Structural and optical properties of CdS:Sn thin films prepared by chemical spray pyrolysis method

CdS and CdS:Sn thin films were successfully deposited on glasssubstrates by spray pyrolysis method. The films were grown atsubstrate temperatures 300 C°. The effects of Sn concentration on thestructural and optical properties were studied.The XRD profiles showed that the films are polycrystalline withhexagonal structure grown preferentially along the (002) axis. Theoptical studies exhibit direct allowed transition. Energy band gapvary from 3.2 to 2.7 eV.


Introduction
Cadmium sulphide (CdS) is a II-VI binary semiconductor material, its actual color is yellow [1] having two crystal structure, cubic and hexagonal phases [2]. CdS is insoluble in water but soluble in acids [3]. It has direct energy gab equal to 2.42 eV at (300 K) so that the blue region of the spectrum is absorbed by the crystal. The intrinsic conductivity of CdS material at room temperature is low because of the high value of its energy gap compared with Silicon 1.12 eV and Germanium 0.66eV [4,5]. CdS is an important material due to its novel properties like high photoconductivity, high index of refraction (2.5) and its high electron affinity [6]. Besides it is one of the most promising materials for application in electronic and optoelectronic devices such as solar cell, photo sensors, laser materials, optical filters, multilayer light emitting, thin film field effect transistors and gas sensors [7,8]. Polycrystalline CdS films are generally used in CdTe, CuInSe 2 (CIS) and CuInGaSe 2 (CIGS) solar cell as a window material for transmitting the light absorbed by CdTe, (CIS) or (CIGS), also as the n-type material for p-n junction of solar cells [6,7]. Cadmium sulfide (CdS) thin film has attracted increased attention in recent years because of its wide direct band gap energy, optical and electrical properties, and stability, which is suitable for application as a window layer in solar cells [9]. Various methods employed for deposition CdS film, such as chemical vapour deposition, sputtering and spray pyrolysis [10], chemical bath deposition (CBD) [11,12], close space sublimation (CSS), and successive ionic layer adsorption and reaction (SILAR) [13].

Experimental
CdS CdS and CdS:Sn films were prepared on glass substrat. The coating was found to be stable and has a very good adhesive properties. The transmission and absorption spectra were obtained over the range (300-1100)nm by UV-VISIBLE recording spectrometer (Shimadzu model UV-160).

Result and discussion 1. Structural properties
The X-ray diffraction patterns of CdS and CdS:Sn thin film grown on glass substrates were prepared by chemical spray pyrolysis method where Sn constration equal to (1, 2, 3)% are shown in Fig. 1 respectively. The thickness of the films have been measured to be (200±4) nm. All XRD patterns reveal polycrystalline nature of prepared samples. Table 1 illustrates bragg's angles, inter planers spacing, and miller indices of CdS and CdS:Sn thin film. From Fig.1a where (Sn=0) corresponding to CdS thin film, it can be observed that the dominant phase was hexagonal with preferential orientation along (002) at 26.5045 º . Also it can be observed an existing of two peaks along (100) and (101) at 24.822, 28.1794, as matched with the ASTM (American Standards for Testing Materials) card of CdS. This is attributed to the nature of chalcogenides of cadmium which normally show the duality in their crystal structure which means that they can be formed with either sphalerite (cubic, zinc blend type) or wurtzite (hexagonal type) [4] or mixed structures [6]. Our results are agree with A. Ashour [14] who found that CdS thin film of hexagonal phases with preferential orientation along (002). From Fig.1b where (Sn =1) it can be observed of CdS:Sn crystals with hexagonal structure with preferential orientation along (002) at 26.82 º for CdS and the dominant orientation for CdS:Sn crystals was along (100), (101) at 24.8654 º , 28.2228 º respectively. From Fig. 1c where (Sn =2) it can be also observed a of CdS:Sn crystals with hexagonal structure and the dominant orientation for CdS crystals was along (002), (100) and (101) at 26.5658, 24.9698, 28.1958. Finally, from Fig. 1d where (Sn =3) corresponding to CdS:Sn thin film, it is clear that the pure hexagonal structure was dominated for all peaks as matched with the ASTM card of CdS with preferential orientation along (002) at 26.1900 º . From Figure Ncesph note that an increase of the Sn less than intensity while has higher intensity of CdS and this explains.   as well as indicates that the lattice constants decrease with the increasing in Sn concentration as listed in Table 2 and calculated from the following equation [15]: (2) where : is the interplaner distance.
The calculated values of lattice constants for CdS and CdS:Sn thin films are in good agreement with ASTM data. It is clear from XRD patterns of CdS and CdS:Sn thin films, that the full width at half maximum (FWHM) increases with the increasing of Sn concentration in these films. This decreasing in (FWHM) indicates an fluctuating in the grain size of CdS and CdS:Sn thin films as given in Table 2 according to Scherrer's formula where the relation between the grain size (D) and (FWHM) is reversal as follows [16]: D .
where : is the full width at half maximum (FWHM) in radian and λ: is the X-ray wavelength (1.5406 Aº). The dislocation density of CdS and CdS:Sn thin films which defined as the length of dislocation lines per unit volume of the crystal was calculated from this equation [17]: (4) The values of the dislocation density of CdS and CdS:Sn thin films are given in Table 2 The values of the strain of CdS and CdS:Sn thin films are given in Table 2.
It is clear from this table that the strain in CdS and CdS:Sn thin films increases with increasing in Sn concentration which can be also deduced from the decreasing in the grain size , where the increasing in the strain and the dislocation density with the decreasing in the grain size is a well-known phenomenon [19].

