Preparation and characterization of ZnS thin films prepared by chemical bath deposition

Materials Science in Semiconductor Processing 16 (2013) 1478–1484

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Preparation and characterization of ZnS thin films prepared by chemical bath deposition Aixiang Wei n, Jun Liu, Mixue Zhuang, Yu Zhao School of Material and Energy, Guangdong University of Technology, Guangzhou 510006, China

a r t i c l e in f o

Keywords: ZnS thin film Chemical bath deposition Reaction mechanism Morphology analysis Optical properties

abstract Zinc sulfide thin films were prepared by chemical bath deposition technique using zinc sulfate (ZnSO4
7H2O) and thiourea [SC(NH2)2] as sources of Zn2+ and S2– ions, and ammonia (NH3) and hydrazine hydrate (N2H4) as complexing agents. The structural, stoichiometric proportion, morphology and optical properties of the ZnS thin films were investigated as a function of thiourea and ammonia concentrations using X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and UV-visible spectrophotometry measurements. The deposition mechanism is discussed. The results reveal that the ZnS films exhibit poor crystallinity. The ammonia concentration had an obvious effect on the surface morphology, optical properties and deposition mechanism. The S/Zn atomic ratio and optical bandgap of the ZnS thin films first increased and then decreased with increasing ammonia or thiourea concentration. & 2013 Elsevier Ltd. All rights reserved.

1. Introduction Record efficiency beyond 20% for Cu(In,Ga)Se2 (CIGS) thin-film solar cells has been achieved using a CdS buffer layer grown by chemical bath deposition (CBD) [1]. However, the bandgap energy of CdS is 2.4 eV and absorption losses occur for light of o520 nm in CIGS solar cells based on a CdS buffer layer. Moreover, Cd is toxic and can cause serious environmental problems. Therefore, it is very important to search for alternative non-toxic and environmentally friendly buffer materials with a wider bandgap that are suitable for industrial production. As an important II–VI semiconductor, ZnS is a promising candidate for replacement of toxic CdS in the buffer layer for CIGS solar cells. A CIGS solar cell with CBD-ZnS(O,OH) as the buffer layer has been realized with efficiency of 18.6% [2]. ZnS films can not only increase responses in the shortwavelength region but can also reduce the band offset of

n Corresponding author: Tel.: +86 20 13924087415; fax: +86 20 39322570. E-mail address: [email protected] (A. Wei).

1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.03.016

the ZnS/ZnO layer, because the ZnS bandgap (3.5–3.7 eV) is close to the ZnO bandgap (3.37 eV). There are many methods for fabricating ZnS thin films, such as CBD, chemical vapor deposition, photochemical deposition, spray pyrolysis, and metal–organic vapor-phase epitaxy. Among these, CBD is the most suitable for producing ZnS thin films for photovoltaic applications because of its efficiency, cost-effectiveness and large-scale capability. Chemical deposition of ZnS thin films has been carried out in aqueous alkaline baths by many workers [3–10]. The complexing agent plays an important role in the formation process. A suitable complexing agent controls the reaction rate by controlling the rate of release of Zn2+ ions. Roy et al. prepared ZnS films using tartaric acid and hydrazine hydrate as complexing agents [3], whereas Long et al. used tri-sodium citrate and hydrazine hydrate [10]. Agawane et al. [11] and Vallejo et al. [12] used tri-sodium citrate and ammonia. However, ammonia and hydrazine hydrate are still popular choices for complexing agents [7–9,13]. Dona and Herrero reported that the quality and growth rate of ZnS thin films were improved by addition of hydrazine hydrate to a reaction system containing ammonia [7]. Oladeji and Chow observed a lack of film growth if

