Indium sulfide (In2S3) as a III-VI group semiconductor compound with α (cubic), β (tetragonal) and γ (trigonal) crystalline phases is an interesting material because of its remarkable properties such as stability, transparency, photoconductive nature, large band gap changing between 2.10 eV  and 3.91 eV , n-type conductivity  and low hazard material compared with cadmium sulfide, cadmium selenide , and cadmium telluride. Among these three crystalline phases, β-In2S3 is the most stable one at room temperature. It is an ideal material used as a layer in CuInS2 (CIS), Cu(In, Ga)Se2 (CIGSe), and CdTe-based thin film solar cells . Many researchers have been trying to tune the optical and electrical properties of In2S3 by doping thin films for thin film solar cells applications. It is well known that metal dopant acts as electron donors in semiconductor thin films  and leads to more electrons available in the valence band.
In the previous work. we deposited In2S3 thin films with different complex agent volume, triethanolamine (TEA), on microscope glass substrates at room temperature using the chemical bath deposition technique (CBD) . There are many works on In2S3 thin films doped with elements such as Sn , Co , A1 , As, Sb or Bi , Na , Cu  and Ag . However, there is no attempt on nickel doping of the In2S3 thin films. In this work, undoped and Ni doped indium sulphide (In2S3:Ni) thin films have been synthesized on indium tin oxide (ITO) coated glass substrates for the first time. The present work is focused on the changes in the structural, morphological, compositional and optical properties of the In2S3 thin film when doped with Ni dopants. Undoped and Ni doped films have been synthesized using a simple and inexpensive SILAR technique which provides easy coating of the samples at room temperature [15,16]. In this technique, thin films are obtained by dipping substrate into separately placed cationic and anionic precursors, and then rinsing with deionized water after each immersion. The deposition rate and the thickness of the film may be easily controlled by changing the deposition cycles of SILAR.
2 Experimental Details
The In2S3:Ni thin films have been deposited on indium tin oxide (ITO) coated glasses with sheet resistances of 9.5 Ω/square (76 mm × 26 mm × 1 mm) at room temperature by SILAR method. For the deposition of these films, the concentration of nickel ions is adjusted by controlling the quantity of nickel chloride in the mixture, varying as 0%, 4%, 5% and 6% (in molar ratio of nickel ions to indium ions). Before the deposition, the substrates have been initially washed by detergent, boiled in deionized water, and cleaned in methanol, acetone and deionized water for 10 min sequentially. Following air drying, the deposition of In2S3:Ni thin films were carried out at room temperature. The ITO substrate was immersed in cation precursor solution, containing 0.1 M indium (III) acetate [In(CH3COO)3; Merck; 99.99% purity; pH ~ 4.02] and 0.1 M nickel (II) chloride (NiCl2 + 6H2O; Merck; ACS. Reag.) for 40 s, and then rinsed with deionized water for 40 s before it was immersed in solutions containing 0.05 M sodium thiosulfate [Na2S2O3 5H2O]; 99.5% - 100.5% purity; pH ~ 10.62] for additional 40 s. The substrate was rinsed again with deionized water for 40 s to remove the unreacted ions. By repeating above SILAR steps for 70 times and 75 times for undoped and doped films respectively, thin film of In2S3:Ni was deposited on ITO substrate.
XRD patterns were carried out using x-ray diffractometer with CuKa radiation (XRD, Bruker/AXS-D8, λ = 1.5406 Å) in the 2θ range from 20° to 70°, whileurface morphology was studied by scanning electron microscope (SEM, EVO40-LEO). The elemental composition of the films wasexamined using energy dispersive x-ray spectroscopy (EDS) attached to the SEM. For the optical transmittance measurements, a Perkin Elmer Lambda 4S UV-Vis spectrophotometer in the wavelength range of 400–1100 nm at room temperature has been used. Film thicknesses (t) were determined gravimetrically with a precision microbalance by assuming the density of bulk In2S3 as 4.845 g cm–3. Film thickness shows a reduction with increasing Ni concentration as seen in Table 1.
Ethical approval: The conducted research is not related to either human or animals use.
3 Results and Discussions
Figure 1 shows the diffraction patterns of ITO substrate and the In2S3:Ni thin films. Deposited films are amorphous in nature. When compared with the standard 2Θ and d values, it can be concluded that the obtained film is cubic In2S3 structure (JCPDS card no. 65-0459). As can be seen in Figure 1, the substrate effect is more dominant in the films deposited at Ni concentration of 0% and 4%. However, the crystallinity of the film deteriorates more with increasing Ni incorporation and results with disappearing of diffraction peaks coming from substrate in the film deposited at 6% Ni dopant. Our XRD results are in agreement with the literature  and  in which Mane et al. reported cubic structure with amorphous nature of In2S3 films on glass substrates using SILAR method.
The surface morphology of ITO substrate and In2S3:Ni thin films are shown in Figure 2. As can be seen, the surface of the films shows granular structure with cracks.
