A comprehensive study on the structural, morphological, compositional and optical properties of ZnxCd1−xS thin films

The absorption loss in cadmium sulfide (CdS) thin films which are widely used as a window layer in a photovoltaic cell limits the efficiency of the device. This issue can be addressed by ZnxCd1−xS thin films due to its tunable band gap nature. Herein, the various composition of ZnxCd1−xS (x=0, 0.15, 0.30, 0.45, 0.70, 0.85, 1) thin films were grown by a vacuum thermal evaporation technique and the characteristics of the films were investigated by varying the composition ‘x’. The x-ray diffraction (XRD) studies displayed that the as-deposited films consist of diffraction peaks from both CdS and ZnS lattice. The formation of ternary ZnxCd1−xS films was verified when the deposited films were subjected to an annealing treatment. The morphology of the films was analyzed using a scanning electron microscope (SEM) and it was observed that the films are uniform, homogeneous and free from any pin-holes and cracks. The presence of Zn, Cd and S elements were quantized using an energy dispersive spectroscopy. Optical studies showed a successful non-linear band gap engineering (2.42–3.49 eV) for the deposited ZnxCd1−xS thin films. All films showed a very high optical transmittance of above 70% in the visible wavelength region.


Introduction
In the present decade, there has been substantial research attention in the II-VI compound semiconductors for the use in opto-electronic devices [1][2][3]. Among these II-VI semiconductors, cadmium and zinc compounds are extensively studied due to their suitability for solar cells applications since they are direct band gap semiconductors and have high absorption coefficients [4]. CdS is a wide band gap material and used extensively as a window material in non-linear optical devices and photovoltaic cells. But the absorption loss of CdS thin films in the blue region limits the efficiency of the photovoltaic cells [5] which can be overcome by using a relatively wider band gap material than CdS. ZnS, on the other hand, is having a relatively wider band gap than that of CdS and crystalizes in with a cubic or hexagonal structure. It is an n-type semiconductor, has a very high transmittance (>85%) in the visible wavelength region and is chemically stable [6]. Zn x Cd 1−x S films can be formed by diffusion of Zn in CdS. The semiconducting behavior of the Zn x Cd 1−x S compound can be tuned between the values corresponding to its pure binaries depending on the device requirements. This makes Zn x Cd 1−x S thin films more interesting for solar cell development [7]. Zn x Cd 1−x S thin film can be utilized as a window layer for p-n junction devices without lattice mismatch based on quaternary materials such as CuIn x Ga 1−x Se 2 [8].

Experimental information
The Zn x Cd 1−x S (x=0, 0.15, 0.30, 0.45, 0.70, 0.85, 1) thin films were deposited onto pre-cleaned glass at a substrate temperature of 373 K by vacuum thermal evaporation method. The thickness of the films was maintained at ∼500 nm for all the compositions. Commercially available ZnS (purity 99.995%, Sigma Aldrich) and CdS (purity 99.999%, Alfa Aesar) powder were mixed proportionally for various compositions and was used as the source material. Gravimetric analysis was employed to determine the thickness of the films. Crosssectional SEM was also used to cross-verify with the gravimetric analysis data. The deposited films were subjected to annealing at 373 K in vacuum for a duration of 2 h in order to get pure Zn x Cd 1−x S phase. XRD (Rigaku MiniFlex 600 with CuKα source having λ=1.5418 Å) analysis was used to examine the structural properties of the deposited films. The surface morphological analysis was characterized using a scanning electron microscope (FE-SEM, Carl Zeiss) and the elemental information of the films was obtained by an energy-dispersive x-ray spectroscopy (EDS) method. A UV-vis spectrometer (SpectraPro 2300i) was utilized to record the optical spectra versus wavelength of the deposited films in the range of 325-750 nm at room temperature.

