Nanostructured Tin Sulfide Thin Films: Preparation via Chemical Bath Deposition and Characterization

INTRODUCTION

A thin film is a material layer with a thickness ranging from fractions of a nanometer (monolayer) to several micrometers [1-5]. The method of “deposition” is used to make thin films. Deposition is a thin film coating technique that involves changing the four states of matter: solid, liquid, vapor, and plasma [6-9]. Any deposition process aims to produce thin films that are repeatable, controlled, and predictable [10]. Large area thin films must be deposited using chemical processes due to factors of simplicity, affordability, and input energy. Chemical methods for the development of thin films include electrodeposition, electroconversion, electrophoresis, electroless, spray pyrolysis, dip growth, and chemical deposition [11-15]. Thin films are used in a wide range of applications, from nanostructures in nano electronics to multi-square-meter coatings on window glasses [16-18]. Due to the purposeful engineering of nanoscale characteristics into the structure, nanostructured thin films and coatings have characteristics that differ from homogeneous materials [١٩, ٢٠].

Because of their prospective use in optoelectronic devices and solar cells, semiconductors from the IV-VI group have gotten a lot of interest [21, 22]. Due to its advantageous optical bandgap energy, high optical absorption coefficient, and high optical transmission, tin monosulfide (SnS) has attracted a lot of interest as an absorber layer in the production of photovoltaic systems in recent years within the IV-VI group [23-26]. Tin monosulfide is a p-type semiconductor made out of inexpensive and non-toxic materials. It may form cubic as well as orthorhombic crystal structures [27]. It has been reported to have a broad bandgap, ranging from 0.9 to 1.8 eV [28]. These attractive properties make it candidate for different application.

In this study, SnS thin film was prepared via CBD route. The structural and morphological properties of prepared thin film was studied via XRD, AFM, SEM, and UV-Vis analysis comprehensively.

MATERIALS AND METHODS

CBD was utilized to synthesize SnS thin films. The bath contained 0.1 M of (SnCl2.2H2O), 0.15 M (C2H5NS) and (Na3C6H5O7) (TSC), that is dissolved in deionized water by 50 ml . Various TSC mixture (0.10 M, 0.15 M, and 0.20 M) concentrations are prepared. Drop by drop aqueous ammonia is added to adjust the solution Ph to 5.8. At room temperature, the mixtures are stirred by a motorized magnetic stirrer. Previous film deposition, commercial microscopic glass slides are cleaned with acetone, methanol and deionized water. The deposition is done for 4 h at 80oC. Then, the substrates are removed from the beaker, washed with deionized water and naturally dried. The deposited films are homogeneous.

The structural properties of the SnS films were studied by XRD. AFM was employed to study the surface of the films. Then optical transmittance are recorded employing UV-Visible spectrophotometer.

RESULTS AND DISCUSSION

Fig. 1, represent XRD styles of SnS films that are grown through the use of TSC (0.10–0.20 M) various concentrations . All peaks of SnS thin films are matched orthorhombic structure. The dominant peak was (111) plane is actually agrees with (ICDD card no.: 39-0354) [29].

The crystallite size D is evaluated from the Debye Scherer’s formula [30-33]:

Where shows (FWHM).

From Fig. 2, it is stated that there is an increase in the D value with the increase of the TSC concentrations. This is consistent with other results that are published [34]. Moreover, the increasing of the average crystalline size with the increase of the TSC concentration may be assigned to the improvement of SnS crystallinity [35, 36].

The following equation estimate the dislocation density (δ) [37-40]:

The following equation estimates the lattice strain (ε) [41-44]:

The values of scrostrain in Table 1 offer an increase with increasing content of TSC. The

Calculated structural parameters are offered in Table. 1

The FWHM is represented in Fig. 2, Grain size, Dislocation density and Strain versus doping content. The inverse relationship between crystallite size and other parameters are noted in the Figure. Fig. 2 demonstrates β, D, δ and ε Strain versus doping.

The SnS thin film surface morphology is studied through the use of AFM, as it is shown in Fig. 3 which shows AFM measurement technique that offers the digital images that allow of surface features quantitative measurements such as root mean square (RMS) or average roughness Ra that are shown in the Table 2. When taking two images with dimensions, Fig. 3 (a1, b2 and c1) 3D, it is noted that the films are regularly distributed in the small granules form connected without any spaces between them. Fig. 3 (a2, b2 and c2) shows curved volumetric distribution for crystalline granules where difference with TSC content from 0.10 M to 0.20 M respectively. From Fig. 3 (a3, b3 and c3), Ra and RMS values of (12.90, 4.39 and 3.76) nm and (14.80, 5. 08 and 4.34) nm, respectively, The above analysis shows that TSC content affect the values of Ra and RMS. Table 2 represent the values of AFM parameters.

A sharp fall in transmittance at various wavelengths, which corresponds to absorption edge is shown in the recorded Optical transmittance spectra. From Fig. 4, it is proved that a TSC content from 0.10 M to 0.20 M respectively. In general, from Fig. 4 it is observed that the optical transmittance is reduced from 95 to 80% with the increase of the TSC content from 0.10 M to 0.20 M. The transmittance shifts toward lower energies that join the content increment of TSC is explained by the fact that the increase of TSC from 0.20 M approaches the structure of bulk material [45]

The absorption coefficient (α) was specified by equation [46-49]:

α = (2.303×A)/t (4)

Where (t) is film thickness, A is constant, Fig. 5. Shows α against the energy of photon (hυ) , it is observed that the films offers a high (α˃104cm-1) which in turn shows direct transitions [50] .

Band gap variation was calculated from transmittance spectra and Tauc’s formula [51-54]:

Where A is a constant, the relations are plotted between (αhν) 2 and hν. From Fig. 6 we can conclude that the bandgap depends on the TSC content, Eg decreases as TSC concentration increases. The bandgap of nanocrystalline SnS films depends on the content of TSC from 0.10 M to 0.20 M its values were 1.45 and 1.25 eV, respectively [55].

Table 1 represent the values of bandgap.

CONCLUSION

The nanostructure tin sulfide films are produced successfully through CBD at 80 oC for 4 h through the use of various concentrations (0.10 M, 0.15 M and 0.20 M) of TSC. The orthorhombic structure of the grown SnS is revealed by the XRD spectra, as it is marked by the dominant peak of (111) plane. The results show that the increment of TSC content cause a decrement in crystalline size and a decrease in strain. The presence of the SnS phase in the deposited thin films decreased gradually with increasing TSC content. AFM image showed that the grain size of the nanoparticles observed in the range of 72.57 nm to 60.35 nm with TSC content, the optical transmittance exhibits excellent optical transparency. The optical transmittance reduced from 95 to 80%, with the increase of TSC content. The SnS films have (α > 104 cm−1). The energy bandgap range between 1.45 eV and 1.25 eV with the increase TSC content.

ACKNOWLEDGMENTS

Authors would appreciate Mustansiriyah University (www. uomustansiriyah.edu.iq) for supporting this work.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.