Influence of deposition parameters on the structural

Journal of Physics and Chemistry of Solids 77 (2015) 14–22

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Influence of deposition parameters on the structural and optical properties of CdS thin films obtained by micro-controlled SILAR deposition K. Gowrish Rao n, V.K. Ashith Department of Physics, Manipal Institute of Technology, Manipal University, Udupi, Karnataka 576104, India

ar t ic l e i nf o

a b s t r a c t

Article history: Received 29 July 2014 Received in revised form 22 August 2014 Accepted 16 September 2014 Available online 28 September 2014

Polycrystalline CdS films were obtained by a micro-controlled SILAR deposition technique, using aqueous solutions of cadmium acetate and thiourea as precursors. The structural and optical properties of the films were found to be influenced by various deposition parameters such as number of immersion cycles, concentration of the precursors and temperature of the solutions. Contrary to the observations made by some researchers, we found that the thickness of the films increased continuously with number of immersion cycles and also with concentration of the precursor solutions. We also found that the films covered the substrates uniformly, without any voids, unlike the films obtained by others. Effect of deposition parameters on thickness, substrate coverage, grain size, chemical composition, optical band gap and other properties of the films is discussed in detail. & 2014 Elsevier Ltd. All rights reserved.

Keywords: A. Chalcogenides A. Semiconductors A. Thin films B. Chemical synthesis

1. Introduction Cadmium sulphide (CdS) is a II–VI group semiconductor which is widely studied because of its potential applications in optoelectronic devices such as lasers, light emitting diodes, photodetectors and solar cells [1–4]. It has many highly desirable properties such as a wide band gap (2.4 eV) at room temperature, good photoconduction, high electron affinity and inexpensive preparation [2,3]. The n-type CdS is already being used as the window material in commercially available p-CdTe/n-CdS hetero-junction solar cells [4]. With the advancement in II–VI semiconductor technology many devices based on CdS such as photo-conducting sensors, Cd þ 2 ion selective sensors, have come up of late [5]. Polycrystalline CdS films have been obtained in the past by almost all wellknown deposition techniques such as electrochemical synthesis [6,7], chemical bath deposition [8, 9], sputtering [10], thermal evaporation [11], chemical vapour deposition [12,13], pulsed laser deposition [14], spray pyrolysis [15,16], dip coating, etc. [17]. The methods of preparation of CdS films have significant impact on the properties of the films [18–20]. Most of the higher-end techniques yield films with excellent characteristics. However, they may not be commercially viable because of their high cost of production. In order to bring down the cost of thin film based devices, it is really important to obtain films from simple and low cost techniques. n

Corresponding author. Fax: þ91 820 2571071. E-mail address: [email protected] (K.G. Rao).

http://dx.doi.org/10.1016/j.jpcs.2014.09.008 0022-3697/& 2014 Elsevier Ltd. All rights reserved.

Chemical deposition techniques such as chemical bath deposition, spray pyrolysis, SILAR etc. are far more cost effective than the vacuum based, higher-end techniques. Although, the films obtained by these techniques are usually amorphous or nanocrystalline, the cost advantage, ease of large scale production and better control over the film properties offered by these techniques have made them very popular. The successive ionic layer adsorption and reaction (SILAR) technique was introduced in mid-1980s. Since then the method has been used to obtain thin films of many semiconductor materials, including some selected II–VI compounds [21]. Basically, the SILAR technique is a refined version of chemical bath deposition. In the conventional chemical bath deposition, the films are obtained from a single chemical bath containing all necessary precursors. In such a case the precipitation, and hence material loss, is inevitable. On the other hand, in SILAR, the anionic and cationic precursors are taken in separate containers to prevent precipitation [22–27]. In a typical SILAR deposition process, the substrate is immersed separately in precursor solutions and rinsed in between with deionized water to get rid of the loosely bound ions. The growth rates of the thin films can be controlled very effectively by fine-tuning the experimental conditions. The number of immersion cycles, the concentration and temperature of the precursor solutions, dip duration etc. have a strong bearing on the properties of the films obtained. Although there are previous reports available on SILAR deposited CdS films, many of the results reported in those are contradictory [28–30] and therefore inconclusive. Some of the authors

