Science and Engineering Applications 1(7) (2016) 92-95
ISSN-2456-2793(Online)
Contents lists available at JFIPS
Science and Engineering Applications
Journal home page: JFIPS
SAEA
Optical and Electrical properties of Sn-Doped Cadmium Oxide Thin
Films Grown by Chemical Bath Deposition Technique
Gbadebo Taofeek Yusuf*, Babatunde Keji Babatola, Abdul Dimeji Ishola
Faculty of Science, Osun State Polytechnic, Iree, Nigeria
Segun Rotimi Oladimeji
School of Vocational and Technical Education, Department of Technical Education, Osun State College of Education, Ila-Orangun, Nigeria
Adebukola Ayoade Adedeji
Electrical/Electronics, Osun State College of technology, Esa-Oke, Nigeria
Joshua Toyin Adeleke
Faculty of Science, Engineering and Technology. Department of Mathematical and Physical Sciences, Physics with Electronics unit. Osun State
University, Osogbo, Nigeria
*Email: [email protected]
Abstract
The structural, optical and electrical properties of the films at different doping concentrations of 0, 2, 4, and 6 % have been investigated using
chemical bath deposition technique. Cadmium acetate di-hydrate was used as precursor for preparation of the CdO thin films and Tin (II) chloride
was employed as a tin source. The XRD patterns revealed a polycrystalline having the characteristic peaks of cubic structure. Transmittance of the
CdO films increases with increasing Sn dopant in the films, and the maximum transmittance of 84% was seen at 4% doping concentration. The
above observation was confirmed by AFM study which depicts uniform and homogeneous, thin films at 2 and 4 % at Sn doping concentrations.
The average bandgap energy of tin oxide was 2.30 eV and this value is bigger than that of pure CdO 2.22eV. This implies that the tin doping can
be used as a regulator of the bandgap of CdO films. Hall mobility, carrier density of Sn: CdO films show variation with Sn: CdO doping
concentration such that carrier concentration and mobility show the maximum at 4% doping concentration. The films grown at 4% in this work is
a promising candidate for solar cell applications.
Keywords: Cadmium oxide, tin, electrical, transmittance, optical band gap.
Received : 18/12/2016
1. Introduction
There is an ever growing interest in the synthesis of low-cost, non-vacuum
transparent conducting oxides (TCOs) with n-type conductivity [1]. In search of
such suitable TCOs, it is found that chemical and physical conditions can be
conveniently incorporated in the CdO thin films to obtain higher transmission and
conductivity [2]. Transparent conducting oxide thin films of (CdO) has been
deposited on to glass substrates using chemical bath deposition (CBD) [3].
Among these TCO, cadmium oxide (CdO) has received considerable attention for
solar cell application due to its low electrical resistivity and high transparency in
the visible range of solar spectrum [4-5]. Different techniques such as sol-gel [6],
DC magnetron sputtering [7], radio frequency sputtering [8], spray pyrolysis [9]
chemical vapor deposition [6], chemical bath deposition [2-3], and pulsed laser
deposition [10-11] have been used to deposit CdO thin film. CdO micro particles
undergo bandgap excitation when exposed to UV-A light and is also selective in
phenol photo degradation [12]. It has a high electrical conductivity that is
attributed to moderate electron mobility and higher carrier concentration due to
the contribution from shallow donors resulting from inherent non-stoichiometry.
Non-stoichiometric undoped CdO thin films usually exhibit low resistivity [13,
16] and n-type degenerate semiconductor with high electrical conductivity (10 2
to 104 S/cm) due to native defects of oxygen vacancies and cadmium interstitials
[6, 14-16].
Generally, the optical and electrical property of the thin films are dependent on
its oxidation state, the amount of dopant materials, and the fabrication process
[14]. It was reported that when CdO is doped with rare earth oxides, its optical
bandgap can be widened [17]. This can be largely achieved by doping it with
foreign impurities [15]. León-Gutiérrez et al. [18] prepared CdO thin films using
chemical bath deposition method. Furthermore, Fluorine-doped CdO thin films
were prepared by sol-gel process with various atomic ratios of Cd: F to improve
its electrical and optical properties [19], it was reported that the 10% fluorinedoped film prepared with pH 8 precursor solution showed the lowest resistivity
of 0.01574 Ω cm [19]. In another research work, the effects of Te-doping on the
structural, morphological and optical properties of the CdO thin films were
studied by [20]. Optical results indicate that 1 % Te-doped CdO thin film has the
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Published online : 2/2/2017
maximum transmittance of about 87 %, and the values of optical energy band gap
increases from 2.50 to 2.64 eV with the increase of Te-doping ratio [21].
