1 Introduction

Metal chalcogenides (sulfides, selenides, tellurides) are important materials for applications such as photo conducting cells, photovoltaic cells, and other opto-electrical devices [1–5]. Among various sulfides, antimony sulfide (Sb₂S₃) thin films have gained special attention due to their potential properties such as high refractive index [6] and well-defined quantum size effects [7]. Moreover, Sb₂S₃ finds applications to the target material of television cameras [8], microwave devices [9], switching devices [10] and in photovoltaic structures [11–14].

Sb₂S₃ thin films are obtained by various techniques such as vacuum evaporation [15–19], precipitation and sintering [20], the three-temperature method [21], the dip and dry method [22], the solution-gas interface technique [23], spray pyrolysis [24–27], electrodeposition technique [28] and chemical bath deposition technique [29–33]. Among them, chemical deposition method is of a special interest, because this method is proved to be the least expensive, low temperature and a non-polluant method. It is also very suitable for making films of large area.

In order to avoid hydrolytic precipitation of antimony (III) basic salts in aqueous media, one has to work in very acidic solutions, which are inconvenient for film deposition. Another way of avoiding basic salt precipitation is to complex the Sb³⁺ ion using a suitable complexing agent such as EDTA and tartric acid, and then another chalcogenide source reagent such as thiourea, thioacetamide or selenosulfate.

In this work, the two required precursors were replaced by a single source reagent, namely sodium thiosulfate, which is both a complexing agent for Sb³⁺ and at the same time a source of sulfide ions upon hydrolysis. The present paper describes the chemical bath deposition of Sb₂S₃ thin films at deposition time ranging from 10 to 60 min and at different temperature from 40 to 70 °C. The structure, morphology and optical properties of these films were analyzed using X-ray diffraction, atomic force microscopy and UV–vis spectrophotometry.

2 Experiments

The formation of antimony sulfide films can be explained by different mechanisms depending on the dissociation of thiosulfate ions [34]. In our experiment, the chemical bath is weakly acidic (pH = 3.8). The dissociation of thiosulfate ions in acidic solution occurs according to the reaction:

Since the thiosulfate is a reducing agent, it may act as an electron donor and reduce the S to S²⁻. The thiosulfate forms with antimony ion (Sb²⁺) a very strong complex Sb₂(S₂O₃)₃, which hydrolyses to form Sb₂S₃ [33] as described by the following chemical reaction:

or

When the ionic product (IP) of Sb³⁺ and S²⁻ ions produced through reactions in the bath is greater than the solubility product (K_sp = 10^−92,77) of Sb₂S₃ [35], the Sb₂S₃ film starts deposition at the substrate.

Orange-yellow antimony sulfide thin films were prepared on tin oxide doped with fluorine SnO₂:F substrates by chemical bath deposition. The SnO₂:F is obtained by spray pyrolysis technique, the composition of the spray solution is:

• • 23 ml of tin chloride, 99%, anhydrous (Acros organics);
• • 5 g of NH₄F (puriss p.a., ACS, ≥98%);
• • 7 ml of bi-distilled water;
• • 970 ml of methanol (puriss p.a., ACS).

The SnO₂ formed on the surface of the glass substrate heated to the temperature 448 °C, results from the following endothermic reaction:

The results of optimization of such layers have been reported in previous works [36,37]. The chemical bath for Sb₂S₃ was prepared in a 100 ml beaker as follows: 650 mg of SbCl₃ (puriss. p.a., ACS reagent, ≥99.0%) dissolved in 10 ml of CH₃COCH₃ (ACS reagent, ≥99.5%). This was followed by the addition of 25 ml of 1 M Na₂S₂O₃ purum p.a., anhydrous, ≥98.0% and 65 ml of bi-distilled water. The SnO₂:F/glass substrates were placed vertically into a hermetical closed deposition cell. The chemical bath was heated, the colorless solution turned into orange-yellow at 35 °C indicating initiation of chemical reaction. The thickness of the films is determined by double weight method, using the relation:

In the chemical bath deposition, the deposition thin film was reported to proceed through a nucleation or incubation period followed by a growth phase and terminal phase. Fig. 1 gives the growth nature of Sb₂S₃ thin films with time period (min). The film thickness increases steadily with dipping time up to 40 min to reach the maximum value of 2.4 μm and the terminal phase in which the growth slows down. The studies were carried out at 40 min and for various bath temperatures varying from 40 to 70°̊C. The obtained layers of Sb₂S₃ were well homogeneous, uniform and compact.

