1.
Introduction
Zinc sulphide (ZnS) is a broadband semiconductor II–VI[1]. It is a potential non-toxic alternative to cadmium sulphide (CdS) as a buffer layer. CdS has been widely used as a buffer layer on Cu(In, Ga)Se2 (CIGSe) absorbers for thin-film solar cell applications. Cell efficiency of 20.3% has been reported using CdS in CIGS solar cells[2].The role of the buffer layer is to ensure trellis matching, protect the junction area from sputtering damage during the deposition of zinc oxide (ZnO), and correct the absorber surface CIGS[3]. However, the quantum efficiency of a CdS / CIGS solar cell descents at the short wavelengths due to the loss of optical absorption by the CdS films. This implies that a greater improvement in the short-circuit current (JSC) can be obtained by replacing the CdS by other wider band gap materials[4].
In addition, the deposition of CdS thin films by the chemical bath deposition (CBD) technique in big scale solar cell production, could pose a serious environmental problem due to the large amount of Cd-containing waste from the process deposit[5].
On the other hand, ZnS is non-toxic, abundant, and inexpensive, making this material an excellent candidate for thin-film solar applications. It is a wide band gap n-type with direct transition, the semiconductor having a bandgap varied between 3.2–3.9 eV at room temperature, therefore is more translucent in the short wavelength region between 350 and 550 nm, compared with CdS. The wider band gap of ZnS allows high-energy incident photons to complete the junction of the window absorber. This will improve the blue response of photovoltaic cells and thus contribute to better cell performance. In addition, the high refractive index of ZnS (n = 2.4) allows it to be used as an antireflection coating or to combine it with a high refractive index material[2, 6]. Therefore, a lot of work on the ZnS thin films has been prepared[7].
For example, chemical bath deposition (CBD) in acid medium[8], magnetron sputtering RF[9], route hydrothermal[10], and sol–gel[11]. Among the techniques mentioned above, radiofrequency (RF) sputtering is more interesting because it is cost-effective and makes it possible to control the stoichiometry and the uniformity of the thin films[12]. In addition to its high deposition rate, this technique is compatible with the CIGS absorber and ZnO window layer depositions by sputtering[2].
In this study, we prepared ZnS thin films using RF cathodic sputtering for use in solar cells as a buffer layer. The influence of the RF-power on the structural optical and morphology properties of the films was investigated.
2.
Experimental procedure
RF sputtering in an Alcatel SCM 451 deposition system equipped with an Alcatel ARF 601 RF generator operating at 13.56 MHz, was used to fabricate ZnS thin films, which was described in previous work[13, 14]. These films were deposited from a high purity ZnS target (99.99%) with a diameter of 10 cm. The substrates used in these studies were corning glass and silicon (100). They were cleaned by acetone and isopropanol in an ultrasonic bath for 20 min and then rinsed with deionized water. After cleaning, the samples were instantly loaded into the deposition chamber, which was pumped at a pressure of 10?6 mbar using an oil diffusion pump. The distance between the target and the substrate is about 10 cm. The target was pre-sputtered for 20 min before the samples were in direct contact with the target[15].
The ZnS films were deposited at different RF powers (100, 150, 200, and 250 W) during the same deposition time 75 min, and Ar: 10?2 mbar gas pressure. The different deposit parameters are summarized in Table 1.
| Parameter | Value |
| Deposition time | Fixed 75 min |
| RF-power (input-reflected) | Varied from 100 to 250 W |
| Target diameter | 10 cm |
| Target-substrate distance | 8 cm |
| Bombardment gas | Argon (Ar) |
| Gas pressure | 0.01 mbar |
| Substrates used | Glass, Si |
| Substrate temperature | 300 K |
Table1.
The deposition parameters of the ZnS thin films.
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| Parameter | Value |
| Deposition time | Fixed 75 min |
| RF-power (input-reflected) | Varied from 100 to 250 W |
| Target diameter | 10 cm |
| Target-substrate distance | 8 cm |
| Bombardment gas | Argon (Ar) |
| Gas pressure | 0.01 mbar |
| Substrates used | Glass, Si |
| Substrate temperature | 300 K |
The structural properties of the obtained ZnS thin films were studied by X-ray diffraction (XRD) with an X-pert MPD diffractometer using CuKα radiation. Scanning electron microscopy TESCAN VEGA3, located at the Cadi Ayyad University Center for Analysis and Characterization, was used to study the surface morphology of the films. The SEM was equipped with an EDX microanalysis system (AMATEC) that was used to examine the composition of films. Optical transmittance measurements were executed using a Shimadzu UV-PC dual beam spectrophotometer in the 200–2000 nm range[14].
