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ffect of tri-sodium citrate concentration on structural, optical andlectrical properties of chemically deposited tin sulfide films
. Godea,∗, E. Gunerib, O. Baglayanc
Department of Physics, Mehmet Akif Ersoy University, 15030 Burdur, TurkeyDepartment of Primary Education, Erciyes University, 38039 Kayseri, TurkeyDepartment of Physics, Anadolu University, 26470 Eskisehir, Turkey
r t i c l e i n f o
rticle history:eceived 31 October 2013eceived in revised form 24 February 2014ccepted 17 April 2014vailable online 26 April 2014
eywords:
a b s t r a c t
Tin sulfide thin films were deposited onto glass substrates by chemical bath deposition. The effects ofmolar concentration of the complexing agent, tri-sodium citrate, on the structural, morphological, opticaland electrical properties of the films were investigated. The films are characterized by X-ray diffraction,scanning electron microscopy, atomic force microscopy, optical absorption spectroscopy and Hall effectmeasurements. Polycrystalline film structure in orthorhombic phase was determined. Flower-like spher-ical grains are observed on the surface. While their average size increased from 345 nm to 750 nm when
−3 −3
in sulfidehin filmshemical bath depositionptical band gapobility
the tri-sodium citrate concentration was increased from 6.4 × 10 M to 8.0 × 10 M, the surface rough-ness varied in an opposite manner from approximately 120.18 nm to 29.36 nm. For these concentrations,optical band gap of the films decreased from 1.40 eV to 1.17 eV, whereas the Hall conductivity, mobilityand carrier concentration of the films increased slightly from 5.91 × 10−5 to 8.78 × 10−5 (� cm)−1, from148 to 228 cm2 V−1 s−1 and from 1.73 × 1012 to 3.59 × 1012 cm−1, respectively.
In recent years, semiconductor tin sulfide (SnS) films havettracted considerable attention because of their numerous advan-ageous properties. For instance, SnS thin films possess large opticalbsorption coefficient of ̨ > 104 cm−1 [1]; a direct energy band gaphanging between 1.10 eV [2] and 2.35 eV [3], which are suitable forhotovoltaic applications; its constituent elements are abundant
n nature and do not give rise to any health and environmentalazards compared to similar materials [4] such as lead and cad-ium compounds. The photoelectric conversion efficiency of the
olar cell fabricated with Cu2ZnSnS4 was measured as 3.14% [5].hese properties facilitate their incorporation in applications, suchs absorber layers in thin film solar cells [6], near infra red detectors7], holographic recording systems [8], anode material in lithiumon batteries [9] and optical sensors [10].
SnS thin films have been produced by various techniques,
uch as spray pyrolysis [11], sputtering [12], vacuum evapora-ion [13], successive ionic layer adsorption and reaction (SILAR)14], solvothermal [15] and chemical bath deposition (CBD) [16].
CBD method is a well-known prevalent low temperature aqueousmethod for directly depositing large-area thin films of semi-conductors. It requires no sophisticated instruments, such asvacuum systems, while the starting chemicals are commonly avail-able and inexpensive. Moreover, the preparative parameters areeasily controlled.
In the production of SnS thin films, use of an appropriate com-plexing agent (i.e. tri-sodium citrate, TSC) is crucial. The effects oftriethanolamine (TEA) as the complexing agent and tin salt concen-tration in the bath on the growth of SnS films by CBD were reportedby Jayasree et al. [17]. Hankare et al. [18] deposited SnS films byCBD using tartaric acid. Mnari et al. [19] produced SnxSy thin filmsusing tri-sodium citrate by CBD and characterized crystallogra-phy, morphology, and chemical properties of the obtained films.Salavati-Niasaria et al. [20] prepared SnS with different nanostruc-ture forms including nanoparticles, nanosheets and nanoflowersvia a simple hydrothermal reaction using thioglycolic acid. Jayas-ree et al. [21] grown SnS films using ethylene diamine tetra-aceticacid (EDTA). SnS microflowers were synthesized with numerousnanoplates by using l-cysteine as the sulfur source and complex-
ing agent by Cai et al. [15]. Although there have been a numberof studies on the growth mechanism, as well as morphologicaland structural properties, reports on influence of the complexingagent on optical parameters (band gap, refractive index, extinction
oefficient real and imaginary dielectric constants) and electri-al properties (conductivity, carrier concentration and mobility) toate are limited.
