1.
Introduction
Semiconductor nanocrystals (NCs) have attracted a great deal of attention during the past few decades. Among them, CdS is the most interesting because it found large interest for various applications in optoelectronics, photovoltaics, catalysis, and biological sensing[1]. CdS is an IIBVIA semiconductor material which possesses excellent physical properties such as: structural and optical properties, making it a good candidate as the buffer layer in CdS/CuInSe2[2], CdS/CdTe[3], CdS/ CIGS[4], CdS/CuS[5] and recently in Cu2ZnSnS4/CdS thin film solar cells[6], photo detectors, gas sensors, optical filters[7], thin films transistors[8], semiconductor lasers[9], and photo electrochemical cells[10]. Several deposition techniques have been used for CdS thin films preparation, namely: thermal evaporation[11], sputtering[12], spray pyrolysis[13], electrodeposi tion[14], chemical bath deposition CBD[15], and pulsed laser deposition[16].
Among these techniques, CBD is simple and inexpensive. It is a technique in which films are deposited on substrates immersed in solutions sources of metallic ions (Cd2+) and chalcogens ions (S2–). A complexing agent (a base) is added to limit the hydrolysis of the metallic ion and to give certain stability to the bath. This method is based on material controlled precipitation. The slow release of the chalcogens ions (S2–) in the solution where free metallic ions (Cd2+) are complexed in low concentration. The formation of uniform CdS film on the substrate takes place when the ionic product [Cd2+]; [S2–] exceeds the solubility product Ksp = 10?28[17]. The cadmium source agents including cadmium sulfate, cadmium acetate, cadmium chloride[18], and cadmium nitrate[19] are commonly used as the Cd source. Cadmium ions were complexed by ammonia, EDTA, and thriethanolamine[9]. While thiourea, sodium sulfide, sodium thiosulfate, and thioacetamide were used as sulfur source agents[17].
In this work, our attention is focused on the elaboration of cadmium sulfide (CdS) thin films and on some of their physical properties using cadmium sulfate as the Cd source and ammonia as the complexing agent.
2.
Experimental details
2.1
Thin film fabrication
Cadmium sulfide thin films have been deposited on glass slides 36 × 14 × 1.6 mm3 by the chemical bath deposition method. Before deposition the substrates are well cleaned using, successively, distilled water, acetone, and methanol followed by a second clean in distilled water for 15 min, and finally they are dried in air.
Fig. 1 shows the scheme of the experimental set-up (CBD) realized to prepare CdS films. CdS films are formed from the reaction between aqueous solutions of 1 M cadmium salt (CdSO4·8/3 H2O), 1 M thiourea (CS(NH2)2) in 60 mL distilled water and 9.5 M ammonia was used as complexing agent. The alkaline solution pH was adjusted to 11 in the beginning of each deposition. The introduction of solutions in the beaker was made in the following order: (1) Dionized water, (2) Cadmium sulfate solution (CdSO4·8/3 H2O), (3) Ammonia or ammonium hydroxide (NH4OH), (4) Thiourea solution (CS(NH2)2).
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Figure1.
(Color online) Diagram of the CBD process. CdS film is formed by a mechanism ion by ion (blue) or a cluster by cluster (red) or both simultaneously. Homogeneous precipitation in solution (green) and heterogeneous precipitation on the substrate (blue and red).
A set of samples was prepared by varying the solution temperature from 55 to 75 °C and by keeping the deposition time fixed at 25 min (Table 1).
| Code of sample | Molarity solutions (mol/L) | Bath temperature (°C) | Deposition time (min) | ||||
| CdSO4·8/3H2 | NH4OH | CS(NH2)2 | |||||
| CdS 1 | 1 M | 9.5 M | 1 M | 55 | 25 | ||
| CdS 2 | 60 | ||||||
| CdS 3 | 65 | ||||||
| CdS 4 | 70 | ||||||
| CdS 5 | 75 | ||||||
Table1.
Deposition parameters of different CdS samples.
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| Code of sample | Molarity solutions (mol/L) | Bath temperature (°C) | Deposition time (min) | ||||
| CdSO4·8/3H2 | NH4OH | CS(NH2)2 | |||||
| CdS 1 | 1 M | 9.5 M | 1 M | 55 | 25 | ||
| CdS 2 | 60 | ||||||
| CdS 3 | 65 | ||||||
| CdS 4 | 70 | ||||||
| CdS 5 | 75 | ||||||
The CdS thin film color was yellow at low bath temperatures and orange at high temperatures. All films enjoyed a good adherence with the glass substrate or the beaker walls; this was tested by adding concentrated hydrochloric acid to the film or using scotch tape; we note that it is difficult to peel the film.
