Assessment of the Annealing Effect on Varied Ligand Deposited Lead Sulphide Thin Film Grown by Chemical Bath Deposition Technique

Abstract

The study of the influence varied concentrations of ligand and annealing of chemically bath deposited PbS thin film has been carried out the influence of ligand on the optical, solid state, morphological and Electrical properties of the deposited thin films has been investigated. The absorbance, transmittance from the UV-VIS spectrophotometer and other parameters such as absorption coefficient, reflectance, energy band gap, optical conductivity indicate that there is a effect of ligand variation on the properties and the absorption coefficient of the films was found to increase as the concentration of ligand as they annealed to the same temperature and in all case they decreases towards longer wavelength region and the energy band gap was observed to increase from 1.61 to 3.71 eV as the concentration of ligand increases. The surface morphology of the SEM micrographs result shows that the deposited thin films are of polycrystalline and the particle grains are evenly distributed across the substrate’s surface indicates that the grain sizes of the films are dependent on the molar concentration of ligand increases while the XRD showed where the preferential plane with highest diffraction intensity is located.

1. Introduction

It has been recently observed that much oxide based and chalcogenide based thin films form the major types of compounds/thin films that have attracted the interest of many researchers due to their anticipated amenability in modern thin film technological applications in this modern period. This observation is based on their unique characteristics which is the root of their application [1]. And apart from these their multipurpose applications, they have characteristic of crystallization with the perovkite structure coupled with their unique behavior in gas-solid interface, heterogeneous catalysis and also their susceptibility in partial substitution in A and B position [2]. They usually have an exceptional capacity of glass forming, a high refractive index, and photonic energy lower than the standards, high photosensitivity, and excellent transmission over the infrared region. These characteristics make them ideal for manufacturing various civil, medical and military applications such as infrared detectors, infrared lens, planar optics, photonics integrated circuit lasers etc. [3]. Coupled with the fact of their abundance on the earth and environmental friendliness, excellent structural, optical and electrical properties, chalcogenide materials are considered to possess excellent absorption coefficient that makes it suitable for use in absorption of photons from sun radiation [4]; they can be synthesized easily with given extensive range of their amenability of their chemical and physical properties.

However, our work here is focusing only on binary sulfur chalcogenides based thin film known to be ternary neither quaternary chalcogenide nor is it on oxide base thin film which has also attracted lots of researchers [5].

Binaries chalcogenide constitute the following; CdS, CdSe, Bi₂S₃, PbSe, PbS, As₂S₃, Ag₂S, Cu₂S, Sb₂Se₃, ZnS, CaS, PbTe, MnS. Some of which have been grown using CBD technique [6] and by other deposition techniques such as pulsed laser technique has been employed to deposit PbS thin film [7]; which is an indication that many other techniques have been implemented by many researchers in the growth of this group of thin film showing it’s friendly to different growth techniques.

Generally, metal dichalcogenides have the formula ME₂, where M = a transition metal and E = S, Se, Te [8] and the most important members are the sulfides. They are always dark diamagnetic solids, insoluble in all solvents, and exhibit semiconducting properties. Some are superconductors [9]. Several metals, mainly for the early metals (Ti, V, Cr, Mn groups) also form trichalcogenides. These materials are usually described as M⁴⁺(E₂²⁻) (E²⁻) (where E = S, Se, Te). A well-known example is niobium triselenide. Amorphous MoS₃ is produced by treatment of tetrathiomolybdate with acid [10]. The process involves combination of electropositive elements thin films combined with chalcogens (S, Se and Te) under the control of ligand. For instance, the growth and optical characterization of Antimony Selenide (Sb₂Se₃) and binaries chalcogenide such as CdS, CdSe, Bi₂S₃, PbSe, PbS, As₂S₃, Ag₂S, Cu₂S, Sb₂Se₃, ZnS, CaS, PbTe, MnS have been grown using different deposition technique including CBD [6, 11, 12]. It’s important to mention here that a more technical approach known as pulsed laser technique has been used to grow PbS in quest to obtain a more refined sample of the film. Also, theoretically and mathematical approach has been used to study and analyze the properties of some of these binary based chalcogenide thin film materials in order to compare the experimental and theoretical analysis [13].

Continuing on the quest for search on optimization of chalcogenide based thin films for applications in harnessing solar energy and for other device applications in this work, we intend to examine the effect of the annealing on varied ligand of chemically bath developed binary PbS thin film and to ascertain its effect on the structural, morphological, electrical and optical properties of thin film.

2. Materials and method

The experimental process and theoretical analysis were carried out as described here in this section.

