Investigation of the Solution-Dependent Zinc Oxide (ZnO) Thin Film Growth Process by the Electrostatic Spray Deposition (ESD) Method

The present study describes a facile route of nanocrystalline (NC) ZnO growth using the solution-dependent ESD method at temperatures ranging from 300˚C to 500˚C. Zinc chloride (ZnCl ₂) was used as the Zn source, and it was dissolved in ethanol (CH ₃CH ₂OH) to prepare the various ESD spray solutions. The crystallographic orientations of the ZnO thin films were evaluated using X-ray diffraction (XRD). The morphologies of the ZnO films were observed by scanning electron microscopy (SEM). In order to check for the material composition of ZnO films, the energy-dispersive X-ray spectroscopy (EDX) analysis was performed. XRD and Raman show that ZnO thin films have a hexagonal wurtzite structure and point out a possible complex reaction mechanism due to the increase in H ₂O ratio in the solution. The microstructural parameters (MIP), namely, lattice parameters, were revealed using Bragg’s law. Meanwhile, other MIPs, such as bond length, positional parameters of the lattice phase, full width at half maximum, crystallite sizes, lattice strain, and lattice dislocation density, were estimated using the Debye-Scherrer method (D-S). The findings indicate that the ratio of added deionized water (H ₂O) suppresses the c-axis crystal growth of ZnO thin films. The adhesion of anions is thought to be responsible for this suppression. The results and analysis gave clues about how to develop a high-quality oxide-based crystal semiconductor that is economically viable for industrial and commercial applications of the ESD technique to semiconductor technology devices.

• Electrostatic Spray Deposition
• Debye-Scherrer Analysis
• ZnO Thin Film

1. Introduction

ZnO nanostructures have been the focus of significant research because their potential for use as a material in various fields, including solar cells [1] , light-emitting diodes [2] [3] , photocatalysis [4] , gas sensors [5] , catalysis [6] , laser diodes [7] , varistors [8] and sensors [9] , is promising. ZnO is a widely recognized n-type semiconductor with a wide band gap of approximately 3.37 eV and a substantial exciton binding energy of around 60 meV at ambient temperature [10] . However, the use of the methodologies for ZnO nanostructures is restricted due to factors such as elevated temperature, precise gas concentration, hazardous chemical reagents, and expensive equipment. Various deposition methods, including wet and dry processes, are employed to create ZnO thin films, such as sputtering [11] , pulsed laser deposition [12] , chemical bath deposition [13] , mist chemical vapor deposition [14] , spray processes [15] [16] , and the sol-gel method [17] [18] . Among these, the spray process is highlighted by its cost-effectiveness and simplicity, as it operates under air conditions, making it suitable for manufacturing on a large scale, utilizing roll-to-roll fabrication methods.

In this study, the current goal is to create a simple and cost-efficient method that can be easily expanded for commercial use for thin film fabrication. ESD is a highly attractive technique, particularly in spray applications, owing to its capability to generate submicron-sized droplets and manipulate their movement using an external electric field source [19] - [26] , as shown in Figure 1(a) . Under the present circumstances, ESD application on thin film fabrication aims fourfold, not only to study the ZnO thin film growth mechanism but also to investigate different crucial MIP analyses of the nanocrystal particle sizes for the development of high-quality crystal growth for oxide-based semiconductor technology. Firstly, the literature review results indicate that the growth mechanism of ZnO thin films using the ESD technique has not been documented within this specific combination of samples and temperature ranges. Secondly, the ESD approach offers user-friendly functionality with significant industrial advantages, including its capacity to effectively cover extensive regions due to its uncomplicated and easily accessible nature. Thirdly, the ESD method enables deposition without requiring a vacuum, is reasonable, and offers relatively easy control of the composition ratio and doping. Fourthly, within the ESD application for thin film fabrication, the functionality of “cone-jet mode” was tried to be revealed for the optimum correlation of the thin film growth mechanism. Both theory and experimental processes in literature [27] - [29] reveal that by adjusting the applied voltage, one can also control the flow rate. Moreover, by applying a high voltage, a stable flow rate can also be obtained; as a result, the spray area can also be generated in a large possible region. The current study tests this phenomenon to fabricate thin films on a large area of the substrate surface. The working principle for this “cone-jet mode” is electrostatic atomization for liquids of relatively high conductivity; the conical form of the meniscus results from a static equilibrium between capillary, hydrostatic, and electrostatic pressures. Furthermore, when conductivity is low, this acceleration zone can begin at the outlet of the capillary, and the conical form is more or less marked depending on the flow rate of the fluid, the size of the capillary, and the applied voltage. In addition, the appropriate high voltage and the corresponding flow rate are mentioned in Table 1 as the ESD parameter.