K.G. Rao, V.K. Ashith / Journal of Physics and Chemistry of Solids 77 (2015) 14–22

may have done the SILAR deposition manually. Hence the consistency and reproducibility of their results is questionable. Extensive research work has to be done in order to conclusively establish these results. In the research work presented here, we have made an attempt to study the correlation between afore mentioned deposition parameters and the properties of SILAR deposited CdS films. A microprocessor controlled SILAR deposition unit was employed to accurately control the deposition process. The results obtained have been compared with those of the other authors.

2. Experimental details CdS thin films were deposited on glass substrates which were cleaned for 10 min in acetone, rinsed with distilled water and dried prior to the deposition. The depositions were carried out in a micro-controlled HOLMARC™ SILAR deposition unit (procured from HOLMARC™ opto-mechatronics, Kochi, India) equipped with independent heaters for the beakers. The unit has provisions to hold up to 6 beakers in a single cycle. Each beaker-holder is connected internally to independent heaters with temperature controllers. These beaker-holders are arranged in a circular manner on the top face of the unit. The substrate holder is attached to one of the arms of a ‘T’ shaped armature (based at

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the centre of this circular arrangement of beakers) such that it is directly above the beakers. During the deposition, the ‘T’ shaped armature rotates about the central vertical axis so that the substrate holder can move from one beaker to the other. It also moves up and down in a vertical plane to dip the substrates in solution. The substrate holder has a programmable stepper motor attached to it. Thus the glass substrate doubles as a stirrer and, when dipped in the solution, it stirs-up the solution to maintain uniform distribution of ions. The rotation speed of ‘T’ armature, substrate dip duration, dip speed, stirring speed etc. can be accurately programmed by the micro-controller unit. During depositions, the SILAR unit was kept inside a transparent, air-tight enclosure to prevent the contamination of the films and also to avoid exposure to harmful vapours that may emanate from the solution while heating. The cationic and anionic precursor solutions used were cadmium acetate (Cd(CH3COO)2) and thiourea ((NH2)2CS) respectively. The deposition of CdS thin films takes place as described below. In the first step, well-cleaned glass substrates were immersed in cationic precursor solution of Cd(CH3COO)2. The cadmium ions get adsorbed to the surface of the substrate. The substrates were then dipped in ammonium hydroxide solution (complexing agent) to form cadmium–ammonia complex. After this, the substrates were rinsed by distilled water to remove loosely adsorbed ions.

Fig. 1. The variation of film thickness with (a) number of immersion cycles, (b) precursor concentration, (c) dip duration (or immersion time) and (d) precursor temperature.

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The chemical reactions involved in this step are as follows:

NH4 OH [aqueous] → NH3 +H2 O [in solution] Cd (CH3 COO)2 [aqueous]+ 4NH3 → Cd (NH3 )24+ [in solution]+ 2CH3 COO− [in solution] Cd (NH3)24+ [in solution] → Cd2 + [adsorbed]+ 4NH3 [in solution]

(1)

In the second step, the substrates were immersed in the anionic precursor thiourea. The sulphur ions react with adsorbed cadmium ions on the substrate to form CdS. The substrates were again dipped in distilled water to remove loosely bound ions. The chemical reactions in this step take place as follows:

(NH2 )2 CS + OH− [in solution] → SH− + CN2 H2 +H2 O [in solution] SH− + OH− [in solution] → S2 − +H2 O [in solution] Cd2 + + S2 − [in solution] → CdS [adsorbed]

number of times. The thickness of the films was determined by gravimetric analysis. The deposition parameters, such as concentration of the precursor, number of cycles, dip duration etc., have strong bearing on the structural and optical properties of the SILAR deposited films. Understanding this correlation between deposition parameters and various properties of the films is therefore very essential. In this work, we have undertaken such an investigation by characterizing films deposited under different conditions. The number of immersion cycles was varied from 50 to 250. The concentration of the precursor solutions was varied from 0.1 M to 0.5 M and dip duration was varied from 50 to 90 s. The films were also deposited under different solution temperatures ranging from room temperature to 343 K. These films were then subjected to structural and optical characterization. The crystal structure of the films was studied by powder XRD (Rigaku Minflex X-ray diffractometer) analysis. The surface morphology of the films was studied by a scanning electron microscope (Philips XL30). Chemical composition of the films was determined by EDAX (Philips) analysis and the optical parameters of the films were determined by a spectrophotometer and an ellipsometer (Ocean Optics).

(2) 3. Results and discussion

The above two steps together constitute one complete cycle of deposition. This cycle was repeated several times to get well adherent and homogeneous CdS thin films. The automated SILAR unit used in our work can perform these cycles specified a certain

The variation of film thickness with number of deposition cycles is shown in Fig. 1a. For this study the concentration of anionic and cationic precursor solutions was kept constant at

Fig. 2. The XRD patterns of CdS films deposited under different conditions.


0.1 M. It can be seen that the thickness increased with the increase in number of cycles. During SILAR deposition, ideally a few layers of atoms get deposited on the substrate in each cycle. As the number of cycles increases, the thickness of the films also increases as long as there is a steady supply of ions. There are some contradictory reports available in literature concerning the effect of deposition cycles on thickness of the films. Garadkar et al. [29] observed increase in thickness upto 130 cycles but it remained constant thereafter. They have used the same precursors and concentration as we have used in our work. This result is in contradiction to the result obtained in our work. We observed continuous increase in the film thickness even beyond 130 cycles and upto 250 cycles. Senthamilselvi et al. [30] have also reported similar increase in thickness upto 100 cycles but using different precursors. In conclusion, we did not observe any peeling effect upto 250 cycles and the concentration of the solution was good enough to maintain steady increase in thickness. Fig. 1b shows the variation in the thickness of the films with the concentration of the precursor solutions. The number of deposition cycles was kept constant at 100 cycles. Again, here, it can be seen that the thickness increased continuously with the concentration. Higher concentration of ions in the solution speeds up the film formation by adsorption. Hence the thickness of the film deposited per cycle increases as the concentration increases. Jadhav et al. [28] have reported a decrease in film thickness after 0.1 M (due to peeling of material) which is again in contradiction to our result.

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In order to determine the effect of dip duration on thickness, four different sets of samples were prepared by dipping the substrates in precursor solutions for different time durations (ranging from 50 s to 90 s). The number of deposition cycles was kept at 100 for all the samples. The variation in thickness with dip duration was found to be only marginal as shown in Fig. 1c. Since in the SILAR technique the film is formed by depositing successive ionic layers, the dip duration has very little effect on the thickness. Once the substrate is covered by a layer of ions of finite thickness, the further adsorption of ions, of the same species, on this layer becomes exceedingly difficult due to the electrostatic repulsion force between the like charges. Hence, even if the substrate is dipped in the solutions for long durations, it would not contribute significantly to the thickness of the film. The XRD patterns of films deposited under various conditions are shown in Fig. 2. In all these cases, the films were found to be polycrystalline. The patterns show two peaks of cubic (zinc blende) phase corresponding to (111) and (311) planes and one peak of hexagonal phase corresponding to (110) planes. This kind of polymorphism is common in cadmium compounds such as CdS and CdTe. The grain size ‘D ’ of the films was calculated using the Debye–Scherrer formula:

D=

λ β cos θ

(3)

where λ is the wavelength of the x-rays, β is the full-width-at-halfmaximum (FWHM) of the peak and θ is the Bragg angle of the

Fig. 3. The variation of grain size of the films with (a) number of immersion cycles, (b) precursor concentration and (c) precursor temperature.

