CHAPTER-VI
INVESTIGATION ON DOUBLY DOPED
(FLUORINE + ANTIMONY) TIN OXIDE FILMS
6.1 INTRODUCTION
SnO2 thin films doped with fluorine or antimony are widely
used in practice as transparent conducting films in the field of
photothermal conversion [1], photovoltaic cells [2],electrochromic
devices
and
low-e
windows
[3],
photocatalysis
[4],
electroluminescence [5], liquid crystal displays [6] and gas sensors
[7].
Eventhough, there are plenty of reports on fluorine doped tin
oxide
and
antimony
doped
tin
oxide
films,
studies
on
fluorine+antimony doped films are very scarce in the literature. Here,
in the present study, simplified spray technique using perfume
atomizer is employed for the deposition of fluorine+antimony doubly
doped tin oxide films from different molar concentrations of the host
precursor with different F doping levels and are characterized to
analyze the structural, electrical, optical, elemental and surface
morphological properties.
6.2 MATERIALS AND METHODS
SnO2:F:Sb films were deposited on to glass substrates using
starting solutions having different molar concentrations of
149
SnCl2.2H2O (0.1-0.7 M in steps of 0.2 M)
with the simultaneous
doping of antimony (2 at. %) and fluorine (20 at. %, 30 at.% and 40
at. %) employing simplified spray pyrolysis technique using perfume
atomizer. The antimony and fluorine doping was achieved by adding
the dopant precursors SbCl3 and NH4F respectively. To enhance the
solubility of tin oxide salt, a very small amount of HCl (5 mL) is added
with the precursor solution and the starting solution is obtained by
diluting it with doubly deionized water of suitable volume. The
substrate temperature (Ts) is maintained at 340
5
C using a
temperature controller with a chromel-alumel thermocouple. Each
spray cycle followed in this study has a spray and a 10 s interval. The
experimental details have been reported elsewhere [8-10]. The
substrates were pre-cleaned ultrasonically with organic solvents and
doubly deionized water. After deposition, the coated substrates were
allowed to naturally cool down to room temperature before being
taken out from the spray chamber.
Structural studies of the spray pyrolysed films was carried out
using an X- ray diffractometer (PANalytical, PW 340/60 X’ pert PRO).
For electrical characterization, the four-point probe with van der Paw
configuration was used. The optical transmission spectra for the
doubly doped thin films were obtained using a UV-Vis NIR double
beam spectrometer (Perkin Elmer-Lamda-35). Surface morphology
and the elemental composition of the samples were investigated by
150
scanning
electron
microscope
(HITACHI
S-3000H)
and
energy
dispersive analysis of X-ray (EDAX) respectively.
6.3 RESULTS AND DISCUSSION
6.3.1 Structural properties
The XRD patterns of the
doubly doped films deposited from
precursor solutions having concentration of 0.1 M with fluorine
doping level 20 at.%, 30 at.% and 40 at.% are shown in Fig.6.1. The
films were polycrystalline in nature with tetragonal structure [11-13].
The film deposited from starting solution having F doping level 20 at.
% has preferential orientation along the (211) plane where as the
films with doping levels 30 and 40 at. % were grown along the (200)
plane. In the present study, at lower F doping level (20 at. %), the
lesser incorporation of F ions into the SnO2 lattice made the
preferential orientation as (211) plane. But at higher F doping level
(30 at. %) the preferential orientation changes from (211) to (200)
plane due to the enhanced incorporation of F ions and the intensity
of the (200) plane found to be increased with the further increase in
the fluorine concentration (40 at. %). Elangovan et al. [14] observed a
similar transition in the preferred orientation from (211) to (200) for
fluorine doping. The sharp and highly intense peaks suggest that the
deposited films have good crystalline nature.
151
Smith et al. [15] have reported that if HCl was added with the
starting solution, the films were highly oriented along (200) plane.
This can be ascribed to the different formation of intermediate
molecules in the starting solution. As mentioned in the experimental
section (section 2) the starting solution in the present study was
prepared with 5 mL of HCl, the above discussed result is an expected
one. The other peaks appeared in the XRD patterns correspond to the
planes (110), (101), (310) and (301).
