The growth of CuS thin films by Spray Pyrolysis
L A Isac1,4, A Duta1, A Kriza2, I A Enesca1, M Nanu1,3 1The Centre: Product Design for Sustainable Development, Transilvania University of Brasov, Eroilor 29, 500036, Romania
2
Inorganic Chemistry Department, Bucharest University, Dumbrava Rosie 23, Romania
3
Delft University of Technology, Julianalaan 236, The Netherlands
Abstract. Thin films of CuS (covellite) were deposited onto TCO (SnO2:F) glass by Spray
Pyrolysis (SP) technique. Aqueous and water:alcohol (ethanol, 1-propanol) solutions of copper(II) chloride and thiourea with different Cu/S molar ratio have been used as precursors. The substrate temperature was varied from 185°C to 285°C. The structural and the morphological characterization of the films has been carried out by Raman spectroscopy and Scanning Electron Microscopy. The X-Ray Diffraction analysis of as-grown films showed the single-phase covellite, with hexagonal crystal structure built around three preferred orientations corresponding to (102), (103) and (110) atomic planes. The dense morphology of CuS films with large crystallites/aggregates suggest that crystal growth is the limiting step in the films deposition, at 235 °C and at 285 °C, from precursors’ solution containing water or mixtures of water:alcohol as solvents. The growth of CuS thin films by spray pyrolysis is favored by increasing both the alcohol concentration and the deposition temperature.
1. Introduction
Due to their structural, electrical and optical properties, copper sulfides (Cu2-xS, x = 0-1) thin films are widely used as semiconductor and/or absorber materials with application in electronics, photovoltaic cells, and tubular solar collectors. Among these, CuS (covellite) thin films are known to exhibit metal-like electrical conductivity and to posses near-ideal solar control characteristics. All these make of CuS thin films deposited onto different substrates (glass, polymers) promising materials for either electric and electronic devices or radiation control coatings (filters), [1].
Covellite, the stoichiometric copper-poor phase (66,46% Cu), is a green-black crystalline solid with a special hexagonal crystalline structure, consisting of layers of planar CuS3 triangles, containing Cu2+ and S2– ions, surrounded (above and bellow) by CuS4 tetrahedral arrangements of Cu+ and S22– ions. The Cu-S distances in tricoordinated Cu2+ are 2.19 Å, while in tetrahedral Cu+ are 2.31 Å. Two-third of the sulfur species are S22– anions and one-third are S2– ions, [2]. The lattice parameters of various crystalline types of covellite are: a = 3.768 – 3.796 Å and c = 16.27(6) – 16.382(5) Å, [3]. The crystalline structure correlated with the surface morphology gives important information on the ratio of the nucleation and growth reactions. These strongly depend on the deposition technique and parameters.
Different techniques have been used for the CuS thin films deposition, such as chemical bath deposition, [4], photochemical deposition, [5], metal organic chemical vapor deposition, [6], doctor Blade, [7] and spray pyrolysis deposition [8]. Spray pyrolysis is a simple and low-cost technique used to deposit thin films at nano-, meso- or microscale.
The present study focuses on the formation (nucleation/growth) of CuS thin films by SPD, correlating the crystalline structure (crystallite size) with the surface morphology (average grain size). These depend on the precursor solution composition and deposition parameters.
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2. Experimental
The CuS thin films were deposited from aqueous and water:alcohol precursors’ solutions containing CuCl2.2H2O (99.9%, Merck), and H2NCSNH2 (99.9%, Sigma Aldrich), with Cu:S molar ratio between 0.33 to 0.5. The concentration of CuCl2.2H2O in the precusors’ solution was varied from 0.1 to 0.2 mol/L. The alcohol solutions contained, in volumes, 20% 1-propanol (Pr, 99%, J. T. Baker) or 20-70% ethanol (Et, 99.8%, J.T. Baker) solved in deionized water (W).
Films were deposited onto pre-heated transparent conductive SnO2:F glass substrates (TCO, Libbey Owens Ford, TEC15/2.3 mm), which were ultrasonically cleaned by successive immersion in ethanol and acetone and dried under a nitrogen gas flow.
The deposition of CuS thin films by chemical spray pyrolysis from precursor solutions was performed using a Camag nozzle, a ceramic hot plate (CERAN 500 ± 1°C) and N2 as carrier gas. During spraying, the substrate temperature (T) was varied from 185 °C to 285 °C, the pressure of the carrier gas was varied from 1 to 1.4 bars and the distance between the spraying nozzle and the heater was 25 cm or 30 cm. The spraying sequences (tsp) were varied from 15 minutes (aqueous solutions) to 40 minutes (W:Et solutions) and the breaks between two pulses varied from 10 to 60 seconds.
The as-deposited films were characterised by X-Ray Diffraction (XRD), Raman spectroscopy and Scanning Electron Microscopy (SEM). XRD patterns of the films were registered by a Bruker D8 Advance Diffractometer with Cu-Kα1 radiation. The Raman scattering measurements were carried out by using a home-built set-up (TU Delft, The Netherlands) containing an excitation source (SpectralPhysics Millenia Nd:YVO4 laser, λ = 532 nm). The Raman set-up also contains neutral density filters for the laser power adjusting and a liquid nitrogen cooled CCD camera (CCD-1100PB) connected to a Spex 340E monochromator, equipped with 1800 grooves/mm grating. The Raman patterns were recorded with a time resolution of 60 s, from different points on the film surface. The SEM analysis was performed using a Jeol JSM-5800LV scanning microscope.