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peak. Since the correctness of the Scherrer formula is questionable in the case of polycrystalline films, the results obtained were later re-confirmed by analysing the SEM images of the films (Fig. 4). The grain size of the films was found to increase (from 16 nm to

149 nm) with the increase in number of deposition cycles (Fig. 3a) and also with the increase in concentration of the precursor solutions (from 57 nm to 152 nm) (Fig. 3b). This improvement in crystallanity is mainly due to increase in thickness of the films.

Fig. 4. The SEM images of CdS films deposited under different conditions.

Table 1 Composition of the CdS films deposited under various conditions. Number of immersion cycles

50 100 150 200 250 50 100 150 200 250 100 100 100 100 100 100 100 100 100

Precursor concentration

Solution temperature (K)

Cadmium acetate (M)

Thiourea (M)

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.3 0.4 0.5 0.1 0.1 0.1 0.1 0.1

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.3 0.4 0.5 0.1 0.1 0.1 0.2 0.3

300 300 300 300 300 300 300 300 300 300 300 300 300 300 323 333 343 300 300

Post-deposition heat treatment

NIL NIL NIL NIL NIL 523 K, 523 K, 523 K, 523 K, 523 K, NIL NIL NIL NIL NIL NIL NIL NIL NIL

2h 2h 2h 2h 2h

Atomic percentage (%) Cd

S

52.03 52.02 52.34 51.87 52.53 51.03 51.11 51.00 51.08 51.63 52.45 52.28 52.17 51.99 52.06 52.43 52.21 50.18 48.11

47.97 47.98 47.66 48.13 47.47 48.97 48.89 49.00 48.92 48.37 47.55 47.72 47.83 48.01 47.94 47.57 47.79 49.82 51.89


The temperature of the precursor solutions also affects the thickness of the films as shown in Fig. 1d. Here, both anionic and cationic solutions (0.1 M) were heated to the same temperature ranging from 300 K to 343 K. Deposition was carried out for 100 cycles. The thickness increased with the increase in temperature. Liu et.al. [31] have reported a similar increase in thickness of the films with solution temperature for chemical bath deposited films. In their work both precursors were taken in a single chemical bath. However, we obtained the same result despite the fact that the anionic and cationic precursors were taken in separate beakers in our work. The increase in temperature increases the mobility of the ions in the solution which in turn speeds up the adsorption process. Hence the thickness of the films deposited per cycle increases. In addition to thickness, the grain size of the films also increased (from 57 nm to 172 nm) with solution temperature as shown in Fig. 3c. Due to the increased mobility of the ions at elevated solution temperatures, the surface mobility of the ions on the surface of the glass substrates will also be high. Thus, the ions can move around on the surface and form grains of larger size. It can be seen from Fig. 2d that the peak heights of both cubic and hexagonal peaks increase with the solution temperature, indicating the improvement in crystallanity. However, the improvement in hexagonal phase (110) is noticeably more than that in the cubic phase. This is consistent with the fact that, in CdS, hexagonal phase is favoured at higher temperatures.

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The SEM images of the films deposited under various conditions are shown in Fig. 4. All these films show good substrate coverage. The substrate coverage of SILAR deposited CdS films in our work is better than that of the chemical bath deposited films reported by Liu et al. [31]. The chemical bath deposited CdS films in their report had considerable amount of voids (for solution kept at room temperature) which reduced only at higher solution temperatures. In our case we found that even the films deposited at room temperature, cover the substrates well, leaving very less or almost no voids. As mentioned earlier, the SEM images of the films confirmed the grain size values obtained from the Scherrer formula. Improvement in grain size is clearly visible in the SEM images of films deposited with higher number of immersion cycles (Fig. 4b), with higher concentration of precursors (Fig. 4c) and at higher precursor temperatures (Fig. 4d). The composition of the films determined from EDAX analysis is given in Table 1. It can be seen that the films obtained from precursor solutions of equal concentration are slightly sulphur deficient. This trend remains intact even if the number of cycles is increased or the concentration of both the solutions is increased to the same extent. The sulphur deficiency of the films is mainly due to the different rates of adsorption of cadmium and sulphur ions. When the concentration of both the precursor solutions is the same, the cadmium ions adsorb slightly more effectively than the sulphur ions. When these non-stoichiometric films were annealed, the composition slightly changed due to the evaporation of excess,