Fig. 6.1 XRD patterns of SnO2:F:Sb films
The crystallite size of the films is calculated using the wellknown Scherrer’s formula [16-17]
(6.1)
152
where
diffractometer,
is the wavelength of X-rays (1.5406 Å) used in the
is the Bragg diffraction angle and
is the full-width
at half-maximum of the preferential orientation peak. From the
estimated crystallite size values (Table 5.1), it is found that the
crystallite size decreases from 64 nm to 45 nm when the fluorine
doping level increases from 20 at. % to 40 at.% which indicates the
increase in the incorporation of fluorine into the SnO2 matrix. Shinde
et al. [18] reported that increase in the fluorine incorporation causes
a decrease in the grain size for fluorine doped zinc oxide films. The
number of crystallites per unit area (N) is estimated using the
following relations N = t/D3, where t is the thickness of the film[19,
20]. As the films are tetragonal in structure, the lattice constants a
and c are calculated using the equation [21, 22]
(6.2)
The calculated structural parameter values are presented in
Table 6.1. It can be seen from the table that the slight increase in the
lattice constants a and c may be due to the increase in the
substitutional incorporation of F- ions in the O2- sites. It is noteworthy
to mention here that the ionic radius of F- (133 pm) is slightly greater
than that of O2- (132 pm). These values are well matched with the
standard JCPDS (Card No: 41-1445) values [23].
153
Table 6.1
Structural and optical parameters of SnO2:F:Sb films
Sb+F doping
level in the
Lattice
constant (Å)
starting
solution (at. %)
a
N
D
Eg (eV)
(nm)
(
1015)
c
2+20
4.745
3.192
64.22
2.831
3.72
2+30
4.746
3.230
47.07
7.504
3.83
2+40
4.748
3.240
44.91
9.382
3.75
D- Crystallite size, N- number of crystallite per unit area, EgBand gap Standard Values [24, 25] of a = 4.737 Å , c= 3.187Å
6.3.2 Electrical properties
The plots of sheet resistance (Rsh) of SnO2:F:Sb films as a
function of fluorine doping level in the spray solution are shown in
Fig. 6.2 and the corresponding values are presented in Table 6.2. For
lower molar concentrations of the starting solution (0.1 M and 0.3 M)
the electrical sheet resistance values show that the Rsh value
decreases initially as the fluorine doping level increases and attains a
minimum value for 30 at.%. Beyond this doping level, the Rsh
increases with the increase in the F doping level. This F doping level
for which the Rsh is minimum is called as critical doping level. The
154
increase in the Rsh after the critical doping level of F content may
represents the solubility limit of F in the tin oxide lattice. The excess
F ions probably increase the disorderliness in the lattice of the film,
as observed by Thangaraju for FTO films [26]. In addition, after the
critical doping level, the fluorine ions act as acceptors by occupying
the interstitial sites [27]. As a result of these two effects, the Rsh value
begins to increase when the doping level exceeds the critical fluorine
doping level.
Fig. 6.2 Variation in sheet resistance as a function of doping level
155
Table 6.2
Electrical sheet resistance of SnO2:F:Sb films
Sb+F doping
Sheet resistance ( / )
level in the
starting solution
0.1 M
0.3 M
0.5 M
0.7 M
2+20
25.36
28.62
5.89
4.6
2+30
9.51
4.98
4.01
1.81
2+40
38.95
14.49
4.63
0.91
(at. %)
But in the case of higher molar concentrations of the starting
solutions (0.5 M and 0.7 M), the films have lesser Rsh value even for
lower doping level (20 at. %) and the Rsh decreases slightly with the
increase in the fluorine doping (30 at. % and 40 at. %). It may be due
to the predominant role of oxygen vacancies over the substitutional
incorporation of F- ions. A similar behaviour was observed in our
previous study [28] for fluorine doped tin oxide films deposited from
different molar concentrations of the precursor solution by spray
pyrolysis technique. Therefore, it is concluded that the Sb doping in
the starting solution does not influence the variation in the Rsh of
F+Sb doped tin oxide films.
6.3.3 Optical properties
The transmission spectra of SnO2:F:Sb (0.3 M) films with
different doping levels of F are shown in Fig.6.3. The average
156
transmission in the visible region is observed as 85%. At the fluorine
doping level of 30 at. %, the film shows a phenomenal reduction in
the optical transmittance in the IR region which may be due to the
increased number of carrier concentration that causes an enhanced
IR scattering [27]. As tin oxide is a direct band gap material, the
absorption coefficient can be calculated from the following relation
[29, 30].
