Download vacuum annealing effect on the structural and optical properties of

VACUUM ANNEALING EFFECT ON THE STRUCTURAL AND
OPTICAL PROPERTIES OF ANTIMONY TRIOXIDE THIN FILMS
N. TIGAU1, S. CONDURACHE-BOTA1, R. DRASOVEAN1, J. CRINGANU1, R. GAVRILA2
1
Dunarea de Jos University of Galati, Faculty of Sciences and Environment,
111 Domneasca Street, 800201, Galati, Romania,
E-mails: [email protected], [email protected], [email protected]
2
National Institute for Research and Development in Microtechnology (IMT-Bucharest),
126A Erou Iancu Nicolae Street, 077190 Bucharest, Romania, E-mail: [email protected]
Received September 4, 2016
Abstract. Antimony trioxide (Sb2O3) thin films were deposited onto glass
substrates held at 25C using the thermal vacuum evaporation technique. The films
were annealed at 250C in vacuum after evaporation. The morpho-structural
properties of the films were studied by X-ray diffraction (XRD) and atomic force
microscopy (AFM), respectively. The optical properties were investigated for both the
as-deposited and annealed film in the wavelength range of 190–1100 nm. The optical
constants such as the refractive index, the absorption coefficient and the energy
bandgap were determined from the transmission spectra, by using Swanepoel’s
method.
Key word: Sb2O3, thin films, annealing effect, structural properties, optical
properties.
1. INTRODUCTION
Sb2O3 is an important member of semiconducting V–VI-type of compounds
because of its large band gap located in the near ultraviolet region. The optical
band gap of senarmontite Sb2O3 it was found to be 3.6 ± 0.1 eV as deduced from
spectroscopic measurements [1]. Due to all the properties presented above,
antimony trioxide has wide applications as transparent conductive material, as
effective catalyst, flame-retardant synergist in plastics, paints, adhesives and textile
back coatings or as functional filter, as component of optical materials and for
visible light photodetectors [2–4].
The authors of the present paper have already analyzed as-deposited and
open-air annealed antimony trioxide thin films deposited on glass substrates
through the thermal vacuum evaporation technique [5–9]. Instead, this paper deals
with the vacuum annealing of such films and with the influence of this type of
thermal treatment on the morpho-structural and optical properties of the films.
Comparisons are made not only between the annealed and as-deposited films, but
also between those annealed in vacuum, as done for this paper and those annealed
in open-air, as studied elsewhere [5].
Romanian Journal of Physics 62, 604 (2017)
Article no. 604
N. Tigau et al.
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2. EXPRIMENTAL
The thickness of the films was inferred through the multiple-beam Fizeau
fringe method by means of a Linnik interferometric microscope operated with
white light [10]. The films resulted to be 542 nm thick. The as-deposited antimony
trioxide thin films were annealed in vacuum as follows: 1st step consisted in a
gradual heating with 4.5 deg./minute up till 250C, the 2nd step: 250C held for one
hour and 3rd step: cooling the films back till room temperature (25C) with the
same rate as for heating. The crystallinity of both the as-deposited and the annealed
films was examined by means of X-ray diffraction (XRD) with CuK radiation
(λ = 1.5418 Å) in the scanning range 2θ = 20–60°. The surface morphology of
Sb2O3 thin films was studied by atomic force microscopy (AFM). The optical
transmission spectra of the films were recorded by using a Perkin Elmer Lambda
35 UV-Vis double beam spectrophotometer, operated in air, at normal incidence,
for the 190–1100 nm spectral range.
3. RESULTS AND DISCUSSION
Figure 1 presents the XRD spectra of both the as-deposited and annealed
Sb2O3 thin films. The presence of individual maxima in the XRD patterns proves
that the films are polycrystalline.
Fig. 1 – XRD patterns of the Sb2O3 thin films: (a) as-deposited at 25C,
(b) annealed in vacuum at 250 C.
Following the indexation made according to the JCPDS database, it results
that the films contain only the cubic phase of Sb2O3. It can be seen that both types
of films contain six families of crystalline planes, namely those with the following
Miller indices: (222), (400), (331), (440), (622) and (551). Since the peak
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Vacuum annealing effect on the properties of Sb2O3 films
Article no. 604
appearing at 2 = 27.68, given by the (222) planes is much higher than the other
XRD peaks, this proves that the films are texturized, having the (222) direction as
preferential orientation. After the thermal treatment, all the XRD peaks get higher,
especially the one given by the (222) planes. This fact indicates crystallinity and
texturizing improvement following the annealing process [10].
The lattice parameter, a, for cubic phase of Sb2O3 thin films was calculated
according to:
a

