In Situ Low-Temperature Chemical Bath Deposition of CdS Thin Films without Thickness Limitation: Structural and Optical Properties

In this work, thin CdS films have been deposited using the chemical bath deposition technique (CBD). Different synthesis parameters, such as number of runs, deposition time, and postannealing temperature, are studied and optimized in order to avoid the supersaturation phenomenon and to achieve a low-temperature growth. CdS thin films, of cubic structure, oriented along the (111) direction with homogenous and smooth surface, have been deposited by using the CBD growth process without any annealing treatment. Based on a set of experimental observations, we show that the solution saturation phenomenon can be avoided if the deposition is performed in several runs at a short deposition time. Throughout the CBD technique, it is then possible not only to overcome any film thickness limitation but also to grow the CdS films in a single technological step at a low temperature and without any postdeposition annealing treatment. CdS films with excellent structural quality and a controllable thickness are obtained when the deposition bath temperature is fixed at 65C. In addition, deposited films exhibit an optical transmittance ranging from 70 to 95% depending on the synthesis parameters, with band gap energy around 2.42 eV. The process developed in this work might be useful for depositing CdS films on flexible substrates.


Introduction
Thin semiconductor films show a great potential for environmental and energy-related applications owing to their abundant unique characteristics [1][2][3][4][5]. Among these films, the CdS ones have drawn the attention of the research community due to their unique optoelectronic properties which include a large direct band gap (2.42 eV at room temperature), excellent optical and electronic properties, and a high chemical stability [6]. In addition, the CdS compound is one of the most suitable partners for heterojunction solar cells as a window layer when used in association with absorber layers like cadmium telluride (CdTe), copper indium gallium selenide/sulfide (CIGS), kesterites (CZTS) [7], or perovskites as an electron transport layer [8][9][10]. CdS also has potential applications in several fields, such as photocatalysis [11], laser [12,13], light-emitting diodes, and field effect transistors [14].
CdS thin films can be synthesized using several deposition techniques through various physical and chemical methods such as molecular beam epitaxy (MBE) [15], sputtering [16], thermal evaporation [17], spray pyrolysis [18], chemical bath deposition [19], successive ionic layer adsorption and reaction [20], and electrodeposition [21]. Among these techniques, chemical bath deposition (CBD) is a simple and cheap method that can produce uniform and adherent large-area films [22]. Hereafter, this method was adopted to prepare the CdS thin films presented in this study. According to previous works, the quality of CdS thin films prepared by the CBD technique depends strongly on various synthesis parameters such as the deposition time [23], the bath and annealing temperature [24,25], the ammonia agent complex [26], and the concentrations of the chemical reagents [27], Unfortunately, when seeking to synthesize CdS thin films by CBD, two major problems are generally encountered: (i) the postannealing treatment, which is a classical essential step for film crystallinity improvement, usually inducing a strong Cd thermal diffusion [28] and prevents the CdS deposition to be extended to flexible substrates [29], and (ii) the film thickness limitation, which is attributed to the solution supersaturation phenomenon [23,30]. To overcome these problems, the synthesis process has been carried out in several runs. Based on experimental measurements, we show that the "number of runs" is a key parameter that affects strongly the structural, optical, and morphological properties of the synthesized CdS thin films. When compared to many reported works [23,[29][30][31], the obtained experimental results show that it is possible to achieve a desired film thickness only by controlling the number of cycles and/or the deposition time. Consequently, when these two parameters are optimized, the problem of supersaturation of the solution, observed in the literature [23,[30][31][32][33], can be avoided. Moreover and contrary to the reported works [24,29,30,34], this technique allows growing in situ films, in a single technological step, at low temperature and without any postdeposition annealing treatment. Thus, the process developed in this work is considered as a relevant candidate when seeking to deposit CdS films on flexible substrates used in the embedded electronic systems.

Experimental Detail
CdS thin films used in this study are grown through heterogeneous reaction, on a glass substrate of 25 mm × 15 mm, by CBD technique. Two solutions named A and B have been firstly prepared separately. Solution A, considered as the cadmium source, is obtained by mixing 10 −2 M of CdCl 2 and 3.6 × 10 −2 M of NH 4 Cl, while solution B, considered as the sulfur source, is the mixture of 1.7 × 10 −2 of SC (NH 2 ) 2 and 3.6 × 10 −2 M of NH 4 Cl. Both mixtures were prepared in water solvent at room temperature. They are secondly individually heated at 45°C in a water bath until they become transparent, then mixed under continuous magnetic stirring (300-400 rpm) to obtain solution C. Before deposition, the glass substrates were ultrasonically cleaned in acetone and ethanol, rinsed in distilled water, dried in air, then vertically immersed into solution C with the help of Plexiglas holders. Our deposition method consists in stabilizing the temperature of the chemical bath (solution C) and substrate at an appropriate value (65°C ± 3°C) and then adding the ammonia drop by drop in order to maintain the pH at approximately 10. Right after, the solution color goes from transparent to orange indicating the start of CdS growth. After an appropriate deposition time, the first run is achieved. The successive runs were performed under the same conditions as the first one. It is worth noting that between two successive runs, the growing films do not undergo any thermal pyrolysis or postannealing treatment, but they are only submitted to an ultrasonic treatment to remove the poorly adhered CdS particles on their surface and then dried in air. The preparation conditions of annealed CdS thin films are presented in Table 1. After the preparation step, the film structure is determined by the PANalytical X'Pert Pro X-Ray Diffractometer, using CuKα (1.5406 Å) radiation source. Scanning electron microscopy coupled to EDS (Quanta 200) is used to observe the surface morphology and to perform the chemical composition analysis of the films. The optical transmittance is measured at room temperature using the Lambda 900 PerkinElmer spectrophotometer in the 300-1100 nm range.

