Thin Solid Films 500 (2006) 4 – 8 www.elsevier.com/locate/tsf
Review
Effect of pH on the properties of ZnS thin films grown by chemical bath deposition T. Ben Nasr a,*, N. Kamoun a, M. Kanzari b, R. Bennaceur a b
a Laboratoire de Physique de la Matie`re Condense´e, Faculte´ des Sciences de Tunis (2092) El Manar, Tunisia Laboratoire de Photovoltaı¨que et des Mate´riaux Semi-conducteurs, Ecole Nationale d’Inge´nieurs de Tunis, Tunis El Manar, Tunisia
Received 7 April 2005; received in revised form 16 September 2005; accepted 14 November 2005 Available online 20 December 2005
Abstract Zinc sulphide thin films have been deposited on glass substrates using the chemical bath deposition technique. The depositions were carried out in the pH range of 10 to 11.5. Structure of these films was characterized by X-ray diffraction and scanning electron microscopy. Optical properties were studied by spectrophotometric measurements. Influence of the increased pH value on structural and optical properties is described and discussed in terms of transmission improvement in the visible range. Transmission spectra indicate a high transmission coefficient (¨70%). The direct band gap energy is found to be about 3.67 eV for the films prepared at pH equal to 11.5. D 2005 Elsevier B.V. All rights reserved. Keywords: Zinc sulphide; Chemical bath deposition; Structural, morphological and optical properties
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . . . . Experiments . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . 3.1. Structural and morphological properties 3.2. Optical properties . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Zinc sulphide (ZnS) is an important II –VI semiconducting material with a wide direct band gap of 3.65 eV in the bulk [1]. It has potential applications in optoelectronic devices such as blue light emitting diodes [2], electroluminescent devices and photovoltaic cells [3]. In thin film solar cells based on CuGaIn(S,Se)2 absorbers, a CdS buffer layer is generally required in order to obtain high conversion efficiency. However, there are toxic hazards with respect to the production and use of the CdS layer. Therefore research in developing Cd* Corresponding author. Tel.: +216 98524797; fax: +216 71885073. E-mail address:
[email protected] (T. Ben Nasr). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.11.030
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4 5 5 5 6 7 8
free buffer layers has been encouraged. This has lead to the investigation of ZnS as a buffer layer in ZnO / ZnS / CuInS2 devices [4]. ZnS has a wider energy band gap than CdS, which results in the transmission of more high-energy photons to the junction, and to the enhancement of the blue response of the photovoltaic cells. Several techniques such as thermal evaporation [5], molecular beam epitaxy [6], metal-organic vapor phase epitaxy [7], chemical vapor deposition [8], spray pyrolysis [9], and chemical bath deposition (CBD) [10] have been used to produce ZnS thin films. Among them, chemical bath deposition is well known as a prevalent low-temperature aqueous technique for depositing large area of semiconductor thin films, the simplest and the most economical one. The CBD process uses a controlled chemical reaction to achieve thin film
T. Ben Nasr et al. / Thin Solid Films 500 (2006) 4 – 8
600
Thickness (nm)
deposition by precipitation. In most of the experimental approaches, substrates are immersed in an alkaline solution containing the chalcogenide source, the metal ion, the added base and a complexing agent. In CBD spontaneous reaction is possible from the liquid phase, whereas in spray pyrolysis technique the reaction takes place from the vapour phase at much higher temperature (300 –600 -C). Using CBD method, a large number of binary compound semiconductors such as CdS, CdSe, PbS, ZnS have been deposited as thin films. Chemical bath deposition of ZnS thin films using NH3 and hydrazine (N2H4) as complexing agents has been reported by Dona and Herrero [11]. In the present work, we report the chemical bath deposition of ZnS thin films and their characterization. The effect of pH on structural, morphological and optical properties of these films is investigated with the objective to optimize the conditions of the deposition process.
5
500
400
300 10.0
10.5
11.0
11.5
pH Fig. 1. Thickness of ZnS thin films deposited at different pH of the chemical bath for a constant dipping time of 3 h.