2 sin 
(h 2  k 2   2 )1/ 2
(1)
where λ is the wavelength of the XRD radiation, h, k and ℓ are the Miller indices
and  is the diffraction angle corresponding to a certain crystalline plane.
It was found that the lattice parameter decreased from 11.163 Å to 11.154 Å
after annealing. The obtained values of the lattice parameter are consistent with the
standard value for the Sb2O3 cubic phase (a = 11.152 Å according to the JCPDS
5-0543).
The grain size of the Sb2O3 crystallites was determined from the XRD
patterns by using the Debye-Scherrer formula [11]:
D
0.9 
,
 cos
(2)
where D is the average diameter of the crystallites within the film, λ is the
wavelength of the XRD radiation, β is the peak width at half maximum (FWHM),
expressed in radians and θ is the Bragg diffraction angle. The calculated grain size
for the as-deposited film is approximately 47.30 nm, which increases to 61.54 nm
after the 250 C annealing. This is a similar behavior with that of the same type of
films annealed in open atmosphere [5]. This was not the case when open-air
annealing was performed to similar Sb2O3 thin films, when grains grew bigger, but
their uniformity didn’t improve visibly after the thermal treatment [5].
The increase of grain size after the annealing treatment could be attributed to
the improvement in the mobility of the surface ad-atoms and to an increase in the
cluster formation leading to the agglomeration of small grains, followed by their
coalesce, leading to the formation of larger grains with improved crystallinity [12].
Figures 2 (a) and (b) show the 5 µm × 5 µm AFM images of the antimony
trioxide films as-deposited, while Figures 2 (c) and (d) exhibit the AFM images of
the same films after their vacuum annealing.
The surface of the as-deposited film is characterized by high-density
columnar morphology. The as-deposited films are continuous, without any voids or
cracks on the surface. The post-deposition thermal treatment clearly changes the
film morphology, as proved by the AFM images. Thus, Figs. 2(c) and 2(d)
demonstrate that following the annealing, the larger crystallites get even bigger,
Article no. 604
N. Tigau et al.
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while the smaller crystallites shrink. This fact is in agreement with the growth
mechanism of polycrystalline semiconducting film [10]. Since the Sb2O3 thin films
under study exhibit a texturized surface, this recommends them for solar cell
applications, where light trapping may be induced and thus, the reflection of
photons incident on the film surface may be significantly inhibited [13].
Fig. 2 – 2D and 3D-AFM images of the Sb2O3 thin films: (a) and (b) as-deposited at 25C;
(c) and (d) annealed in vacuum at 250C.
After annealing, the average roughness Ra increases from 4.13 to 17.28 and
the root mean square roughness Rrms increases from 6.34 to 22.21 nm (Table 1).
This is a significantly different behavior of the thermal treatment in vacuum of
antimony trioxide thin films as compared to open-air annealing, since in the latter
case, a significant decrease of the root mean square roughness happened [5].
The increase of surface roughness following the vacuum annealing could be
due to the increase in grain size and to their more closer packing, as it was proved
by XRD analysis, which induced a reduction of grain boundaries [14]. Thus, the
AFM analysis proves that vacuum annealing changed significantly the morphology
of the films.
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Vacuum annealing effect on the properties of Sb2O3 films
Article no. 604
Table 1
Morpho-structural and optical parameters of the Sb2O3 thin films
Sb2O3 thin
films
as-deposited
annealed in
vacuum
t
[oC]
25
D
[nm]
47.30
Ra
[nm]
4.13
Rrms
[nm]
17.28
S0
[1012 m-2]
67.11
λ0
[nm]
173.18
250
61.54
6.34
22.21
88.15
166.48
The optical studies were carried out on both for the as-deposited and the
annealed Sb2O3 thin films, by using transmittance and reflectance versus
wavelength measurements. The resulting transmission and reflection spectra are
shown in Figure 3.
Fig. 3 – Transmission and reflection spectra of the as-deposited
and annealed in vacuum Sb2O3 thin films.
The presence of the interference fringes indicates that the films are uniform,
with smooth surfaces. As a consequence of the thermal treatment, it can be seen
from Fig. 3 that the optical transmittance increases, reaching as high as 75 % in the
near-infrared region. The higher transmittance observed for the annealed films as
compared to the as-deposited films can be attributed to the crystallinity
improvement and subsequent less scattering effects, as well as to lower defect
density near the band edge [10, 15]. A fast decrease in the transmittance at lower
wavelengths results from the excitation of charge carriers across the optical band
gap [16].
Swanepoel’s algorithm [17, 18] was applied in order to infer the most
important optical constants of the films, namely the refraction index, n, the
extinction coefficient, k, and the absorption coefficient, α.
Article no. 604
N. Tigau et al.
6
In the region of weak and medium absorption, the transmittance decreases
mainly due to the effect of absorption coefficient. In this case, the refractive index
of the films can be calculated by using the next equation [17, 18]:
n
N  N 2  ns2 ,
(3)
where
n 2  1 2ns (TM  Tm )
,
N s

