Optical and Structural Properties of Chemical Bath Deposited Cadmium Sulphur Selenide (CdS 1–x Se x (0 ≤ x ≤ 1)) Thin Films

The tuneable band gap property of Cadmium-sulphur-selenide (CdS 1–x Se x ) thin film makes it an appropriate material for a wide range of optoelectronic applications and this has aroused a lot of interest. In this paper, we report the study of Cadmium-sulphur-selenide (CdS 1–x Se x ) thin films, successfully grown on commercial glass slide substrate by the chemical bath deposition technique. The effect of selenium content (x value) on the structural, and some optical properties have been studied. The bath solution contained cadmium acetate dehydrate [Cd(CH 3 COO) 2 ·2H 2 O], sodium selenosulphate [Na 2 SeSO 3 ] and thiourea [CS(NH 2 ) 2 ] were used as the sources of Cd 2+ , Se 2− and S 2+ , re-spectively. Tartaric acid (C 4 H 6 O 6 ) was used as a complexing agent. The pH of the solution was adjusted to 12 by drop-wise addition of ammonia. The bath temperature was kept at 90˚C for a deposition time of 1 hour. Post deposition annealing processes of the thin films were performed in a furnace at a temperature of 400˚C for two hours. Both as-deposited and annealed films were characterised by Powder X-Ray Diffraction, Scanning Electron Microscopy, UV-Visible Optical Absorption Spectroscopy and Energy Dispersive X-Ray Analysis. Optical absorption data analysis indicates that direct allowed transi-tions occur in the films. The band gap of the as-deposited CdS 1–x Se x decreased linearly from 2.34 eV to 1.48 eV, with increasing selenium content, and in the annealed samples, decreased from 1.84 eV to 1.36 eV. X-ray diffraction measurements revealed, that pure CdS, and CdSe had mixed hexagonal and cubic phases. All the remaining ternary compounds were composed of cubic CdS and hexagonal CdSe phases. The annealed samples showed well defined and more intense peaks, suggesting an improvement in crystallinity. The average grain size increased slightly with increasing selenium content. SEM micrographs showed that the films were compact with a smooth texture and good coverage across the entire area of the substrate.


Introduction
Extensive studies have been carried out on CdS and CdSe semiconducting compounds, either as thin films or nanoparticles. This is due to the interesting properties they exhibit such as, high absorption coefficients in the visible and infrared part of the electromagnetic spectrum, good electrical properties (e.g. carrier mobility and lifetime) and the possibility of obtaining adjustable n-or p-type conductivity by doping [1]. Both compounds have direct band gaps with values suitable for photovoltaic applications. In addition, the formation of a homogeneous solid solution over the entire compositional range (0 ≤ x ≤ 1), by the combination of these two compounds allow the production of very interesting ternary CdS 1-x Se x systems [2]. Such mixed compositions are of interest mainly because they allow tuning of the semiconductor properties (most commonly bandgap and, therefore, spectral sensitivity). These are considered very important for a wide range of optoelectronic applications.
There are several techniques available for the fabrication of binary and ternary semiconductor materials in varying sizes. Among these techniques, CBD offers a simple and inexpensive route to deposit semiconductor nanostructures and thin films. CBD is currently one of the fastest growing techniques of materials research since this deposition method is straightforward, inexpensive and yields high quality materials that are interesting for applications in various optical and electronic devices. CBD is becoming an important deposition technique for thin films of compound materials like chalcogenides [3]. Another attractive feature of the CBD process is the ease with which alloys can be generated without the use of any sophisticated instrumentation and process control [4]. Post deposition annealing is also an important technique in thin film fabrication, which can help enhance the properties of the thin film by changing the microstructure and phases.
As a follow up to our earlier publication, Hone et al. [5], we reported the chemical bath deposition of CdSe thin films, from baths containing Cadmium acetate as the source of cadmium ions and tartaric acid as a complexing agent. The bath temperature was 80˚C, pH of 10 and a deposition time of ninety minutes. We further explained that, this was the first time cadmium acetate was being used as the source of cadmium ions in combination with tartaric acid as a complexing agent for the deposition of CdSe thin films. In this present work, the deposition parameters reported in Hone et al. [5], have been modified to synthesize the ternary CdS 1-x Se x (0 ≤ x ≤ 1). The optical and structural properties of the as-deposited and annealed films are studied. To the best of our knowledge, this work provides a new synthesis route for the chemical bath deposition of CdS 1-x Se x (0 ≤ x ≤ 1) thin films.

Materials and Methods
The starting chemicals used for the deposition were 0. The 0.25 M sodium selenosulphate (Na 2 SeSO 3 ) was prepared by dissolving 3.088 g of Na 2 SO 3 in distilled water and transferring into a 100 ml volumetric flask. Distilled water was then added to make up the final volume of 100 ml. The solution was then shaken thoroughly to have a uniform sodium sulphite solution. This was then heated to a temperature of 80˚C with continuous stirring.

Optical Properties Studies
Generally, the optical properties of the semiconductors, especially the absorption spectra and optical band gaps, are intensively affected by their band structure, which is basically dominated by the microstructure [6]. The absorbance spectra of the CdS 1-x Se x (0 ≤ x ≤ 1) thin films are shown in Figure 1. The spectra were taken within the wavelength range of 200 to 1100 nm at room temperature.
From Figure 1, the samples exhibit a strong and broad absorption edge in the range of 450 to 700 nm. It can also be observed from the optical spectra that there is a shift in the fundamental absorption edge towards longer wavelengths (red shift) with increasing selenium content. This suggests a decrease in band gap which may be attributed to an increase in grain size as sulfur is substituted by selenium [7]. It can also be observed that the absorbance of CdS is higher than CdSe. This is because CdS deposited under these conditions resulted in thicker films. A similar observation was also reported by Hodes [3].
The absorbance spectra of the annealed films also follow a similar pattern.

