SYNTHESIS AND CHARACTERIZATION OF CHEMICAL BATH DEPOSITED COPPER DOPED LEAD SULFIDE THIN FILMS

. The effects of copper concentration on the properties of copper lead sulfide (Cu � Pb ��� S) thin films have been reported. Chemical bath deposition (CBD) method was used to deposit the ternary material on soda-lime substrates. Here, the Cu � Pb ��� S films were grown using copper(II) chloride, lead nitrate, and thiourea as sources of copper (Cu), lead (Pb) and sulfur (S), respectively. The grown films were examined using X-ray diffractometer (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, UV-spectrophotometer and four-point (4P) probes. XRD measurements revealed a polycrystalline material with strong adherent to the substrates. SEM readings indicated various grain-like crystalline morphologies with different sizes that change with copper concentrations. The EDX studies revealed that the deposited material consist of copper, lead and sulfur in various compositions. The transmittance was high in the near infrared regions of the electromagnetic radiation and, also decreases with copper concentrations. The band gap (E g ) varies from 2.05 to 2.50 eV with increase in copper concentrations. The study shows that CBD is an excellent method in depositing good quality films for device applications.


INTRODUCTION
Lead sulfide (PbS) is an importance semiconducting material that belongs to group IV-VI of the periodic table [1,2]. It possesses excellent optoelectronic properties and finds applications in light emitting diodes, photovoltaic cells, cathode-luminescence and electroluminescent displays, optical switch multilayer dielectric filters, sensor, lasers and catalysis [1]. At room temperature, PbS possesses narrow band gaps of about 0.4-1.35 eV in the UV region [3,4]. It can virtually be deposited with all chemical and physical methods. Lead sulfide appears as a black coloured crystal or powder with a density of 7.60 gcm -3 and molecular mass of 239.30 gmol -1 . It crystallizes in halite (cubic) structures with the (lattice) constant a = 5.936 Å [5]. Recently, investigation into new photovoltaic materials with improved efficiency have assumed a considerable interest and researchers are investigating to understand the characteristics of PbS films for opto-electronic applications [6]. Doping with elements like copper has shown to enhance the structural, optical and electrical properties of PbS thin films.
Copper doped lead sulfide (Cu Pb S) thin films have attracted a great deal of attention due to its applications. Usually, wide ranges of solubility of lead and copper ions in ternary copper lead sulfide compound can be expected, as these ions can fill either substitutional or interstitial positions of the resulting films [7]. The structure of copper lead sulfide thin film may not be unstoichiometric, as the vacancies control the conductivity type which may causes variations in conductivity [8]. These properties of ternary copper doped lead sulphide thin films have attracted great interest in the fundamental studies on the films. These properties depend on the preparation methods as well as parameters like substrates, impurity level and post-deposition processing [1].
Thin films of miscible systems have been grown utilizing different deposition techniques and these techniques have their different deficiencies [9]. The application of CBD technique has received much attraction due to its great potential to fabricate high quality films [2,10]. It is a preparative method well-suited for large scale production which is cost effective comparing with other deposition methods [10]. The method was used in the past to synthesize crystalline and polycrystalline binary sulfides and oxides [11]. Recently, CBD technique is desired to synthesize ternary and quarternary materials for device applications.
Reports on the fabrication and characterization of various binary copper-sulfide and leadsulfide thin films using different deposition techniques are relatively common in the literature [1][2][3], but work on their ternary compound of copper lead sulfide (Cu Pb S) is less common. In this work, we investigate the effect of copper concentrations on the properties of Cu Pb S thin films using the chemical bath deposition (CBD) technique. The interest in ternary materials comes from the possibility of engineering the elemental parameters with deposition. Then again, ternary compounds are attracting increasing attention due to their good performances with moderately simple chemistry and the absence of environmental concerns associated with their uses. This research presents the synthesis of the ternary Cu Pb S thin films via the CBD technique. In addition, the effects of copper concentrations on the properties of Cu Pb S thin films, which, hitherto has not been studied were examined.

Deposition of CuxPb1-xS thin films
Solutions of 1.0 M of copper(II) chloride, lead nitrate and thiourea were prepared from accurately weighed analytical solids. 25 mL of each solution was taken into a separate beaker to form 75 mL of the solution. Then 25 mL of distilled water was used to top the solution to 100 mL. It was then stirred for about 30 min for proper harmonization of the ions. During the deposition, EDTA disodium salt was used as a complexing agent. 25% of ammonia solution was added drop-wise using a burette to maintain the pH of the solution at 9.0. Constant volumes of lead nitrate, thiourea, ammonia solution and the EDTA disodium was maintained. When doping with copper, varying volumes of uniform concentration of CuCl2.2H2O solutions was used by changing the concentration of х in Cu Pb S nanostructured thin films; where, Cu and Pb are the number of moles of copper(II) chloride and lead nitrate respectively. However, the value of x varied from 0.0 to 0.4 using the relation Cu Pb S. Table 1 indicates how the deposition conditions were varied.