(6.3)
By
extrapolating
the
Tauc’s
plots
drawn
between
the
(αhυ)2vshυ, we can estimate the optical band gap of the SnO2:F:Sb
films. The estimated Eg values are in the range of 3.72 to 3.83 eV are
presented in Table 6.1.
Fig.6.3 Variation in transmittance as a function of doping level
157
From the table it is observed that the widening of optical band
gap at 30 at. % of fluorine is associated with the Moss-Burstein (MB)
effect [31-33]. According to this effect, when the carrier concentration
is high, the conduction band is partly filled up and the transition
takes place only for photons of higher energy.
6.3.4 Elemental analysis
Energy dispersive X-ray spectroscopy (EDS) is used to obtain
the composition of the films. The EDS spectrum of the SnO2:F:Sb (0.3
M) film deposited with 30 at. % of F doping is shown in Fig.6.4. The
EDAX profiles show that the films contain the Sn, Sb, O and Al
except F. The Al in the deposited films may be resulted from the glass
substrate. The difficulty in observing the presence of F through the
EDAX analysis is mainly due to the lesser amount of F available on
the surface of the film.
Fig.6.4 EDS spectra of the SnO2:F:Sb film
158
Generally, the presence of F in the ultimate product (film) is very low
when compared with that available in the starting precursor solution.
Fukano et al. [34] observed only 0.74% of F in the deposited SnO2:F
film, eventhough the amount of F content in the staring solution is
50%. Hence, the absence of F in our EDS profile is an expected
result. In fact, in the case of fluorine doped tin oxide films, different
research groups have failed in the determination of the F content due
to several factors such as low content of fluorine in the films, the
restricted detection limit of the corresponding techniques and the
high evaporation rate of fluorine which is enhanced by the electron or
photon bombardment used in the Auger electron spectroscopy (AES)
and XPS techniques, respectively [35]. But, the comparison between
the Rsh of undoped and doubly doped films (Table 6.3) confirms the
incorporation of F.
Table 6.3
Sheet resistance of undoped and doubly doped films
S. No.
Precursor
concentration
(M)
Rsh of the
undoped tin
oxide film
(k / )
1.52
Minimum Rsh of the
(Sb+F)doped film and the
corresponding doping
level ( / )
9.51 (2+30at. %)
1
0.1
2
0.3
1.33
4.98 (2+30at. %)
3
0.5
1.19
4.01 (2+30at. %)
4
0.7
1.02
0.91(2+40at. %)
6.3.5 Surface morphology
The SEM images of SnO2:F:Sb(0.3 M) films prepared with
different concentrations are shown in Fig. 6.5
159
Fig. 6.5 SEM images of SnO2:F:Sb films.
From the Fig.6.5,it is observed that the microstructure of the
films is altered by the level of fluorine doping. At low fluorine
concentration (20 at. %), the size of the grains is not uniform and are
found to be in the range of 100-500 nm (Fig. 6.5a). When the fluorine
doping level increases to 30 at. %, the grains become smaller and
uniform in size and the surface looks smooth with good packing
density (Fig. 6.5b). At the doping level of 40 at. %, the surface has
several clusters of grains which may be due to the excess fluorine
incorporation (Fig. 6.5c).
160
REFERENCES
[1]. W.A. Badawy, H.H. Afify and E.M.
Elgiar. J. Electrochem. Soc.,
137 (1990) 1592.
[2]. U. Chaudhuri, K. Ramkumar, M. Satyam. J. Appl. Phys., 66 (1989)
1748.
[3]. R. Cinnsealac, G. Boschloo, S.N. Rao, D. Fitzmaurice. Sol. Energy
Mater. Sol. Cells, 57 (1999) 107.
[4]. K. Vinodgopal, P.V. Kamat. Environ. Sci. Technol., 29 (1995) 841.
[5]. Y. Arai, M. Ishii, H. Shinohara, S. Yamazaki. IEEE Electr. Dev. Lett.,
12 (1991) 460.
[6]. S. Kumar, B. Drevillon. J. Appl. Phys., 65 (1989) 3023.
