ZnS thin films deposited by spray pyrolysis technique

phys. stat. sol. (c) 2, No. 3, 1161– 1166 (2005) / DOI 10.1002/pssc.200460651 ZnS thin films deposited by spray pyrolysis technique T. Dedova, M. Krunks*, O. Volobujeva, and I. Oja Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Received 6 September 2004, accepted 8 September 2004 Published online 18 February 2005 PACS 61.10.Nz, 68.37.Hk, 68.55.–a, 78.30.Fs, 81.05.Dz, 81.15.Rs ZnS thin films were prepared by spray pyrolysis technique using aqueous solutions of zinc chloride and thiourea at molar ratio of 1:1, 1:2 and 2:1. The films were characterized by XRD, FTIR, SEM and EDX. The phase composition, structure and morphology of sprayed films are controlled by both, the substrate temperature and the precursors molar ratio in the solution. The films deposited below 380 ºC are amorphous and contain thermal decomposition residues of Zn(tu)2Cl2. Chlorine content becomes negligible at 500 ºC, carbon and nitrogen in the form of zinc cyanamide are present in a trace amount up to 540 ºC. Highly (002) orientated ZnS films with wurtzite structure and closely stoichiometric composition could be grown at temperatures close to 500 ºC using Zn:S molar ratio of 1:2 in spray solution. The film consists of well-shaped hexagonal prisms with size of 100 nm. The spray of equimolar solution (Zn:S = 1:1) results in ZnS films with smaller grain size (~25 nm) and slightly Zn-rich composition at 500 ºC. Higher temperature (530 ºC) is needed to grow the films with wurtzite structure. The use of Zn-rich (Zn/S>1) solution results in the films consisting of ZnO and ZnS phases above 400 ºC. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction ZnS is a semiconductor compound suitable for a number of optoelectronic devices, such as solar cells [1], antireflective coatings [2], electroluminescent displays [3], optical sensors [4], etc. ZnS can be obtained as polycrystalline thin film by several deposition techniques: reactive sputtering [1], chemical vapour deposition [5], atomic layer epitaxy [6], chemical bath deposition [2, 7] and spray pyrolysis [8– 12]. Spray pyrolysis is considered as a cheap technique to produce large area thin films [3, 13]. To deposit ZnS thin films by spray pyrolysis usually ZnCl2 and SC(NH2)2 (tu) are used as initial chemicals [3, 8, 9, 12, 14]. Most experiments have been performed at ZnCl2 and tu molar ratio of 1:1 [3, 8, 9, 14, 15]. It has been found that the films are amorphous at growth temperatures below 300 ºC [8, 9, 14]. Crystalline films with sphalerite structure or the mixture of sphalerite and wurtzite phases have been prepared at temperatures close to 450 ºC [8, 9]. ZnS films deposited at optimal conditions have been found to contain C, O and Cl beside Zn and S, whereas the films were slightly sulphur deficient (Zn/S=1.18) [8]. The post-deposition annealing at 470-500 ºC in air led to the appearance of additional ZnO phase, whereas annealing in vacuum or nitrogen atmosphere improved the crystallinity without the oxidation [8, 9]. The optical band gap values have been reported to be 3.35-3.46 eV [14] or 3.3-3.52 eV [8] depending on the deposition temperature. So far, there is no systematic study on the influence of the precursors molar ratio on the properties of spray-deposited ZnS films. It is known from the earlier studies that in an aqueous solution ZnCl2 and thiocarbamide form 1:2 complex - dichlorobis(thiocarbamide)zinc(II) [16, 17]. Zinc sulphide forms upon * Corresponding author: e-mail: [email protected]; Phone: +372 620 3363, Fax: +372 620 2798 © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1162 T. Dedova et al.: ZnS thin films deposited by spray pyrolysis technique the thermal decomposition of Zn(tu)2(Cl)2 [17]. In our previous paper we reported that the molar ratio of the precursors has significant effect on optical properties of spray deposited ZnS films and the film with the band gap energy of 3.67 eV could be prepared from 1:2 solution [12]. For this reason in the present work the influence of ZnCl2 molar ratio in solution and the deposition temperature on the phase composition, structural and morphological properties are studied. 