Characterization techniques
The crystalline characterization was measured using D8 High Resolution X-ray diffractometer with wavelength of λ = 1.5406 Å. Morphological and elemental compositions were observed by JEOL JSM-7600F Scanning electron microscopy (SEM). Optical measurements were done through a (UV-1800S) Spectrophotometer in a wavelength range of 350 to 800 nm at room temperature, while four-point probes were used to determine the electrical properties.

X-ray crystallization
The crystallization patterns of the films are illustrated in Figure 1. The pattern of sample A (PbS) is illustrated in Figure 1 It is also observed that there is one preferred growth orientation of (222) plane, with intensity that is reliant on the preparation parameters. Moreover, the absence of any other peaks shows that the material is free of impurities. Figure 1 From the analysis of Cu Pb S thin films, it showed that the intensities of the peak at 2θ = 30.3 increases with copper concentrations. This increase in peak intensity may result from the induced structural disorder within the deposited films as a result of copper impurities in Cu Pb S system [12]. The occurrence of well-defined and sharp peaks confirms the polycrystalline nature of the deposited films [13,14]. Polycrystalline thin films orientations are generally determined by the processing parameters. The changes evident in the structure occur due to surface diffusion as well as grain boundaries migration of two smaller nuclei of different orientation to form a larger one [10].
The crystalline size (D) of the films was calculated from the X-ray diffraction (222) reflection using the Debye-Scherer's relation (2) [15] = where β is the measured FWHM (in radiant), θ is the Bragg angel of the peak, the λ is the X-ray diffraction wavelength, D is the effective crystalline size. The calculated D was 21.144, 22.788, 23.266 and 26.084 nm for the various samples. This shows that the D varies with concentration but the enlargement was more evident when the concentration was increased from Cu . Pb . S to Cu . Pb . S.

Morphological study
The variations in surface morphology at different doping concentrations are illustrated in Figure  2. As indicated, the substrates were well covered with films of different morphological structures. For Sample A (PbS), the SEM micrograph shows cloudy images, which confirms a polycrystalline grains formation within the films. It was also, seen, that copper concentration highly influences the surface morphologies of the samples, as Sample B shows rougher films with the creation of some compact grain-like features. Evidently, the gradual formation of grain-like features was due to the increase nucleation rate as the doping concentration is increased. Further increase in doping concentrations (Samples C and D), increased the creation of more grains. Therefore, maintaining the non-uniform and close-packed morphology over the substrate. Moreover, the increased formations of black and white grains were also observed, which originated from the variation of lead and copper content within the Cu Pb S matrix. This may explain the coexistence of lead and copper in the Cu Pb S system as indicated from the x-ray diffractometry (XRD) measurement. The larger grains were found to detach into smaller and well-organized quasispherical polycrystalline grains (Sample D). This development parallel the fact that increasing the deposition concentration results in a number of grains diffusing and coalescing simultaneously to form sizable crystalline grains with defined crystallographic morphologies as revealed from the scanning electrode microscopy (SEM) studies [15]. Furthermore, the different morphological structures exhibited by this material may be the direct results of the various phases and elemental compositions of the material suggested from the XRD studies.

Elemental analysis
The elemental analysis of the undoped lead sulphide (PbS) thin films demonstrate that the lead (Pb) atomic percentage is 20.34 and that of sulfur (S) is 49.40 as indicated in Figure 3(a). The S/(Cu + Pb) atomic ratio is 2.43 as shown in Table 2, which indicates that the sulfur atoms are in excess and could occupy either the interstitial sites or at the regular sites by creating equivalent number of lead (Pb) vacancies. Also indicated in the energy dispersive X-ray (EDX) spectrum in Figure 3(a), is the large amount of oxygen within the deposited film. This large amount of oxygen may be attributed to the abundance of oxygen in the starting material as well as the migration of these elements (oxygen) from the glass slide to the films during deposition [15]. Figures 3 (b-d) illustrate the EDX spectrums of copper doped lead sulfide thin films. When the intrinsic lead sulfide (PbS) is doped with Cu ions, the copper ions will successfully occupy the Pb 2+ sites, thus reducing the S/(Cu + Pb) ratio below 1.0, as indicated in Table 2. This means that more copper atoms can diffuse into the PbS lattice without altering the host crystal structure as indicated from the X-ray diffractometry (XRD) measurements. The EDX measurements also showed that the elemental compositions of the doped material consist of copper (Cu), lead (Pb) and sulfur (S), which imply that the Cu Pb S film is non-stoichiometric. It is reasonable to assume that the increase in copper concentration raises the rate of nucleation of atoms in the precursor and, after that, the elements in the Cu Pb S system.