In this work, structural, morphological, optical and electricalroperties of SnS thin films grown by the CBD method usinghree different TSC concentrations of 6.4 × 10−3 M, 7.2 × 10−3 Mnd 8.0 × 10−3 M are investigated. In addition, suitability of therown films as absorber layers for use in photovoltaic solar cellpplications is discussed.
. Experimental details
SnS thin films were deposited on glass substrates76 × 26 × 1 mm3) by the CBD method. Deposition of the filmsnitiates from the following precursor solution: 0.5 g of SnCl2·H2Oissolved in 2.5 ml of acetone, which is then mixed in 6 ml of 3.7 Mf triethanolamine [(HOCH2CH2)3N]. After that, the non-toxicomplexing agent tri-sodium citrate (C6H5Na3O7) was used inhree different concentrations as 6.4 × 10−3 M, 7.2 × 10−3 M and.0 × 10−3 M. Then, 4 ml 1 M of thioacetamide (CH3CSNH2) and
ml 4 M of ammonia (NH3) were added such that the total solutionolume was completed to 50 ml by adding de-ionized water.he films were obtained at room temperature (30 ◦C) after 24 h.ollowing deposition, the SnS films were washed with de-ionizedater and dried in air. The obtained films were homogeneous, and
heir colors changed from light to dark grey with increasing TSConcentration.
For the X-ray powder diffraction experiments, a ‘Rigaku RadB’iffractometer with Cu K� radiation (� = 0.154 049 nm) was usedt a scanning speed of 0.02◦ s−1 in 2� ranging from 10◦ to 60◦. Theorphology of the films was characterized by using an EVO40-LEO
canning electron microscope (SEM) at a magnification of 30 000×
nder an operating voltage of 20 kV. The film thickness, roughnessnd section analysis of the samples were performed using a VEECOultimode 8 model atomic force microscope (AFM). Transmis-
ion and absorption measurements were carried out with a ‘Perkin
Fig. 1. XRD patterns and SEM micrographs for SnS thin films grown under different
ience 318 (2014) 227–233
Elmer Lambda 2S’ UV–vis spectrophotometer in the 400–1100 nmwavelength region using a non-coated glass as the reference beam.Hall measurements were performed in a HS-3000 Manual Ver 3.5system, using Van der Pauw geometry, at a constant magnetic fieldof 0.54 T. In order to reduce the errors in the calculations, espe-cially for the electrical studies, the sample geometry was fixed toa symmetrical square shape (10 × 10 mm2). Indium contacts havebeen introduced on film surfaces in order to carry out Hall mea-surements.
3. Results and discussion
The effect of TSC concentration on the structure of SnS thin filmswas investigated by means of XRD. Fig. 1(a), (c) and (e) shows theXRD patterns of SnS thin films deposited for three different TSCconcentrations in the reaction bath. All films exhibit only one peakcorresponding to the (1 1 1) plane of SnS with orthorhombic latticestructure. A broad hump between 20◦ ≤ 2� ≤ 40◦ of which is dueto the amorphous glass substrate. Table 1 shows that the observedXRD data is in good agreement with standard data (PDF no. 33-1375) for SnS film and with the data reported by Subramanian et al.[22] and Ghosh et al. [23,24]. The Miller indices (h k l), the observedand calculated interplanar spacing (d), the Bragg angles (2�) andthe lattice parameters (a, b and c) of the diffraction lines comparedwith standard values are listed in Table 1. The lattice parametersof the films deposited for each TSC concentration were calculatedusing the dh k l values (inter-planar spacing) for the orthorhombicstructure, which is given by [25]:
1
d2h k l
= h2
a2+ k2
b2+ l2
c2(1)
where (h k l) are the Miller indices of the plane concerned, and a,b and c are the lattice parameters. These values were found to bea ≈ 0.431 nm, b ≈ 1.120 nm and c ≈ 0.399 nm for the orthorhombicunit cell.