3. Results and discussion
Previous studies, [8], have shown that the composition, morphology, electrical and optical properties of CuS thin films, deposited by spray pyrolysis, can be tailored by changing the composition of precursors’ solution (Cu/S molar ratio, type of solvent) and deposition parameters, especially the substrate temperature. According to these studies, mono- and polycrystalline thin films of covellite with hexagonal structure can be obtained using an excess of sulfur (Cu:S = 1:3, 1:4, 1:5) and water:alcohol solvents in the spraying solution, at deposition temperatures between 150 °C and 285 °C. In the present study, the correlation of crystalline structure (crystallite size) with the surface morphology (average grain/aggregate size), depending on the composition of precursor solution and deposition temperature (table 1), is reported.
Table 1. The chemical deposition parameters for CuS thin films obtained by SPD. Test Solvent W:Pr (Et) (V) Cu:S CuCl2 (mol/L) T (°C ) p (bar) tsp (min) A 10:0 1:3 0.2 235 1 15 B 1:1(Pr) 1:2.5 0.1 285 1.4 30 C 1:4(Pr) 1:2 0.1 185 1.4 30 D 7:3(Et) 1:3 0.2 235 1 40 E 1:4(Et) 1:3 0.2 235 1 40
also observed, but is weak compared to the (102) or (110) peaks. The various orientations of grains or crystallites in the CuS thin films can be attributed to the different precursors’ solution composition (aqueous and W:Pr(Et) solvents), with predominant influence in the growth, and different deposition temperature, with strong influence in the nucleation step. The TCO substrate influence can also be suspected.
Figure 1. XRD patterns of CuS thin films deposited on TCO substrates.
Figure 2. Raman spectra of CuS thin films deposited on TCO substrates. In the Raman spectra (figure 2), the high intensity peaks recorded at 470 cm–1 (film A) and at 469 cm–1 (film E) can be attributed to the formation of hexagonal CuS, by correlation with the XRD results. Raman spectrum of film B contains an intensive and sharp band at around 474 cm–1, which can be also attributed to the formation of covellite with hexagonal crystal structure. The shift at lower wavenumber values (compared to the reference 474 cm–1, [9]) may be linked with the vacancies in the CuS lattice, induced during the film growth.
Table 2. The correlation of crystal structure with surface morphology for the CuS thin films obtained by SPD.
The average crystallite size, D, was calculated by using the Scherrer formula, [10]:
β
θ
λ
cos 9 . 0 = Dwhere λ is the wavelength of Cu-Kα1 radiation (0.15406 nm), β is the broadening of diffraction line measured at half maximum intensity (in radians) and θ is the diffraction angle. The calculated average crystallite sizes in CuS thin films are given in table 2.
The SEM pictures of the CuS films, figure 3, show that the films are relatively uniform, homogenous, with the average grain/aggregate size (
d
) varying from 250 nm to 350 nm and texture fiber. The deposited CuS films are denser when mixtures of water and alcohol (Pr, Et) are used as solvents in precursors’ solution. The addition of alcohols in solvent lowers the surface tension and the dimensions of precursor drops in the aerosol, allowing the fast evaporation of the solvent and thus a higher reaction rate.
(A) (B)
(D) (E) Figure 3. SEM images of CuS deposited on TCO substrate by SPD.
The differences in surface morphology of the CuS films prepared from precursors’ solutions containing aqueous (sample A) and W:Et solvents (sample D), can be explained as a consequence of increasing the crystal growth rate and obtaining of larger aggregates, which contain around 13 small crystallites (sample D). Increasing the ethanol concentration in precursors’ solution, the average crystallites size also increases: from 22.82 nm to 37.35 nm, for the orientation corresponding to the (102) planes, and from 19.69 nm to 22.96 nm along (103) atomic planes. The average crystallite size for the orientation corresponding to (110) planes has the same value (24.16 nm), suggesting that the crystal growth along this orientation plane is irrelevant. At deposition temperatures lower than 235 ºC, the alcohol concentration increase determines the fast consumption of H2O solvent and favors the decreasing of copper complexes stability, therefore a higher nucleation rate of the metal ion on the substrate.
4. Conclusions
Homogenous and relatively uniform thin films of covellite (CuS) with hexagonal crystalline structure are deposited at different temperatures (T = 185 – 285 ºC), by chemically spray pyrolysis, from aqueous and water:alcohol (Et, Pr) solutions containing CuCl2 and thiourea as precursors.
The crystalline structure (crystallite size) correlated with the surface morphology (average grain/aggregate size) gives information on the film growth, which can be controlled by adding alcohol(s) in precursors’ solution and by varying the deposition temperature. Generally, the addition of alcohols in aqueous precursors’ solution mainly favors the obtaining of porous films with larger aggregates, suggesting that crystal nucleation is the limiting step, if the deposition temperature is maintained at lower values (T = 185 – 235 ºC). According with this study, the addition of ethanol in aqueous spraying solutions determines the obtaining of aggregates containing 5 -15 crystallites. This suggests that the crystal growth is the limiting step in the obtaining of CuS thin films at 235 ºC. Increasing the concentration of the alcohol in the aqueous precursors’ solution, dense CuS films with large crystallites and aggregates are obtained, confirming that the crystal growth is the limiting step, when the deposition takes place in the temperature domain up to 285 ºC.
Acknowledgments
The authors thank for the support given by the Inorganic Chemistry Department in the Delft University of Technology, The Netherlands, especially to Prof. dr. J. Schoonman, in developing of the experimental part of this study and also to the Socrates-Erasmus Program.
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