Impact of Basic Bath on Optical and Electrical Characteristics of Zinc Sulfide (ZnS) Thin Films

1. INTRODUCTION

In comparison to bulk structured materials, nanostructures have recently emerged as a possible alternative. Regardless of their size, they induce a vast amount of potential applications for device fabrications which can be fruitful for everyday use [1, 2]. Thin films have distinguished themselves from other nanostructures over the years due to their remarkable properties. All optoelectronic devices can be employed by using thin films because of their optical and electrical properties [3, 4]. A valuable wide bandgap semiconductor that attracts the attention of the semiconductor industry is zinc sulfide (ZnS). ZnS belongs to an important family of semiconductors (II-IV) offering a 3.7 eV energy gap and a large absorption coefficient which is suitable for fabricating anti-reflection coatings [5–7].

ZnS thin films are a highly favorable candidate for applications in optoelectronic devices, for example, electroluminescent devices and photovoltaic (PV) cells [8]. This is because of their promising attributes towards wide range bandgap and n-type conductivity. These properties enable the transmission of highly energetic photons reaching to junction, thus improving the reaction of blue response in PV cells [9]. Moreover, ZnS thin film is a well-known semiconducting material for potential applications in light-emitting diodes that operate well in the blue region owing to its huge direct energy band gap. Most of the reports available regarding the energy bandgap properties of ZnS films demonstrate that in its cubic form along with lattice constant, a=0.541nm; c=2.49nm, it exhibits a direct energy gap of 3.68eV at 295K. Conversely, in the case of a hexagonal with a direct energy band gap between 3.74 and 3.87 eV at 300 K with a lattice constant of a = 0.382 nm and c = 2.49 nm. The optical properties of ZnS films including their n (refractive index) and k (extinction coefficient) values have been calculated and studied [10].

Over the last decade, several studies reported on the fabrication of ZnS films by chemical and physical techniques [11–13]. Several methods including thermal evaporation, sputtering, sol-gel, and CBD (chemical bath deposition) have all been investigated to synthesize ZnS thin films [14–17]. The CBD method is a well-known deposition technique that stands out among all others due to its cost-effectiveness, providing a non-vacuum atmosphere and allowing large-area deposition. This technique is particularly suitable as it doesn't involve the use of any advanced instruments. Moreover, CBD offers the flexibility of using a variety of substrates that can be utilized in the deposition process at low temperatures. Low-temperature deposition procedure not only reduces the layer of oxides on the substrates but also helps to prevent the film from substrate corrosion [18].

In the current work, a chemical method that included the CBD technique was employed to fabricate ZnS thin films. This research aims to study the necessary conditions to deposit high-quality ZnS thin films from CBD. Moreover, it intends to calculate their optical properties including transmission, refractive index, energy bandgap, and extinction coefficient. Furthermore, it strives to optimize the conditions of the deposition technique for ZnS thin films.

2. SYNTHESIS DETAILS

The most advantageous method for developing thin films on a variety of surfaces is CBD. This method is mostly utilized for the synthesis of thin films, such as zinc sulfide, zinc oxide, zinc selenide, and lead sulfide because of its amazing benefits and low cost of production. CBD also synthesizes ternary composite [19]. In this study, this method was utilized for the synthesis of ZnS thin films. Zinc acetate and thiourea were taken as precursors. Zn2+ ion from zinc acetate while S2- from thiourea were used. Zinc acetate was mixed in DI water under continuous stirring at room temperature. Similarly, thiourea was mixed in DI water under continuous stirring to form a homogeneous solution. Both solutions were mixed under continuous stirring for 2 hours. Ammonia was added dropwise to adjust the pH of the solution. Properly cleaned glass slides were adjusted in the holder and dipped in the beaker for the deposition of ZnS thin films. A series of these thin films was deposited by varying bath temperature from 25oC to 125oC for 1 hour. These thin films were dried at room temperature. The synthesis detail is shown in Figure 1.

Figure 1. Synthesis Detail of ZnS Thin Films by Varying Bath Temperature

3. RESULTS AND DISCUSSION

Figure 2 (a-e) depicts the XRD pattern of the prepared ZnS thin films with varying temperatures via CBD. All the planes (1014), (100), (103), (1010), (104), (1011), (2013), (2014) of ZnS thin films clearly indicate the formation of hexagonal phase matched with JCPDS card no 01-072-0162 under all conditions.

