Anisotropic Role on Zinc Oxide (ZnO) Thin Film Growth Processes of Solution-Dependent Novel Electrostatic Spray Deposition (ESD)

This study systematically investigated an anisotropic correlation on structural properties (STP) for the 0.005 M (m/l) and 0.015 M (m/l) hydrochloric acid (HCl) effect on the ZnO growth mechanism, using the solution-dependent novel ESD technique across a range of temperatures with varying the volumes of deionized water (H₂O). For ZnO thin film recitation, zinc chloride (ZnCl₂) has been used as a precursor and dissolves it in ethanol (CH₃CH₂OH) at a concentration of 0.1 M to create a zinc complex molecule. X-ray diffraction (XRD) images confirmed that the ZnO nanoparticles exhibited a clean wurtzite crystal structure. Additionally, Bragg’s law and the Debye-Scherrer (D-S) methods were utilized for the detailed microlevel investigation, namely, lattice parameters and lattice crystallite. The STP result analysis disclosed the existence of an anisotropic countenance, which correlates with the excessive presence of leaving group (OH^-) compounds within the framework of ESD. The consequences also provided insight into how to develop an oxide-based crystal semiconductor that is economically viable for industrial and commercial applications of ESD-deposited highly efficient semiconductor technology devices, specifically the growth mechanism for ZnO.

Introduction

With numerous typical properties, including wurtzite structure, transparency in the visible range [1], light-emitting diodes [2], [3], photocatalysis [4], gas sensors [5], catalysis [6], laser diodes [7], varistors [8], and sensors [9], ZnO, commonly referred to as an II-VI semiconductor in material science, is a significant and promising material. With a large exciton binding energy of about 60 (meV) at room temperature and a massive band gap of about 3.37 (eV), ZnO is a well-known n-type semiconductor [10]. There are a lot of well-known ways to make ZnO thin films. Some are dry, like sputtering [11] and pulsed laser deposition [12], and some are wet, like chemical bath deposition [13], mist chemical vapor deposition [14], and spray pyrolysis [15].

Furthermore, the ESD approach is not only user-friendly but also offers significant industrial advantages, including the capacity to cover extensive regions due to its simplicity and comprehensibility. This method facilitates non-vacuum deposition, is cost-effective, and offers precise control over composition ratios and doping levels [16]–[18]. Now, the question arises about how to create a high-quality oxide semiconductor using the ESD route. However, this study aims fourfold to investigate not only the ZnO thin film growth mechanism within the framework of the ESD development technique but also use different STP properties analysis of the nanocrystal particle sizes for considering oxide-based semiconductor technology. Firstly, literature review conclusions indicate that a comprehensive investigation utilizing these frameworks for the progress mechanism of ZnO thin films via the ESD fetch has not been documented within this specific combination of samples and temperature range. Secondly, ESD approach offers not only user-friendly functionality but also significant industrial advantages, including its capacity to effectively cover extensive regions due to its uncomplicated and easily accessible nature. Thirdly, ESD method enables deposition without requiring a vacuum, is reasonable, and offers relatively easy control of the composition ratio and doping. Fourthly, comparing the impact of anisotropy of changing the H₂O volume ration in HCl mixed spray solution on the formation of ZnO thin films using the innovative ESD approach. This work represents a first step in developing cost-effective and easy-to-use technologies that allow visible light to pass through without obstruction using the novel ESD method for oxide-based semiconductor research studies.

Methodological Approaches

ZnCl₂ (98% Assay, ZnCl₂ = 136 g), Lot. LEH7398, Mfg. Date: 2021.09, FUJIFILM Wako Pure Chemical Corporation, was the zinc source used in this investigation. The solutes were deionized water (H₂O) and ethanol (CH₃CH₂OH (99.5%), Cat. No. 14033-70, FW: 46.07, KANTO CHEMICAL CO, INC). Not only H₂O, but also HCl were used as a doping material for this study. ZnO films were developed gradually on a conductive In₂O₃: Sn (ITiO) coated alkali-free glass substrate utilizing an electric field applied spray pyrolysis process known as ESD. Fig. 1a–c displays the ESD experimental diagram. For the ESD spray approach, 20 ml of six (6) distinct spray solutions were made by varying the HCl concentrations and H₂O volume ratio. First, 20 ml of CH_₃CH_₂OH was measured and put into a glass beaker. Next, 0.2726 g of ZnCl_₂ was added to the glass beaker along with CH_₃CH_₂OH (density of CH_₃CH_₂OH 0.78945 g/cm^³). The solution was heated on the magnetic stirrer upper hot plate (ADVANTEC SR/350) for up to 20 minutes to make the precursor solution homogeneous (Fig. 1c). A temperature of 150°C was considered for the magnetic stirrer hot plate device. For the second 20 ml solutions, 20% H_₂O (4 ml) and 80% CH_₃CH_₂OH (16 ml) were taken in two different glass beakers. Then, the same amount of ZnCl_₂ was added to the CH_₃CH_₂OH-containing glass beaker. Next, follow the specified procedure and time to achieve a homogeneous solution. Then, H_₂O was added and heated for up to 10 minutes. Finally, 0.005 M HCl was added. Next, we heated the solution for 10 minutes. Mixing the HCl into the solution did not change the concentrations of the combined 80% CH_₃CH_₂OH (16 ml) and 20% H_₂O (4 ml). After following the same steps, the third 20 ml solution was made by mixing 10 ml of CH_₃CH_₂OH with 50 ml of water and adding 0.015 M HCl.