Furthermore, [22] prepared cadmium oxide thin films of zinc with different
concentrations of Zn-doping levels i.e. (0, 2, 4, 6, and 8 at %). The band gap value
increases with Zn doping and reaches a maximum of 2.65 eV for the film coated
with 6 at% Zn doping and for further higher doping concentration it decreases
[22].
Tin (Sn) is another dopant that if doped with cadmium oxide could improve both
its optical and electrical properties of the materials [1]. One of the most favored
dopants is tin which substitutes the cadmium cations. Sn was chosen in this work
as dopant to control and enhance the properties of CdO thin films. Sn 4+ ion has
four valence electrons (5s2, 5p2) and the ionic radius of Sn4+ ion (0.69 Å) is
slightly smaller than that of Cd2+ ions (0.95 Å), thus we expect that Sn4+ ions
doping in CdO will lead to improvement in electrical conductivity by increasing
electron concentration [4]. Also, Sn2+ has an ionic radius of 83pm while Cd2+ has
109 pm. Since Sn2+ has lower ionic radius than that of Cd, there would be
considerable enhancement in its electrical property [24]. This is achievable by a
shift in the optical band gap along with the increase in transparency of CdO films
[23]. CdO thin films have been doped with tin to improve optical and electrical
properties in the literature. [5, 23] fabricated tin-doped cadmium oxide (CdO:
Sn) using atmospheric pressure metalorganic chemical vapour deposition
technique, the result showed highly transparent and conductive CdO: Sn thin
films are promising for photovoltaic applications [5]. The effect of doping on the
physical properties of CdO thin films was reported by a Zheng et al., [24].
There are a number of techniques that can also be employed to grow cadmium
oxide (CdO) including Chemical Vapor Deposition [6], SILAR [25], Thermal
evaporation [16], Sol-Gel Process, [19-20][26][34], SPEED [27], DC Reactive
Magnetron Sputtering [28-29], Spray Pyrolysis [8-9, 22, 30-33, 37], Pulsed Laser
Deposition [9, 27, 36]. The control over the properties of cadmium oxide through
chemical route has attracted increasing interest and can be easily achievable
through chemical bath deposition technique. CBD [12, 16] is the most successful
and best method to obtain low cost CdO thin films that have optimal features for
photovoltaic device applications because of it is an efficient, cost effective, and
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Gbadebo Taofeek Yusuf et al., Science and Engineering Applications 1(7) (2016) 92-95
availability. This technique was however used to deposit high quality CdO films
by controlling its optical and electrical properties for solar cells applications.
2 Experimental Details
Cadmium Oxide (CdO) thin films were deposited onto clean glass substrates
using the Chemical Bath Deposition (CBD) technique. Cadmium acetate dihydrate (Merck 99.9%) was used as precursor for preparation of the CdO thin
films as Cd2+ion source and Tin (II) chloride was employed as a tin source. All
chemicals used were of Analytical Grade. Before deposition of the films, glass
substrates were cleaned thoroughly with industrial soap, rinsed with deionized
water followed by acetone, then rinsed again with deionized water and dried [19].
This process was carried out to ensure clean surface essentially for the formation
of nucleation centres that is required for thin film deposition. 0.2 M of cadmium
acetate di-hydrate were dissolved in 0.25 M of methanol with a constant magnetic
stirring. 35 ml of deionized water was added to the solution until the clear
homogeneous solution was obtained. For Sn doping, Tin (II) chloride di-hydrate
was then dissolved in the solution at different percent (0, 2, 4 and 6 %). The final
solution was stirred for 1 h and kept at 85 oC temperature. A clear and
homogeneous solution was seen. The cleaned substrates were then inserted and
held vertically in the reaction bath for 24 hours for the deposition to complete
[29]. The Film thickness was determined by gravimetric weight difference
method using high precision electronic balance.
UV-VIS Spectrophotometer (Perkin Elmer) was used to characterize the optical
properties of the films. The electrical characterization of the samples was carried
out by the Hall Effect measurement using the Vander-Paw configuration at room
temperature. Crystalline phases of CdO films were characterized by X-ray
diffraction using a Rigaku X-ray diffraction (XRD) system (with 40kV, 30 mA
CuKα radiation, λ = 0.154406 nm). The XRD pattern of the films for 2θ values
are recorded at a range from 15 to 85o. The XRD peaks were compared with
JCPDS cards for phase identification. The AFM images of the CDO films
deposited at different doping conventions were measured with AA 3000,
Angstrom Advanced Inc.