2
TM Tm
(4)
where ns is the refractive index of the glass substrate at a certain wavelength, while
TM and Tm are the transmission maximum and minimum of envelope functions,
respectively, at the same wavelength. The refractive index of glass substrate, ns was
found to be nearly frequency independent and equal to 1.50.
In the strong absorption region, where the interference fringes disappear, the
dependence of refractive index, n as a function of wavelength, λ, can be fitted with
Sellmeier’s dispersion equation [19–21]:
n  1
S0 022
[2  0 2 ]
,
(5)
where S0 is the average oscillator strength, λ0 is the average oscillator wavelength
and λ is the wavelength of incident radiation. In order to apply this model, we
assumed that the material is composed of individual dipole oscillators, which are
set to forced vibrations by the incident photons [19]. The resulting values of the
dispersion parameters (S0 and λ0) are presented in the Table 1 given above.
The spectral dependences of the refractive index of the as-deposited and
annealed Sb2O3 thin films are shown in Figure 4. The values of the refractive index
are decreasing with increasing wavelength, proving a normal dispersion behavior
of the studied films. Since there is a good fit between Sellmeier’s model and the
experimental data, it results that the single-oscillator model is adequate to describe
the dispersion behavior of this material in the investigated spectral range. It can be
noticed from Fig. 4 that the refractive index of the analyzed films increases after
annealing, which may be attributed to the increase in surface roughness [22].
The extinction coefficient, k can be calculated by using the next formula
[17, 18]:
k

1
ln  ,
4 d  x 
where d is the thickness of the film and x is the absorbance.
(6)
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Vacuum annealing effect on the properties of Sb2O3 films
Article no. 604
In what the extinction coefficient is concerned it was found that its average
value in the visible and near infrared region is found to be approximately 10-2. Such
low values represent a qualitative indication of the excellent surface smoothness of
the analyzed films [19]. The fact that the extinction coefficient decreases after
annealing proves that the fraction of light lost due to scattering and to absorbance
processes decreases after annealing, as related to crystallinity improvement [23].
Fig. 4 – The dispersion curves for the refractive index of both the as-deposited and the annealed
in vacuum Sb2O3 thin films.
In the region of weak and medium absorption the absorbance, x, can be
calculated by [17, 18]:
2
E  [ EM
 (n 2  1)3 (n 2  ns4 )]1 / 2
x M
(n  1)3 (n  ns2 )
(7)
where [17, 25]:
EM 
8n 2ns
 (n 2  1)(n 2  ns2 )
TM
(8)
In the strong absorption region, where the interference maxima and minima
converge to a single curve T0, the absorbance, x, is given by [17]:
x 
(n  1)3 (n  ns2 )
T0 .
16n 2ns
The absorption coefficient, α, was calculated by means of [17, 18]:
(9)
Article no. 604
N. Tigau et al.

4 k

.
8
(10)
Another consequence of the vacuum annealing is the shift towards shorter
wavelengths of the position of the fundamental absorption edge, corresponding to
the electron excitation from the valence band to the conduction band, which can be
used to determine the nature and the value of the optical band gap.
In order to study the type of optical transitions happening within the films, as
well as to assess the value of optical energy band gap, Tauc’s equation was used
[24]:
( h )r  A(h  Eg )
(11)
where A is a parameter that depends on the transition probability, Eg is the energy
band gap of the material, while the exponent ‘r’ depends on the type of electronic
transition. Thus, r = 1/2 for indirect allowed transitions, while r = 2 for direct
allowed transitions. A good fit between the experimental data and ( h )2 versus
h plots, shown in Fig. 5, indicates that the direct allowed transitions represent the
mechanism responsible for the optical absorption in the Sb2O3 thin films under
study. The direct optical band gap energy can be determined from the intercept of
the extrapolated linear part of ( h )2 curve with the h axis [25].
Fig. 5 – Plots of (αhυ)2 vs. hυ of the as-deposited and the annealed in vacuum Sb2O3 thin films.
It can be easily seen from Fig. 5 that annealing increases the optical band gap
energy of the studied films from 3.58 eV to 3.73 eV. The values of the energy band
gap for both the as-deposited and vacuum-annealed antimony trioxide thin films
are in good agreement with those found in the literature [1, 6] and correspond to
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Vacuum annealing effect on the properties of Sb2O3 films
Article no. 604
the fact that the optical band gap energy of a certain thin film is affected by its
crystallinity changes, following the thermal treatment [12].
4. CONCLUSIONS
Antimony trioxide (Sb2O3) thin films were deposited through vacuum
thermal evaporation on glass substrates kept at room temperature (25C) and then
they were annealed in vacuum at 250C, for one hour. Morpho-structural and
optical analysis of both the as-deposited and vacuum-annealed films were
performed and relevant parameters were inferred and compared between the two
types of studied films, but also with similar films either annealed in open air or
deposited at higher substrate temperatures.
The X-ray diffraction analysis showed that both the as-deposited and the
annealed Sb2O3 thin films are polycrystalline and have a cubic structure. The grain
size of the as-deposited films, calculated from XRD patterns, increases from
47.30 nm to 61.54 nm after annealing. The AFM micrographs revealed that the
surfaces of the films are smooth and pinhole free. The annealing in vacuum
improved the crystallinity and increases the surface roughness of films, as well as
the optical transmittance in the visible and near infrared regions. It was found that
the dispersion of the refractive index obeys the single oscillator model. The optical
band gap energy, determined from the variation of the absorption coefficient with
wavelength, increases from 3.58 eV for as-deposited films to 3.73 eV for the
vacuum-annealed Sb2O3 thin films.
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