2.2
Films characterization techniques
CdS film thickness (d) was measured with a gravimetric weight difference method using a sensitive electronic microbalance and employing the following relation[20]:
$$d = frac{{{m_2}-{m_1}}}{{ ho S}}, $$  | (1) |
where m1 (g) and m2 (g) are respectively the sample mass before and after deposition, ρ is the CdS density (4.84 g/cm3) and S (cm2) is the film area. The thickness results were confirmed by using a KLA-TENCOR P6a type profilometer.
An X’PERT Powder, PANalytical X-ray diffractometer operating at 40 kV and 40 mA, in a 2θ scanning range from 20°–70° with Cu Kα radiation of wavelength 1.5406 ? was used for films structural analysis. The films crystallite sizes (D), deformation or strain (ε), dislocation density (δ) and number of crystallites per unit area (N) of CdS films were deduced from XRD analysis.
An UV-3101 PC-SHIMADZU double beam spectrophotometer was used for optical study in the wavelength range 200–800 nm. The optical properties studied include: transmittance (T%), energy band gap (Eg) and Urbach energy (EU). The refractive index was deduced from an ellipsometric carried by the Jobin-Yvon Horiba ellipsometer.
3.
Results and discussion
3.1
Kinetic and growth mechanisms
The obtained films thicknesses are ranged from 250 to 500 nm with increasing bath temperatures (Fig. 2). Generally, film thickness is the most reported parameter rather than the deposition rate. From thickness variation, we deduced the growth rate which is the film thickness ratio on deposition time. In Fig. 2, we have also reported the variation of CdS thin films deposition rate as a function of bath temperature. As seen, the deposition rate is an increasing function of bath temperature indicating a significant dependence of the growth rate on temperature. The highest growth rate, equal to 19.46 nm/min, was obtained for films grown at 65 °C; however, at low temperature the deposition rate is around 10 nm/min. As shown, the influence of the temperature can be divided, at least, in three regions:
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Figure2.
(Color online) Variations of CdS thin films thickness and growth rate as a function of bath temperatures.
? Region I (nucleation and growth) low temperatures range (< 65 °C). In this range, the growth rate increases quasi-linearly, as a function of bath temperature. The deposition rate obtained at 65 °C is equal to 19.46 nm/min, and it is, approximately, two times higher than that obtained at 55 °C. This influence is due to the activation of chemical reactions between species, which contribute to films formation from the alkaline solution. Actually, films growth by the CBD technique may occur through two mechanisms such as ion by ion or cluster by cluster. Indeed, the bath temperature motivates, on one hand, the thiourea decomposition (Eq. (2)), which is responsible for sulfur chalcogenide ion (S2–) production:
$${ m{SC}}{left( {{ m{N}}{{ m{H}}_2}} ight)_2} + 2{ m{H}}{{ m{O}}^-} = {{ m{S}}^{2-}} + { m{C}}{{ m{H}}_2}{{ m{N}}_2} + 2{{ m{H}}_{ m{2}}}{ m{O}}, $$  | (2) |
and on the other hand, it releases metal ions (Cd2+) by dissociation of complexing compound Cd(NH3)42+ according to the following equation:
$${ m{Cd}}left( {{ m{N}}{{ m{H}}_3}} ight)_4^{2 + } = { m{C}}{{ m{d}}^{2 + }} + 4{ m{N}}{{ m{H}}_3}. $$  | (3) |
The formation of this compound is an intermediate reaction, necessary to control (Cd2+) ions hydrolysis and to give a certain stability to the bath (Eq. (4)). Therefore, the ammonium hydroxide solution addition in the bath is primordial.
$$begin{split}& ,, {{ m{NH}}_4^ + ,, { m{ + ,, H}}{{ m{O}}^{ m{- }}} ,, =,, { m{ N}}{{ m{H}}_{ m{3}}},, +,, {{ m{H}}_{ m{2}}}{ m{O, }}}& quad ,,{{ m{CdS}}{{ m{O}}_{ m{4}}} ,, { m{ = ,, C}}{{ m{d}}^{{ m{2 + }}}}{ m{,, +,, SO}}_4^{2- }, }& {{ m{C}}{{ m{d}}^{{ m{2 + }}}},, { m{ +,, 4N}}{{ m{H}}_{ m{3}}},, { m{ = }},, {{left[{{ m{Cd}}{{left( {{ m{N}}{{ m{H}}_{ m{3}}}} ight)}_{ m{4}}}} ight]}^{{ m{2 + }}}}.}end{split}$$  | (4) |
Consequently, the obtained free ions Cd2+ and S2- react between them by ionic reaction (mechanism ion by ion) to forming nucleuses or molecules of CdS (Eq. (5)).
