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 65°C. 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.
Thin semiconductor films show a great potential for environmental and energy-related applications owing to their abundant unique characteristics [
CdS thin films can be synthesized using several deposition techniques through various physical and chemical methods such as molecular beam epitaxy (MBE) [
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 [
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 CdCl2 and 3.6 × 10−2 M of NH4Cl, while solution B, considered as the sulfur source, is the mixture of 1.7 × 10−2 of SC (NH2)2 and 3.6 × 10−2 M of NH4Cl. 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
Conditions for preparing the CdS films.
Sample | Annealing temperature (°C) | Deposition time (min) | Number of runs |
---|---|---|---|
A | As-deposited | 5 | 3 |
B | 100 | 5 | 3 |
C | 300 | 5 | 3 |
D | 400 | 5 | 3 |
E | 550 | 5 | 3 |
In the chemical bath deposition, the ammonia is a complexing agent which controls the release of metal (Cd2+) and sulfur (S2−) ions in the alkaline solution. The classical growth mechanism can be summarized by the following chemical reactions [ The solution of amino-cadmium complex equilibrium:
The formation of Hydrolysis of thiourea in an alkaline medium:
Cadmium sulfide formation:
The global reaction of CdS formation can be summarized as
Figure
X-ray diffractograms of CdS thin films prepared in different runs and deposition times: (a) 1 min, (b) 5 min, and (c) 15 min.
The variation of the crystalline ratio (
(a) Variation of the crystalline ratio as a function of number of runs and deposition time. Schematic representation of the growth states: (b) the colloidal solution state and (c) the colloidal precipitate state.
Figure
X-ray diffraction analysis of CdS thin films. (a) XRD spectra of the as-deposited and annealed CdS films. (b) Variation of the maximum intensity of the (111) peak as a function of the annealing temperature. (c) Variation of the grain size with the annealing temperature.
According to the XRD data of the CdS (111) plan, the crystallite size can be calculated by the following Scherrer formula:
To investigate the effect of annealing on the morphological characteristics of CdS thin films, Figure
SEM micrographs of CdS thin films: (a) as deposited (sample A) and (b) annealed at 400°C (sample D).
Figure
SEM micrographs and cross-sectional view of CdS thin films: (a) 1 run, (b) 3 runs, and (c) 5 runs with the deposition time of 5 min.
Variation of the film thickness as a function of the number of runs and deposition time.
Results of EDS analysis performed on the CdS structure (spectrum “a”) and agglomerated particle (spectrum “b”) are presented in Figure
EDS analysis performed on the CdS structure (spectrum “a”) and agglomerated particle (spectrum b).
The compositional analyses of the CdS structure and agglomerated particles.
Atomic % | Weight % | |||
---|---|---|---|---|
Cd | S | Cd | S | |
CdS structure | 50.72 | 49.27 | 78.29 | 21.70 |
Agglomerated particles | 50.59 | 49.40 | 78.22 | 21.77 |
Based on [
The refractive index (
Figure
UV-Vis transmission spectra and the gap energy of as-deposited (sample A) and annealed CdS thin films (samples: B, C, D, and E).
Thickness and average transmission values of the as-deposited (sample A) and annealed CdS thin films (samples B, C, D, and E).
Sample | Thickness (nm) | |
---|---|---|
A | 254 ± 4 | 82 |
B | 251 ± 4 | 81 |
C | 253 ± 4 | 80 |
D | 259 ± 4 | 79 |
E | 219 ± 4 | 70 |
The optical band gap (
Figure
UV-Vis transmission spectra of CdS thin films synthesized at different number of runs and deposition times: (a) 1 min, (b) 5 min, and (c) 15 min.
Figure
Therefore, the results presented in Figure
The band gap values (
The band gap values in eV of the prepared CdS thin films as a function of the number of runs and deposition time.
CdS films | 1 min | 5 min | 15 min |
---|---|---|---|
1 run | 2.62 | 2.44 | 2.45 |
2 runs | 2.49 | 2.43 | 2.44 |
3 runs | 2.45 | 2.42 | 2.43 |
4 runs | 2.43 | 2.42 | 2.42 |
5 runs | 2.42 | 2.41 | 2.42 |
The absorbance spectra of the CdS thin films annealed at different temperatures are presented in Figure
The absorbance spectra of (a) the as deposited and annealed films and (b) the films deposited in different numbers of runs.
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.
The data used to support the findings of this study are included within the article.
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.
The authors declare that they have no conflicts of interest.
The authors wish to thank gratefully the National Center of Scientific and Technical Research (CNRST) and the staff of the UATRS Division, for the use of their equipment and technical assistance.