The X-ray reflectance spectra were acquired in a station equipped with a SIEMENS M386-X-A3 direction finder whose θ/2θ rotations are decoupled. The acquisition and control of the data is controlled by software. This system also allows X-ray diffraction measurements in classical or grazing incidence on the samples studied[16]. FTIR measurements were carried out with a Vertex 70 (Bruker) spectrometer.
3.
Results and discussion
3.1
Optical properties of ZnS thin films
3.1.1
Optical transmittance
The optical transmission spectra in the wavelength range 200 to 2000 nm of ZnS films deposited at different RF powers were revealed in Fig. 1. All spectra have well-contrasting interference fringes indicating a smooth surface. The fringes observed may be attributed to the interference between the waves reflected by the film-film and film-substrate interfaces. The films formed at different RF ??powers between 100 and 200 W have a relatively high average transmission in the visible and near infrared regions (80%). The optical transparency of our thin ZnS films is comparable to that of other ZnS thin films prepared by sputtering[11].
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Figure1.
(Color online) The transmittance of ZnS thin films deposit at different RF power.
On the other hand, the average transmission of films deposited at 250 W drops to 70% on average and has a small shoulder in the absorption zone. In addition, a small shift of the absorption edge to higher wavelengths is also observed and can be attributed to the increase in RF power[12]. These results are in agreement with other transmittances of thin films, which confirms that they depend on their thickness and their morphology[17].
3.1.2
Thickness and refractive index
In the case where the film is condensed on the one hand and smooth on the other hand, the multiple reflections of the light are between the lower surface in contact with the substrate and the free surface of the thin film.
The thickness of the films (d) and the refractive index (n) were obtained from the optical transmission measured according to the method described[18]. The refractive index (n) is calculated from the interference fringes observed in the full transmission region (transparency) and in the low/medium absorption regions by the Swanepoel method. As mentioned above, the interference fringes result from the interference of waves reflected from the upper and lower sides of the film (air-film and film-substrate interfaces), as demonstrated by the authors[19]. The refractive index of the film on a substrate is given by:
$n = {left[ {M + {{left( {{M^2} - n_{ m{s}}^2} ight)}^{frac{1}{2}}}} ight]^{1/2}},$  | (1) |
where
$M = frac{{2{n_{ m{s}}}}}{{{T_{ m{m}}}}} + frac{{n_{ m{s}}^2 + 1}}{2},$  | (2) |
Tm is the maximum transmittance for a given fringe and
${n_{ m{s}}} = frac{1}{{{T_{ m{S}}}}} + sqrt {frac{1}{{{T_{ m{S}}}}} + 1} ,$  | (3) |
is the substrate refractive index obtained from the measured transmittance Ts of the substrate.
The films thickness is then calculated from the following equation:
$d = frac{{{lambda _1}{lambda _2}}}{{2left( {{lambda _1}{n_2} - {lambda _2}{n_1}} ight)}},$  | (4) |
where n1 and n2 are the refractive index respectively at wavelength λ1 and λ2.
The thickness (d) and the refractive index (n) of the ZnS thin films were revealed respectively in Figs. 2 and 3. The film thickness increased from 185.43 to 768.45 nm when the power RF increased from 100 to 250 W, while the refractive index n decreased from 2.25 to 1.65. The increase in film thickness could avoid the damage caused by the sputtering process of the window optic ZnO in the manufacture of solar cells. Therefore, optimizing the thickness of the buffer layer is essential to improve the overall performance of the solar cells. The lower refractive index of the films deposited at 250 W is responsible for the small fringe contrast observed in the transmission spectrum.
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Figure2.
Refractive index variation of ZnS thin films with different RF power.
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Figure3.
The thickness variation of ZnS thin films with the RF power.
3.1.3
Band gap
In the spectral curve where the light is absorbed, and knowing the thickness of the thin film ZnS, we can determine the absorption coefficient α for each value of the transmittance T in (%), which corresponds to an energy by the law from Beer-lambert[20].
The values of the absorption coefficient α are calculated in the region of high absorption[4, 20] using the relation:
$alpha = frac{1}{d} ln {frac{1}{{{T_{ m{n}}}}}} ,$  | (5) |
where d is the thickness and Tn is the film transmittance in this region.
