Effect of Thickness on Structural, Morphological, and Optical Properties for Nanocrystalline Thin Films of Cd1-xSnxS, for Optoelectronic Applications

thin films have a large Abstract Thin films of Sn1-xCdxS nanocrystals (X=0,0.7, and 1) with different thicknesses 200, 300, and 400 nm were prepared on glass bases at 300 C by the participation method and chemical spray pyrolysis method (CSP). The effect of the concentration of Sn and thickness on the structural and optical properties of the prepared films was studied. The films were characterized to evaluate the structure, transmittance


Introduction
There is considerable interest in the field of transparent semiconducting materials such as CdS, SnS, etc. for use in a variety of applications, including architectural windows, solar cells, heat reflectors, light transparent electrodes, thin-films photovoltaic devices, and many other optoelectronic devices 1 .CdS and SnS are both promising materials for solar cells.CdS, which belongs to II-VI compound semiconductors 2,3 , is an n-type semiconductor with a direct band gap of (2.4eV), CdS is used as a window material for heterojunction thin films solar cells [4][5][6][7][8] .It has also applications in light-emitting diodes (LED), gas detectors photovoltaic cells, nonlinear optics, and thin film transistors 7,9,10 .SnS is one of the Tin chalcogenide layered semiconductors in group IV-VI, SnS, SnSe is a promising material for solar energy conversion 11, 12 .SnS films are highly suitable for any application in several of solid-state devices, such as photovoltaic, photoelectrochemical(PEC), photoconductive cells, and intercalation battery systems 13,14 .In addition, SnS thin films have a large optical absorption coefficient (> 104 cm-1) [13][14][15] .It Baghdad Science Journal is a p-type window layer heterojunction device [16][17][18] .SnS materials have an optical energy gap for direct transitions of 1.6 eV.
Various methods employed for the deposition of CdS and SnS films are chemical bath deposition 19,20 , chemical vapour deposition 1 , electrochemical deposition [21][22][23] , rf sputtering, vacuum evaporation 1 , and spray pyrolysis method 24,25 .Amongst all the deposition methods, the spray pyrolysis (SP) method is a simple, convenient, and low-cost method for large-area deposition of many binaries, ternary, and quaternary semiconducting films with varying anion and cation concentrations.In this study, thin films of Cd1-xSnxS nanocrystals were prepared using the thermal chemical spray method and their structural and optical properties were studied by studying the effect of changing thickness on their properties to obtain the best properties, which we were able to use in the manufacture of solar cells.

Materials and Methods
Cd1-xSnxS nanocrystalline thin films were prepared on glass substrates at 300°C substrate temperature by the CSP method using aqueous solutions.The spray solution (prepared using the participant method ) consisted of (by volume) 0.05M cadmium chloride (CdCl 2 .H 2 O), thiourea (H 2 NCSNH 2 ), and 0.05 tin chloride (SnCl 2 .2H 2 O) solutions and sodium hydroxide (NaOH) to ensure maximum growth of medium alkaline.Tin solution or cadmium solution or together Tin and cadmium acetate solutions were heated up by 45°C with magnetic stirring principle.The alkaline NaOH solution was then added drop by drop to reach 10 pH.The color of the CdS solution in 30 minutes was light yellow.Then its color grew darker as the reaction time rose until it changed completely from dark yellow to orange.The composition of Cd1-xSnxS films changed from pure SnS to pure CdS (x=0, 0.7, and1).The glass substrates were soaked in chromic acid, cleaned in isopropyl alcohol, rinsed in distilled water at each step, and dried in air.The chemical spray-head-to-substrate distance was fixed approximately at 30 cm.Nitrogen was used as the carrier gas during spraying.The glass substrates were heated by an electrical heater and control of substrate temperature was done using a chromealumen thermocouple.The thickness of the films was measured using the weighted method.The structural properties of the films were studied using XRD analysis and it was performed by a Rigaku Xray diffractometer system using CuKα radiation with the wavelength of λ=1.5406A ο .The morphological properties of the films were investigated using CSPMAA3000 AFM.Optical transmittance spectra of the films were carried out using the Shimadzu UV-160 (UV-Visible-NIR Spectrophotometer) system covering the spectral range from 200 to 1100 nm.

Structural characterization
The structural properties of the Cd1-xSnxS films have been investigated by XRD patterns.The XRD patterns of the samples are given in Fig. 1.The spectra have been obtained by scanning angle 2θ in the range from 20° to 60°.The existence of multiple eight diffraction peaks, and sulphide phases in the diffraction patterns indicates the polycrystalline nature of the Cd1-xSnxS.It is seen that the crystallinity of the CdS films is better than that of other films.It should be noted that the XRD patterns exhibit clear dependence on Sn concentration.The CdS film has been crystallized in a hexagonal ( JCPDS Card no:96-101-1055) with the preferential orientation of(002) as shown in Fig. 1a.The intensity of the peak corresponding to the CdS phase decreases as SnS concentration increases as shown in Fig. 1.cThe SnS phase becomes dominant with lower Cd content.The SnS film has been crystallized in an orthorhombic structure (JCPDS Card no: 96-900-8296) with the preferential orientation of (021) as shown Describes the spectrum X-ray diffraction of films (SnxCd1-xS) thin when changing thickness, as evidenced by the way that the increase in thickness leads to increase the height of the peaks and increase the intensity of which indicates that the change in thickness affects the installation of the film, and this is due to the difference in the number of atoms forming the film from one area to another.The crystallite size of all prepared samples was determined for three peaks with the greatest intensity based on the following Scherrer equation Where λ is the wavelength of the x-ray (Å), β is FWHM (radian) is the intrinsic full width at Half Maximum of the Peak and θ is the Bragg's diffraction angle of the respective XRD Peak.The dislocation density (δ) is calculated using 27 δ=1/D2.It is interesting to note that the crystallite size of Cd 1-x Sn x S thin films improves and the defects like dislocation density decrease with the increase of film thickness.This may be due to the improvement in crystallinity in the films with the increase in film thickness.The crystallite size (D) and dislocation density (δ) for different thicknesses are shown in Table 1.AFM technology provides digital images that quantitatively evaluate surface characteristics, including grain size (nm) and roughness average (nm).Fig. 2 demonstrates the spherical shapes for all samples studied.The images also show a noncompact surface which is not smooth.The grain size (average diameter) obtained from AFM measurements are listed in Table 2.The sizes of nanoparticles obtained from the AFM images appear bigger than the values obtained from XRD measurements 28 .Those results can be interpreted for several reasons; the first explanation is that the nanoparticles tend to form aggregates on the surface during deposition.The second explanation is related to the shape of the tip AFM which may cause misleading cross-sectional views of the sample.The results show that the grain size of CdS is larger than other samples which are consistent with X-ray results.The optical properties of all films with different thicknesses 200,300 and 400 nm have been determined by using the transmittance (T) and absorbance (A) spectrum in the region (220-1100) nm.