The Optical Band Gap
The energy band gap and transition type was determined using the relationship given by Stern [8]; where v is the frequency, h is the Planck's constant, K is a constant while n carries the value of either 1 for direct transition or 4 for indirect transition. Since CdS and CdSe have direct transitions, it follows that their mixed compositions are also direct transitions, hence, n = 1. Figure 2 shows (Ahv) 2 as a function of   [5]. Several investigators have offered various explanations for the decrease in band gap after annealing. Ezugwu et al. [9] explained that, the temperature dependence parameters that affect the band gap are reorganization of the film, change in the crystallite size of the film and self-oxidation. According to Chikwenze and Nnabuchi [10], reorganization of the films occurs at all annealing temperatures, which may then fill the voids in the films resulting in denser films and lower band gaps. They further explained that as the size of a semiconductor becomes larger, its direct allowed band gap decreases. In this work, the annealed films appeared to be darker as compared to their as-deposited counterparts. This change in colour is a manifestation of a change in grain size.
The decrease in band gaps of the annealed films could be attributed to an increase in grain size as a result of smaller grains coalescing to form larger grains during the annealing process. This supports the explanations offered by Ezugwu et al. [9] and Chikwenze and Nnabuchi [10].

X-Ray Diffraction Studies
The X-ray diffractograms of CdS, CdSe, CdS 0.9 Se 0.1 (as-deposited and annealed),        have mixed hexagonal and cubic phases. Singh and Bhushan [1] have suggested that, the pure CdS thin film might be described as consisting of different atomic layers of CdS in cubic as well as hexagonal phases. A similar explanation can be used for the pure CdSe thin film. Hodes [3] carried out an extensive study on the chemical bath deposition of CdS, and CdSe. The study examined the crystal structure, optical and electrical properties of the thin films, as well as variants of the deposition process. On the crystal structure of CdS, the author reported that the energy difference between the hexagonal (wurtzite) and cubic (zincblende) phases is very small (the former is slightly more stable) hence it is not surprising to observe a mixture of these phases. Such mixed phases were also reported in CdSe thin films deposited from baths containing selenosulphate as the Se source.
The author also explained that in view of the lack of a convincing explanation of why one or the other crystal structure is formed, differences in preparation from the standard bath (if any) may be responsible for the crystal modification obtained.
It was also observed that all the CdS 1-x Se x ternary films consisted of cubic CdS and hexagonal CdSe. Typical examples are shown in Figure 8 and Figure 9.
Langer et al. [11], described a possible structure for this observation. According The lattice constant of cubic CdS was computed from Equation (2) using data from the (311)c plane, whilst that of the hexagonal CdSe was computed from Equation (3), using data from the (011)h plane. The plots are shown in Figure   10 and Figure 11.
From Figure 10, it can be observed that lattice constant of CdS generally increases with Se percentage concentration indicating a pull by the surrounding CdSe leading to expansion. It can also be observed from Figure 11 that the lattice constant of CdSe also decreases with Se percentage concentration which shows compression by the CdS neighbours. Such a model could explain the uniform shift of absorption edge with variation in composition [11].
A simpler model describing the solid solutions as consisting of a statistical distribution of CdSe and CdS with respect to their overall concentrations was also mentioned by Langer et al. [11]. Open Journal of Applied Sciences Figure 10. Variation of lattice constant "a" of (311)c CdS with Se mole concentration "x" for the as-deposited samples. Figure 11. Variation of lattice constants "a" and "c" of (011)h CdSe with Se mole concentration "x" for the as-deposited samples.
Calculation of the average grain size of all the compositions CdS 1-x Se x , (0 ≤ x ≤ 1) was done using the Scherrer [12] formula which is given as: where D is the crystallite size, λ is the X-ray wavelength, β full width at half maximum (FWHM) or integral breadth, θ is the Bragg angle and K is a constant [13].
A plot of the variation in crystallite size against selenium composition is shown in Figure 12.
From the graph the average grain size increased slightly with the Se content.
This supports observations made in the optical analyses that the decrease in energy band gap of the films with increasing Se content can be attributed to an Open Journal of Applied Sciences  increase in grain size. Other structural parameters such as variation in lattice constants, shown in Figure 10 and Figure 11, may also contribute to the observed Open Journal of Applied Sciences

Conclusion
A new synthesis route has been used to successfully deposit CdS 1-x Se x (0 ≤ x ≤ 1), thin films by the chemical bath deposition technique. X-ray diffraction measurements revealed, that pure CdS, and CdSe had mixed hexagonal and cubic phases. All the remaining ternary compounds were composed of cubic CdS and hexagonal CdSe phases. There was no phase changes observed in the annealed samples, however, they showed well defined and more intense peaks, suggesting an improvement in crystallinity. The average grain size increased slightly with increasing Selenium content. Structural parameters such as, lattice constants and average grain size varied with increasing selenium content. SEM micrographs showed that the films were compact with a smooth texture and good coverage across the entire area of the substrate. Optical Absorption Spectroscopy showed that Vergard's law [14] was well established and the optical band gap of the CdS 1-x Se x thin films can be controlled in an optimal region of 1 -1.5 eV for photovoltaic application. Any future work would include further characterization techniques such as Photoluminescence measurements and further studies on the surface morphology.