[7]. V. Brinzari, G. Korotcenkov, V. Golovanov, J. Schwank, V. Lantto,
S. Saukko. Thin Solid Films, 408 (2002) 51.
[8]. L.
Chinnappa,
K.
Ravichandran,
K.
Saravanakumar,
N. Jabenabegum, B. Sakthivel. Surf. Eng., 27 (2011) 770.
[9]. G. Muruganantham, K. Ravichandran, S. Sriram, B. Sakthivel.
J. Optoelectron. Adv. Mater., 14 (2012) 277.
[10]. K.
Ravichandran,
G.
Muruganantham,
P. Philominathan. J. Ovonic Res., 5
B.
Sakthivel,
(2009) 63.
[11]. P.S. Shewale, S.I. Patil, M.D. Uplane. Semicond. Sci. Technol., 25
(2010) 115008.
[12]. H. Kim, R.C.Y. Auyeng, A. Pique. Thin Solid Films, 516 (2008)
5052.
161
[13]. V. Senthil Kumar, P. Vickaman. J. Mater Sci.: Mater. Electron., 21
(2010)578.
[14]. E. Elangovan, K. Ramamurthi. Appl. Surf. Sci., 249 (2005) 183.
[15]. A. Smith, J.M. Laurent, D.S. Smith, J.P. Bonnet, R.R. Clemente.
Thin Solid Films, 266 (1995) 20.
[16]. K. Ravichandran and P. Philominathan. Solar Energy, 82 (2008)
1062.
[17]. B.D. Cullity. (1978), Elements of x-ray diffraction, 2ndedn. AddisonWesley Publishing Company, Reading, MA.
[18]. S.S.
Shinde,
P.S.
Shinde,
S.M.
Pawar,
A.V.
Moholkar,
C.H. Bhosale, K.Y. Rajpure. Sol. State Sci., 10 (2008) 1209.
[19]. K. Ravichandran, P. Philominathan. Appl. Surf. Sci., 255 (2009)
5736.
[20]. K. Ravichandran, B. Sakthivel. P. Philominathan, Cryst. Res.
Technol., 45 (2010) 292.
[21]. G.
Muruganantham,
K.
Ravichandran,
K.
Saravanakumar,
A.T. Ravichandran, B. Sakthivel. Superlatt. Microstruct., 50 (2011)
722.
[22]. E. Fazio, F. Neri, R. Ruggeri, G. Sabatino, S. Trusso, G. Mannino,
Appl. Surf. Sci., 257 (2011) 2520.
[23]. E.V.A. Premalal, N. Dematage, S. Kaneko, A. Konnom. Thin Solid
Films, 520 (2012) 6813.
[24]. JCPDS:
International centre for Diffraction Data,
41 – 1445, (1997).
162
Card
No.
[25]. J.V. Smith (E.d.). X-ray Powder Data File, ASTM Spec. Tech. Publ.
48 L, Inorganic:5-0467 and 6-0416, 1962.
[26]. B. Thangaraju. Thin Solid Films, 402 (2002) 71.
[27]. K. Ravichandran G. Muruganantham, B. Sakthivel.
Physica B,
404 (2009) 4299.
[28]. L.
Chinnappa,
K.
Ravichandran,
K.
Saravanakumar,
G. Muruganantham, B. Sakthivel. J. Mater. Sci.: Mater. Electron.,
22 (2011) 1827.
[29]. J. Jeong, D.-S Na, B.-J Lee, H.-J. Song, H.-G. Kim, Current Appl.
Phys., 12 (2012) 303.
[30]. J. Tauc, R. Grigorovici, A. Vancee. Phys. Stat. Sol., 15 (1966) 627.
[31]. G.
Muruganantham,
K.
Ravichandran,
K.
Saravanakumar,
P. Philominathan, B. Sakthivel. Surf. Eng., 27 (2011) 376.
[32]. E. Burstein, Phys. Rev., 93 (1954) 632.
[33]. T.S. Moss, Proc. Phys. Soc. London. Sect. B 67 (1954) 775.
[34]. T. Fukano and T. Motohiro. Sol. Energy. Sol. Cells, 82 (2004) 567.
[35]. P. Ji, H. Byun, D. Jin and L.J. Kee. J. Elect. Ceramis., 23 (2009)
506.
163