Fig. 5. The optical properties of CdS films deposited for different numbers of immersion cycles: (a) absorbance, (b) Tauc plots, (c) extinction coefficient k and (d) refractive index n.

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unpaired cadmium ions but the films still remained non-stoichiometric. However, when the concentration of sulphur precursor solution is increased above that of the cadmium, the films became stoichiometric. In this case, the increased concentration of sulphur ions helps to offset the difference in rate of adsorption. When cadmium-acetate-to-thiourea ratio was 0.1:0.2 M, the films were almost stoichiometric. The sulphur content in the film increased above that of cadmium when concentration of thiourea was increased to 0.3 M. The optical properties of CdS films deposited under various conditions are shown in Figs. 5–7. The films show high absorption for wavelengths below mid-visible band. The bandgap values found from Tauc plots varied from 2.3 eV to 2.4 eV. The values slightly deviate from the bulk bandgap value of 2.4 eV. Such small deviations in the values of band gaps can be mainly attributed to the variations in stoichiometry and the fine grain structure of the film. The value of bandgap of the films approached the bulk value of 2.4 eV as the grain size of the films improved. The refractive index n and extinction coefficients k of the films (in the wavelength range 400–800 nm) are shown also in Figs. 5–7 as functions of photon energy. The refractive index varied only marginally from 2.65 (at lower wavelengths) to 2.33 (at higher wavelengths). Meanwhile, the extinction coefficient k varied from 0.75 to 0.4. These observed values of n and k are very close to that of bulk CdS material and also to that observed by other authors [32, 33].

The reflectivity R, for an incidence angle of 90°, was determined from the n and k values using the formula [32]

R=

(n − 1)2 + k 2 (n + 1)2 + k 2

(4)

The values of reflectivity of the films varied from 0.23 (near 400 nm) to 0.19 (near 800 nm).

4. Conclusions CdS films obtained from the micro-controlled SILAR unit showed uniform substrate coverage. Films were well adherent to the substrate. The thickness of the films can be controlled by various parameters such as the number of immersion cycles, concentration of the precursors and the temperature of the solutions. The thickness increased continuously with number of immersion cycles and the concentration of the solutions, contrary to the observations of some of the other researchers. Significant improvement was observed in grain size of the films with the increase in the number of immersion cycles and also with the increase in concentration and temperature of the precursor solutions. Equal concentration of anionic and cationic precursors yielded non-stoichiometric films. Stoichiometric films were

Fig. 6. The optical properties of CdS films deposited from precursor solutions of different concentrations: (a) absorbance, (b) Tauc plots, (c) extinction coefficient k and (d) refractive index n.


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Fig. 7. The optical properties of CdS films deposited from precursor solutions kept at different temperatures. (a) absorbance, (b) Tauc plots, (c) extinction coefficient k and (d) refractive index n.

obtained for the cadmium acetate concentration of 0.1 M and thiourea concentration of 0.2 M at room temperature. The optical bandgap of the films deviates slightly from the bulk value due to the fine grain structure of the films. As the grain size increased, the bandgap approached the bulk value of 2.4 eV

Acknowledgement The research work presented here is fully funded by Manipal Institute of Technology and Manipal University, Manipal, India. Part of the characterization work was done in Central Instrumentation Facility, MIT Innovation Centre, Manipal University.

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