Furthermore, from the physics perspective, Coulomb forces caused charged droplets to repel each other, preventing them from colliding during the spray on the ITIO substrates. These features allow for the creation of dense and homogeneous thin films using ESD to cover the large area on the surface of the ITIO substrates. The crystal quality of ZnO thin films deposited through ESD was evaluated to determine the suitability of this technique, particularly for semiconductor thin-film deposition. Another key point of this study is to investigate the effects of the addition of H₂O on the crystal characteristics and structural properties of ZnO thin films. By providing the fundamental understanding of these processes, this work focuses on laying the foundation for the practical application of ZnO thin films as well as p-n junction/solar cell devices and other electronic applications, ultimately contributing to the advancement of semiconductor thin-film deposition technology using ESD within a very simple framework of fabrication. Further study is required to develop ESD as a novel fabrication process for thin films, which offers high-quality, cost-effective, and very easy to maintain compared to other growth processes and commercial fabrication methods [20] - [26] .

This paper follows a structured format. Section 2 provides a concise experimental technique conducted for the work. The results and discussion are presented in Section 3. We conclude and make remarks on this innovative fabrication technique in Section 4.

2. Experimental Procedure

The ESD setup is illustrated in Figures 1(a)-(d) . The zinc source used in this study was ZnCl₂ (98% Assay ZnCl₂). The solutes were CH₃CH₂OH (99.5%, pure) and deionized water (H₂O). ZnO thin films were deposited using the electric field applied spray pyrolysis, named ESD, on the conductive ITiO-coated al-kali-free glass substrates. 20 ml of three different spray solutions were prepared by changing the H₂O ratios. To prepare the first solution with a 0.1 M concentration, 20 ml of CH₃CH₂OH was taken in a glass beaker. The density of CH₃CH₂OH was 0.78945 g/cm³. Then 0.2726 g ZnCl₂ was added with the CH₃CH₂OH in the glass beaker. To make the solution homogeneous, it was heated up to 20 minutes in the magnetic stirrer with a hot plate (ADVANTEC SR/350) ( Figure 1(c) ). The temperature was considered 150˚C for the magnetic stirrer with a hot plate. Two separate glass beakers were used to prepare the second solution, which consisted of 80% CH₃CH₂OH (16 ml) and 20% H₂O (4 ml), respectively. Next, the same amount of ZnCl₂ was added to the glass beaker along with 16 ml of CH₃CH₂OH. The same procedure was applied to prepare the homogeneous spray solution. Then 4 ml of H₂O was poured into the solution containing CH₃CH₂OH and ZnCl₂. The mixture of the solution was heated for an additional 10 minutes using a magnetic stirrer with a hot plate. For the third ESD spray solution, 50% CH₃CH₂OH (10 ml of CH₃CH₂OH) and 50% H₂O (10 ml of H₂O) were taken in two different glass beakers. We followed similar procedures to create a homogenous solution.

Figure 1

Figure 1. Experimental setup of ESD.

Table 1

Table 1. Applied ESD parameters for fabrication.