The optical band gap, Eg is calculated by using the Tauc relation, assuming a direct transition between the valence and conduction bands[18, 21]. The model of optical absorption in semiconductors relates the absorption coefficient α to the energy of photons hν as follows:
${left( {alpha hnu } ight)^{1/n}} = Aleft( {hnu - E_{ m g}} ight),$  | (6) |
where A is constant, h is Planck’s constant, and Eg is the band gap width. n is a parameter that depends on the nature of the transition. For direct permitted transitions n = 1/2, while for indirect allowed transitions n = 2, and for direct forbidden transitions n = 3/2.
Fig. 4 shows the curve of (αhν)2 as a function of the incident radiation energy hν (eV). The band gap values ??(Eg) obtained from the linear connection and the extrapolation are 3.5, 3.46, 3.37, and 3.29 eV for ZnS thin films grown at 100, 150, 200, and 250 W, respectively. The band gap decreased slightly as the RF deposition power increased and remained slightly below the bulk material value (3.6 eV). This decrease is due to the increase in crystallite size of thin films, also asymptotically the band gap value of the bulk material when the thickness films are improved[22].
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Figure4.
(Color online) Direct gap optics of the ZnS thin films.
3.2
Reflectometry of X-ray
The experimental measurements of X-ray reflectometry have been carried out on these thin films of ZnS; the results obtained are illustrated in Fig. 5. As can be seen, the reflectance of the films presents a critical angle for the total internal reflection αc[14], which corresponds to the inflexion point of the specter (Fig. 5(b)).
This angle is interrelated to the volume density of the material by using the following equation:
${alpha _{ m{c}}} = sqrt {frac{{{lambda ^2}{e^2}Nleft( {Z + Delta {f^{'}}} ight)}}{{4{pi ^2}{varepsilon _0}m{c^2}}}} ,$  ight)}}{{4{pi ^2}{varepsilon _0}m{c^2}}}} ,$'/> | (7) |
where λ = 0.15418 nm is the RX source (CuKα), Δf’ is the dispersion factor, N(Z + Δf’) is the electronic density, Z is atomic number of ZnS, N is the atomic density, and the parameter
m{e}}} = sqrt {frac{{{e^2}}}{{4pi {varepsilon _0}m{c^2}}}} $
Fig. 5 shows that the zinc sulfide films deposited at 100 and 200 W have the same critical angle αc = 0.28°. From Eq. (8) we can directly calculate the density electronic N(Z+
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Figure5.
(Color online) (a) Derivative of the reflectance (dR/dα). (b) X-ray reflectometry diagram of ZnS samples.
${alpha _{ m{c}}} = sqrt {frac{{{lambda ^2}{e^2}{N_{ m{A}}}left( {Z + Delta {f^{'}}} ight)}}{{4{pi ^2}{varepsilon _0}m{c^2}M}}} ho ,$  ight)}}{{4{pi ^2}{varepsilon _0}m{c^2}M}}} ho ,$ '/> | (8) |
where ρ is volume density, M is the molar mass, and NA is Avogadro’s number.
If the atomic number Z of ZnS is much greater than
3.3
Structural properties of ZnS films
Fig. 6 shows the XRD diagrams of thin films of ZnS deposited by RF-sputtering at different powers ranging from 100 to 250 W. These spectra exhibit a large major peak at 2θ ≈ 28.50° with an intensity depending on the RF power. The peak is attributed to the (111) plane of the zinc blende structure, suggesting that the films have a preferential growth orientation in this direction. As the RF power is increased to 200 W, the peak intensity has increased significantly. An additional increase in power at 250 W resulted in a slight decrease in peak intensity. This decrease may be due to the scattered particle during deposition. The highest maximum intensity was observed for the ZnS film grown at 200 W, indicating that these films have the best crystallinity[8, 24].
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Figure6.
(Color online) XRD diagrams of thin films of ZnS deposited at different RF power.