2.1 Experimental procedure

The chemical bath was prepared by sequential addition of 2.5 ml (0.5 M) of lead acetate trihydrate ($\mathit{Pb}{\left({\mathit{CH}}_3\mathit{COO}\right)}_2.3H_{2}O$), 2.5 ml (2 M) of sodium hydroxide ($\mathit{\text{NaOH}}$) to neutralize the acidity of the solution, 3 ml (1 M) of thiourea (${\mathit{CH}}_4{N}_{2}S$), while tri-ethanolamine (TEA) as ligand was varied from 0.5 M, 0.8 M to 1.0 M according to Table 1. The growth process was carried out at room temperature of (27°C) within a specified period. After preparing the solution as stated above, distilled water was added until the volume of the solution reached 70 ml and the PH of the solution was measured using digital PH meter and kept constant at 11. After which the bath solution was then thoroughly stirred on a magnetic stirrer for 30 min to ensure homogeneity. The cleaned and dried substrate was incited and clamped firmly on a vertical position using retort stand and clip and then lowered into the 100 ml beaker containing the solution inside the beaker with aluminum foil caped on it as a cover for the top of the beaker in order to prevent dust or unwanted particles from entering the solution. The deposition time was kept constant throughout for 12 h for each of the samples labeled T₁, T₂ and T₃ for which the ligand, TEA concentration is 0.5 M, 0.8 M and 1.0 M respectively. At the end of the specified period of the deposition, the three samples were taken out of the bath, rinsed in distilled water and dried in an open furnace at moderate temperature for 3 min to remove residual water content and other possible adsorbed surface impurities. The films were observed to be homogeneous and well adhered to the substrate. Table 1 gives the details of the concentration of the ligand, (TEA) used for different samples [14, 15].

Samples	Conc. of ligand (TEA) in (mole)
T1 (W1)	0.5
T2 (W2)	0.8
T3 (W3)	1.0

Table 1.

The Samples showing the arranged sample according to the ligand concentration.

The optical and solid-state properties to be studied as a thin film characterization includes absorbance, transmittance, reflectance, absorption coefficient, extinction coefficient, refractive index, optical conductivity, dielectric constant, and energy band gap. The study of the solid-state properties of the film would give one an idea of these characteristics which arise as result of the interaction between photon energy and the structure of the thin film or between the energy configuration and other optical constants e.g., refractive index and extinction coefficient of the material [15].

2.2 Theoretical procedure

The optical and solid-state properties were deduced using some of the outlined formulas as given here, for instance, the absorbance A absorbance/transmittance spectra of the films T are obtained directly using spectrophotometer whose relation is given in Eq. (1) [16] although they can be obtained theoretically from the following relations

$${log}_{10}\left(\frac{I}{I_{\circ }}\right)={log}_{10}\left(\frac{1}{T}\right)=2-logT\left(\%\right)$$E1

where I represent the incident radiation flux and $I_{\circ }$ stands for the transmitted flux of the photon energy

$$T={10}^{-A}x100\%$$E2

Where A is the absorbance.

2.3 Transmittance (T)

Accordingly, transmittance is simply defined as the ratio of the transmitted flux to that of the incident flux I mathematically it is expressed as [17]

$$T=\frac{I}{I_{\circ }}$$E3

2.4 Reflectance (R)

Reflectance is determined mathematically from the relation:

$$A+T+R=1$$E4

Where R is the reflectance, A is the absorbance, and T the transmittance. This equation is valid on the account of the principle of energy conservation [16].

2.5 Absorption and extinction coefficient, $\left(\alpha \right)$ and (K)

Accordingly, the absorbance and absorption coefficient could be calculated in the fundamental absorption region using Lambert law:

$$ln\frac{I}{I_{\circ }}=2.303A=\mathit{\alpha d},$$E5

$$\alpha =\frac{2.303A}{d}$$E6

$$k=\frac{\mathit{\alpha \lambda }}{4\pi }$$E7

Where $\lambda$ is the wavelength of the incident photon.

2.6 Refractive index and optical conductance (n) and $\left(\sigma \right)$

From the transmittance result analysis, the refractive index n of the sample could be calculated using the following relations;

$$n=\frac{1}{T}+{\left(\frac{1}{T-1}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\text{or}n=\frac{1+\sqrt{R}}{1-\sqrt{R}}$$E8

where R is defined as [18];

$$R=\frac{{\left(n-1\right)}^2+k^2}{{\left(n+1\right)}^2+k^2}$$E9

and

$$\sigma =\mathit{\alpha nc}{\epsilon }_o$$E10

3. Results/discussion

Figure 1 is the experimental set up used for the deposition process of the film below.

Figure 1.

The three samples clamped with Retort stand during the growth.

Figures 2 and 3 presents the absorbance and transmittance of thin film and Figures 4–6 depicts the Tuac plots ${\left(\mathit{\alpha h\nu }\right)}^2{\left(\mathit{eV}/\mathit{cm}\right)}^2$ as a function of photon energy (eV) for sample W_1, ${\left(\mathit{\alpha h\nu }\right)}^2{\left(\mathit{eV}/\mathit{cm}\right)}^2$ as a function of photon energy (eV) for sample W₂ and ${\left(\mathit{\alpha h\nu }\right)}^2{\left(\mathit{eV}/\mathit{cm}\right)}^2$ as a function of photon energy (eV) for sample W₃. Figures 7–9 showcased the refractive index plot as a function of photon energy in eV while the optical conductance for each of the samples is presented in Figures 10–12 respectively.

Figure 2.

Absorbance as a function of wavelength for the three samples.

Figure 3.

Transmittance against wave length.

Figure 4.