SI. No.	Parameter	Values
1.	Distance between nozzles and the hot plate top	3.5 cm
2.	Flow rate	2.0 ml/h
3.	Deposition time	5 min
4.	Applied voltage	8 kV

The distance between the nozzle and the substrate was 3.5 cm. The precursor solution was pumped through a 0.26 mm diameter metallic nozzle at a flow rate of 2.0 mL/h. The hotplate maintained the surface of the conductive substrate at 300˚C - 500˚C, thereby facilitating the rapid evaporation of the solvent. In this experiment, the main solvent was CH₃CH₂OH, which has a boiling point of 78.37˚C. The conductive substrate was heated on the hot plate for 10 min before the deposition. A voltage of 8 kV was applied between the metallic nozzle and the conductive substrate to form the Taylor cone [27] - [29] , after which ESD was carried out for 5 minutes. After the deposition, the sample was kept on the hotplate for 10 minutes to evaporate the solvent thoroughly. For simplification and understanding, the ESD parameters are listed in Table 1 of this investigation. The crystallographic orientations and morphologies of the ZnO films were observed by XRD, SEM, EDX, and Raman, respectively. Furthermore, nanoscale-level studies mainly lattice parameters, a (Å) , and c (Å) [20] [30] , bond length, (Zn-O bond (Å)) [30] [31] , and, positional parameter, μ , were investigate to understand the preferential growth of ZnO thin films and how behavior is changing and affected by ESD with changing the H₂O ratios. The average crystallite size, D (Å) was calculated from the XRD peak width of (002) based on the (D-S) method (equation (8)) [25] - [30] . Peak width analysis yields two primary properties, lattice strain ε (%) [20] [32] and dislocation distribution δ (*10¹⁸ m²) [32] [33] , which have been studied in detail and are correlated to the basic crystal growth mechanism [34] [35] . We will discuss the detailed mathematical formulation of these MIP properties in the next section.

3. Results and Discussion

3.1. Characterization

ZnO crystallizes in the wurtzite structure, where oxygen atoms are distributed in a hexagonal close-packed pattern and zinc atoms occupy half of the tetrahedral positions. The Zn and O atoms exhibit tetrahedral coordination with each other, resulting in an identical position. The Zn structure is characterized by an open configuration, where all the octahedral sites and half of the tetrahedral sites are unoccupied. According to Bragg’s law [32] [36] ,

$n\lambda =2d\sin \theta$ (1)

where n is the order of diffraction (usually n = 1), $\lambda$ is the X-ray wavelength and $d$ is the spacing between planes of given Miller indices h, k, and l. In the ZnO hexagonal structure, the plane spacing $d$ is related to the lattice constants a, c, and the Miller indices by the following relation [20] [30] ,

$\frac{1}{d_{hkl}^2}=\frac{4}{3}\left(\frac{h^2+k^2+hk}{a^2}\right)+\frac{l^2}{c^2}$ (2)

Considering the first-order approximation, n = 1, Equation (2) can be written:

${\sin }^2\theta =\frac{{\lambda }^2}{4a^2}\left[\frac{4}{3}\left(h^2+k^2+hk\right)+\frac{a^2}{c^2}{l}^2\right]$ (3)

Following Equation (3), the lattice constant a (Å) for the (100) plane is calculated by [30]

$a=\frac{\lambda }{\sqrt{3}\sin {\theta }_{100}}$ (4)

Similarly, for the (002) plane, the lattice constant c (Å) is calculated by Equation (3) [30]

$c=\frac{\lambda }{\sin {\theta }_{002}}$ (5)

Moreover, the microstructure parameter, namely bond length (Zn-O) (Å), correlated with the lattice parameter, a, and c is also estimated. The equation used to estimate the bond length for the ZnO thin films is [20] [37]

$L={\left(\frac{a^2}{3c^2}+{\left(0.5-\mu \right)}^2*c^2\right)}^{1/2}$ (6)

where (µ) is the positional parameter of the wurtzite structure that indicates the extent of atom displacement relative to the following plane in the c axis, as expressed with Equation (7) [20] [31]

$\mu =\frac{a^2}{3c^2}+0.25$ (7)

The average crystallite size was calculated from the XRD peak width of (002) based on the Debye–Scherrer equation [31] - [43]

$D=\frac{K\lambda }{\beta \cos \theta }$ (8)

where β is the integral half width, K is a constant equal to 0.90, λ is the wavelength of the incident X-ray (λ = 0.1540 nm), D is the crystallite size, and θ is the Bragg angle.