Optical analysis
The transmittance spectra and wavelength plot is shown in Figure 4. It was found that the films had a very high transmittance of above 75% in the visible and near infrared regions. Sample C had the least transmittance in the infrared region while sample D had the highest in same region. Also, copper concentrations were decreasing with transmittance, except for sample D. This decrease may result from the formation of copper doped lead sulfide nanocrystals, which are increasing with transparency [16]. Therefore, reduction of copper concentrations makes the films more transparent. The high transmittance observed was due to the improved crystallinity of the films, which in turn, reduces the defects in the grains. Films of such high transmittance could be used for several applications, including solar cells, transparent conducting materials in light emitting diodes (LEDs) and photonic devices [17,18]. The optical absorbance against wavelength is indicated in Figure 5. It was observed that the absorption was high in the ultraviolet region of the spectra. Also, from the spectra, the absorbance increases with increasing copper concentrations. An indication of an improve transmission and crystallinity within the films [19]. Generally, the absorbance values were seen to be low throughout the visible and near infrared regions and range between 0.02 and 0.12. The refractive index (n) against photon energy is indicated in Figure 6. From the graph, the refractive indexes were decreasing with photon energy. The highest refractive index was established to be approximately 2.02 in the high energy region. However, as the doping concentration is increased, the average refractive indexes fall, and rise within the values of 1.57 and 2.02 (Table 2). This trend in 'n' was possibly due to the trapped photon energy within the deposited thin films [13,16]. The direct band gap (Eg) of the films was estimated by plotting (∝hv) 2 against photon energy (hv). Figure 7(a-d) shows the plot of various samples. For all compositions, the Eg increases with copper concentration as depicted in Table 2. The variation in Eg with copper concentration was seen to be somewhat linear which is due to the Eg of copper sulfide (CuS) being greater than that of lead sulfide (PbS) thin films ( Table 2). This increase toward higher energy side is in accordance with the trend reported in literature for ternary materials with two binary constituents [10]. The increase in Eg shows an improvement in structural crystallinity or phase transformation taking place within the films as a result of increased copper concentrations, as indicated in the XRD and SEM measurements [20]. Also, it could be due to the famous Burstein-Moss effect [21]. When copper atoms are deeply doped into lead sulfide (PbS) thin films, the lower levels in the conduction bands are occupied by electrons resulting in an increased Fermi level and then the Eg as well [22]. Another possibility for the Eg increase could be credited to the existence of secondary phases in small quantity that may not be detected within the films [23].

Electrical (I-V) characterization of the thin films
The I-V study was done using the four-point probe technique. The sheet resistivity and conductivity of Cu Pb S thin films were measured and the results were indicated in Figure 8. From Figure 8, the electrical resistivity at very low concentration was valued at 10 3 Ωcm and then increases to the order of 10 5 Ωcm. The resistivity were also increasing with copper concentration which shows that copper doping influences the electrical conductivity of Cu Pb S thin films. However, as x increases beyond 0.2 (Sample B), the resistivity begins to increase. This may be credited to the nature of the films deteriorating with further increase in the dopant concentration [15]. The nature of the observed conductivity is owed to the improved concentration of the charge carriers within the films system [24]. This reflects the expansion and pairing of grains to form larger crystallites within the Cu Pb S system.

CONCLUSION
Ternary copper doped lead sulfide (Cu Pb S) thin films were achieved through the CBD techniques. The route offers a unique technique in preparing copper doped lead sulfide thin films utilizing common bath solutions. Structural, elemental, morphological, optical as well as the I-V characteristics were directly influenced by the dopant concentrations. The EDX measurement corroborated that the films comprise of copper (Cu), lead (Pb) and sulfur (S) in various elemental compositions. SEM revealed an agglomeration of grains with different morphological forms. The films show high transmittance of above 75% in the UV region with an energy gap that increased from 2.05 to 2.50 eV with copper concentrations. Thus, confirming the suitability of Cu Pb S thin films for various device applications. Since, a surface-sensitive quantitative spectroscopic method based on the photoelectric effect called X-ray photoelectron spectroscopy (XPS) can identify the elements present in a material (its elemental composition) or that are present on its surface, as well as their chemical state, general electronic structure, and density of the electronic states in the material. It is therefore suggested that future studies be conducted on XPS data (which will reveal the oxidation states of elements) as well as XPS depth profiling. Also, the Hall Effect measurements were not considered during this study due to limitations in the device used for the study. However, these will be considered in future scope so as to determine the types of carrier concentrations, mobility of the carriers as well as the hall coefficients of the material wherein the four point probes cannot provide such a feature.