A. Wei et al. / Materials Science in Semiconductor Processing 16 (2013) 1478–1484

hydrazine hydrate or ammonia was used as the only complexing agent [9]. Two distinct models (ion-by-ion and cluster-by-cluster) have been discussed for the CBD growth mechanism [11,14]. The reaction mechanism for CdS thin films is believed to be ion-by-ion growth. Although there have been many studies of CBD-ZnS, no definitive mechanism has been established. Some researchers have proposed that ZnS film growth entails adsorption of colloidal particles from solution onto the substrate (cluster-by-cluster mechanism) [15,16], but others suggested that the growth mechanism involves heterogeneous nucleation on the substrate surface and subsequent growth of nuclei in an ion-by-ion process [17,18]. The morphology, stoichiometric composition and structure of ZnS thin films strongly depend on the growth mechanism. ZnS thin films with differing morphology, such as fibrous-like [19], microscale [20] and nanoscale structures [21,22], have been obtained by CBD. We prepared ZnS thin films by CBD using ammonia and hydrazine hydrate as complexing agents, and zinc sulfate and thiourea as sources of Zn2+ and S2– ions. The effects of thiourea and ammonia concentrations on the surface morphology, optical properties and deposition mechanism for ZnS thin films were investigated. We previously observed that hydrazine hydrate is always necessary to obtain ZnS and ZnSe thin films by CBD [23]. However, the exact role of hydrazinehydrate in the chemical deposition of ZnS and ZnSe thin films remains uncertain. We hypothesized that hydrazine hydrate might play a complexing and/or catalytic role in the deposition process and thus improves the growth rate, compactness and adherence of CBD-ZnS and -ZnSe thin films. 2. Experimental details Soda-lime glass slides were used as substrates for deposition of ZnS thin films. Before deposition, the substrates were ultrasonically cleaned sequentially in acetone, ethanol, and deionized water for 15 min each and rinsed with deionized water between each step. Clean substrates were finally dried in air. ZnS thin films were deposited onto the substrates by CBD using zinc sulfate, hydrazine hydrate, ammonia, thiourea powder and deionized water as precursor materials. In a typical synthesis process, 100 ml of reaction solution was obtained by mixing 20 ml of 0.5 M zinc sulfate,

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5 ml of 80% hydrazine hydrate, 5 ml of 25% ammonia, 5.70 g of thiourea powder and 70 ml of deionized water. First, zinc sulfate solution was mixed with hydrazine hydrate and ammonia in a 100-ml beaker under continuous stirring. Initially, the solution was milky and turbid due to formation of Zn(OH)2 in suspension. The solution became clearer with stirring. Then deionized water and thiourea powder were added to the solution and the mixture was stirred under ambient conditions for 5 min. The pre-cleaned substrates were vertically immersed into the reaction solution. The beaker was sealed to prevent evaporation. CBD was performed at a temperature of 70 1C for 2 h in the bath. After deposition, the substrates were removed from the solution and rinsed in deionized water to remove loosely adherent ZnS particles from the surfaces before they were dried in air. To study the effect of thiourea and ammonia concentrations on the properties of ZnS thin films, two sample series were prepared by varying the thiourea concentration or the ammonia concentration separately while keeping the other reagent concentrations constant. The deposition parameters for different samples are listed in Table 1. The samples obtained were characterized by means of X-ray diffraction (XRD; D/MAX-UItima IV, Rigaku), scanning electron microscopy (SEM; S-3400N,Hitachi), energydispersive spectroscopy (EDS; S-3400N, Hitachi) and UVvisible spectroscopy (Lambda 850 UV/Vis spectrometer, Perkin Elmer). XRD patterns were obtained using a Cu Kα radiation source (λ ¼ 0.1542 nm) over diffraction angles ranging from 201 to 801 at a scanning speed of 61 min−1 with a grazing angle of 0.51. Optical transmission measurements were performed using a UV-Vis spectrometer at room temperature in the wavelength range from 300 to 900 nm. 3. Results and discussion 3.1. Reaction mechanism for ZnS deposition O'Brien and McAleese proposed two possible reaction mechanisms for the growth of ZnS films [14]. The first is a cluster-by-cluster process, in which colloidal ZnS particles preformed in solution via a homogeneous reaction agglomerate and then adsorb on the surface to form a film. Hence, the film consists of larger spherical particles and exhibits poor compactness and adherence. The second

Table 1 Deposition parameters for various samples. Sample

a1 a2 a3 a4 b1 b2 b3 b4

Concentration (M)

Temperature

Deposition

ZnSO4

N2H4·H2O

NH3·H2O

SC(NH2)2

(1C)

time (min)

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.82 0.82 0.82 0.82 0.82 0.82 0.82 0.82