$${ m{C}}{{ m{d}}^{{ m{2 + }}}}{ m{ + }}{{ m{S}}^{{ m{2-}}}}{ m{ = CdS}}{ m{.}}$$  | (5) |
These nuclei cover the substrate surface (nucleation or incubation period) and become stable when the solubility product (Ksp = 10?28) of CdS in the alkaline bath is reached. Then, the growth is carried out on these first nuclei. Simultaneously with the production of a new nucleus, a thin film is formed by the substrate coverage.
The calculated activation energy of film growth, in the low temperature region, as shown in the insert of Fig. 2 is equal to 0.72 eV (~71 kJ/mol), this is close to the formation enthalpy 74.2 kcal/mol of Cd2+[21]. Thereafter, one can conclude that the formation of Cd2+ (reaction 3) controls the films formation. This is consistent with the fact that an ammonia complexing agent is necessary for slow release of Cd2+ ions that control the film formation. Moreover, the growth is dominated by the ion by ion process in this low temperature range.
However, the thiourea decomposition and Cd2+ ions release become faster by increasing the bath temperature from 55 to 65 °C. This causes the increase of free ions (Cd2+ and S2–) concentrations in the bath, and therefore, the increase in the growth rate (Vd) and films thicknesses (Fig. 2).
During the experiments in this temperature range, a heterogeneous precipitate formation on the substrates is clearly seen. It is denser compared to the precipitate in solution (homogeneous precipitation).
? Region II (saturation), at 65 °C, the growth rate reaches its saturation value (~ 20 nm/min); due to the strongest decomposition of cadmium sulfate and thiourea. One can consider the temperature of 65 °C as the critical temperature corresponding to the appearance of the mixed growth process, where both mechanisms ion by ion and cluster by cluster coexist.
? Region III (reduction) (T > 65°C): At the temperatures higher than 65 °C, we remark a fast formation of CdS in clusters form in the solution as well as on the immersed substrate. Once the reactants are exhausted, the growth starts to slow down and stops. The films growth is carried out, probably by the cluster by cluster mechanism. The film formation passes, initially, by the production of an intermediate phase Cd(OH)2 coming from the dissolution of the ammonium hydroxide (Eq. (6)),
$$n;{ m{C}}{{ m{d}}^{{ m{2 + }}}}{ m{ + }}2n{ m{ H}}{{ m{O}}^- } = {left[{{ m{Cd}}{{left( {{ m{OH}}} ight)}_{ m{2}}}} ight]_n}, $$  | (6) |
and secondly, by the replacement of HO– by S2– in order to form CdS (Eq. (7)),
$${left[{{ m{Cd}}{{left( {{ m{OH}}} ight)}_{ m{2}}}} ight]_n} + n{ m{ }}{{ m{S}}^{2 -}} = n{ m{ CdS}} + 2n{ m{ H}}{{ m{O}}^ -}.$$  | (7) |
By increasing the temperature from 65 to 75 °C, a decrease in the growth rate is observed (Fig. 2) accompanied by film thickness reduction. This is due to the impoverishment of the solution just after the first minutes of deposition. This is due to the fast consumption of reactants. In fact, at high temperatures (above 65 °C), we feel a pungent odor of ammonia (NH3), evaporated during the chemical reactions, which led us to limit the bath temperature at 75 °C. The presence of NH3 in the bath is essential to control the chemical reactions and to stop the creation of colloids in solution. Moreover, the evaporation of NH3 and therefore, HO– formation in the solution, proves that film growth is controlled cluster by cluster as a dominant mechanism. From another point of view, the reduction of growth rate and film thickness may have other origins such as: the dissolution of unstable nuclei adsorbed on the film surface, or the formed film peeling due its thickness increasing.
The variation of the growth rate has been reported and studied extensively by several authors as a function of different deposition parameters such as: bath temperature[22], deposition time[22], pH of the solution[23] and thiourea[24] or cadmium concentrations[25]. The shape of this variation is an intrinsic characteristic of the CBD technique, contrary to the other deposition techniques of thin films[17].
