Abstract: ZnS thin films on glass substrate have been prepared by home built spray pyrolysis technique at 220, 250 and 280°C. Aqueous solution of 50 mL containing zinc acetate dihydrate and thiourea salts of 1:1 M is sprayed as fine mist at a pressure of 2 kg cm-2 with flow rate of 3 mL min-1 on preheated glass substrate. Film obtained at 220°C found to be amorphous and polycrystalline at 250 and 280°C with preferential orientation along (111) plane. X-ray line broadening technique is utilized to determine the grain size and microstrain of the ZnS film. Compositional studies indicate the presence of oxygen for the film prepared at 250°C. The effect of substrate temperature on crystalline structure, surface morphology and composition were analyzed and reported.
INTRODUCTION
ZnS is a key material for cathode-ray tubes (Bredol and Merikhi, 1998), light-emitting diodes (Tetsuya et al., 2001), thin film electroluminescence (Kavanagh et al., 2004) and in photovoltaic cells (Shao et al., 2003; Fathy et al., 2004). For the preparation of ZnS film various techniques such as molecular beam epitaxy (Zhang et al., 2004), RF sputtering (Ghosh et al., 2007), MOCVD (Abounadi et al., 1994), MOVPE (Briot et al., 1994), pulsed-laser-deposition (McLaughlin et al., 1993; Yano et al., 2003), chemical bath deposition (Cheng et al., 2003) and spray pyrolysis (Ashour et al., 1994) are being used. Among the various techniques spray pyrolysis has its advantage due to its inexpensive, capability of depositing homogeneous materials and ease to incorporate in industrial production line (Martin et al., 2002). In this method, a fine mist of precursor solution with the help of neutral or reactive gas is sprayed on to the preheated glass substrate. The film formation depends on the process of droplet landing, reaction and solvent evaporation which are related to droplet size and momentum. Moreover, the quality of spray deposited film depends on initial precursor solution concentration, carrier gas pressure and substrate temperature. The spray pyrolysis has well suited for preparing semiconducting films of desired stoichiometry on large scale (Mooney and Radding, 1982). In the present study the effect of deposition temperature in the growth of ZnS film on glass substrate is analysed and reported.
MATERIALS AND METHODS
Zinc Sulphide thin films were prepared on glass substrate from aqueous solution containing zinc acetate dihydrate [Zn(CH3COO)2.2H2O] and thiourea [CS(NH2)2] in concentration ratio of 0.02:0.02 M. The solution was sprayed as fine mist at an angle of 45° onto preheated glass substrate kept at a distance of 40 cm from the spray nozzle. Prior to deposition, the substrate was cleaned with acetone and deionised water. Compressed dry air is used as carrier gas with a pressure of 2 kg cm-2 and the spray rate of the solution was maintained at 3 mL min-1. To avoid excessive cooling of the substrates, successive spraying process was carried out with time period of 10 sec between two bursts. Substrate temperature was controlled by a chrome-nickel thermocouple fed to a temperature controller with an accuracy of ±1°C. The temperature on top side of the substrate is measured by placing thermocouple on a reference glass substrate kept nearer to the coating substrate to measure the exact temperature. Film thickness was estimated using stylus profilometer (Mitutoyo SJ-301). To investigate the microstructural detail of the film, PANalytical X-ray diffractometer (Model X’per PRO) using Ni-filtered CuKα radiation (λ = 1.54056 Å) was employed with generator setting of 30 mA and 40 kV. Continuous scanning was applied with a speed of 10° min-1. A range of 2θ from 20 to 80° was scanned from a fixed slit type, so that all possible diffraction peaks could be detected. Surface morphology and elemental analysis of the films were investigated using HITACHI Scanning Electron Microscope (Model S-3000H) with an accelerating potential of 18 kV. Prior to imaging, the films were sputtered with thin gold film to enhance the emission of secondary electron for better imaging.
RESULTS AND DISCUSSION
Structural analysis: Figure 1 shows the reduction of film thickness as substrate temperature increases. This is because, the vaporization of precursor occurs before reaching the substrate and subsequently lesser deposition and hence thickness.
Figure 2a-c show the X-ray diffraction (XRD) pattern of ZnS film prepared at different temperatures. XRD pattern of film prepared at 220°C does not show peaks, indicating amorphous nature. But film prepared at 250 and 280°C shows peaks and matches with standard JCPDS card (05-0556) of cubic structure with preferential orientation along (111) plane. The other peaks are indexed as (220) (311) and (222). The presence of different peaks indicates polycrystalline nature of film.
