-Si:H precursor during the annealing.
This stimulates the dissolution of Si atoms into the Al and
the consequent re-arrangement into a Si crystalline
structure, resulting in a higher growth rate and larger
grains in the poly-Si films. The suggestion is based on the
fact that the higher degree of disorder creates energetically
more favourable conditions for the transformation of the a-
Si:H film into a poly-Si one. It is known that the H
concentration in a-Si:H films decreases with increasing
substrate temperature, so that the precursors deposited at
RT (without heating) should have a higher H content [13].
In this case, the higher effusing quantity of hydrogen
probably creates conditions for an increased diffusion rate
of Al and Si. Thus, intermixing of Al and Si would take
place within the bulk of the resulting poly-Si film, which
would inhibit the exchange between Al and Si and would
lead to a lower rate of crystallite growth and a smaller
grain size. The deposition of the precursor layers, at
temperatures > 350o C, would result in a reduced H content
and in better short range order of the a-Si:H films [10],
which would slow down the re-arrangement into the
crystalline structure – the poly-Si films would again have
grains of a smaller size.
25 30 35 40 45 50 55 60
glass/Al/a-Si:H
PH=0.1 Pa
4000C
3500C
3000C
2500C
RT
Intensity, (arb. un.)
2 Θ, [deg.]
Fig.3. XRD spectra of the set of poly-Si samples
deposited at different Tsa-Si and PH2 = 0.1 Pa.
XRD spectra of the set of sample deposited at
different Tsa-Si and PH2 = 0.1 Pa are presented in Fig. 3.
This set of samples was chosen because it had a relatively
larger grain size (the narrowest FWHM of the Si-Si TO
Raman band). Only one peak of reflection in the XRD
spectra at about 2θ = 28.39 - 28.480 is observed for the
poly-Si films, which correspond to the (111) preferential
orientation. The position of this peak in the c-Si is at 2θ =
28.480. The intensity of the peak is higher
1082 D. Dimova-Malinovska, O. Angelov, M. Sendova-Vassileva, V. Mikli
(a) (b)
(c) (d)
Fig 4. Optical micrograph images of poly-Si films using precursor layers of a-Si:H deposited with PH2=0 Pa at different Tsa-Si :
RT (a), 250oC (b), 3000 C (c) and 400oC (d).
(a) (b)
(c) (d)
Fig 5. Optical micrograph images of poly-Si films using precursor layers of a-Si:H deposited with PH2 = 0 .05 Pa at
different Tsa-Si : RT (a), 250oC (b), 3000 C (c) and 400oC (d).
100
μ
m
100
μ
m
Correlation between the UV-reflectance spectra and the structure of poly-Si films obtained by Aluminium ... 1083
(a) (b)
(c) (d)
Fig 6. Optical micrograph images of poly-Si films using precursor layers of a-Si:H deposited with PH2=0.1 Pa at different
Tsa-Si : RT (a), 300oC (b), 3500C(c), and 400oC (d).
(a) (b)
(c) (d)
Fig 7. Optical micrograph images of poly-Si films using precursor layers of a-Si:H deposited with PH2=0.2 Pa at
different Tsa-Si : RT (a), 300oC (b), 3500C(c), and 400oC (d).
and is at 2θ = 28.480 in the poly-Si films obtained from a
precursor deposited at Tsa-Si = 300 0C. This observation is
an indication of better crystalline quality. The FWHM of
the (111) peak in the XRD spectra decreases with
increasing Tsa-Si. This demonstrates an increase of the
average grain size with preferential (111) orientation
which is in agreement with the tendencies in Raman
spectra. The position of the (111) peak of the sample
deposited at Tsa-Si =3500 C (2θ = 28.480) coincides with the
c-Si one. The peak shifts to the lower2θ (28.390) for the
samples deposited at the other Tsa-Si, which could be due to
the tensiel stress in the films and is in agreement with the
conclusions from the Raman spectra.
Optical micrographs images of the surface of poly-Si
films obtained from a-Si and a-Si:H precursors, deposited
at different Tsa-Si, are shown in Figs. 4 - 7. Images of the
100
μ
m
100
μ
m
1084 D. Dimova-Malinovska, O. Angelov, M. Sendova-Vassileva, V. Mikli
samples obtained from unhydrogenated amorphous Si
layer precursors deposited at Tsa-Si < 2500C have a smooth
surface. Those deposited at substrate temperatures of 250,
300 and 400oC exhibit a high density of Si precipitates
(hillocks or islands) on the poly-Si surface. However, in
the case of poly-Si, prepared from a hydrogenated
amorphous silicon precursor, silicon islands appear on the
surface only for precursors deposited at the higher
temperatures, Tsa-Si = 300 0C (for PH2 = 0.05 Pa), 3500C(
for PH2 = 0.01 Pa) and 400 0C (for PH2 = 0.2 Pa). The
substrate temperature of the a-Si:H precursor at which the
precipitates appear on the poly-Si film surfaces increases
with increasing hydrogen partial pressure. The presence of
numerous Si islands on the poly-Si surface after AIC has
been reported when the Al precursor is thicker than he
amorphous silicon one, and in the case of precursors of a-
Si:H with high short range disorder and of microcrystalline
Si films deposited by PE CVD methods [1,14]. It is
possible to suppose that the growth of the silicon hillocks
in our case is a result of the better structural order in the a-
Si:H precursors deposited at higher temperature. The
influence of the hydrogen concentration in a-Si:H should
be taken in account, as well, because it depends on the Ts
and the hydrogen pressure during the deposition. The
conditions of deposition of the precursor layers define the
balance between the rate of nucleation and of the crystal
growth and the circumstances for the preparation of poly-
Si with a smooth surface.