2 Experimental ZnS films were prepared using the aqueous solutions containing zinc chloride (ZnCl2, Aldrich, purity 98%) and thiocarbamide ((NH2)2CS, Merck, ≥ 98%). Deionized water was used as a solvent. The molar ratio of the zinc and sulphur sources of Zn: S = 1:1, 1:2 and 2:1 were used, whereas the concentration of ZnCl2 was kept constant at 0.05 mol/l in stock solution. The precursor solution was pulverized onto the substrates placed on soldered tin bath. Commercial glass (30x30x1 mm3) and n-type Si (100) wafers were used as substrates. The deposition temperature was varied from 200 ºC to 600 ºC using electronic temperature controller and hold with accuracy of ± 10 ºC. Compressed air was used as a carrier gas with the flow rate of 8 l/min. The solution spray rate was maintained at 2.5 ml/min. The structural characterization of deposited films was carried out by X-ray diffraction (XRD) technique on Bruker AXS D5005 diffractometer (monochromatic Cu Kα radiation, λ=1.54056 Ǻ). The XRD patterns were recorded in 2θ interval from 20 up to 60 deg with the step 0.04 deg and counting time 2 s/step. The reflections were identified by JCPDS files. IR spectra were collected in the spectrum region of 4000 - 400 cm–1 at a resolution of 4 cm–1 on Perkin-Elmer FTIR spectrophotometer (model GX1) using KBr pellets technique. The surface morphology and film cross-section were characterized by scanning electron microscopy (SEM) on a LEO SUPRA. Elemental analysis was performed on LEO Supra 35 equipped with the Röntec EDX XFlash 3001 detector and the Röntec QuanTax QX2 software. 3 Results and discussion 3.1 Phase composition and structure of the sprayed ZnS films according to XRD study XRD patterns of ZnS films sprayed at different temperatures using the Zn:S molar ratios of 1:1 and 1:2 are presented in Figs. 1 and 2, respectively. The growth temperatures (marked as Ts in the figures) around 400 ºC result in broad diffraction peaks belonging to ZnS with sphalerite structure (PDF 05- W(002) 0 Ts= 490 C 20 W(103) 0 Ts= 530 C 0 S(111) Ts= 490 C 0 Ts= 400 C W(110) 0 Ts=530 C Zn:S=1:1 W(100) Intensity, a. u. W(103) Zn:S=1:2 W(110) Intensity, a. u. W(002) 0 Ts= 400 C 30 40 50 60 2θ, deg Fig. 1 XRD patterns of sprayed films from the solution with the precursors ratio Zn:S=1:1 at growth temperatures of 400, 490 and 530 ºC. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 20 30 40 50 60 2θ, deg Fig. 2 XRD patterns of sprayed films from the solution with the precursors ratio of Zn:S=1:2 at growth temperatures of 400, 490 and 530 ºC. phys. stat. sol. (c) 2, No. 3 (2005) / www.pss-c.com 1163 0566). The peaks shape indicates the films poor crystallinity. The observed results are in accordance with the data published earlier [8, 14]. ZnS films grown from 1:2 solution at temperatures above 450 ºC crystallize in wurtzite structure as proved by distinct appearance of the (002) and (103) peaks of hexagonal crystal structure (PDF 36-1450) (Fig. 2). The crystallites in the film are (002) orientated. The use of equimolar solution and the deposition temperature of 490 ºC results in a diffraction pattern exhibiting only one diffraction peak at 2θ=28.5 deg, which could be assigned to both, the (002) peak of wurtzite or the (111) peak of sphalerite (Fig. 1). The film obviously has sphalerite structure as no peaks of the wurtzite phase was observed. The further increase of the deposition temperature up to 530 ºC results in ZnS films crystallised in hexagonal structure as all recorded diffraction peaks are characteristic for the wurtzite phase (Fig. 1). The formation of ZnS with wurtzite structure has been recorded for the films prepared at 450 ºC using ALD technique [6] and close to 500 ºC by spray pyrolysis method [14]. These temperatures are significantly lower than reported for sphalerite-wurtzite polymorphic transition [17]. The films were also deposited from Zn-rich solutions using Zn:S molar ratio of 2:1. The replicas of ZnO phase could be clearly distinguished in addition to the broad replicas of ZnS if deposited at 400 ºC (Fig. 3). This temperature is considerably lower than that corresponding to the oxidation reaction of ZnS with formation of ZnO. The 1:1 spray solution, mainly used to prepare ZnS films by spray technique [8, 9] and in particular, 2:1 solution, both contain ZnCl2 in excess to form the intermediate zinc complex Zn(tu)2Cl2 [16, 17]. Therefore, ZnCl2 originated phases could be present