0.7 0.7 0.7 0.7 0.1 0.4 0.7 1.2

0.15 0.45 0.75 1.05 0.75 0.75 0.75 0.75

70 70 70 70 70 70 70 70

120 120 120 120 120 120 120 120

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3.2. Structural analysis ZnS can exist in a cubic (zinc-blende type) or hexagonal (wurtzite) structure. Oladeji and Chow reported that chemically deposited ZnS thin films are highly disordered materials but can be transformed into a wurtzite-2H phase by annealing [9]. Göde et al. obtained an amorphous film at 60–70 1C and a wurtzite-2H phase at 80 1C [6]. Fig. 2 shows XRD patterns for ZnS films after deposition, after annealing at 200 1C for 2 h, and after multiple depositions. The film preparation conditions were as listed for sample a3 in Table 1. Multiple deposition was carried out by placing the substrate into fresh solution and allowing growth to proceed for 2 h, and this process was repeated three times. The XRD pattern for the as-deposited film only shows a wide diffraction peak in the range 20–301. There is also no obvious diffraction peak for the ZnS film annealed at 200 1C. There are three diffraction peaks corresponding to (111), (220) and (311) for the multiple deposition sample. This indicates that this ZnS thin film was a pure cubic phase with a zinc-blende structure. The cross-sectional SEM image of a typical ZnS thin film (sample a3) shown in Fig. 3 indicates a film thickness of approximately 80 nm. In several studies, very thin ZnS films showed no discernible XRD peaks [4,10,24]. XRD analysis was only possible after multiple depositions to increase the thickness. A wide (111) peak corresponding to

(311)

(220)

Intensity(a.u.)

is an ion-by-ion process, in which ions condense on the reacting surface to form a film via a heterogeneous reaction. Hence, the film consists of very small particles with excellent compactness and adherence. Ion-by-ion growth involves heterogeneous nucleation on the substrate surface and subsequent growth of nuclei. In general, OH– first adsorbs onto the substrate and then water-soluble [Zn (L)n]2+ complex ions react with OH– to release Zn2+ and form Zn(OH)2, which serves as nuclei on which thiourea is adsorbed and hydrolyzed to form a ZnS film. In fact, a mixed mechanism is typically involved in the growth of ZnS films (Fig. 1). It is critical to restrain the cluster-by-cluster mechanism and encourage the ion-byion process for the formation of high-quality ZnS films. The process depends on suitable deposition parameters, such as the bath temperature, pH of the reaction solution and the concentration of the reacting species.

(111)

Fig. 1. Growth mechanism for ZnS thin films in aqueous alkaline solution.

Multiple deposited films (Unannealed)

Annealed films

As-deposited films

20

30

40

50 2θ(deg.)

60

70

80

Fig. 2. XRD patterns for as-deposited, annealed and multiple-deposited ZnS thin films (sample a3).

Fig. 3. Cross-sectional SEM images of a typical as-deposited ZnS thin film (sample a3).

a cubic structure was observed for the multiple deposition film. However, the peak position is shifted from the typical position of 28.5671 (PDF card 65-9585) to 29.1851; similar results were reported by Oliva et al. [25] and Chen et al. [26]. This difference may be attributed to lattice distortion by dopant oxygen atoms. No peaks characteristic for

A. Wei et al. / Materials Science in Semiconductor Processing 16 (2013) 1478–1484

impurity phases, such as ZnO and Zn(OH)2, were detected for any of our ZnS films. According to the Debye–Scherrer formula, the grain size of a thin film can be determined as: D¼

Kλ βcosθ

ð1Þ

where Κ is the Scherrer constant (0.94), λ is the radiation wavelength (λ¼0.1542 nm), β is the full width at half maximum of the (111) diffraction peak and θ is the Bragg angle. The grain size of our ZnS films calculated using this equation was approximately 5 nm. Therefore, we conclude that our ZnS thin films deposited by CBD exhibited poor crystallinity. 3.3. Morphology SEM images of as-deposited ZnS thin films prepared using different thiourea concentrations are shown in Fig. 4. It is evident that the films became more uniform and dense with increasing thiourea concentration. The film grown using 0.15 M thiourea consists of small villiform structural grains and has a rough surface. The films grown using 0.45, 0.75 and 1.05 M thiourea are very dense and smooth and have no defined grain boundaries. Thus, the thiourea concentration has no obvious effect on the surface morphology of ZnS thin films. SEM images of as-deposited ZnS thin films prepared using different ammonia concentrations are shown in Fig. 5. It is clear that the ammonia concentration has an obvious effect on the surface morphology of the films. For 0.1 M ammonia, the film is composed of a large number of uniform but not very dense spherical particles, which have a diameter of approximately 110 nm. The film grown using 0.4 M ammonia contains spherical particles, but the