Results and Discussions
In the chemical bath deposition, the ammonia is a complexing agent which controls the release of metal (Cd 2+ ) and sulfur (S 2− ) ions in the alkaline solution. The classical growth mechanism can be summarized by the following chemical reactions [35,36]: (1) The solution of amino-cadmium complex equilibrium: The formation of Cd NH 3 2+ 4 prevents the precipitation of Cd OH 2 .
(2) Hydrolysis of thiourea in an alkaline medium: where SH − ions are in equilibrium with water and give S 2− ions according to this equation: (3) Cadmium sulfide formation:  Figure 1 displays the XRD patterns of CdS thin films deposited in several runs (1-5 runs) at different deposition times (1-15 min). Each deposition is characterized by the number of runs and the deposition time of each run. Only the peak at 2θ equal to 26.81°, corresponding to the (111) plan of the CdS cubic structure, is observed for all the films. It is obvious that the intensity of the observed peak depends strongly on the number of runs and/or the deposition time. The highest intensity is recorded when the process is performed in 5 runs of 5 min. To do better, the variation of the crystalline quality as a function of these two parameters is examined by means of the crystalline ratio (R) defined by the following equation: where I run number, deposition time is the (111) peak intensity at a given number of runs and deposition time and I 1 run, 1 min is the lowest (111) peak intensity obtained for the sample synthesized in a single run during one minute. The variation of the crystalline ratio (R), deduced from Figure 1, as a function of the number of runs and deposition time is illustrated in Figure 2(a). It shows clearly that, regardless of the deposition time, all the CdS films deposited in only one run present low crystalline ratios indicating the poor crystallinity of the films. This behavior can be attributed to the amorphous structure of the glass substrate [37,38]. However, the figure shows also that the (111) peak intensity of each sample increases speedily as the number of runs is  21 22 23 24 25 26 27 28 29 30 18 19 20 21 22 23 24 25 26 27 28 29 30 18 19 20 21 22  3 International Journal of Photoenergy increased. The observed improvement of film crystallinity is attributed to (i) the increase in the material quantity as the number of runs is increased and/or (ii) the deposition is performed on a buffer layer which has already been crystallized. As far as the "deposition time" axis is concerned, two regions can be obviously distinguished in Figure 2(a): (i) a colloidal solution state region is observed when the deposition time is less than 5 min, in which the CdS thin films are formed throughout a nucleation growth mechanism [39]. In fact, just after the addition of ammonia, nuclei are firstly formed on the substrate surface, then grown by diffusion and finally closely adsorbed to form the film (see Figure 2(b)). In this region, an increase of the crystalline ratio is observed as the deposition time increases up to 5 min, which represents the optimal deposition time. This comportment has been also reported by many authors [23,30]. (ii) A colloidal precipitate state region takes place when the deposition time exceeds 5 min. It is initiated by the appearance of the solution supersaturation phenomenon, where precipitated particles come to pulverize relatively the growing film (see Figure 2(c)), leading to the reduction of both the film thickness and the crystalline ratio as experimentally observed. A similar behavior has been observed by Tec-Yam et al. [40]. Based on these results, we can state that deposition in successive runs, with deposition time less or equal to 5 min, is preferred to avoid the solution supersaturation phenomenon. According to Figure 2(a), 5 min is considered as the optimal deposition time which allows the formation of CdS layers with the best crystallinity. Figure 3(a) illustrates XRD patterns corresponding to the as-deposited (sample A) and annealed CdS thin films in air atmosphere (samples: B, C, D, and E) during 1 hour. A highly intense single peak at 26.55°is simultaneously observed for the as-deposited film and also for annealed ones at temperatures lower or equal to 400°C. In general, this atomic position can be attributed to the hexagonal or cubic structure of the CdS phase. However, compared to JCPDS card number 80-0019 and as suggested by Mahdi et al. [41], the absence of (100) and (101) peaks in the pattern (Figure 1(a)) confirms the formation of a (111)-oriented CdS cubic structure. As   International Journal of Photoenergy illustrated in Figure 3(b), only a slight increase of the (111) intensity is noticed when the temperature is increased up to 400°C. This suggests that the crystallization of the asdeposited film is achieved throughout the surface reaction kinetic of the Cd 2+ and S 2− ions under the effect of bath temperature and substrate which is kept at 65°C. By contrast, the film annealed at temperature of 550°C shows a dramatic reduction of CdS (111) intensity and the appearance of other major peaks located at the positions of 32.83°and 38.07°. When compared to the JCPDS card number 78-0653, the two later peaks indicate the formation of CdO phases which are attributed to CdS oxidation. This oxidation took place because the annealing was performed in oxygen ambient [16]. Subsequently, we can state that the annealing treatment slightly improves the crystallinity when the temperature is below 400°C; nevertheless, it becomes destructive and leads to the formation of other phases when the temperature is above 400°C. According to the XRD data of the CdS (111) plan, the crystallite size can be calculated by the following Scherrer formula: where λ (1.5406 Å) is the X-ray wavelength, θ is the Bragg angle, and β is the full width at half maximum (FWHM)  Figure 3(c) shows the variation of the CdS crystallite with the annealing temperature. No remarkable change in the mean crystallite size (about 30 nm) is observed when the annealing temperature is increased up to 400°C. Meanwhile, sample E, which is annealed at 550°C, exhibits a very small crystallite size indicating a drastic decomposition phenomenon of CdS into the CdO phase. These observations show clearly that our process permits to deposit CdS films with good crystallinity without any subsequent annealing treatment. Thus, it is then possible to manage CdS deposition at a lower temperature on substrates, even those which are instable at high temperatures, particularly the flexible ones.