2. Experiments ZnS thin films can be prepared from decomposition of thiourea in an alkaline solution containing a zinc salt and a suitable complexing agent which allows to obtain a soluble species of Zn2+ in this medium. One of these complexing agents is NH3, following the reaction [12]: 2þ ZnðNH3 Þ2þ 4 T4NH3 þ Zn :
ZnCl2 is used as the Zn2+ source, and thiourea supplies S2 ions through hydrolysis in alkaline medium [11]: SCðNH2 Þ2 þ OH TSH þ CH2 N2 þ H2 O SH þ OH TS2 þ H2 O:
Hydrochloride acid solution is added to the chemical bath to adjust the pH from 10 to 11.5 under the control of a pH meter. Layer thickness is estimated by the double weight method using an ultraprecision balance. Fig. 1 shows the variation of ZnS film thickness as a function of the pH value. The thickness increases from 320 to 600 nm when the pH decreases respectively from 11.5 to 10. X-ray diffraction (XRD) spectra are recorded with an automated Bruker D8 advance X-ray diffractometer with CuKa radiations (40 kV and 30 mA) with 2u ranging from 20- to 60-. Scanning electron microscopy images are carried out using a Cambridge S360 microscope operating at acceleration voltage of 20 kV. The optical properties are studied with a UV-VISNIR spectrophotometer (Shimadzu UV-3101PC).
Complex and sulphide ions migrate to the substrate surface, where they react to form ZnS [13]:
3. Results and discussion
2 ZnðNH3 Þ2þ 4 þ S TZnS þ 4NH3
3.1. Structural and morphological properties
During ZnS film deposition according to the above reaction, the formation of Zn(OH)2 occurs as a competitive process in the bath. NH3 is a source of OH ions through the following reaction: NH3 þ H2 OTNH4 þ þ OH : Hydrazine is a usual complementary complexing agent for the chemical bath deposition of ZnS. It improves homogeneity, and specularity of the layer, as well as its growth rate. Hydrazine could potentially act as a bridging ligand and may facilitate surface binding. ZnS thin films are prepared by CBD method on glass substrates. The chemical bath is an aqueous solution of zinc chloride (ZnCl2: 0.077 mol L 1), thiourea (SC (NH2)2: 0.071 mol L 1), ammonia (NH3: 1.39 mol L 1) and hydrazine ((NH2)2: 2.29 mol L 1). All samples reported here correspond to 3 h deposition time at a bath temperature of 90 -C. The film deposition is carried out with the same bath basic composition.
Zinc sulphide exists in sphalerite, cubic and hexagonal forms. The cubic form is stable at room temperature, while the less dense hexagonal form (wurtzite) is stable above 1020 -C at atmospheric pressure [14]. However, some authors have observed hexagonal structure for ZnS films obtained by CBD [15]. XRD measurements are performed to follow the change of layer crystallinity induced by the pH of the chemical bath. Fig. 2 shows the diffraction patterns of polycrystalline ZnS films deposited at different pH ranging from 10 to 11.5. The broad hump in the 2u range of 20 –30- is due to the glass substrate. At pH = 11.5 no diffraction peak is discernable (Fig. 2a), which indicates highly disordered layer. Broad peaks corresponding to improved crystallinity start to appear, while the pH decreases from 11.5 to 10. Two main peaks can be observed at the diffraction angles of 28.8- and 47.7- on the XRD spectrum obtained on the ZnS films prepared at pH 10 (Fig. 2d). These two peaks are assigned to both cubic and hexagonal phases of the planes (111)cub/(002)hex and (220)cub/ (110)hex. The other characteristic peaks of (100), (101), (102),
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T. Ben Nasr et al. / Thin Solid Films 500 (2006) 4 – 8
Intensity (Arb. Unit)
(111)cub/(002)hex
(220)cub/(110)hex
(d)
(c)
(b)
(a) 20
30
40
50
transmission spectra of ZnS thin films obtained at different pH are displayed in Fig. 4. The broad cut off towards short wavelengths indicates the onset of intrinsic inter-band absorption in the ZnS. An increase of the transmission values over the whole spectral range is observed with increasing pH values. In the transparency region, transmission is the largest (T å55– 71%) at pH = 11.5. At pH = 10.99 and pH = 10.31 transmission is ¨60% in the near infrared region and ¨35 – 48% in the visible region. At pH = 10 transmission varies from 20 to 46%. However there is no detectable variation of the short wavelength absorption edge with the pH. The decrease of the transmission coefficient with decreasing pH can be interpreted by an increase of the film thickness as reported in Fig. 1. Films grown at pH = 10 appear to have the best crystallinity but also the smallest transmission in the transparency region. In Fig. 4d one notices the presence of interference fringes which confirm that, the ZnS film thickness is almost constant at pH = 10. In fact, the effect of pH on the transmission spectra is related to the growth conditions. An increase of the pH leads
A
60
2θ (deg.) Fig. 2. X-ray diffraction patterns of ZnS thin films deposited on glass substrates at different solution pH, (a): 11.5, (b): 10.99, (c): 10.31 and (d): 10.