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surface of these particles is covered with fine particles of a few nanometers in diameter. The films grown using 0.7 and 1.2 M ammonia are composed of uniform and dense fine particles with a diameter of 10–20 nm. However, there were some cracks in the thin film deposited using 1.2 M ammonia and the films deposited using 0.7 M ammonia is denser and smoother than the sample prepared using 1.2 M ammonia. The results can be explained according to kinetics and reaction mechanism for ZnS deposition. The change in the molar NH3/N2H4 ratio resulting from adjustment of the ammonia concentration strongly affects the concentrations of [Zn(NH3)4]2+, [Zn(N2H4)3]2+ and [Zn(NH3)3]2+ ions. The concentrations of these ions control the rate of Zn2+ generation and thus the deposition mechanism for ZnS films. The stability constant for [Zn(NH3)4]2+, [Zn(NH3)3]2+ and [Zn(N2H4)3]2+ is 108.9, 106.6 and 105.5, respectively [9]. A higher stability constant means that Zn2+ release will be slower from these complexes and the rate of ZnS deposition will be very slow. A lower stability constant means that Zn2+ release will be faster and thus ZnS and Zn(OH)2 colloidal particles can easily form and precipitate out in the solution. For an ammonia concentration of 0.1 M, the molar NH3/N2H4 ratio in the reaction solution is 0.12 and [Zn(N2H4)3]2+ ions are predominant. The colloidal particles formed by homogeneous reaction in solution were gradually adsorbed on the substrate and finally become films, as shown in Fig. 5. In other words, the cluster-by-cluster mechanism for ZnS film growth predominates. For ammonia concentrations of 0.7 and 1.2M (NH3/N2H4 ratio 0.85 or 1.46), [Zn(NH3)3]2+ ions play an important role in the deposition process. ZnS film growth on the substrate involves an ion-by-ion mechanism and thus the films are denser and smoother. For 0.4 M ammonia, the film consists of large colloidal particles and fine particles. The larger

Fig. 4. SEM images of as-deposited ZnS films prepared with different thiourea concentrations.

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particles are formed by aggregation of ZnS colloid particles in solution (cluster-by-cluster deposition). By contrast, a few nanometer-sized particles are formed via ion-by-ion reactions on the surface and strongly adhere to it. We can conclude that an optimal NH3/N2H4 ratio is necessary for the formation of high-quality ZnS thin films.

3.4. Elemental analysis The sulfur and zinc content in as-deposited ZnS films was determined by EDS analysis. The results are shown in Table 2. The results indicate that the thiourea and ammonia concentrations markedly affect the S/Zn ratio in the films. ZnS thin films prepared by CBD were zinc-rich. The coexistence of Zn(OH)2 and ZnO is a possible reason for this. Hydrolysis of thiourea provides S2+ ions, and thus the thiourea concentration affects the concentration of S2+ ions during deposition. The pH of the reaction solution strongly depends on the ammonia concentration. Before deposition, the pH values for reaction solutions containing 0.1, 0.4, 0.7 and 1.2 M were 9.3, 9.7, 10.2, and 10.6 and decreased to 8.3, 8.8, 9.2 and 9.4, respectively, after deposition. It is possible that several soluble and insoluble Zn2+ species exist in aqueous alkaline solution. When the

pH of the reaction solution is less than 7.5 or greater than 2− 11.4, soluble species of Zn2+ or ZnO2 , respectively, are present [27]. At pH 7.5–11.4, insoluble Zn(OH)2 may also be present. This is the reason for the high amount of Zn(OH)2 colloids suspended in the reaction solutions. Sahraei et al. prepared ZnS thin films in a weak acidic medium as a new CBD route, and obtained films with a composition much closer to stoichiometric ZnS because of significantly lower concentrations of Zn(OH)2 and ZnO species or organic impurities in the films [24]. There is competition between homogeneous and heterogeneous reactions in the formation of ZnS films. Depending on the growth mechanism, Zn(OH)2 colloids may form directly in the reaction bath at lower ammonia concentrations because Zn2+ ions are easily released, and thus more Zn(OH)2 may exist in the film. Otherwise, since Zn(OH)2 serves as nuclei in ion-by-ion deposition, it will also probably be present in films if it is not completely consumed during film growth.

3.5. Optical properties Fig. 6a shows optical transmittance spectra for asdeposited ZnS films for the wavelength range 300–900 nm.

Fig. 5. SEM images of as-deposited ZnS films prepared with different ammonia concentrations.

Table 2 Content of S and Zn in as-deposited ZnS films prepared with different thiourea and ammonia concentrations. Thiourea concentration

S (at.%) Zn (at.%) S/Zn Bandgap (eV)

Ammonia concentration

0.15 M

0.45 M

0.75 M

1.05 M

0.1 M

0.4 M

0.7 M

1.2 M

34.52 65.48 0.53 3.83

41.13 58.87 0.70 3.84

44.13 55.87 0.79 3.87

41.34 58.66 0.70 3.86

33.47 66.53 0.50 3.76

35.56 64.44 0.55 3.83

44.13 55.87 0.79 3.87

40.50 59.50 0.68 3.84

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5

100

4

80 a1 60

a4 20 100 80

b1 b2

60

a2 a4

1 5 4

b1

3

b2 b3

b3

40

a1 a3

2

a3

(αhν)2×103(eVcm-1)2

Transmission/%

3

a2

40

2

b4 20 0 300

1483

b4

1 400

500 600 700 Wavelength/nm

800

900

0 3.0

3.2

3.4

3.6 3.8 hν(eV)

4.0

4.2

Fig. 6. (a) Optical transmittance spectra and (b) (αhν)2 versus hν curves for as-deposited ZnS films prepared with different thiourea and ammonia concentrations. a1, a2, a3 and a4 correspond to thiourea concentrations of 0.15, 0.45, 0.75 and 1.05 M; b1, b2, b3 and b4 correspond to ammonia concentrations of 0.1, 0.4, 0.7 and 1.2 M.