Figure 1. ESD experimental setup and ZnO crystal structure.

The nozzle was 3.5 cm away from the surface of the substrate during the time of the experiment. With a flow rate of 2.0 ml per hour, the precursor solution was circulated through a metallic nozzle with a diameter of 0.26 ml. Ethanol, which has a boiling point of 78°C (173°F), was used as the solvent in this experiment. A voltage of 8 kV was applied between the metallic nozzle and the conductive substrate to form the Taylor cone, after which ESD was carried out for 3 min. After applying a voltage of 8 kV between the metallic nozzle and the conductive substrate in order to initiate the formation of the Taylor cone, ESD was carried out for a duration of three minutes. ESD temperature ranges of 300°C, 400°C, and 500°C, respectively, followed the ZnO thin film development procedure. ZnO film crystallographic orientations were assessed utilizing X-ray diffraction (XRD; Rigaku Ultima IV, Rigaku SmartLab). Using Bragg’s law and the Debye–Scherrer (D-S) methods, the microstructural parameters of ZnO thin films, specifically the lattice parameters, a (Å) and c (Å), and the crystallite size, D (Å) [19]–[23], were computed numerically. The ESD technique utilized the XRD diffraction plane to perform these calculations. The fundamental crystal growth theory correlates with these calculated results [24]. The equation involved of this estimation were given below (1)–(3):

(1) $$\begin{array}{r}a=\frac{\lambda }{\sqrt{3}sin{\theta }_{100}}\end{array}$$

(2) $$\begin{array}{r}c=\frac{\lambda }{sin{\theta }_{002}}\end{array}$$

(3) $$\begin{array}{r}D=\frac{0.9\lambda }{\beta cos\theta }\end{array}$$

where a, c, λ, θ, β are lattice parameter along a-axis (a), and lattice parameter along c-axis (c), lambda as the wavelength (1.54 Å), theta as angle (diffraction angle), beta as angle (in radian), respectively. Equation (3) represents the Debye-Scherrer formula.

Results and Discussion

X-ray Diffraction (XRD) Analysis

At temperatures of 300°C, 400°C, and 500°C, respectively, the XRD experiment was conducted to investigate the STP of the ZnO thin film fabrication cycle utilizing the ESD methodology. Six distinct spray solutions were prepared by dissolving ZnCl₂ in CH₃CH₂OH at a concentration of 0.1 M (m/l). These solutions were utilized to investigate the impact of 0.005 M and 0.015 M HCl on the growth mechanism of ZnO thin films using the ESD method. The H₂O ratio varied during the experiments. XRD patterns of the ZnO thin films on

ITiO substrate for 0.005 M and 0.015 HCl are shown in Figs. 2a–c and 3a–c.

Figure 2. XRD patterns with doping of 0.005 M HCl at different volume ratios of H₂O: (a) H₂O 0%, (b) H₂O 20%, (c) H₂O 50%, and temperatures, respectively.

Figure 3. (a–c). XRD patterns with doping of 0.015M HCl at different volume ratios of H₂O: (a) 0%, (b) 20%, and (c) 50%, and temperatures, respectively.

The H₂O volume ratios were 0%, 20% and 50% in the ESD, respectively.

Figs. 2a and 3a present the outcomes for the 0.005 M and 0.015 M HCl doping without the contribution of deionized H₂O volume in the solutions.