The compositional analysis of In2S3:Ni thin films was performed by EDS and the results are listed in Table 2. The EDS results confirm the presence of O, In, S and Ni elements in the synthesized films. It can be seen that the deposited films show excess of oxygen increasing with increasing Ni content. This is an expected result because of the aqueous solution method used. Kamoun et al.  reported that the presence of Al leads to an increment in adsorption of oxygen in the sample. The average ratio for atomic percentage of S/ (In+Ni) is 1.18 for deposited films and it decreases to a value of 0.40 with increasing Ni dopant.
In order to examine the effect of doping on the optical properties of the films, the absorbance and transmittance measurements were performed at room temperature in the wavelength range 400–1100 nm as shown in Figure 3. The films exhibit average transmittance between 47% and 74% and in the visible region.
In order to calculate the band gap of the films, optical absorption of the films wass studied at room temperature The absorption coefficient (α) of the films was calculated from the normalized transmittance (T) data using the formula:
where t is the film thickness. The band gap energy (Eg) is determined using the Tauc’s relation:
where K is a constant depending on transition probability, hv is a photon energy, Eg is the optical band gap, n is an index that characterizes the optical absorption process and theoretically equal to 1/2 for allowed direct transition and 2 for indirect transition .Figure 4 shows the plot of (ahv)2 versus photon energy for the films. The optical band gaps of the films were determined from the intercept of (ahv)2 versus hv curves and obtained Eg values are given in Table 3. The optical band gaps of the films decrease from 2.33 eV to 1.61 eV and shift towards the red region as the Ni concentration in the films increases, where as Barreau et al. [3,l2] found that the optical band gaps of In2S3 thin films, grown by physical vapor deposition, increased from 2.10 eV to 2.95 eV with increasing Na concentration. The shifting in Eg of the films may be attributed to the band shrinkage effect. The narrowing band gap energy is due to the existence of Ni impurities in the In2S3 structure, which induce the introduction of shallow donor levels due to doping. This red shift of Eg could be an increase in carrier concentration with Ni doping. Moreover, a red shift is attributed to a reduction in S concentration (an increase in sulfur vacancies), detected from compositional analysis in Table 2, leads to a reduction in the optical energy gap for In2S3: Ni thin films. The measured band gaps are consistent with reported values 2.3 eV . Timoumi et al.  reported a band gap changing between 1.61 eV and 2.19 eV for In2S3 thin films deposited on glass substrates by vacuum thermal evaporation method.
The transmittance and absorbance data were used to calculate the optical constants such as refractive index (n), extinction coefficient (k), real (ε1) and imaginary (ε2) parts of dielectric constant. The n is described by the Fresnel formulae :
where R is the reflectance and k is given by αλ/4π. The dependence of n and k on wavelength for the films are shown in Figures 5(a) and (b), respectively and results are listed in Table 3. At λ = 600 nm, the n value of deposited films first increases from 1.84 to 2.33 and then decreases to the value of 1.82 with Ni concentration. The present n values are also in good agreement with the literature for In2S3 thin films (1.6–1.84)  and (2.5) . Esmaili et al.  found the n value between 1.0 and 2.5 for In2S3:Cu thin films deposited by CBD. The k values are consistent with our previous data (0.33–0.72)  but they are lower than Ref.  in which In2S3:Cu thin films were deposited by CBD. At λ = 600 nm, the k value varies between 0.18 and 0.43 with the increase in Ni concentration. The k values are consistent with the reported data obtained by Kaleel et al.  in which In2S3:Cu thin films were obtained by spray pyrolysis method.
Figures 5(c) and (d) show the plots of ε1 and ε1, irrespectively. At λ =600 nm, the ε1 value first increases from 3.35 to 5.26 and then decreases to the value of 3.26 with Ni concentration. The ε2 value varies between 0.64 and 2.00 as seen in Table 3. Both ε1 and ε2 values are consistent with the previous data [7,23].
In2S3:Ni thin films are deposited on ITO substrate by SILAR technique. The SILAR is a simple and economic technique and useful for large area thin film depositions with low cost. The effect of Ni dopant on the structural, morphological, optical and electrical properties is studied. The S/(In+Ni) ratio in the films decreases from 1.18 to 0.40 with Ni content. The XRD result shows that deposited films have cubic structure with amorphous nature of In2S3 and their crystallinity deteriorates with increasing doping concentration. The Eg values of the films are affected from the incorporation of Ni and resulted a reduction with Ni doping. The optical parameters such as refractive index, extinction coefficient, real and imaginary parts of dielectric constant were determined. These results show that the Ni doping modifies the structural, morphological and optical properties of the In2S3 thin films.
This work has been supported by the Management Unit of Scientific Research Projects of Mehmet Akif Ersoy University (project no. 0201-NAP-13).
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About the article
Published Online: 2018-08-20
Conflict of interest: Authors state no conflict of interest.
Citation Information: Open Chemistry, Volume 16, Issue 1, Pages 757–762, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2018-0089.
© 2018 Fatma Göde, Serdar Ünlü, published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0