Results
3.1. X-ray studies X-ray diffraction analysis was carried out to obtain information about the various crystallographic aspects of the Zn x Cd 1−x S films. Figure 1 displays the XRD spectra of the Zn 0.45 Cd 0.55 S thin film deposited at a substrate temperature of 373 K prior to the annealing treatment. The two distinct peaks at 2θ=26.46°and 28.52°in figure 1, indicate the improper formation of ternary Zn x Cd 1−x S alloy. Therefore, all the compositions of the deposited films were subjected to an annealing treatment at 373 K in vacuum for a duration of 2 h. The XRD spectra of the annealed films shown in figure 2 revealed the formation of single phase Zn x Cd 1−x S without any secondary phases (JCPDS Card No. 00-024-1137). The sharp peak (figure 2(a)) labeled as (002) at 2θ=26.46°c orresponds to hexagonal CdS thin films [27] whereas the peak at 2θ=28.52°(figure 2(g)) confirms the polycrystalline nature of cubic ZnS films [28]. The obtained XRD patterns suggest that the Zn x Cd 1−x S films change from a hexagonal crystal structure to a cubic crystal structure when the zinc concentration is>45% in the films. From figure 3, it can be observed that the predominant peak slightly shifts towards higher Bragg angles upon rising Zn concentration in the films. Patidar D et al [4] obtained a predominant peak of Zn x Cd 1−x S films at 2θ=∼33°which is higher when compared to the results reported here. The presence of a single peak for Zn x Cd 1−x S films was also observed by Li W et al [23] and Zakira M et al [25].  Using Scherrer's formula, the crystallite size was calculated.
where, 'β' is 'full width at half maximum of the diffraction peak', 'λ' is 'wavelength of the x-ray used' and 'θ' is the 'Bragg diffraction angle'.  For the cubic phase, the lattice parameter 'a' was calculated using: whereas for the hexagonal crystal system, the lattice parameters 'a' and 'c' were determined using: From table 1, it can be noticed that the crystallite size of the (002) predominant peak reduces as Zn content in the Zn x Cd 1−x S thin films increases up to 0.45 in the hexagonal phase which may be due to the stress produced prior to phase change from hexagonal to cubic crystal structure. The lattice parameter, as well as the interplanar spacing of the hexagonal and cubic crystal system, decreases with increasing Zn concentration in the films. A similar decrease in the lattice parameter and interplanar spacing was also observed for Zn x Cd 1−x S films deposited using co-evaporation technique as reported by W Li et al [23]. This is attributed to higher ionic radii of Cd 2+ (0.95 Å) compared to that of Zn 2+ (0.74 Å) in the crystal lattice. The decrease in the lattice parameter and interplanar distance thereby concludes the successful substitution of Cd 2+ by Zn 2+ and hence verifies the development of the Zn x Cd 1−x S films.

Optical studies
The Zn x Cd 1−x S thin films were subjected to a UV-Vis spectrometer at room temperature to study its optical properties. The transmittance spectra obtained for the Zn x Cd 1−x S films are presented in figure 6. It can be seen that all the films are having a transmittance of above 70% and the transmittance of the films rises as the Zn content in the film increases. The presence of the interference pattern in the transmittance spectra at a higher wavelength region indicates the uniform thickness of the films. It can also be observed that as the Zn concentration in the films increases, the absorption edge moves to lower wavelength values, as reported in other investigations [20]. This further confirms the formation of Zn x Cd 1−x S ternary compound as suggested by the XRD analysis. Using Tauc's relation, the optical band gap energy 'Eg' of the deposited films [29] was determined.
where 'B' is a constant and the value of 'n' is ½ for allowed direct transition type.   Figure 7 shows the Tauc's plot of the Zn x Cd 1−x S thin films. The band gap was estimated from the absorption spectra of the films. The band gap energy values calculated differs from 2.42-3.49 eV as the composition 'x' increases from 0.0 to 1.0. Therefore, it can be mentioned that a successful band gap engineering has been achieved for the thermally evaporated Zn x Cd 1−x S thin films. The optical band gap for the corresponding binary elements i.e., ZnS and CdS was found to be 3.49 eV and 2.42 eV respectively, as reported in our previous studies [27,28]. The obtained band gap values of the films are presented in table 2. Figure 8 shows the variation of band gap over the range of composition 'x'. By fitting the obtained band gap values with the below quadratic relation, the degree of the non-linear variation of the films can be calculated [26]: where 'c' is called 'bowing parameter', which is characteristic of a particular alloy system, and 'x' is the composition of the film. Equation