in higher amount and might appear at lower temperatures than in the films produced from 1:2 solutions. XRD study confirms that the ZnO phase is clearly present in the film deposited at 600 ºC from 1:1 solution onto silicon substrates whereas in the case of 1:2 solution it is not detected (Fig. 4). ZnO ZnS ZnO Intensity, a. u. ZnO ZnS Ts= 540 C 0 Ts= 500 C 0 Ts= 400 C Intensity, a. u. 0 Si substrate ZnO Zn:S in solution 1:1 1:2 20 30 40 50 60 2 θ, deg Fig. 3 XRD patterns of sprayed films from the solution with the precursors ratio of Zn:S=2:1 at growth temperatures 400, 500 and 540 ºC. 20 30 40 50 60 2 θ, deg Fig. 4 XRD patterns of ZnS films deposited onto Si substrate at 600 ºC using the Zn:S ratio of 1:1 (1) and 1:2 (2) in solution. ZnS films from 1:2 solutions on glass substrates distinctly show the formation of wurtzite structure and sharper diffraction peaks than that from 1:1 solution (Figs. 1 and 2). The increase of the deposition temperature leads to narrower diffraction peaks indicating the increase in crystallite size. For example, the mean crystallite size in ZnS film from 1:2 solution is 60 nm and 73 nm deposited at 490 ºC and 530 ºC , respectively, as calculated from the FHWM of the (002) peak. Using the 1:1 solution and the deposition temperature of 490 ºC the mean crystallite size is around 30 nm. Consequently both, the growth temperature and molar ratio of precursors has an effect on the crystallite size in sprayed films. 3.2 Phase composition of sprayed films according to FTIR study The deposition of ZnCl2 and tu aqueous solution at low substrate temperatures (below 400 ºC) results in amorphous layers according to XRD. In the present study FTIR technique was applied to characterize these coatings. The film deposited at 200 ºC shows the FTIR spectrum (Fig. 5) similar to that recorded for the Zn(tu)2Cl2 heated up to 200 ºC [17]. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1164 T. Dedova et al.: ZnS thin films deposited by spray pyrolysis technique o Zn:S = 1:2 TS = 380 C TS = 320 C 660 2180 2030 o TS = 240 C 2100 1660 o TS = 200 C 2100 Zn:S = 1:2 Transmittance, a. u. Transmittance, a. u. o Zn:S = 1:1 Zn:S = 2:1 2060 1620 905 730 1660 3496 4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 -1 Wavenumber, cm Fig. 5 FTIR spectra of ZnS films deposited from the solution with the precursors ratio of Zn:S=1:2 at growth temperatures 200 ºC, 240 ºC and 320 ºC Wavenumber, cm 1000 500 -1 Fig. 6 FTIR spectra of ZnS films deposited at 380 ºC using the Zn:S ratio of 1:2 (1), 1:1 (2) and 2:1 (3) in solution. Compared to Zn(tu)2Cl2 [17], the rearrangement of the complex is occurred. The strong absorption close to 2100 cm–1 could be assigned to thiocyanate (SCN–) and hydrogen bonded RNH3+ groups [17, 20]. Obviously the isomerisation reaction of thiourea into ammonium thiocyanate is occurred. The IR spectra of the film deposited at 240 ºC and the complex compound heated up to 230 ºC are similar [17]. The characteristic vibrations of Zn complex are not yet disappeared. The deposition at 320 ºC results in rearrangement of the film composition as double band at 2100 cm–1 is splitted into two absorption bands at 2180 and 2030 cm–1. The absorption at 1660 cm–1 is significantly weakened whereas the broad band centred at 1620 cm–1 is still present. New absorptions appear at 1080, 729 and 660 cm–1. The absorptions in the region of 2000-2200 cm–1 and at 660 cm–1 could belong to cyanamide (NCN2–) group and around 730 cm–1 to metal coordinated thiocyanate ion (SCN–) vibrations [17, 20, 21]. FTIR spectra of the films grown at 380 ºC using different Zn:S ratios in the solution are presented in Fig. 6. The films from 1:1 and 1:2 solution show similar spectra whereas the spectrum of the film from Zn-rich solution is different. The strong absorptions in the regions of 3200-3600 cm–1 and 1580-1690 cm– 1 refer that OH– group containing residues are present. Intensive vibrations appear in the region of 8501000 cm–1 and 700-800 cm–1 which are difficult to interpret. Due to very good adhesion of the films to the glass substrate at deposition temperatures higher than 400 ºC, the films were deposited onto the silicon slices for IR characterization. The films deposited at 460-540 ºC show quite poor FTIR spectrum indicating significant purification effect. However, weak absorptions at 2050 and 670 cm–1 indicate that Zn cyanamide residues in low concentrations are present in the films deposited below 540 ºC. According to IR spectroscopic study, amorphous films deposited at temperatures below 400 ºC contain thermal decomposition products of the complex compound. The deposition at temperatures higher than 400 ºC leads to crystalline films containing significantly reduced amount of residues. The results of the FTIR study are in good correspondence with XPS results confirming that carbon containing residues are present in spray deposited ZnS films up to 500ºC [8, 14]. 