Morphological Study.
To investigate the effect of annealing on the morphological characteristics of CdS thin films, Figure 4 presents the surface SEM micrographs of samples A (Figure 4(a)) and D (Figure 4(b)). In both samples, homogeneous distributions of nanometric grains cover uniformly the surfaces. Grains are smooth and dense, and no evident morphological change is observed before and after the annealing treatment. Figure 5 presents the SEM micrographs (a, b, and c) and the cross-sectional views of CdS thin films prepared at various numbers of runs (1, 3, and 5 runs). The obtained optimal deposition time of 5 min has been chosen. The micrographs show dense structures and smooth and relatively void-free surfaces. In each sample, the grains are well-defined, spherical, and homogeneously sized. It is worth noting that the increase in the number of runs has no significant effect on the grain sizes. Meanwhile, a few agglomerated particles are still observed on the surface; this fact is probably attributed to the slight saturated solution in the end of deposition time (for each run). Based on the cross sections SEM images, the film thickness values of 180, 320, and 580 nm have been measured for 1, 3, and 5 runs, respectively. These last values are very close to that obtained from transmittance spectra ( Figure 6).

Energy-Dispersive Spectroscopy (EDS) Analysis.
Results of EDS analysis performed on the CdS structure (spectrum "a") and agglomerated particle (spectrum "b") are presented in Figure 7. Table 2 reports the average chemical composition (weight and atomic percentage) of the two areas. It appears that both areas are constituted of Cd and S with some additional peaks of Si, O, Al, Na, and Mg which are attributed to the glass substrate chemical elements. It is clear that the average atomic percentage of S/Cd for both studied zones is in a near stoichiometric ratio of approximately 0.97.

Optical Characterizations
3.4.1. Thickness Film Calculation. Based on [42,43], the thickness of the films (d) is calculated from the transmission interference fringes between 530 and 1100 nm using the following equation: where n 1 and n 2 are the refractive indices corresponding to two adjacent maxima (or minima) at wavelengths of λ 1 and λ 2 , respectively. M = 1 for two adjacent extremes of transmittance spectrum (max-max or min-min), and M = 1/2 for two adjacent unlike extremes (max-min or min-max) [44].
The refractive index (n) of the film over the transparent region can be calculated using [42,43] where s is the refractive index of the substrate (in our case, s = 1 51) and N is given by [45] N = 2s T max − T min T max T min + s 2 + 1 2 10 2 휇m