The transmittance of all samples is 470% at 350–900 nm. Therefore, the films are suitable buffer layers for replacing CdS in CIGS-based solar cells. The optical bandgap of ZnS films can be obtained from optical data using the Tauc relationship [28] αhv ¼ kðhv−Eg Þn

ð2Þ

where α is the absorption coefficient, k is a constant, Eg is the optical bandgap and n is 1/2 for a direct-bandgap semiconductor. Fig. 6b shows (αhν)2 versus (hν) plots for as-deposited ZnS films prepared at different thiourea and ammonia concentrations. Extrapolation of the linear portion of the curve to (αhν)2 ¼0 gives the optical bandgap. The optical bandgap of ZnS thin films varied in the range 3.76–3.87 eV. The optical bandgap first increased and then decreased with increasing ammonia or thiourea concentration. 4. Conclusions ZnS thin films were prepared by CBD on glass substrates using different thiourea and ammonia concentrations. The molar NH3/N2H4 ratio affected the surface morphology, optical properties and deposition mechanism for ZnS thin films. For a lower NH3/N2H4 ratio, film growth was via a cluster-by-cluster mechanism. For a larger NH3/N2H4 ratio, the predominant growth mechanism was an ion-by-ion process. For a moderate NH3/N2H4 ratio (e.g., 0.49), the two distinct growth mechanisms coexisted. The atomic S/Zn ratio for films deposited under optimal conditions was 0.79. The coexistence of Zn(OH)2 and ZnO is a possible reason for this zinc-rich composition. The optical transmittance of ZnS thin films was 470% in the wavelength range 350–900 nm. The optical bandgap calculated from transmission spectra was 3.76–3.87 eV. ZnS thin films obtained by CBD showed poor

crystallinity. The optimal conditions for deposition of highquality ZnS thin films were a thiourea concentration of 0.75 M and a molar NH3/N2H4 ratio of 0.8–1.2.

Acknowledgements We gratefully acknowledge financial support through a Guangdong Province and Ministry of Education Cooperation Project (Grant No. 2011A090200003) and a Project from the Science and Technology Department of Guangzhou City (Grant No. 12C52111614). References [1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovoltaics 19 (2011) 894–897. [2] M.A. Contreras, T. Nakada, M. Hongo, A.O. Pudov, J.R. Sites, in: Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, 2003, pp. 570–573. [3] P. Roy, J.R. Ota, S.K. Srivastava, Thin Solid Films 515 (2006) 1912–1917. [4] M. Lodar, E.J. Popovici, I. Baldea, R. Grecu, E. Indrea, J. Alloys Compd. 434–435 (2007) 697–700. [5] C. Hubert, N. Naghavi, B. Canava, A. Etcheberry, D. Lincot, Thin Solid Films 515 (2007) 6032–6035. [6] F. Göde, C. Gümüs, M. Zor, J. Cryst. Growth 299 (2007) 136–141. [7] J.M. Dona, J. Herrero, J. Electrochem. Soc. 141 (1994) 205–210. [8] J. Vidal, O. Vigil, O. De Melo, N. Lopez, O. Zelaya-Angel, Mater. Chem. Phys. 61 (1999) 139–142. [9] I.O. Oladeji, L. Chow, Thin Solid Films 474 (2005) 77–83. [10] F. Long, W.M. Wang, Z. Cui, L.Z. Fan, Z. Zou, T. Jia, Chem. Phys. Lett. 462 (2008) 84–87. [11] G.L. Agawane, S.W. Shin, A.V. Moholkar, K.V. Gurav, J.H. Yun, J.Y. Lee, J.H. Kim, J. Alloys Compd. 535 (2012) 53–61. [12] W. Vallejo, C. Quiñones, G. Gordillo, J. Phys. Chem. Solids 73 (2012) 573–578. [13] S.M. Salim, A.H. Eid, A.M. Salem, H.M.A.b.o.u. El-Kair, Surf. Interface Anal. 44 (2012) 1214–1218.

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