3.3 Morphology and elemental composition SEM micrographs of ZnS films deposited at Ts = 500 ºC using spray solution with Zn:S molar ratio of 1:2, 1:1, 2:1 are shown in Fig. 7 (a-c). The films deposited from 1:1 and 1:2 solution show uniform surface and dense structure. It could be seen that ZnS film from the 1:2 solution are consisting of wellshaped hexagonal prisms with the size of 100 nm. ZnS film from 1:1 solution consists of smaller grains with the size about 25-30 nm. The use of Zn-rich solution results in ununiform surface as shown in Fig. 7c. Large crystallites are formed on smooth surface. According to EDX analysis the crystals contain very low amount of sulphur © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim phys. stat. sol. (c) 2, No. 3 (2005) / www.pss-c.com 1165 (Zn/S >10). As the concentration of chlorine is beyond the detection limit and absorptions of possible organic residues are low according to the FTIR study, then obviously oxygen is present in a high quantity. The flat area contains Zn and sulphur and still Zn/S > 1. EDX results are in accordance with that recorded by XRD as the films from Zn-rich solutions contain crystalline phases of ZnS and ZnO. a) b) c) Fig. 7 SEM micrographs of ZnS films deposited at Ts = 500 ºC using the Zn:S ratio of 1:2 (a), 1:1 (b) and 2:1 (c) in solution. According to EDX analysis Zn/S atomic ratios in the films are 1.01 and 1.11 for the films deposited at 500 ºC from 1:2 and 1:1 solution, respectively. It could be concluded that ZnS films from 1:1 solution contain an another phase in addition to ZnS which upon annealing turns into crystalline ZnO phase as shown in Fig. 4. The purity of the films was characterized also by the amount of chlorine. Chlorine concentration as high as 5 mass % was recorded in the films deposited close to 400 ºC. Its amount has been found to decrease by the deposition temperature and falls beyond the detection limit if deposited at 500 ºC independent of the solution composition. 4 Conclusions The films deposited from aqueous solution containing ZnCl2 and SC(NH2)2 at temperatures below 380 °C are amorphous and contain residues as thermal decomposition products of the precursor complex Zn(tu)2Cl2. The increase in the deposition temperature decreases the content of residues and promotes the formation of crystalline films. ZnS films free from chlorine could be deposited around 500 °C, whereas a trace amount of cyanamide group containing residues are present up to 540 °C. The phase composition, crystallinity and morphology of ZnS films prepared by spray pyrolysis technique are controlled by both, growth temperature and precursors molar ratio in spray solution. It is shown that highly c-axis orientated ZnS (wurtzite) films with closely stoichiometric composition and reasonable chemical purity could be grown by spray technique at moderate temperatures around 500 °C using ZnCl2 to SC(NH2)2 molar ratio of 1:2. The use of equimolar solutions results in slightly Zn-rich composition and smaller crystallites. The higher deposition temperatures are needed to grow the films with wurtzite structure. The Zn-rich solutions are unsuitable for deposition as resulting in the mixture of ZnS and ZnO phases. Acknowledgements This work is supported by the Estonian Science Foundation grant No. 5612 References [1] [2] [3] L.-X. Shao, K.-H. Chang, and H.-L. Hwang, Appl. Surf. Sci. 212-213, 305 (2003). U. Gangopadhyay, K. Kim, D. Mangalaraj, and J. Yi, Appl. Surf. Sci. 230, 364 (2004). El Hichou, M. Addou, J.L. Budendorff, J. Ebothé, El Idrissi, and M. Troyon, Semicond. Sci. Technol. 19, 230 (2004). © 2005 WILEY-VCH Verlag GmbH & Co. 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