3 Results and Discussions
Fig. 1 shows the X-ray diffraction pattern of CDO films deposited at different Sn
doping concentrations. All CdO thin films were transparent, have good adhesion
to substrate, better uniformity, free from pinhole and were stable for long period
when kept in atmosphere. The XRD revealed diffraction peaks at (111), (200),
and (220) which corresponds to polycrystalline having the characteristic peaks of
cubic structure of CdO (JCPDS Card No. 73-2245. Other disenable planes are
(311) and (222) but with lower intensities. The CdO films have (111) plane as
preferred orientation. Similar observation was also reported by [2].
doping levels, average transmittance are: 75 %, 83%, 84%, and 74 % respectively.
Lowest value of transmittance observed at 6% doping level may be due to excess
Sn atom in the Sn: CDO interstitial sites. On the other hand, low value of
transmittance observed at 0% may be attributed to disproportionate of Sn atom to
balance Cd atom in the Sn: CdO interstitial sites. The films deposited at 2% and
4% have transmittance above 80% and is due to correct proportion of Sn atom to
replace Cd atoms in their intestinal sites. They can therefore be useful for window
layer of a solar cell.
Fig. 2: Transmittance curve of CdO thin films at different tin concentrations
Fig 3 shows the AFM analysis of the films grown at different Sn concentrations
in CdO films. All images exhibit granular morphologies for CdO samples with
grain sizes ranging from 0.21 μm to 0.46 μm and the average thickness of the
nanostructures calculated are found to be about 150 nm for pure CdO film. It is
observed that the doping of tin stimulate an obvious changes of grain size [6].
Furthermore, the influence of incorporation of tin on surface morphology of the
samples can be clearly seen. The thin films with 2% and 4 % doping
concentrations are uniform and homogeneous, with few cracks or few defects [7].
While doping levels 0% and 6% doping concentrations exhibit poor, nongranular, non-uniform and generally poor CdO films. The change of grain size
means change in crystalline quality and total grain boundary fraction in the films.
The change of particle size can be attributed to the difference in ionic radius
between cadmium and tin [8]. On the other hand, results agree with XRD
crystallite size calculations. Electronegativity of these elements are 1.76 and 1.87
respectively. The changes in morphologies may be concerned with different
electronegativity of dopants which affect the thermodynamically stable
development mechanism and free surfaces of the crystal faces of CdO [30].
Fig. 1: XRD at different Sn doping concentrations
(a) 0% of Sn) (b) 2% of Sn (c) 4% of Sn (d) 6% of Sn
Fig. 2 shows the transmission curve of CdO films deposited as a function of Sn
doping concentrations. At 300-550 nm. Average transmittance in the visible
region generally increases as Sn doping level increases. At 0%, 2%, 4%, 6%
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(a)
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Gbadebo Taofeek Yusuf et al., Science and Engineering Applications 1(7) (2016) 92-95
(b)
Sn doped CdO film. The average optical bandgap energy of pure CdO film was
calculated to be 2.30eV. The optical band gap for 0%, 2%, 4%, 6% are: 2.27,
2.29, and 2.33 eV. The optical bandgap energy of tin-doped CdO films increased
with the increase in the tin-doping with maximum at 4 at%. Further increase in
concentration to 6% leads to decrease in optical bandgap. It is consequence of the
structural improvement and the size quantization which leads to change in optical
properties of the metal oxide nanostructures [15]. Moreover, the average bandgap
energy of tin oxide obtained in this work which about (2.30eV) is bigger than that
of pure CdO 2.22 used in the present work. The bandgap of tin-doped CdO which
is bigger than the bandgap of pure CdO means that the tin doping can be used as
a regulator of the bandgap of CdO films. Similar types of behavior of change in
band gap after Sn, Ga and Cu doping were reported in the literature [6]. They
have described increase in bandgap after doping. Higher band gaps are required
for any optoelectronic application, particularly for solar cell applications. The
blue-shift of energy band gap appeared due to electrons populated states within
the conduction band which pushes the Fermi level to higher energy and related to
the Burstein–Moss effect [24, 25].Change in the band structure of the films and
creation of new donor levels in the forbidden band gap can also be observed that
the band gap starts to increase as Sn doping increases. This may also be due to
Burstein–Moss effect. This effect occurs when the carrier concentration exceeds
conduction band edge density of states, which corresponds to degenerate doping
in semiconductors. As the doping concentration increases, more and more donor
states are produced which pushes Fermi level higher in energy [24, 26]. The
Moss–Burstein shift in the band-gap value is given by equation (1).