TSC concentrations: (a, b) 6.4 × 10−3 M, (c, d) 7.2 × 10−3 M, (e, f) 8.0 × 10−3 M.
F. Gode et al. / Applied Surface Science 318 (2014) 227–233 229
Table 1Comparison of XRD results for the grown SnS films with the standard values.
Material (SnS) (h k l) 2� (◦) d (nm) a (nm) b (nm) c (nm)
Fig. 2. Two and three dimensional views of AFM images of the SnS films grown under TSC concentrations of (a, b) 6.4 × 10−3 M, (c, d) 7.2 × 10−3 M and (e, f) 8.0 × 10−3 M.
230 F. Gode et al. / Applied Surface Science 318 (2014) 227–233
ferent
dbtobdippscfi
aiciini
Fig. 3. Section analysis of AFM images for SnS thin films prepared with dif
Fig. 1(b), (d) and (f) shows SEM micrographs of SnS filmseposited at the considered TSC concentrations recorded in theackscattered mode at 30 000× magnifications. It can be seenhat film deposited using 6.4 × 10−3 M TSC concentration consistsf flower-like spherical grains with significant vacant space inetween, Fig. 1(b). The grains start to coalesce for the film pro-uced at 7.2 × 10−3 M TSC concentration, Fig. 1(d). Further increase
n the TSC concentration to 8.0 × 10−3 M results in almost disap-earing of the vacancies, Fig. 1(f). This is due to the fact that theorosity of the films is inversely related to the TSC concentrationince larger amounts of the citrate ions hold more Sn2+ ions, as theoncentration is increased. Formation of similar grains on SnS thinlm surfaces is also observed in our previous work [16].
Two-dimensional AFM images in Fig. 2(a)–(c) show that theverage grain size of the SnS particles increases from approx-mately 345 nm to approximately 750 nm with increasing TSConcentration, as listed in Table 2. Three-dimensional AFM images
n Fig. 2(d)–(f) reveal that the surface roughness decreases withncreasing TSC concentration. The root mean square (RMS) rough-ess for SnS films decreased from 120.180 nm to 29.356 nm with
ncreasing TSC concentration. Indeed, as shown in Fig. 2(f), the
TSC concentrations (a) 6.4 × 10−3 M, (b) 7.2 × 10−3 M and (c) 8.0 × 10−3 M.
film surface becomes smoothest when the TSC concentration is8.0 × 10−3 M, while the mean grain size is highest as calculated fromAFM data (approximately 750 nm).
To measure the film thickness by AFM, we produce a steplike structure of the films by applying scratch marks on it witha sharp tool. As the material of the film was softer compared tothe substrate, we assume that the scratches completely removethe film material on the substrate and a step structure is pro-duced at the junction of a scratched and unscratched film wherethe heights of the steps represent the film thickness. Several lat-eral AFM scans across the steps were taken and an average ofthem was calculated to obtain the film thickness. The film thick-nesses calculated from the AFM images increased approximatelyfrom 229 nm to 500 nm, as shown in Fig. 3 and listed in Table 2.From the XRD, SEM and AFM results, the uniformity of SnS thinfilms is found to be improved with increasing TSC concentrationand denser films could be produced by increasing the concentra-
tion.
The dependence of transmission and reflectance spectra of theSnS films is shown in Fig. 4(a) and (b), respectively. The opticalabsorption coefficient (˛) was calculated to be on the order of
F. Gode et al. / Applied Surface Sc
400 500 600 700 800 90 0 100 0 11000
10
20
30
40
50
6.4 x10-3
M TSC
7.2 x10-3
M TSC
8.0 x10-3
M TSC
Tra
nsm
itta
nce
(%
)
Waveleng th (n m)
(a)
400 500 600 700 800 90 0 100 0 11000
10
20
30
40
50
60
70
80
90
6.4x10-3
M TSC
7.2x10-3
M TSC
8.0x10-3
M TSC
Ref
lect
ance
(%
)
Waveleng th (n m)
(b)
Fw
1l
T
wtfibr
˛
wp(vvcv
TA
ig. 4. The spectral variation of (a) transmittance and (b) reflectance of SnS filmsith various TSC concentration.