2.Optical properties
The transmittance spectrum of CdS and CdS:Sn thin films where Sn equal (1, 2, 3)% are shown in Fig. 2. It is clear from this figure that the transmittance increases with increasing in Sn concentration which can be also deduced from the changing in the films' colure, from yellowish color for x=0 (CdS thin film) to dark yellowish for (CdS:Sn thin film). The absorbance spectrums of CdS:Sn thin films where Sn equal (1, 2, 3)% are shown in Fig. 3. It is clear that as the Sn concentration increases the absorbance of CdS:Sn thin films is decreased.  The absorption coefficient () of CdS and CdS:Sn thin films, was determined by using Eq.(6).
The variation of the absorption coefficient of CdS and CdS:Sn thin films with the wavelength for Sn constretion equal to ( 0, 1, 2 and 3)% is shown in Fig. 4. It can be noticed that the value of the absorption coefficient of CdS and CdS:Sn thin films is of the order of (10 4 ) cm -1 which supports the direct band gap nature of the semiconductor [20]. Our result agrees with Yadav and Masumdar [21] and also agree with Muthukumaran and Muthusamy [22]. It is also clear that the absorption coefficient of CdS and CdS:Sn thin films decreases with the increase in Sn concentration.This is attributed to the decrease in the absorbance of CdS and CdS:Sn thin films with the increase in Sn concentration causing an increment in their absorption coefficient where the relation between the absorbance and absorption coefficient is proportional at constant thickness according to Eq.(6).

Fig. 4: Absorption coefficient of CdS and CdS:Sn thin film.
The optical energy gap values (E g ) for CdS:Sn thin films prepared by chemical spray pyrolysis methode have been determined from the region of the high absorption at the fundamental absorption edge of these films by using Tauc equation [23]: where hυ: is the incident photon energy in eV, Β : is a constant depends on the nature of the material (properties of its valence and conduction band) [39] and r: is a constant depends on the nature of the transition between the top of the valence band and bottom of the conduction band. This equation is used to find the type of the optical transition by plotting the relations (αhυ) 2 , (αhυ) 1/2 , (αhυ) 2/3 and (αhυ) 1/3 versus photon energy (hυ) and select the optimum linear part. It is found that the first relation yields linear dependence, which describes the allowed direct transition, then E g was determined by the extrapolation of the portion at ( α=0 ) as shown in The refractive index ° of CdS and CdS:Sn thin films have been determined by using the following equation [24]: where R: is the reflectance of the films and k ο : is the extinction coefficient. The variation of the refractive index as a function of the wavelength for CdS CdS:Sn, thin films is illustrated in Fig.6. It is clear from this figure that the refractive index decreases with the increasing in the wavelength of the incident photon. Also it can be observed, that the refractive index of CdS and CdS:Sn thin films decreases with the increasing in the Sn concentration. This decreasing is attributed to the decreasing in the grain size of the films with the increasing in the Sn concentration which interns causing an increment in the compactness of the films which in turns reduces the speed of light in the material of the thin film and then leads to an increasing in the refractive index. Where ° varies according to the grain size even if the crystalline structure is itself of the material.   The extinction coefficient (k°) have been determined by using the following equation [24]: where : is the wavelength of the incident photon. It is clear from this equation that k° depends on α and has a similar behavior to α. Fig. 7 illustrates the variation of the extinction coefficient of CdS and CdS:Sn thin films with the wavelength, with the increasing in the Sn concentration the extinction coefficient k° increases. This is attributed to the increasing in the absorption coefficient due to the increasing of the depth of donor levels associated with sulfur vacancies and these levels will be available for the photons to be absorbed causing an increment in the absorbance and leads to increase in the absorption coefficient. Therefore k° will increase with the increasing in the Sn concentration since it has a similar behavior to α and depends on it.

Conclusions
CdS and CdS:Sn thin film thin films were successfully deposited using spray pyrolysis method at substrate temperatures 300 °C. The crystalline and hexagonal CdS and CdS:Sn thin films with only (002). Optical studied indicates that CdS and CdS:Sn thin films exhibit direct band gap and band gap energy decreases with increase in Sn concentration. The variation of band gap energies from 3.2 to 2.7 eV.