An XRD experiment has been performed to characterize the structural properties of the ZnO thin film growth at 300˚C, 400˚C, and 500˚C temperatures, respectively. Figures 2(a)-(c) demonstrate the XRD pattern of the ZnO thin films on conductive ITiO substrate. Figure 2(a) represents the XRD results without mixing the H₂O ratio in the spray solution. The data analysis reveals that the fabricated thin film exhibits a hexagonal polycrystalline structure in the wurtzite phase, with the preferential directions being (100), (002), and (101) compared to the JCPDS 36-1451 card for ZnO . Significantly, the (002) plane at 400˚C and 500˚C temperatures exhibits a strong peak at the diffraction angle around 34.50˚C, respectively. At 300˚C temperature, the (100) and (102) planes exhibit very weak peaks compared to 400˚C and 500˚C temperatures. Furthermore, the strong peak for the (100), (002), and (101) planes only appeared at the 500˚C temperature of 0% H₂O ratio. Moreover, the consecutive peak appearance of the (002) plane in the observation corroborated the fabrication of ZnO thin films with a hexagonal wurtzite structure [32] . Figure 2(b) illustrates for 20% H₂O ratio, and it shows that there is a strong peak for the (002) plane at 400˚C and 500˚C, respectively. It is also observed that the peak strength for 400˚C temperature is weaker than that of 500˚C temperature. In addition, the peak position (500˚C temperature) shifted slightly to the right, which suggests a decrease in the lattice constant along the c-axis at high temperatures. The peak strength for the (100) plane is found to be weak at temperatures ranging from 400˚C to 500˚C. The other peak positions corresponding to ZnO thin film growth on the ITIO substrates are also observed at the (101) and (102) planes. Interestingly, the peak intensity (102) plane is observed to be stronger compared to Figure 2(a) . Gradually, it has become stronger by increasing the H₂O ratio to 50%. This change may be attributed to the transition of the crystal phase from wurtzite to another phase caused by the excessive H₂O ratio. From Raman analysis, it can be correlated that increasing the H₂O ratio suppresses the wurtzite phase, which is elaborately discussed in the corresponding section. Figure 2(c) shows the results for a 50% H₂O ratio to ESD spray solution. At 500˚C, a similar diffraction peak orientation exists for ZnO thin films in the (100), (002), and (102) planes. The (102) plane had a significantly greater peak strength compared to other planes as the temperature increased to 500˚C, as mentioned earlier, due to the H₂O effect. However, the ZnO thin film exhibited a physically significant change at 400˚C in the (100) and (102) planes. The peak intensity on the plane (002) was observed to decrease progressively as the H₂O ratio increased. This complexity may be attributed to the intricate reaction formation that occurs during thin film fabrication using the ESD technique among the [OH]⁻ and [Cl]⁻ ionic state reactions, for which we proposed the reaction mechanism in our previous work with the HCl doping effect [23] . We expect that a similar complex reaction mechanism may have occurred in the present study. Moreover, anions might preferentially adsorb onto specific crystal faces and inhibit their growth. At 300˚C, a very small and weak peak could be seen on the crystal planes (100), (002), (101), and (102). Furthermore, a distinct maximum is not observed for the (002) plane in the case of a 50% H₂O ratio. On the other hand, the (002) orientation of the hexagonal wurtzite structure has a small surface energy, which results in a higher growth rate, according to the basic crystal growth theory [37] . But the opposite trend is seen in the higher H₂O ratio than the ESD technique. This trend also explains the possible anisotropy of the fabrication process. The XRD measurements confirm that the deposited thin films exhibit a significant orientation along the c-axis (002), with variations in peak strength almost identical to those reported in the JCPDS 36-1451 card for ZnO .

Figure 2

Figure 2. XRD patterns of fabricated ZnO thin film according to the weight ratio of H 2 O.

Figure 3

Figure 3. SEM patterns of ZnO thin film according to 0%, 20%, and 50% weight ratio of H 2 O in the solution at 500˚C.

Figure 4

Figure 4. EDX patterns of ZnO film according to 0%, 20%, 50% weight ratio of H 2 O in ESD solution at 500˚C.