06 cm−1 in the wavelength range of 400–1100 nm using the fol-owing equation [26]:
= (1 − R)2exp (˛t) (2)
here T is the transmittance, R is the reflectance and t is the filmhickness. The variation of ̨ with wavelength (�) is analyzed tond out the nature of the electronic transition across the opticaland gap. The nature of the transition was determined using theelation:
h� = B(
h� − Eg
)n(3)
here B is a constant, h is the Planck’s constant, � is the frequency ofhotons and n equals to ½ for direct band gap [27]. Band gap energyEg) can be obtained by extrapolating the straight line of (˛h�)2
s. h� curve to intercept the horizontal h� axis. The calculated Eg
alues of the samples, as depicted in Fig. 5(a)–(c), for different TSConcentrations are listed in Table 2. It can be seen from Fig. 5 that thealues of the optical band gap are observed to decrease from 1.40 eV
able 2 summary of optical parameters of the SnS films at the wavelength of 600 nm.
to 1.17 eV as the TSC concentration increases from 6.4 × 10−3 M to8.0 × 10−3 M, as shown in Fig. 5(a)–(c). The observed decrease inband gap width with increasing TSC concentration can be attributedto quantum confinement in the films. This is consistent with resultsreported in literature by Jakhar et al. [28] and by Jain and Arun [29].The cross-sectional AFM images also showed that the thicknessesof the SnS thin films increased with increasing TSC concentration.The increase in film thickness of the SnS thin films is assigned tothe higher concentration of Sn-complexing-agent bonded ions thatmay have exhibited Sn–S bonding in the reaction bath and thusincreased the thickness of the SnS thin film [30]. Band gap energiesof SnS films listed in Table 2 are also in good agreement with thereported data on SnS thin films [2,24,31].
Determining the refractive index of semiconductor is importantfor optical wave guiding in optoelectronic structures such as het-erojunction laser diodes, optical amplifiers or optical fibers. Hencemany attempts have been made to correlate the energy band gap tothe optical refractive index of semiconductors. The reflectance (R),extinction coefficient (k) and the refractive index (n) of a solid at acertain constant wavelength (�) are related through the followingequations [26]:
k = ˛�
4�(4)
R = (n − 1)2 + k2
(n + 1)2 + k2(5)
Using these relations, the values of the refractive index andextinction coefficient are calculated from the absorbance and trans-mittance data. Fig. 6 shows that n is found to decrease from 12.50to 5.04 at the wavelength of 600 nm with increasing TSC concentra-tion. Considering the spectral range of 400–1100 nm, Nwofe et al.[32] reported a refraction index variation between 6.25 and 5.49and our results are comparable to their values. Also it can be seenfrom Table 2 that the refractive index was higher for lower filmthickness. Such a behavior has been observed in other reportedworks by Swaneopeol [33] and Ahmed et al. [34]. The observedextinction coefficient decreased from 0.41 to 0.09 at spectral rangeof 600 nm, with increasing TSC concentration, see Table 2. The real(ε1) and imaginary (ε2) parts of the complex dielectric constant aregiven by [35]:
ε1 = n2 − k2 (6)
ε2 = 2nk (7)
With the help of Equations (6) and (7), the real (ε1) and imag-inary (ε2) parts of the dielectric constant decrease from 156.00 to25.43 and from 10.36 to 0.88, respectively, with increasing TSC con-centration, see Table 2. The extinction coefficients were also higherfor lower film thicknesses. The present real and imaginary dielectricconstants values are consistent with reported data [36].