Fig. 2a exhibits strong peaks at the (002) plane, and (102) planes. This peak observation clearly confirms the ZnO thin film growth, without the H₂O mixing into the solution at a concentration of 0.005M HCl. Fig. 3a also observed a pattern at (002) plane for the ESD spray solution with a 0.015M HCl concentration. Fig. 3a shows that the peak strength at 500°C is stronger in the (002) plane compared to the 0.005M HCl solution. Figs. 2b and 3b showed the results for 0.005 M and 0.015 M HCl in the spray solution with a 20% H₂O ratio, respectively. The peak intensities reveal the appearance of the (002), and (102) planes for the 0.005M HCl added ESD solution. But, for the 0.015 M HCl solution peak only seen at (100) plane which is very weak. In Fig. 2b, the 0.005 M HCl solution has a stronger peak intensity in the (002) plane at the temperature of 500°C than the 0.005 M HCl doping solution shown in Fig. 2a. Surprisingly, for the high-concentration HCl solution of 0.015 M, disappear the peak for the (002) plane at 500 °C. This may be the complex formation effect in the spray solution due to H₂O and the high concentration of acid at high temperatures perspective. The results for 0.005 M and 0.015 M HCl in the spray solution with a 50% H₂O ratio are shown in Figs. 2c and 3c in sequential order. For 0.005 M HCl case, the peak intensities indicate the presence of the (100), (002), (101), and (102) planes where the strong peak appears at (002) plane for the 500 °C, and weak peak at (102) plane. On the other hand, (100), (102) plane for the 400°C temperature little strong compared to 20% H₂0 ration ESD sample at 400°C temperature. For 300°C, very weak peak intensities were found at (100) and (102) planes. Compared to Fig. 2a and Fig. 2b, Fig. 2c shows a higher peak intensity in the (002) plane at 500°C for the 0.005 M HCl solution. This behavior occurs gradually as the H₂O ratio increases in the spray solution with a low-concentration 0.005 M HCl. The peak intensity in the (100) plane for the 0.015 M HCl solution for both the 50% and 20% H₂O ratios has shown very small change. These behaviors can be understood to analyze how their lattice constant parameter changes. It will be discussed in detail in the lattice parameter analysis section.

In Fig. 2a–c shows a consecutive and gradually increasing peak intensity nature for the 0.005M of HCl samples by the ESD at the diffraction angle around 34.80°C for 500°C temperature. Peaks at the (100) and (102) planes were found to have low intensities in the temperature range of 300°C to 400°C; however, the observation’s emergence of the (002) plane supported the development of ZnO thin films with a hexagonal wurtzite structure [21]. On the other hand, in Fig. 3a–c for the 0.015 M HCl mixing results clearly shows the peaks for the (002) plane. Sudden droops of the peak intensity are found for the 20% of H₂O ratio samples. The peaks for the (002) plane is strong again, for the 0.015 M HCl mixing samples with 50% of H₂O ratio samples. In addition, the peak intensity for the (100) plane is slightly stronger for the 0.015 M HCl mixing samples compared to 0.005 M HCl mixing samples from the ESD under the investigated temperature ranges. But, the consistency of increasing peak intensity results is observed for the (002) plane for the 0.005 M of HCl mixing samples which indicates the gradual ZnO thin film growth. According to the XRD measurements, all the fabricated thin films show a notable orientation along the c-axis (002) plane, and for the 0.005 M of HCl mixing results, moderately strong results are shown along the (102) plane with compared to 0.015 M HCl doping results. Increasing the temperatures as well as the film thickness of ESD-produced films results in a sharper and more intense for the (002) plane of diffraction peak [16], accompanied by a decrease in the full width at half maximum (FWHM). Later, in the discussion of Fig. 3, this issue will be addressed. The c-axis orientation of ZnO thin films is determined by the basic concept of crystal formation [24]. The (002) plane of ZnO has the lowest surface energy, according to this notion, and this is a major factor in ascertaining the crystal structure [19], [24].

Lattice Parameters Analysis

In the preceding section, XRD results indicated that the growth of ZnO thin films was quantitatively improved for HCl doping at low concentrations compared to high concentrations. Only the low concentration of HCl doping results will be outlined in detail for effective STP analysis purposes. Figs. 4a–c and 5a–c underline the change of the lattice parameter, a (Å), and the lattice parameter, c (Å), for the (100) plane and the (002) plane, respectively [23], [24]. The findings of the lattice parameter study revealed a quite consistent behavior change. The lattice parameters a (Å) and c (Å) increase when the temperature rises from 300°C to 500°C, as shown in Fig. 4a, b and Fig. 5a, b. This proportional increasing behavior is the same reflection compared to the lattice crystallite D (Å) (Fig. 6). The sharp increases for a and c were found for 0% and 20% of the H₂O mixed solution samples obtained from ESD for the 0.005 M HCl. Conversely, 50% of the H₂O volume mixed result shows the decreasing pattern, with the whole range of temperature for a (Å). However, a decreasing nature is observed in the lattice parameter, c (Å), for the (002) plane from 300°C to 400°C, followed by a slight increase to 500°C. The underlying principle of crystal development states that the (002) plane of ZnO has the least amount of energy on its surface, and this is important in determining the structure of the crystal [19], [24]. The results of this study are very consistent with the ESD-induced growth mechanism of ZnO thin films. This behavior satisfies the growth mechanism of the hexagonal wurtzite structure of ZnO thin films by the novel ESD study.