3.2
Structural analysis
In Fig. 3, we presented the X-ray diffraction patterns (XRD) of CdS thin films deposited at different bath temperatures from 55 to 75 °C. The XRD spectrum of CdS film deposited at 55 °C indicates that this film has an amorphous structure. Film structure becomes polycrystalline while prepared at higher temperatures. The diffraction peaks recorded in film deposited at 65 °C are assigned to the cubic or hexagonal structure C(111)/H (002), H(103) and C(222)/H(004) located at the angles: 25.4°, 48.1° and 54.17°, respectively. These peaks identifications are based on the standard files (JCPDS data: 80-0019 and 41-1049) respectively. The same results were mentioned by several authors[26–28]. These spectra show also the presence of a broad peak in the form of a bump, indicating that films are composed of small crystallites (cubic or hexagonal) embedded in amorphous tissue. It is noted that the intensity of the peak located at 25.4° increases with the bath temperature (up to 65 °C) and then it decreases. This result is in good agreement with the variation of the films thicknesses as a function of the bath temperature.
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Figure3.
(Color online) X-ray diffraction spectra of CdS films prepared at different bath temperatures.
The crystallite size of CdS film was calculated from the middle-height width of the peak C(111)/H(002) using the Scherrer equation for all samples prepared by this technique (Eq. (8))[29]:
$$D = frac{{0.94lambda }}{{Delta left( {2{theta _{hkl}}} ight) cdot cos {theta _{hkl}}}}, $$  | (8) |
where D: crystallite size, Δ(2θhkl) = β: full width at half maximum (FWHM) of the diffraction peaks, θ: diffraction angle, λ: wavelength of the λKα ray (Cu) = 1.54 ?.
Fig. 4 represents the variation of the growth rate and the crystallite size of CdS films as a function of bath temperature. The crystallite size varies inversely to the growth rate variation, suggesting that the bath temperature affects the crystallite size through its influence on the growth kinetics. In other terms, the film growth rate becomes relatively high with increasing bath temperature (from 55 to 65 °C), which causes the creation of a large number of nucleation centers (rapid nucleation). Consequently, the high concentration of juxtaposed nuclei limits their enlargement and forms small crystallites. While, in the case of the low growth rate the concentration of the formed nuclei on the substrate surface is small. Thereafter, nuclei have enough space to expand and to form larger crystallites.
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Figure4.
(Color online) Growth rate and crystallite size variation as a function of bath temperature.
Fig. 5 illustrates the evolution of the growth rate and films strain (ε). The latter was calculated using the following equation[20]:
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Figure5.
(Color online) Evolution of the growth rate and the strain with different bath temperatures.
$$varepsilon = frac{{Delta left( {2{theta _{hkl}}} ight) cdot cos theta }}{4}, $$  | (9) |
with: Δ (2θhkl) = β: width at half maximum of the diffraction peaks C(111) / H(002).
In Table 2 we have reported the different values of dislocations density and crystallites numbers per unit of surface for CdS films calculated using the following equations, respectively[20]:
$$delta = frac{1}{{{D^2}}}.$$  | (10) |
$$N = frac{d}{{{D^3}}}.$$  | (11) |
As shown, the strain and the growth rate variations, as functions of bath temperature, have the same trends. As previously mentioned, the growth rate rise by increasing bath temperature (from 60 to 65 °C) causes the appearance of strain in CdS films network. By further increasing the temperature from 65 to 75 °C, the films are more organized due to the enhancement of atoms mobility on the growing film surface. This is accompanied by the strain reduction and the crystallite size enlargement. We can, also, deduce the nature of the deformation or strain (extension or compression) from the shift direction of the intense peak C(111)/H(002) (Fig. 3). As depicted in Fig. 6, the strongest peak shifts towards the lower diffraction angles indicating that the deformations are compressive.
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Figure6.
(Color online) Deformation nature determination.
However, the strain varies oppositely to the crystallite size variation (Fig. 7). The increase in the crystallites number and the reduction of their size, with increasing the bath temperature (60 to 65 °C), are at the origin of the appearance of a large density of grain boundaries and dislocations (see Table 2). These defects yield to strain increase in the films network. However, the enlargement of crystallites size in the temperature range from 65 to 75 °C causes the reduction of grain boundaries and thereafter, the reduction in films strain and dislocation (Table 2). The largest crystallite and the lowest strain were recorded in film prepared at a higher solution temperature of 75 °C.
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Figure7.
(Color online) Crystallite size and strain variation as a function of bath temperatures.
| Sample | Bath temperature (°C) | Density of dislocations (10?4lines/nm2) | Number of crystallites/unit of surface (10?4nm?2) |
| CdS 1 | 55 | – | – |
| CdS 2 | 60 | 15.25 | 220.53 |
| CdS 3 | 65 | 25.507 | 626.86 |
| CdS 4 | 70 | 1.92 | 9.107 |
| CdS 5 | 75 | 1.657 | 5.91 |
Table2.