The XRD spectra were obtained by means of X’Pert PRO Alpha-1 using CoKα monochromatic radiation and the instrumental broadening is negligible. The wavelength, accelerating voltage and current were 1.789 Å, 40 KV and 20 mA, respectively, with 2θ ranging from 0 to 70°. The scanning electron microscopy (SEM) was carried out with a Quanta 200. Transmittance and reflectance measurements at near normal incidence were performed, at room temperature, over a large spectral range (0.2–2 μm) on Sb₂S₃ films by a Varian cary 5000 UV–vis/NIR spectrophotometer.

3 Results and discussion

The deposited Sb₂S₃ films were adherent, uniform, crack-free and covered well over the substrate. The XRD patterns (Fig. 2) of the films deposited at different temperatures in the range of 40 to 70 °C were analyzed. It was found that the Sb₂S₃ film material is polycrystalline. The labeled peaks in the XRD were compared to the standard JCPDS powder diffraction data set #74-1046 for Sb₂S₃. The crystallized material is thus identified as orthorhombic Sb₂S₃. A dominant plane (120) was assigned to the Sb₂S₃ material at 2θ = 19.10° for films grown at 40, 50 and 60 °C. However, this peak disappears at 70 °C. Peak intensity of the main plane (120) is found to increase from 40 to 50 °C and decrease from 50 to 70 °C until disappearance. Other characteristic peaks (200), (111), (211) and (301) of stibnite structure were present in all X-ray spectra with low arbitrary intensity compared to the (120) peak. We noted also the presence of planes (111) and (210) assigned to the substrate (SnO₂/glass). In order to study the grain size of the Sb₂S₃ thin film particles, films prepared for 40 min at 50 °C, which present a maximum intensity of (120) plane, are studied by taking the slow-scan XRD pattern around the (120) peak. The grain size is estimated using Scherrer's relation:

SEM is used for studying the surface morphology of the films. Fig. 3 shows SEM images at ×40,000 and ×10,000 of Sb₂S₃ film grown at 50 °C for 40 min. In the micrographs, a spherical shape of the grains is obvious in our case as illustrated in Fig. 3. The grains are homogenous and well distributed indicating the formation of uniform thin films. Presence of some irregular distribution of overgrowth of Sb₂S₃ surface particles are observed in these films (Fig. 3a). Formation of agglomerated surface particles was reported as a common morphological characteristic in Sb₂S₃ films grown by this method [33,35,38]. The grain size was evaluated from SEM images and the approximate value is around 0.20 μm. The value is greater than the value obtained by XRD analysis. The reason may be that SEM is a tool for surface analysis in the level of micrometers while XRD extracts average information from larger area and entire film thickness.

Fig. 4 shows the optical transmittance and reflectance spectra for a 250 nm thick crystalline antimony sulphide thin film grown on SnO₂/glass substrate for 40 min at 50 °C. The broad cut-off of the transmission spectra towards short wavelengths indicates the onset of inter-band absorption in the Sb₂S₃ layers, while the large wavelengths correspond to absorption edge of the SnO₂:(F) underlayer due to the free carries. The corrected transmittance for reflection losses at the air film interface (also displayed in Fig. 4) is calculated using the following equation:

Assuming that the reflection is predominantly from the air-film interface where:

In semiconductors, (hα^ν) is proportional to (hν − E_g)^1/n where E_g is the band gap energy and n = 2, 2/3, 1/2 or 1/3 for direct allowed, direct forbidden, indirect allowed or indirect forbidden optical transitions, respectively [39]. Thus, a plot of (hνα)ⁿ versus (hν) would yield a linear plot, where the band gap value can be determined from the intercept with the photon energy axis. Concerning crystalline antimony sulphide films, the best straight line fit has been found for values of n = 2, meaning a direct allowed transition between the valence and the conduction band. Thus, Fig. 5 shows plot (hνα)² versus (hν) for the Sb₂S₃ thin film grown on SnO₂/glass substrate for 40 min at 50 °C. The extrapolation of the data point to the photon energy axis where α² = 0 illustrates the presence of direct gap located near 2.30 eV for this material. Such a value agrees well with the values of 2.26 eV reported for crystalline Sb₂S₃ thin films [31], but it is relatively higher than the value of 1.73 eV obtained for samples annealed in nitrogen atmosphere [30,33].

For normal reflectance [40], we have:

4 Conclusion

A simple chemical method was used for the deposition of Sb₂S₃ thin films on SnO₂/glass substrates. X-ray diffraction studies show that the deposited films at 50 °C for 40 min were well crystallized with the stibnite structure. Scanning electron micrographs show homogenous and well distributed spherical grains indicating the formation of uniform thin films. Optical measurements show that the films present a well-defined direct band gap of about 2.30 eV. The calculated refractive index agrees well with the reported value 2.75. Further work is required to improve the quality of the Sb₂S₃ films in order to use them as potential materials for the active layer in various solid-state devices, such as thin film solar cells.