The spectra of the UV hemispherical reflectance of
the obtained poly-Si films are presented in Fig. 8.
250 300 350 400 450
0
10
20
30
40
50
60
70
4000C3000C
3500C
2500C
RT
glass/Al/a-Si:H
PH=0 Pa
c-Si
Rd, (%)
λ, [nm] 250 300 350 400 450
0
10
20
30
40
50
60
70 glass/Al/a-Si:H
PH=0.05 Pa
4000C
3500C
3000C
2500C
RT
c-Si
Rd, (%)
λ, [nm]
(a) (b)
250 300 350 400 450
0
10
20
30
40
50
60
70 glass/Al/a-Si:H
PH=0.1 Pa
4000C
3000C
2500CRT
3500C
c-Si
Rd, (%)
λ, [nm] 250 300 350 400 450
0
10
20
30
40
50
60
70 glass/Al/a-Si:H
PH=0.2 Pa
4000C
2500C
RT
3500C
3000C
c-Si
Rd, (%)
λ, [nm]
(c) (d)
Fig. 8. UV reflectance spectra of poly-Si films obtained by AIC from a-Si:H precursors deposited at different TS and
with different H2 partial pressures: 0 (a), 0.05 ( b) 0.1 (c) and 0.2Pa (d) .
The corresponding spectra of c-Si polished wafer
measured inder the same conditions is shown, too. The
two maxima in the spefctrum of single-crystalline Si at
280 nm and 365 nm are caused by optical interband
transition at the X-poimt (band E2) and along the Г-L axis
(band E1) of the Brillouin zone, respectively [15,16]. The
deviation from the UV-R spectrum of bulk crystalline Si is
related to the long-range order deterioration or
amorphisation of the material [15]. The intensity of the E2
band (280nm) decreases and that of the E1 (375nm)
increases with long range disorder, according to the theory
of long range order relaxation effects. Additionally, at
short wavelengths, in particular at 280 nm, the reflectance
is largely determined by the high value of the absorption
coefficient (α >106 cm-1) corresponding to a penetration
depth of less than 10 nm. Imperfect crystallinity in the
near-surface region will cause a broadening and height
reduction of this maximum [15, 16]. It is seen from Fig. 8a
Correlation between the UV-reflectance spectra and the structure of poly-Si films obtained by Aluminium ... 1085
that for the poly-Si films obtained from a-Si deposited at
Ts > 2500 C without hydrogen (PH2 = 0 Pa), the UV-R is
very low in the whole photon range, and smeared out
maxima are observed. The maximal value of the UV-
reflection and the best expressed maxima of E1 and E2
bands are observed for the case of a a-Si:H precursor
deposited at Tsa-Si = 3000 C and PH2 = 0.05 and 0.1 Pa.
However, in the case of PH2 = 0.1 Pa, the values are the
highest and closer to the reflection of the c-Si polished
wafer – these samples have better long range order and
better structure of the near surface region. This is in an
agreement with the XRD and Raman spectra, and with the
surface morphology observed from optical light
microscopy images.
4. Conclusions
The study of the influence of amorphous silicon
precursor layers, deposited at different substrate
temperatures by magnetron sputtering, in an atmosphere
with and without H2, on the structural properties of poly-Si
films obtained by AIC of glass/Al/a-Si(a-Si:H) structures
has been performed by Raman spectroscopy, XRD, UV-
reflectance spectra and optical light microscopy. The
results showed that poly-Si with larger grain sizes were
obtained using a precursor of a-Si:H deposited at a
moderate Tsa-Si = 250-300oC and 0.05 and 0.1 Pa H2
pressures in the sputtering chamber. The tensile stress in
the poly-Si films decreased with increasing temperature of
deposition of the amorphous silicon precursor. Poly-Si
films with smooth surfaces were obtained from an a-Si
precursor deposited at Tsa-Si = 300oC and from an a-Si:H
one at Tsa-Si = 400oC. The differences in the structural
properties of the poly-Si films could be explained by the
different structural order and different content of hydrogen
in the precursor a-Si (a-Si:H) layers deposited under
different conditions.
The complex study of the structural properties, such
as short and long range ordering and the quality of the
structure of the surface area of poly-Si films prepared by
AIC demonstrated a good correlation between the data
obtained from XRD, Raman spectroscopy, optical
microscopy and UV-reflectance spectra. Poly-Si films
with better structural properties were obtained by AIC
using a- Si:H precursor layers deposited at a moderate
hydrogen partial pressure and Tsa-Si about 3000 C.
The results were explained by the influence of the
presence of hydrogen in the precursor a-Si:H layer on the
process of AIC.
Acknowledgements
This work is performed with financial support from
the Bulgarian National Scientific Fund by project X-1503.
The Raman measurements were supported by project
“Support for research activities in Universities”
programme.
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*Corresponding author: doriana@phys.bas.bg