(103) and (200) planes of hexagonal are not present in all spectra. Therefore, it can be concluded that we have prepared ZnS films having cubic structure (h-ZnS) whatever the tested pH. For a comparison, see J. M. Dona and J. Vidal [11,12] who obtained amorphous or poorly crystalline ZnS thin films by CBD method. Applying the Scherrer formula [16] to the (111) diffraction peak of spectrum 2d the average crystallite size is found to be about 14.8 nm. SEM brings microscopic information of the surface structure and roughness. In this work, it appears to be a helpful technique to specify the growth mode via the study of a surface roughness, and to determine the effect of the pH on the film morphology. Fig. 3 shows surface topography of ZnS layers obtained at two different pH. Semispherical grains are uniformly distributed at the surface. A slight increase of the grain size follows the pH decreasing but does not improve the surface roughness. Concerning the nucleation stage film growth proceeds by nucleation of crystallites, then forming grains which coalesce to cover the entire substrate surface and to show a dense structure. These results are consistent with previous reports [17].
1 µm B
3.2. Optical properties Transmission measurements are performed at normal incidence over a large spectral range (300 to 1800 nm). The
Fig. 3. SEM images of ZnS thin films grown at different solution pH, (A): 10, (B): 11.5.
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Table 1 Band gap values of the ZnS thin films for different pH of the chemical bath
Transmission (%)
a b c
60
7
d
ZnS films prepared using
Optical band gap (T0.02 eV)
pH pH pH pH
3.67 3.81 3.88 3.78
11.50 10.99 10.31 10.00
40
20
0 400
800
1200
1600
2000
Wavelength (nm) Fig. 4. Optical transmission spectra of ZnS thin films deposited at different pH of the chemical bath, (a): 11.5, (b): 10.99, (c): 10.31 and (d): 10.
to more OH ions in the solution, which tend to combine readily with zinc without leaving enough zinc for ZnS growth on the substrate. In other words, slow growth rates keep enough Zn in the solution on the form of Zn(OH)2. This result is also indicated by thickness measurements which show thinner layer at greater pH (Fig. 1). To summarize, for the same deposition time, films grown at low growth rate (pH = 11.5) are thinner and have lower absorption, whereas, films grown at higher growth rate (pH = 10) are thicker and have higher absorption. 15 pH = 11.55 pH = 10.99 pH = 10.31 pH = 10
(α)2 (x 109 cm-2) 5
0 2.8
3.0
a2 ¼ Aðhm EgÞ where A is a constant. The band gap energy is obtained by extrapolating the linear portion of the plot to the crossing with hm axis. Determined band gap energies are listed in Table 1. At pH = 11.5, Eg equals 3.67 eV and it is close to the band gap of bulk ZnS (3.65 eV) [1]. However, the film with the best crystalline structure, according to the XRD pattern (Fig. 2), has band gap energy equal to 3.78 eV. One would expect that a more ordered structure will have a band gap closer to the single crystal value. This shift may be due to the CBD technique. Band gap energy varies from 3.67 to 3.88 eV, which closely agree with the values reported for ZnS thin films obtained by CBD [11,19] and by metal organic vapour phase epitaxy (MOVPE) [20]. 4. Conclusion
10
2.6
Based on the optical transmission measurements, the square of absorption coefficient (a 2) is plotted as a function of photon energy (hm) in Fig. 5. It can be seen that the films have a steep optical absorption feature, indicating good homogeneity in the shape and size of the grains as well as low defect density near the band edge. a 2 varies almost linearly with hm above the band gap energy (Eg). Thus, the following equation can be applied for a direct inter-band transition [18]:
3.2
3.4
3.6
3.8
4.0
4.2
hν (eV) Fig. 5. (a 2) versus (hm) plots of ZnS thin films for different pH of the chemical bath.
Zinc sulphide thin films are prepared on glass substrates by the CBD technique. There is a good agreement between XRD, SEM, and optical results. These studies show that the pH contributes noticeably to the growth and to the structure of deposited films. It is particularly observed that the best crystallinity of the ZnS thin films is obtained at pH = 10. The decreasing of the pH value from 10.99 to 10 is related with the increasing of the (111) diffraction peak intensity. The optical transmission coefficient is found to increase when the pH increases from 10 to 11.5. This may be interpreted by the decrease of the film thickness. ZnS film prepared with pH 11.5 shows a high transmission coefficient (¨70%) and a wide band gap of 3.67 eV. From these studies we are able to optimize the process in order to produce the layer suitable for optical window in solar cells. For example, in the solar cell ZnS/CuInS2 that we are still investigating, CuInS2 is the absorber material which will be grown by spray pyrolysis technique [21]. Annealing at high temperature under nitrogen gaz or in vacuum is under investigation in order to improve the crystallinity of these films.
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