Transmittance, Thickness, Optical Band Gap, and
Absorbance of CdS Thin Films. Figure 8 shows the optical transmittance spectra of CdS thin films in the wavelength range 300-2000 nm. The A, B, C, D, and E samples are highly transparent (~80%) in the visible and nearinfrared regions of the electromagnetic spectrum and present a sharp cut-off at approximately 550 nm. Therefore, no additional improvement of the film transmittance is achieved by the annealing step. The measured average transmittances and thicknesses of the as-deposited film and the annealed ones are reported in Table 3. The high average transparency (~80%) and the appearing interference fringes in the visible and near-infrared regions of all CdS films (except for 550°C) attest that the light is less   International Journal of Photoenergy scattered and the film surfaces are smooth and homogeneous with a very similar thickness of about 254 nm [44]. When the annealed temperature reaches 550°C, again drastic decreases of the optical transmittance (70%) and thickness (219 nm) are noticed. As revealed above by XRD analysis, the observed changes at high annealing temperature are attributed to the decomposition of the CdS film to the CdO phase.
The optical band gap (E g ) of CdS films was estimated by assuming a direct transition between valence and conduction  Figure 7: EDS analysis performed on the CdS structure (spectrum "a") and agglomerated particle (spectrum b).   International Journal of Photoenergy bands from the plot of αhν 2 as a function of photon energy hν according to the expression where hν is the photon energy, α is the absorption coefficient, and A is a constant. E g is determined by extrapolating the linear portion of the spectrum to zero absorption coefficients. The intercept on the energy axis gives the value of the band gap energy. As it can be deduced from Figure 8, no significant change in the optical band gap value (around 2.41 eV) is observed between the as-deposited and annealed films (up to 400°C). However, the band gap energy of 2.24 eV obtained for the film annealed at 550°C matches well with the reported band gap value of CdO [46]. Figure 9 displays the transmission spectra, in wavelengths ranging from 300 to 1100 nm, of CdS thins films prepared at various numbers of runs and deposition times. When increasing the number of runs from 1 to 5, the average transmission at wavelength greater than 500 nm is increased from 55 to 91%, 80 to 94%, and 74 to 86% when the deposition times are 1 min (Figure 9(a)), 5 min (Figure 9(b)), and 15 min (Figure 9(c)), respectively. The highest optical transmittance (94%) is recorded when the deposition is performed in 4 runs of 5 min. By contrast, all CdS thin films deposited in whatever the deposition time (1, 5, or 15 min) exhibit a strong transmission tail in the 300-500 nm range, indicating that the films are thinner with a poor crystallinity [47]. However, when the number of runs is varied from 2 to 5, all spectra show a sharp fall at the band edge which is shifted towards the longer wavelengths as the number of runs increases. This observed shift is likely due to the increase in the film thickness as it has been reported by many authors [43,44]. However, the perceived interference fringes attest to the film quality [44,48] and help to estimate the thickness of CdS layer.  9 International Journal of Photoenergy of runs is 1, 2, 3, 4, and 5 runs, respectively. (ii) The film thickness is slightly reduced when the time is varied from 5 to 15 min. This strange behavior is due to the solution supersaturation phenomenon which occurred at the colloidal precipitate state, where precipitated particles come to relatively pulverize the growing film.
Therefore, the results presented in Figure 6 are of great importance for the development of CdS films with a desired thickness without any supersaturation solution phenomenon. On the one hand, 5 min is recommended as the optimized deposition time. On the other hand, the number of runs can be varied to reach the desired thickness film.
The band gap values (E g ) of CdS thin films prepared at different numbers of runs and deposition times are reported in Table 4. A clear reduction of the band gap is noticed as the deposition time is increased up to 5 min for all numbers of runs. This observed change is likely attributed to (i) the increase in film thickness [43] (Figure 6) and/or (ii) the improvement of film crystallinity [36].
The absorbance spectra of the CdS thin films annealed at different temperatures are presented in Figure 10(a), while those of films prepared at different numbers of runs of 5 min are illustrated in Figure 10(b). No obvious difference is observed between the absorbance spectra of the asdeposited and the annealed films up to 400°C. Conversely, the film annealed at 550°C exhibits a drastic absorbance reduction between 300 and 450 nm, indicating a phase transition. On the other hand, Figure 10(b) indicates an increase in the absorbance intensity as the number of runs is increased. As revealed by the XRD results, this behavior is likely ascribed to the film crystallinity betterment.

Conclusion
CdS thin films, with good structural and morphological qualities, have been successfully synthesized using the CBD technique without any postannealing treatment. The band gap energy was found to be around 2.42 eV with 70 to 95% of optical transmittance in the visible range. The main finding of this work is to show experimentally that performing CBD deposition in "several runs of optimized time" allows avoiding the supersaturation solution phenomenon which constitutes the major problem uncounted when seeking to control the thickness of the deposited films. Therefore, by adopting this based CBD process, it is possible not only to overcome any film thickness limitation but also to grow the CdS films in a single technological step at a low solution temperature (60°C) as well. We believe that this technique paves the way to deposit thin layers on several flexible substrates requested in the embedded electronic field.

Data Availability
The data used to support the findings of this study are included within the article.

Disclosure
The research was carried out in the framework of a PhD thesis at Mohammed V University-Faculty of Science, in collaboration with the National Centre of Scientific and Technical Research (CNRST), Rabat, Morocco.