Dislocation density and number of crystallites per unit of surface, calculated for CdS films prepared at different bath temperatures.
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| Sample | Bath temperature (°C) | Density of dislocations (10?4lines/nm2) | Number of crystallites/unit of surface (10?4nm?2) |
| CdS 1 | 55 | – | – |
| CdS 2 | 60 | 15.25 | 220.53 |
| CdS 3 | 65 | 25.507 | 626.86 |
| CdS 4 | 70 | 1.92 | 9.107 |
| CdS 5 | 75 | 1.657 | 5.91 |
3.3
Optical analysis
Optical characterizations were based on the transmission spectroscopy in the UV–visible wavelength region. Fig. 8 presents the variation of transmittance as a function of the wavelength for the samples prepared at different bath temperatures. All films exhibit a relatively high transmittance ranged between 55% and 80% in the visible range. This result supports the application of CdS films as a buffer (window) layer in solar cells[30]. In these spectra, we note the absence of the interference fringes. Therefore, we deduce that the film surfaces are rough. With increasing the solution temperature from 55 to 65 °C, film transmittance decreases due to the film thickness decrease.
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Figure8.
(Color online) Spectra of transmittance as a function of wavelength of CdS films deposited at different bath temperatures.
The film deposited at 75 °C is the best candidate for use as a buffer layer in thin films based solar cells, because of its high transmittance on one hand, and of its thickness on the other hand. Since its thickness is not too thick to prevent the free electrons flowing from the transparent layer towards the absorber layer, and not too thin to cause an electrical short-circuit between these two layers.
The variation of CdS films refractive index (n) measured at 600 nm as a function of bath temperature is reported in Fig. 9. The refractive index varies in the range of 1.85 to 2.5; these values are in good agreement with those reported in Ref. [22]. As can be seen, with increasing solution temperature, the film refractive index increases, indicating the film densification, which is consistent with the crystallite size.
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Figure9.
Variation of CdS film refractive index (at 600 nm) with the bath temperature.
The exploitation of the transmittance spectra enables the calculation of the optical band gap (Eg) and the Urbach energy (EU) of CdS films. Some structural defects are manifested by the appearance of continuous localized states near the band edges, they form band tails at the borders of the valence and the conduction bands. They are known as disorder, the width of the band tail is also named Urbach energy. The band gaps and Urbach energy of the prepared CdS thin films are determined from Eq. (12)[29] and Eq. (13)[31] relations, respectively:
$$alpha hnu = A{(hnu-{E_{ m g}})^n}, $$  | (12) |
$$alpha = {alpha _0}exp frac{{hnu }}{{{E_{ m U}}}}, $$  | (13) |
where A is a constant, Eg is the band gap of the material,
In Fig. 10 we report the variations of these two parameters: optical band gap and disorder. As seen, these two quantities vary in an opposite manner with increasing the bath temperature. The value of the films optical band gap passes by a minimum at 65 °C corresponding to the highest disorder, and that in turn is synonymous with the highest growth rate due to the mixed growth mechanism or the change of growth mechanism from ion by ion into cluster by cluster. However, for the film prepared at 75 °C, the value of the band gap is ~ 2.2 eV, close to the CdS bulk material, while its optical disorder and strain are less weak compared with those of our other prepared films. It is well-known that large disorder in the film network yields to the formation of small crystallites. The largest crystallite size (77.67 nm) and the lowest disorder (180.28 meV) are measured in the film prepared at 75 °C.
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Figure10.
(Color online) Dependence of the optical band gap and the Urbach energy according to the bath temperature.
4.
Conclusion
CdS thin films were prepared by the chemical bath deposition technique. In the present work we have investigated the bath temperature effect on film growth and physical properties. The bath temperature effect was divided into three regions: (a) a low region where the deposition rate increases almost linearly with the solution temperature, where the calculation of the deposition activation suggests that the release of complexed Cd2+ ions controls the growth rate and thereafter the ion by ion is the dominant growth process; (b) a medium range at 65 °C, which corresponds to the growth saturation range; and (c) a high temperature where the film thickness is reduced due to the reactants consumption. The films structural and optical properties are improved with increasing solution temperature. We inferred that the solution temperature of 75 °C can be considered as an optimal for CdS film preparation by CBD with better optical and structural characteristics required as a buffer layer in solar cells.