The average crystallite sizes (D) of the films were calculated using Scherrer's formula:
$D = frac{{0.9lambda }}{{Delta left( {2theta } ight)cos left( {theta '} ight)}},$  ight)}},$'/> | (9) |
where λ is the wavelength of X-rays (0.154 06 nm), Δ(2θ) is the total width at half height (FWHM) of the diffraction peak of the film at 2θ, and θ is the angle of incident diffraction. For ZnS films, the FWHM values decreased from 0.384° to 0.141° when the deposition power increased from 100 to 200 W and then increased 0.154° to 250 W, this indicating a small deterioration in the crystallinity of the films. The average crystallite sizes, as shown in Fig. 7, were approximately 11.3, 25.5, 48.4, and 44.6 nm for samples deposited at 100, 150, 200, and 250 W, respectively. These results suggest that the crystallinity of the films improves as the RF power increases up to 200 W. This behavior is generally attributed to the increased mobility of the sputtered species on the substrate surface such as RF power and, therefore, the kinetics energy of the pulverized species increases[16]. The decrease in crystallinity above 200 W can be attributed to the re-sputtering effect at high RF powers because the kinetic energy of the sputtered species is high[25].
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Figure7.
Progression of crystallite size of ZnS thin films.
3.4
Characteristic morphology of the films
Fig. 8 illustrates a scanning electron microscopy (SEM) image of ZnS films deposited on a Si substrate (100) by 200 W RF sputtering. This image shows that our thin ZnS layer has a smooth and homogeneous surface, and reveals small grains of average size less than 50 nm.
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Figure8.
SEM image of ZnS thin films deposited on Si substrate by RF sputtering at power of 200 W.
Fig. 9 shows a ray spectrum on a coupled MEB-X-Analysis apparatus working in dispersive energy (EDS), typical of a ZnS film deposited on a silicon substrate. In order to determine the ZnS thin film chemical composition obtained, we performed E.D.X analysis at the points of the surface of the thin films.
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Figure9.
(Color online) EDX spectra of a ZnS thin film deposited at 200 W.
As can be seen, the spectrum has characteristic peaks of ZnS in addition to the silicon substrate peak (Si–Ka)[26]. A quantitative analysis of the spectrum (Table 2) reveals that the films are almost stoichiometric (slight excess of sulfur).
| Element | Weight (%) | Atomic (%) | Net Int. | Error (%) | K ratio |
| SiK | 35.98 | 48.60 | 1822.60 | 5.68 | 0.2229 |
| S K | 23.66 | 27.99 | 1019.15 | 5.69 | 0.1480 |
| ZnK | 40.35 | 23.41 | 429.43 | 3.28 | 0.3774 |
Table2.
Composition of the deposited ZnS films.
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| Element | Weight (%) | Atomic (%) | Net Int. | Error (%) | K ratio |
| SiK | 35.98 | 48.60 | 1822.60 | 5.68 | 0.2229 |
| S K | 23.66 | 27.99 | 1019.15 | 5.69 | 0.1480 |
| ZnK | 40.35 | 23.41 | 429.43 | 3.28 | 0.3774 |
3.5
Spectroscopy FT-IR analysis
The FT-IR spectra of the ZnS thin films were achieved at a wavenumber extending from 4000 to 400 cm?1 (Fig. 10). The spectrum clearly shows an absorption band at 3656– 3184.89 cm?1 attributed to the O–H stretching mode that is observed in all samples due to the adsorbed water on the film surface[27]. In addition, the broad absorption bands at 1082 cm?1 and 1118 cm?1 are ascribed to the stretching of the asymmetric vibrations of the Zn–S bonds[26]. The sharp point at 465.01 cm?1 is specific of the ZnS symmetrical bending mode[28].
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Figure10.
(Color online) FTIR Spectra of ZnS thin films for different RF power.
The broad peak appearing at 631.46 cm?1 is due to the Si–H stretching vibration assigned to the substrate band (corresponding to silicon)[29]. The presence of O–H and Si–H bonds shows that the films are a little polluted by hydrogen, which may be the result of the water vapor absorbed during exposure to air (a few days) before analysis[30].
4.
Conclusion
ZnS thin films of thicknesses from 185 to 780 nm were prepared by RF sputtering on glass and on plane-oriented Si substrates (100). X-ray diffraction patterns, SEM and EDX analysis revealed that the surface nature of thin films is read, homogeneous, and nanocrystalline on the cubic Zinc blende phase structure with a preferred growth in the (111) direction, in order to find a good crystallite size of the order of 48.5 nm for the thin film deposited at the RF power 200 W.
UV–visible measurement showed that ZnS films are highly transparent in the 200–2000 nm wavelength range. The band gap values ??ranged from 3.29 to 3.5 eV for ZnS. These latter values ??are therefore confirmed that the ZnS thin film is well adapted to the material of the absorber film to replace the CdS film such as an optical window.