Next, the surface morphologies of the ZnO thin films were investigated. A representative SEM image of the undoped ZnO thin film is shown in Figure 3(a) , revealing that typically 45 - 50 nm grains were grown on the conductive ITIO surface of the ZnO thin film, with each grain being of slightly different size and not agglomerated. Therefore, flat and dense polycrystalline ZnO thin films can be deposited by ESD owing to the mono-dispersion, self-dispersion, and non-agglomeration of the charged droplets [20] . The surface morphologies of the H₂O-added ZnO films were slightly different from those without the additive, regardless of the H₂O ratio. However, unidentified particles of a few micrometers were observed only in the ZnO film with 0% H₂O content, as shown in Figure 3(a) . These could be unreacted precursors or the zinc oxychloride phase. This result correlated with the change in the composition ratio of H₂O obtained via EDX ( Figure 4 ). Additionally, our previous research indicated that oxide thin films fabricated by ESD were extremely flat, boosting an average roughness of approximately 1 nm [19] . Therefore, ESD emerges as a viable deposition method for generating thin films characterized by superior flatness and uniformity, with quality on par with that of dry processes.

3.2. Temperature Effect on Lattice Parameters

The estimated lattice parameters a (Å) and c (Å), using Equations (4) and (5) for this study, are plotted in Figures 5(a)-(c) and Figures 6(a)-(c) , respectively. It is well known that the lattice parameters are temperature dependent, i.e., an increase in temperature leads to expansion of the lattice [31] . Figures 5(a)-(c) showed the changing effect of lattice parameter, a (Å) for the (100) plane, besides Figures 6(a)-(c) lattice parameter, c (Å) for the (002) plane; with changing the water ratios and the temperature range from 300˚C to 500˚C. The lattice parameter analysis results showed a very consistent change in behavior. In Figure 5(a) , the lattice parameter, a (Å), is slightly decreasing from 300˚C to 400˚C temperature, and then it starts to increase from 400˚C to 500˚C temperature. But, the aptitude of lattice parameter, c (Å), is increasing from 300˚C to 400˚C, and then a very constant increasing behavior is observed from 400˚C to 500˚C in Figure 6(a) . Figure 5(b) illustrates the result for 20% H₂O mixing in the fabrication process, and the lattice parameter, a (Å), is increasing from 300˚C to 500˚C. But, the tendency of lattice parameter, c (Å), is increasing from 300˚C to 400˚C, and then a decreasing behavior is observed from 400˚C to 500˚C in Figure 6(b) . Moreover, the results for a 50% H₂O ratio showed that the lattice parameter, a (Å), is increasing from 300˚C to 400˚C temperature, and then it starts to decrease from 400˚C to 500˚C temperature in Figure 5(c) . On the other hand, the lattice parameter, c (Å), is slightly decreasing from 300˚C to 400˚C temperature, and then a very sharp decreasing behavior is found from 400˚C to 500˚C temperature in Figure 6(c) . The consistent results can be seen only for the nonmixing of H₂O with the ESD-fabricated thin films. Some experimental results also claim that the possible complex reaction formation may occur due to excessive mixing of H₂O during the fabrication process of ZnO thin film [45] . But the proper reason is not clear yet. Due to this background, further study may be conducted on the different aspects of composition with the H₂O ratio. The lattice parameter ratio, (c/a), is also listed in Table 2 . Similar results are seen for a (Å), c (Å), and (c/a) to those reported in the (JCPDS 36-1451) card [44] .

Figure 5

Figure 5. The trend of lattice parameter a (Å) of ZnO film according to the weight ratio of H 2 O in the ESD solution at 300˚C, 400˚C, and 500˚C, respectively.

Figure 6

Figure 6. The trend of lattice parameter c (Å) of ZnO film according to the weight ratio of H 2 O in the solution deposited by ESD at 300˚C, 400˚C, and 500˚C, respectively.