Table2. Variation of grain size and average diameter of Cd x Sn
The optical transmittance spectra of the prepared samples are shown in Fig. 3.The figure shows that the permeability decreases with increasing thickness for all prepared samples.The transmittance shows two distinct regions, first at short wavelengths of less than 500 nanometers, where the transmittance suddenly increases with increasing wavelength.This phenomenon is attributed to the pack-pack transition, and the behavior of transitions appears directly in this region, while the second area, is larger than 500 nanometers.We notice that the curve tends to saturate.This is agreed upon by the researcher 29 , as well as the researcher 30 , while Fig. 4 shows the absorption spectrum.

S ,(c) SnS Nanocrystalline thin films with different thicknesses
The absorption spectra of Cd 1-x Sn x S nanocrystalline thin films are shown in Fig. 3.It was observed that the absorption edge shifts towards longer wavelengths with increasing thickness, and this indicates a decrease in the energy gap with increasing thickness.Absorption increases with increasing thickness, and this is because increasing thickness increases the number of atoms, which provides absorption instances for many photons.It has been observed that after (400-500) nanometers there are no absorption peaks, and this stems from the high permeability of the membranes in the visible spectrum region, to observe the behavior.The opposite behavior of permeability.Agreed with the researcher 29 .a The absorption coefficient (α) associated with the strong absorption region of the films was calculated from absorbance (A) and the film thickness (t) using the relation: α = 2.3026 A/t.The absorption coefficient of the Cd1-xSnxS nanocrystalline films for the different thickness films increases with thickness as shown in Fig. 5.
The figure notes the value of the absorption coefficient was (α > 10 4 ) cm -1 .This indicates that the transition takes place between the extended levels in the valence band and the extended levels in the conduction band.We also notice an increase in the values of the absorption coefficient with decreasing thickness at high optical energies, at low optical energies we notice a great convergence in the values of the absorption coefficient, and this is what we also observed in the absorbance.Likewise, the absorbance values converge greatly at high wavelengths.This is what researchers agreed upon 29 and 31 .Likewise, the researcher 32 .

The optical band gap energy values of the Cd 1-
x Sn x S films have been evaluated using the relation between absorption coefficient (α) and incident photon energy (hѵ) by using Tauc eq. 33: αhѵ=B o (hѵ-E g ) r ……2 Fig. 6 and Table 3 show the variation of energy band gab of Cd x Sn 1-x S Nanocrystalline thin films with thickness and concentration of Sn  Fig. 6 and Table 3 show the variation of the energy band gap of CdxSn1-xS Nanocrystalline thin films with the thicknesses of the films and concentration of Sn.It was found that the optical energy gap values decrease with increasing thickness and also with increasing Sn concentration.
The extinction coefficient (k o ) has been determined by using the following equation 34 : Where, α:is the absorption coefficient and λ:is the wavelength of the incident photon.
It is clear from this equation that ko depends on α and has a similar behavior to α. Fig. 6, illustrates the variation of the extinction coefficient of Cd1-xSnxS thin films with the wavelength for x (=0,0.7,1).
From the figure, it is noted that the extinction coefficient changes with the change in thickness.
The figure notes that the relationship is inverse between the extinction coefficient and the thickness, as the extinction coefficient increases with the increase in thickness, because the extinction coefficient behaves the same way as the absorption coefficient, and this is consistent with the results of the researcher 29 .Likewise, the researcher 31 , and also the researcher 32 .

Conclusion
To prepare the Cd 1-x Sn x S nanocrystalline films use first the participant method then use the chemical spray pyrolysis method at 300°C substrate temperature.Films have been characterized using optical and structural measurements.XRD patterns of the films by found polycrystalline.Optical studies indicate that Cd 1-x Sn x S thin films exhibit a direct band gap which strongly depends on the Sn concentration and the thickness of the prepared nanocrystalline films.It was found that the optical energy gap values decrease with increasing the Sn concentrations and with thickness.From the results obtained, it is clear that the prepared material can be used to manufacture the solar systems in taking into account the efficiency of the solar system, whether it is a solar thermal collector or a solar cell which are used as building facades, as well make the other tests to study the possibility of using the prepared samples in the manufacture of the sensors and the effect of changing thickness on the parameters of the gas sensors.