Figure 2. XRD Patterns of ZnS thin Films at Bath Temperatures (a) 25(b) 50 (c) 75 (d)100 (e)125

For the analysis of several parameters, such as crystal growth, phase orientation, and the type of phase of thin films XRD was utilized. For the determination of crystallite size, dislocation density, and strain, Scherer formula [20] was used which is shown below in equation 1,

D= kλ/βcosθ   (1)

where D is the average of nano-crystallite size [21], k depicts shape factor (0.90), λ represents wavelength radiation (0.15406nm) of Cu Kα [22], β shows full width at half maximum (FWHM) [23], and θ is the Bragg's angle of the most preferred orientation plane [24].

Dislocation density is the inverse of crystallite size and it is calculated by the formula given below in equation 2 [25],

δ= 1/D²   (2)

Strain is calculated by using the formula [26] given below in equation 3,

Strain = β/(4 tanθ)   (3)

Parameters calculated from eqs 1-3 are given below in Table 1.

Table 1. Parameters Calculated from XRD of ZnS thin Films

The use of ZnS in optoelectronics devices and many other applications requires high transparency of light in the visible region of wavelength 400-800nm [27]. The transmittance spectra of series samples a,b,c,d, and e against wavelength are shown in Figure 3.

Figure 3. Transmission Spectra of ZnS Thin Films with Varying Temperature

In the region of transparency, the value of transmittance can be found by using the free carrier of absorption. The value of transmission increased with temperature increase. The difference between sample ‘b' and sample ‘a' was studied. The transmission values of 36%, 65%, 62%, 32%, and 14% were observed at varying deposition temperatures, that is, 25℃, 50℃, 75℃, 100℃, and 125℃ given in Table 2.

Table 2. Percentage Transmission of ZnS Thin Films

In optical applications, energy band gaps are very important. The series of sample ZnS thin films synthesized by the CBD method had their energy band gap measured from the information data collected by spectroscopic ellipsometry [28]. Tauc's relation was utilized to calculate the optical band gap for each sample at various conditions. Absorption coefficient (α) with the optical band gap was calculated using the relation in equation 4 [29] given below,

α = (k/hv)( hν – Eg )ⁿ (4)

where k represents constant, h represents Planck's constant [30], hν shows photon energy, E_g is band energy, and n are numbers that illustrate the nature of electron transition between the valance band and conduction band. If n = 2, it represents the semiconductor direct band gap.

To calculate band energy, (α)² against photon energy was plotted as linear to absorption coefficient that reported a direct band gap transition of the prepared material. Tauc's plots [31] were used to calculate band energy and its nature for all samples represented in Table 3.

Figure 4 confirmed the energy band gap of ZnS thin films at varying temperatures. The observed band gap varies from 3.86-3.99eV. The maximum band gap of 3.99eV was observed at 100

Figure 4. Band Gap of ZnS Thin Films with Varying Deposition Temperature  (a) 25℃ (b) 50℃ (c)75℃ (d)100℃ and (e)125℃

Table 3. Energy Band Gap of ZnS Thin Films

Figure 5. Variation in Band Gap with Temperature

The variations in refractive index with wavelength at all conditions are presented in Figure 5. Notably, with the addition of ammonia, the value of the refractive index increases [32]. To calculate optical quantities in the visible region and UV region, Cauchy's relation was used. Cauchy's equation is given below in equation 5 [33].

n(λ) = n(λ) = A +B/λ2 + C/λ2 (5)

In this equation, A, B, and C are represented as constants.

The extinction coefficient is an important optical parameter [34]. It measures the materials that absorb light with a given wavelength [35]. It has a direct relation to the absorption of light. The extinction coefficient varies with the transmission of light. If transmission has a greater value, then k values are smaller. This shows that the material with a small k value has high transparency [36].