Figure 4. Lattice parameter, a (Å) with doping of 0.005M HCl at different volume ratios of H₂O: (a) 0%, (b) 20%, and (c) 50%, and temperatures, respectively.

Figure 5. Lattice parameter, c (Å) with doping of 0.005M HCl at different volume ratios of H₂O: (a) 0%, (b) 20%, and (c) 50%, and temperatures, respectively.

Figure 6. Lattice crystallite, D (Å) with doping of 0.005M HCl and different volume ratios of H₂O at (a) 300 °C, (b) 400 °C, and (c) 500 °C, respectively.

Crystallite Analysis

A comprehensive investigation of the crystallite, D (Å) properties of the ZnO nanoparticles under study is shown in Fig. 6a–c. Specifically, it examines the mean D (Å). Similarly, the analysis is conducted for the 0.005M HCl samples, different aspects of the analysis. The results were computed utilizing (3) specified in the experimental section. The average D (Å), of the ZnO thin films was calculated by applying the Debye–Scherrer’s formula [19], where D represents the average crystallite size (nm), λ is the wavelength of incident radiation (λ = 1.54 Å) and β is (FWHM) of the peak corrected for instrumental broadening and θ is the Bragg diffraction angle [25]. The measured crystallite size of the films is observed to rise proportionally with temperature. This behavior is identical for both a 0% H₂O mixed solution and upon a small amount (20%) of H₂O mixture solution. However, a quick decrease followed by a subsequent increase in the concentration of a substantial quantity of H₂O-mixed solution was observed in Fig. 6c for the 50% of H₂O mixture solution. Almost similar changing behavior were observed compared to crystallite, D (Å) and lattice parameters a (Å), and c (Å). In relation to the oxide-based semiconductor growth approach, this might be regarded as a consistent aspect of the ESD method.

Finally, the systematic evaluation of ZnO thin films was demonstrated in Fig. 2a–c, and Fig. 3a–c by the XRD diffraction pattern. The XRD results show systematically how ZnO thin film formed on the conductive ITiO-coated alkali-free glass base by the novel ESD approach. The effect of varying the H₂O ratio in both low and high acidic solutions of ESD spray has significantly influenced the crystal growth of the (100) and (002) diffraction planes. The morphing pattern points to the entity of an anisotropic feature in the ZnO thin film growth’s STP characteristics. This is perhaps due to the complex compound formation in the ESD spray solution. It was seen that the lattice parameters (a (Å), c (Å)) and crystallite (D (Å)) showed that increasing trend for 0% and 20% H₂O ratio of the ZnO growth thin films. Conversely, samples with a 50% mixed H₂O ratio exhibited a decreasing trend, followed by an increasing trend in both cases.

After analyzing the current progressive, we proposed the formation of a complex compound of the chloride chemical reaction that occurred in the solution. The excessive presence of the compound element in the solution impedes the growth of the ZnO thin film of the ITiO substrate. Moreover, this proposed chemical reaction mechanism may be effective in the ESD solution due to excessive H₂O and other organic compounds. In addition, polycrystalline thin film materials inherently contain a certain amount of excess volume in the grain boundaries, the crystal lattice distortion resulting from the stress field induced by the excess volume is an intrinsic structural characteristic. To ensure this reaction mechanism and produce good quality crystals of the oxide-based thin film by the ESD technique, further study is required (Fig. 7).

Figure 7. Proposed reaction mechanism for the fabrication on ZnO thin film growth mechanism by ESD.

Conclusion

The innovative electrostatic spray deposition (ESD) approach for the next generation of ZnO thin films on ITIO substrates, along with the solution dependency and analysis of these results, led us to make the following concluding remarks:

1. This study elucidated how an excessive H₂O ratio transforms the structural attributes from an isotropic to an anisotropic nature.

2. In contrast to usual methods, this anisotropic phenomenon shows that there are two significant correlations. First, adding different amounts of H₂O and low-acidic (0.005 M) spray solutions helps crystals grow on the (002) diffraction plane. Secondly, the diffraction plane (100) experiences the influence of a fluctuating H₂O ratio and a high-acidic (0.015 M) spray.

3. The relationship between the complex formation of the ESD solution and anisotropy is primarily revealed within this framework. Perhaps this is related to the optimum amount of the leaving group (OH-) compounds present in the ESD spray solutions.

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

Further investigation is also needed to determine the relationship among other structural and solar cell properties, and the anisotropic phenomena by the novel ESD technique.