.
∆𝐸 =
ℏ2
2𝑚 ∗
2
(3𝜋 2 𝑛)3
(1)
Where ħ, is the reduced Planck constant, m*, is the effective electron mass with
respect to the free electron mass and n, is the free electron concentration in a
single valley.
(c)
Fig. 4: Plots of (αhυ)2 versus photon energy hυ for CdO: Sn thin films
(d)
Fig 3: AFM image at (a) 0 % Sn (b) 2 % Sn (c) 4 % Sn (d) 6 % Sn
Fig 4 shows the optical band gap of the CdO films grown at different Sn
concentrations. According to theoretical and practical results, CdO film is known
to be a direct-allowed semiconductor. The optical band gap of Sn doped CdO thin
films was estimated by extrapolation of the linear portion of the (𝛼ℎ𝑣)2 versus for
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Fig. 5 shows the variation of hall mobility, carrier density and resistivity of Sn:
CdO films. It can be seen that the electrical parameters were strongly influenced
by Sn doping in the CdO films. As the Sn concentration increases from 0 to 6%,
the resistivity decreases from 5.56 × 10 −2𝛺𝑐𝑚 to the minimum value of 3.01 ×
10-4 𝛺𝑐𝑚 this value is slightly lower than the obtained resistivity using pulsed
laser method [21] and that of a report which obtained (3.1 × 10 −4 𝛺𝑐𝑚 with metal
organic chemical vapor deposition [27]. Similarly, there is relative improvement
on the carrier concentration results. The improvement may originate from the
replacement of Cd2+ions by Sn4+ions in the CdO lattice. The maximum mobility
4m2/Vs was obtained at 4%. Sn concentration as well as lowest resistivity. As is
well known, resistivity is proportional to the reciprocal of the product of carrier
concentration and mobility. The reduction of electrical resistivity may due to
reduction of voids and improved carrier concentration. The electrical conductivity
and mobility is more sensitive for grain interface than defects of crystals. It is
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Gbadebo Taofeek Yusuf et al., Science and Engineering Applications 1(7) (2016) 92-95
observed that carrier concentration and mobility initially increase with maximum
at 4% Sn: CdO doping concentration. Resistivity decreases with minimum value
of 3.1 × 10−4 Ωcm at 4% Sn: CdO doping concentration which corresponds to
22 ×1021 cm-3 and mobility of 15× cm2 V-1s-1 respectively [28, 35, 36]. These
values indicate the maximum value for optimum concentration for possible
applications. Hall measurements indicate the CdO films to be of n-type
semiconductors. The initial decrease in resistivity is attributed to an increase in
free carrier concentration due to the substitutional incorporation of Sn ions at O 2−
anion sites. However, for films with higher doping concentration, the free carrier
concentration saturates and the mobility slightly decreases due to the formation
of Cd–Sn complexes in the grain boundaries, leading to a slight increase in the
resistivity of the CdO films [29, 37].
Fig. 5: Variation in carrier density (N), Hall mobility 𝜇 H, and resistivity (𝜌) in
Sn:CdO doping concentrations
4 Conclusion
This work investigates the optical and electrical properties of CdO thin films
doped with different concentrations of CdO: Sn (0, 2, 4, and 6 %) using chemical
bath deposition technique. The XRD patterns revealed a polycrystalline having
the characteristic peaks of cubic structure of CdO. Transmittance of the CdO films
increases with increasing Sn dopant in the films, and the average transmittance in
the visible region (at 300-550 nm) has been found (75 %, 83%, 85%, 74 %) for
the tin doping (0, 2, 4, 6 % at) respectively. These observations were confirmed
with AFM studies which exhibits granular morphologies which roughness which
varied with doping content. The grain size of the films ranges from 0.21μm to
0.46 μm based on Sn: CdO doping concentrations. Hall mobility, carrier density
of Sn: CdO films show variation with Sn: CdO doping concentration such that
carrier concentration and mobility increase until maximum at 4% Sn: CdO,
additional increase in concentration led to decrease in carrier concentration and
mobility. Resistivity decreases with minimum at 4% Sn: CdO doping and
increases with further increase in doping concentration.
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