ZnS crystals exist in two forms, cubic and hexagonal. Cubic form is stable at low temperature while hexagonal phase is stable at high temperature. Hexagonal or mixed cubic-hexagonal phase has been reported (Daranfed et al., 2009) for spray deposited ZnS film.
| Fig. 1: | Plot of ZnS film thickness vs. deposition temperature |
However, most of the spray deposited ZnS film (Ashour et al., 1994; Lopez et al., 2005) shows single cubic phase.
In the XRD pattern the peaks are broadened with respect to standard JCPDS, indicating that the film is composed of smaller ZnS crystallites confirming the polycrystallinity. Materials in polycrystalline form generally consist of grains with boundaries where crystallites of different orientations meet. Hence, grain boundary contains those atoms that have been perturbed from their original lattice sites giving rise to microstrains between grains. To analyse the crystallite size and microstrain, X-ray line broadening technique is utilized with the following assumptions: the size effect is of Cauchy and microstrains is of Gaussian type.
| Fig. 2(a-c): | XRD pattern of ZnS thin film prepared at, (a) 220°C (b) 250°C and (c) 280°C |
| Fig. 3(a-c): | SEM image of ZnS thin film prepared at (a) 220°C (b) 250°C and (c) 300°C |
Based on these, the apparent grain size and strain is related with integral breadth. The calculated integral breadth values, βfC, βfG of film profile function for each Bragg reflection, are then utilized to determine grain size (Dhkl) and microstrain (εhkl) using Eq. 1 and 2. It is found that the grain size and microstrain decrease from 45-30 nm and 1.5x1016-0.2x1015 lines m-2 as substrate temperature increases from 220-280°C. This decreasing trend indicates the formation of better crystallinity of ZnS film and good adhesiveness with glass substrate:
| (1) |
| (2) |
Grain morphology and energy dispersive spectroscopy (EDS) studies: Figure 3a-c show the Scanning Electron Micrograph (SEM) of ZnS film deposited at different temperature. No clear particulate morphology is found on the surfaces of ZnS film prepared at 220°C. But some traces of buckled particles are evident. This may be attributed to improper decomposition of precursor salts. Film prepared at 250°C shows well defined grains randomly oriented on the substrate and is agreement with the observed polycrystallinity in XRD pattern. Also, each grain shows a coalescence of nanometer sized crystallites. The crystallite size is found to be 40-50 nm which is comparable with that of calculated value from XRD studies. The surface also shows the presence of voids. This indicates that the film is not well adherent and uniformly coated on the substrate. SEM image of ZnS film prepared at 280°C has nanograins packed closely to each other indicating good adhesiveness and uniform deposition over the substrate. In addition, large coarse aggregates of ZnS were missing. This indicates that the precursor is completely decomposed to form crystalline ZnS film. However, further increase in substrate temperature shows lesser film deposition with powdery nature and poor adhesive. It may be due to the vaporization of fine mist before reaching the substrate (Perednis and Gauckler, 2005). Also, ZnS films were prepared by varying precursor solution concentration for various substrate temperatures. In most of the cases, a mixed polycrystalline ZnO and ZnS or amorphous film are obtained. Therefore, it is understood that for the chosen spray condition, the optimum substrate temperature and precursor concentration is 280°C and 0.02:0.02 M, respectively. In addition, the quantitative elemental analysis of ZnS film prepared at 250°C is carried out using EDS coupled with SEM unit and is shown in Fig. 4.
It indicates the presence of silicon and oxygen peak apart from zinc and sulphur element in the film. The presence of Si peak is due to glass substrate and the oxygen peak may be due to air which is used as carrier gas. Oxygen from carrier gas may diffuse through voids in the film and oxidize during the film formation.
| Fig. 4: | Energy dispersive spectrum of ZnS thin film prepared at 250°C |
The atom percentage of elements for oxygen, zinc and sulphur were 2.37, 48.40 and 49.23%, respectively. Thus the average ratio of atom percentage of Zn and S was 1:0.98, indicating that the film is in nearly stoichiometric ratio whereas EDS of ZnS film (not shown) prepared at 280°C is nearly stoichiometric with absence of oxygen peak.
CONCLUSION
ZnS thin films were spray deposited on glass substrate from zinc acetate and thiourea as precursor salts. Film prepared above 250°C is polycrystalline in nature with preferential orientation along (111) plane. SEM studies indicate the presence of grains randomly oriented on the substrate and also confirms the polycrystallinity of the film. The morphology and elements present in the film strongly depends on substrate temperature. Films prepared at 280°C have closely packed nanograins and absence of oxygen, whereas film prepared at 250°C has voids and presence of oxygen. Thus, it is concluded that 280°C found to be optimum temperature to prepare ZnS film composed of nanograins in the chosen spray condition.
ACKNOWLEDGMENT
The authors acknowledge Prof. K. Ramesh, Department of English, SASTRA University, Thanjavur for his valuable suggestions given in preparing the manuscript.