3.3. Temperature Effect on Lattice Bond Length

The detailed estimated results of the positional parameter µ are listed in Table 2 . A random changing trend is observed for µ with the H₂O ratio. So, accurate prediction related to the atomic displacement and the structural correlation may be difficult at this point. In Figure 7(a) , for the 0% H₂O ratio ESD sample, the nature of bond length, (Zn-O) (Å), shows a very small decrease from 300˚C to 400˚C temperature, and then it starts to increase from 400˚C to 500˚C temperature. On the other hand, Figure 7(b) illustrates the results for a 20% H₂O ratio and an increase from 300˚C to 400˚C temperature, then a very constant nature was observed from 400˚C to 500˚C temperature. This is correlated to the decreasing lattice constant parameter, c (Å), in this temperature range. The results for the 50% H₂O ratio showed the decreasing nature from 400˚C to 500˚C temperature ( Figure 7(c) ). This is due to both the lattice constant parameter, a (Å), and c (Å) being found to decrease in nature except for the 300˚C to 400˚C temperature range of lattice constant parameter, a (Å). The results obtained for ZnO bond length (Zn-O) (Å) for this study are found to be in good agreement with the 1.98 (Å) reported in the literature [31] [32] .

Figure 7

Figure 7. Changing phenomena of bond length L (Å) of ZnO thin film according to the weight ratio of H 2 O at different temperatures.

3.4. Temperature Effect on Crystallite

It is known that a perfect crystal would ideally continue infinitely in all directions, but in reality, all crystals are imperfect since they have a limited size [32] . The broadening of diffraction peaks in materials occurs due to the deviation from perfect crystallinity. Crystallite size, D (Å), refers to the dimensions of a domain that exhibits coherent diffraction. It is important to note that the crystallite size of particles is typically different from their particle size because of the existence of polycrystalline aggregates. Lattice strain ε (%) is a measure of the distribution of lattice constants (a (Å), and c (Å)) arising from crystal imperfections, such as lattice dislocation, δ (*10¹⁶ m²). The X-ray line broadening is used for the investigation of dislocation distribution, δ (*10¹⁶ m²). In the present study, it was found that the ZnO (002) plane diffraction peak is much stronger than the ZnO (101) peak ( Figures 2(a)-(c) ). This indicates that the formation of ZnO nanocrystals has a preferential crystallographic (002) orientation. The average crystallite size calculated for synthesized ZnO nanoparticles was 47.77 nm by the ESD technique.

Figure 8

Figure 8. The trend of (a-c) lattice crystallite D (Å) of ZnO thin film according to the weight ratio of H 2 O at different temperatures.

Table 2

Table 2. Microstructural parameters with changing H 2 O ratio.

H2O Ratio	Temperature (˚C)	Lattice Constant Ratio [(c/a)]	Positional Parameter (μ)	FWHM	Dislocation Density (d*1016) [m−2]	Lattice Strain (ε) %
0%	300	1.598	0.381	0.0413	4.412	0.242
	400	1.634	0.374	0.2707	4.271	0.243
	500	1.583	0.383	0.3382	4.264	0.242
20%	300	1.634	0.375	0.0263	4.307	0.242
	400	1.667	0.370	0.329	4.077	0.243
	500	1.585	0.383	0.2468	4.351	0.242
50%	300	1.64	0.374	0.1323	4.105	0.243
	400	1.612	0.378	0.0511	4.115	0.243
	500	1.604	0.381	0.2351	4.330	0.242

The crystallite size is assumed to be the size of a coherently diffracting domain, and it is not necessarily the same as particle size. The measured crystallite size D (Å) ( Figures 8(a)-(c) ) of the films is observed to exhibit similar behavior compared to the lattice constant parameter c (Å) ( Figures 6(a)-(c) ), temperature 300˚C to 500˚C. This behavior is consistent and supports the growth mechanism of ZnO thin film [32] [37] by the ESD technique. The dislocation density (δ), and lattice strain (ε), FWHM of the deposited ZnO thin films are listed in Table 2 . It should be noted that a higher dislocation density (δ) is always significantly important for the fabrication of high-quality crystal-thin films [31] . In the present study, the route of thin film fabrication by ESD has proven the potential impact for thin film fabrication, and in the future, this device fabrication could also be implemented for solar cells and other optoelectronic device applications for technological advancement. Besides tuning the ESD parameters, the dislocation density (δ) could also be tuned which is another potential impact for easy device usage.