Refractive index and extinction coefficient against the wavelength of sample “a” is depicted below in Figure 6. The sample was synthesized at a temperature of 25℃. The refractive index was observed at 2.19 and the extinction coefficient was observed at 0.008. To observe the refractive index (n) and extinction coefficient (k) for sample “b”, the sample was prepared at a temperature of 50℃. The refractive index value was 2.12 for sample “b”, whereas the extinction coefficient value was 0.066. Similarly, sample “c” was prepared at a temperature of 75℃. The refractive index value was 1.62 and the extinction coefficient value was observed as 0.055. It shows that with temperature increases, refractive index value also increases. Sample “d” was prepared at a temperature of 100℃. The refractive index value and extinction coefficient value were observed. The value of refractive index was 2.16 and that of extinction coefficient was 0.06. Similarly, sample “e” was prepared at a temperature of 125℃, as shown in Table 4. The value of the refractive index was observed as 2.14 and the value of the extinction coefficient was observed as 0.073. This shows that the sample has a small value of refractive index and extinction coefficient.

Figure 6. n and k of ZnS Thin films with Varying Deposition Temperature  (a) 25℃(b) 50℃ (c)75℃ (d)100℃ (e)125℃

Table 4. n and k for All Samples

The dielectric constant of ZnS thin films synthesized at various temperatures was calculated using the equations 6 [37] and 7 [38] given below.

ε₁ = n² – k²  (6)

ε₂ = 2nk  (7)

Where ε₁ shows the real part of dielectrics and ε₂ shows the imaginary part of the optical dielectric constant for ZnS thin films.

Figure 7 shows the real and imaginary parts of thin films of the dielectric constant on the wavelength. The plot shows that with temperature increases the values of the optical dielectric constant decrease. Dielectric constants are listed in Table 5.

Figure 7. ε₁ & ε₂ of ZnS Thin Films with Varying Deposition Temperature (a) 25 (b) 50 (c) 75 (d) 100 (e) 125

Table 5. Optical Dielectric Constant of Samples

The measured electrical properties included resistivity and conductivity. The resistivity of ZnS thin films was measured via the Hall effect [39] and the Van der Pauw method [40]. In this method, there are four probes; a current of 1mA is applied through the 1 and 3 probe and voltage is calculated with the 2  and 4 probe [41]. The applied magnetic field and the applied voltage are 1T and 0.9V, respectively [42]. Voltage is measured in the presence and absence of a magnetic field, respectively. Thus, if the applied voltage sign is positive, then only a single sample is A n-type and all the remaining samples comprise p-type carriers [43]. Resistivity and conductivity are calculated by using the equations  [44–46],

R = V/I  (8)

ρ = RA/l  (9)

σ = 1/ ρ  (10)

where R is resistance, ρ is resistivity, and σ is the conductivity of the sample [47]. For the ZnS thin films, resistivity and conductivity are depicted in Figure 8. The figure shows that by increasing bath temperature resistivity of the ZnS thin films, resistance increases and conductivity decreases.

Figure 8. ρ & σ  of ZnS Thin Films Varying Deposition Temperature (a) 25℃ (b) 50℃ (c) 75℃ (d) 100℃ (e) 125℃

4. CONCLUSION

ZnS (zinc sulfide) is among the most significant II-VI semiconductor materials due to its distinct optical properties. An electroless, CBD technique was used for the synthesis of ZnS thin films. The films were deposited on the glass substrates. The deposition temperature was varied as 25℃, 50℃, 75℃, 100℃, and 125℃. ZnS thin films were utilised by employing XRD (X-ray Diffractometer) and spectroscopic ellipsometry. XRD analysis confirmed the hexagonal structure of these thin films. Ellipsometry results showed high transmission (∼65%) in the visible part of the electromagnetic spectrum for thin films prepared with 50℃ deposition temperature and high refractive index (at λ =550nm) ∼2.04. Diversity in the direct band gap 3.86–3.99 eV was also observed for the prepared thin films. The study concludes that the enhanced optical characteristics of ZnS thin films are appropriate for effective optoelectronic applications.

CONFLICT OF INTEREST

The authors of the manuscript have no financial or non-financial conflict of interest in the subject matter or materials discussed in this manuscript.

DATA AVALIABILITY STATEMENT

Data availability is not applicable as no new data was created.