Table 3lists the Hall coefficients, such as carrier concentration, mobil-
ity, resistivity and conductivity of the SnS thin films with differentTSC concentrations. The carrier concentration and mobility esti-
mated by Hall effect measurements were found to increase from1.73 × 1012 cm−3 to 3.59 × 1012 cm−3 and from 148 cm2 V−1 s−1 to228 cm2 V−1 s−1 with increasing TSC concentration. Devika et al.[37] reported the use of Sb as a dopant that lowers the hole
Fig. 5. (˛h�)2 versus h� plot for the grown SnS films deposited under different TSCconcentrations (a) 6.4 × 10−3 M, (b) 7.2 × 10−3 M and (c) 8.0 × 10−3 M.
Table 3A summary of the electrical properties of the SnS films.
Fig. 6. Dependence of refractive index of SnS films on wavelength as a function ofTSC concentration.
concentration of SnS to be less than 1014 cm−3. Moreover, the car-rier concentration values are lower than those reported in relatedworks (∼1015 cm−3) [38,39]. The reduction of carrier density can beattributed to the scattering agents of charge carriers, such as ionizedimpurities, neutral impurities (defects, vacancies and interstitials),dislocations and grain boundaries [40]. Therefore, the ability to con-trol carrier concentration of SnS with the addition of tri-sodiumcitrate in reaction bath could improve SnS-based thin film solarcells and, in general, could broaden the utility of SnS as an optoelec-tronic semiconductor outside the field of photovoltaics. Moreover,increase in mobility can be understood by considering the increaseof the grain sizes and decrease of the grain boundaries. Mea-sured mobility values are higher than the previously reported data[38,39]. The electrical resistivity decreased from 4.71 × 104 � cmto 2.05 × 104 � cm with increasing TSC concentration. These valuesare lower than the reported data [41]. However, measured electricalconductivity increased from 4.51 × 10−5 to 8.78 × 10−5 (� cm)−1
with increasing TSC concentration. The conductivity values arehigher than that of other reported SnS films deposited by CBD(10−6) [42,43], and SILAR (10−7) [44].
4. Conclusion
SnS thin films are prepared with three different tri-sodiumcitrate concentration (6.4 × 10−3 M, 7.2 × 10−3 M and 8.0 × 10−3 M)on glass substrates at room temperature for 24 h by the chemi-cal bath deposition technique. The structural, electrical and opticalproperties of SnS thin films deposited have been studied as afunction of the tri-sodium citrate ligand concentration. The X-ray powder diffraction pattern was used to characterize SnS thinfilms. The parameters of the orthorhombic unit cell are found tobe a ≈ 0.431 nm, b ≈ 1.120 nm and c ≈ 0.399 nm. The mean grainsize of the films increases with increasing tri-sodium citrate con-centration, as deduced from AFM data. Optical absorption andtransmission of SnS thin films are measured over the 400–1100 nmspectral region to derive the optical parameters. The band gapenergy decreased from 1.40 eV to 1.17 eV with the increase intri-sodium citrate concentration. By SEM and AFM analysis, it isindicated that an increase in tri-sodium citrate concentration leadsto an improvement in the uniformity of SnS thin films, diminu-
tion of the vacancies, reduction in rms surface roughness, as wellas increase in the grain size and thickness. Moreover, Hall mea-surements show that electrical conductivity, carrier concentration
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F. Gode et al. / Applied Surf
nd mobility values increase with tri-sodium citrate concentra-ion.
To sum up, the above-mentioned results suggest that the varia-ion in optical band gap and electrical resistivity with the addition ofri-sodium citrate in reaction bath confirms improved morphologyf SnS thin films which can be utilized in solar energy conver-ion devices. These results may be compared with reported datay Ubale et al. [45] in which influence of the complexing agentNa2-EDTA) on the structural, morphological, electrical and opticalroperties of chemically deposited FeSe thin films was investigated.
cknowledgments
This work was fully supported by the Mehmet Akif Ersoy Univer-ity Scientific Research Projects Coordination Unit under the projectumber 0120-NAP-10. The authors would like to thank Dr. A. Cicek
or reading this manuscript and Dr. R. Demir for helpful discussions.
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