Plot of ${\left(\mathit{\alpha h\nu }\right)}^2{\left(\mathit{eV}/\mathit{cm}\right)}^2$ as a function of photon energy (eV) for sample W₁.

Figure 5.

Plot of ${\left(\mathit{\alpha h\nu }\right)}^2{\left(\mathit{eV}/\mathit{cm}\right)}^2$ as a function of photon energy (eV) for sample W₂.

Figure 6.

Plot of ${\left(\mathit{\alpha h\nu }\right)}^2{\left(\mathit{eV}/\mathit{cm}\right)}^2$ as a function of photon energy (eV) for sample W₃.

Figure 7.

Refractive index, n as a function photon energy (eV) for W₁.

Figure 8.

Refractive index, n as a function photon energy (eV) for W₂.

Figure 9.

Refractive index, n as a function photon energy (eV) for W₃.

Figure 10.

Optical conductance, $\alpha$ as a function photon energy (eV) for W₁.

Figure 11.

Optical conductance, $\alpha$ as a function photon energy (eV) for W₂.

Figure 12.

Optical conductance, $\alpha$ as a function photon energy (eV) for W₃.

Up to date SEM investigation was carried out using SEM as presented in Figures 13–15 for the entire samples with the Dispersive X-ray Spectroscopy (EDAX) analysis depicted in Figure 16.

Figure 13.

SEM showing the annealed morphological structure of W₁.

Figure 14.

SEM showing the annealed morphological structure of W₂.

Figure 15.

SEM showing the annealed morphological structure of W₃.

Figure 16.

XRD showing the structural behavior of the PbS thin film for W₁, W₂ and W₃.

3.1 Optical and solid-state feature of PbS thin film

From the results the spectral absorbance as seen in Figure 2 reveals that the film has higher absorbance within the ultraviolet region and then decreases within the optical and near infrared region of EM wave spectrum for the two samples whose ligand concentration is less than 1.0 mole whereas the sample whose ligand is 1.0 mole low absorbance for all the spectra. On the other hand, the transmittance appears to be high within the infrared region which is the opposite as showcased in absorption characteristics as the samples with less when compared with the Transmittance. With the 1.0 mole ligand concentration it appears to have less transmittance as in Figures 2 and 3 respectively which is reflection of the influence of the ligand concentration coupled with the effect of the annealing. The energy band gap for the three samples as shown in Figures 4‣6 ranged between 3.35 eV and 4.00 eV indicating that the bad gap widens as the ligand was varied coupled with annealing though PbS material is characterized to be a direct band gap with its good feature in energy conversion that made the thin film good candidate for use in solar cell coupled also with its unique corrosion resistant coating characteristics.

3.2 Structural and morphological feature of PbS thin film

The morphological and structural features are shown in Figures 13–16 are obtained by SEM and XRD respectively and it is observed that the thin film are covered by spherical grain size that increased with decrease in ligand concentration [19] coupled with the annealing which enhanced spherical orientation of the grain size which as in Figures 13–15. The XRD indicates that the thin film has diffraction peak at the plane at (200), (111), (222), (220) and (311) with the preferential prominent peak occurring at (200) with three close lying reflections at (111), (222) and (220) as shown in Figure 16. This is as an indication of polycrystalline nature of the film and this varied with the variation of concentration of ligand [20, 21]. The highest intensity occurred at $1.30x{10}^3\mathit{au}$ and at 30° and the observation here is that crystalline orientation decreased with the increase in the concentration of the ligand coupled with the annealing effect which has contributed in the enhancement of the grain size as reported by [22]. The elemental composition of the film is shown on Figure 17 which is the EDAX analysis indicates that only Pb and S are prominent the constituent elements in the in the materials which then culminates the fact that there is no other element present in the thin film and this serves an indication that chemical bath deposition of PbS produced relatively pure thin film of PbS.

Figure 17.

EDAX spectrum of PbS thin film.

At the end of the processes, the samples were annealed at the temperature 80° and the taken to the laboratory for optical, structural and morphological characterization and analysis.

4. Conclusion

The use of varied ligand concentration has been employed to deposit PbS thin film using chemical bath deposition technique under the same time frame and the result presented in this work and from the results, it was observed that there were slight variations in some of the properties as analyzed which have clearly indicated the effect of variation of ligand concentration and the annealing effect on the various properties of the thin film as indicated in figures presented. In a similar manner, it was clearly revealed that the energy band gap of the thin film widened while the transmittance also increased with increase in the concentration of the ligand coupled with obvious indication of variation of grain sizes as indicated in the surface morphology as obtained from the SEM and, XRD and EDAX spectral analysis show that the chemical bath deposited PbS thin film is polycrystalline and relatively pure as it does not contain any impurity element it with little indication of the effect of annealing on the properties of thin film [14]. From the result so far, it could be deduced that generally, good quality of PbS thin film could be grown using CBD with varied ligand because report had already been given that increase in the annealing temperature increases the grain size with enhancement in the strength of the material along with reduction in dislocation density [23, 24]. One of the major attraction in PbS thin film is that the properties of the thin film can be controlled by different deposition conditions such as PH, bath temperature, and ligand concentration just as it is common with so many other chalcogenide based thin films.