3.5. Raman Spectroscopy

Raman spectroscopy can provide insights into several complex molecular phenomena in a substance, including the identification of multiple phases within the same material. Moreover, it facilitated the examination of transitions from amorphous to crystalline phases, diverse flaws, and stress conditions. The current experiment demonstrated the development of thin ZnO films with a hexagonal wurtzite structure. The literature states that ZnO is a semiconductor material with has $c_{6v}^4$ spatial symmetry. Group theory states that there are A₁ + 2E₂ + E₁ modes that are related to ZnO, which has a total of four atoms in each unit cell for its hexagonal wurtzite structure . This arrangement results in 12 phonon branches, with nine being optical modes and three being acoustic modes. Figure 9 shows the Raman spectrum of the ZnO thin film for 500˚C temperature of exchanging H₂O ratio. The Raman peaks around 475 cm⁻¹ and 780 cm⁻¹ were assigned to ZnO E₂ (high) and A₁ longitudinal optical (LO) modes, respectively, for 0% H₂O. Furthermore, other multiphonon modes were also observed, such as transverse phonon (TA), low-order phonon (E₁), mixed phonon (TA + LA), and second-order phonon for longitudinal phonon (A₂). This phenomenon decreases as the H₂O ratio increases to 20% and 50%. Only the effective Raman phenomena are observed for the 0% H₂O ratio. Raman spectroscopy of the E₂ phonons plays a significant role in the study of residual stress in ZnO thin film crystals; that residual stress correlates with the complex growth mechanism in wurtzite structure crystals [45] . This study observes that the E₂ phonon mode gradually decreases as the H₂O ratios increase [37] ( Figures 9(a)-(c) ). A decrease in the E₂ phonon frequency is attributed to tensile stress, while its increase is attributed to compressive stress. The study under this investigation, the E₂ vibration mode at 475 cm⁻¹ is characteristic of the wurtzite phase, and its value is higher than 439 cm⁻¹ for the tensile stress-free bulk ZnO thin film , suggesting that the fabricated ZnO thin films are exhibiting low tensile stress for the 0% H₂O. On the other hand, 20% and 50% H₂O ratio mixing fabricated ZnO thin film possesses very high tensile stress. Due to this high tensile stress, the corresponding Raman peaks are found to be very weak and have disappeared, respectively. This analysis provides us with two possibilities. First, the tensile stress probably originated from a mismatch in the thermal expansion coefficient of the ZnO thin film (4.75 × 10⁻⁶ K⁻¹) and the glass substrate (2.60 × 10⁻⁶ K⁻¹). This mismatch may correlate with crystal defects and anisotropy of the thin films [40] . Second, the possible existence of complex reaction formation due to excessive H₂O carried to the excessive amount of the leaving group [OH]⁻ compound. As a result, the kinetics of ZnO nanoparticles or/ dipole moment changing tendency may occur in the fabrication process . Moreover, the A₁ (LO) mode at 780 cm⁻¹ originates from defects such as oxygen vacancies and Zn interstitials [39] , and its relatively low intensity peak indicates a relatively low density of defects in the ZnO thin films. This corroborates the experimental XRD analysis by the ESD technique ( Figures 2(a)-(c) ). The weak peak at 260 cm⁻¹ was attributed to the second-order Raman processes for 0% H₂O ratio. In addition, the second-order Raman modes are increased with increasing the H₂O ratios. Further study is required to understand this lattice anisotropy and the complex reaction mechanism of ESD with different solution formats.

Figure 9

Figure 9. Raman spectra of the ZnO thin films of ZnCl 2 and CH 3 CH 2 OH precursor solution by ESD at 500˚C temperature, ESD sample with micro-ring structures.

4. Conclusions

The ESD approach for the next generation of ZnO thin films on conductive ITIO substrates, along with the solution dependency analysis of these studies, led us to make the following concluding remarks.

These findings highlight the capability of ESD to enhance semiconductor thin-film technology, facilitating more economical and scalable production techniques.

Acknowledgements

The authors express their sincere appreciation to ESD research group members at Sugiyama Laboratory at Tokyo University of Science for their support in completing the experiments and for the constructive discussions.

Authors’ Contributions

F.I.A.: Conceptualization, investigation, writing-original draft, plotting figures, methodology, software, review & editing; M.S.: Supervising, writing-original draft, review & editing.

Data Availability

The raw/processed data required to reproduce these findings can be shared upon request.

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