Electrodeposition of Nanostructured ZnO Thin Film: A Review

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Zinc oxide has several favourable properties such as good transparency in the visible and high infrared spectrum, high electron mobility, wide and the direct band gap, large exciton binding energy, high thermal conductivity and strong room temperature luminescence. These properties are used in many applications such as transparent electrodes of thin film solar cells for light transmission and the extraction of photocurrent, heat-protecting windows, transparent oxide thin-film transistors, light-emitting diodes, Varistors, piezoelectric devices, etc. Crystalline zinc oxide is thermochromic in nature i.e. changing from white to yellow colour when heated and return to the earlier state on cooling.

ZnO thin films are deposited by the several conventional growth techniques such as DC or RF magnetron sputtering (^{[7, 8, 9]}), Metalorganic chemical vapor deposition (^{[10, 11]}), pulsed laser deposition, spray pyrolysis (^{[12, 13, 14, 15]}), chemical bath deposition (^{[16, 17, 18]}), molecular beam epitaxy and cathodic electrodeposition method. The cathodic electrodeposition method is emerging as an efficient nanotechnology for the production of ZnO thin films and nanostructures because of its simplicity, low cost equipments, and suitable for large area substrate ^[3].

2. ZnO Thin Film by Electrodeposition Methods

The cathodic electrodeposition technique has been recognized as the effective method for the production of ZnO thin films due to its simplicity, low temperature process, high deposition rate, low cost technique and suitability for large area substrate. This technique used a very low cathode voltage or current to produce the ZnO thin film on any conductive substrate such as a transparent conducting oxide (^{[3, 19, 20]}) or any other metal plate (^{[1, 21]}). In this deposition technique, the film thickness, morphology and optical properties can be controlled by the various operating parameters: current density, applied potential, deposition time and the concentration of the electrolytic bath. Generally, in this deposition method, zinc nitrate solutions (^{[19, 20, 22, 23]}) or zinc chloride solutions (^{[1, 3, 24, 25]}) are used as a precursor.

2.1. ZnO Thin Film Deposition from Zinc Chloride Solutions

The two main mechanisms that take place in the solutions are the precipitation and the electrochemical reactions at the cathode surface or the working electrode. The fundamental reaction that leads to the formation of ZnO is the electro generation of a base and the electro-precipitation takes place in the presence of zinc ions in the solution ^[26]. The reaction that takes place with the molecular oxygen can be summarized as

(1)

(2)

A. Goux et al. ^[26] studied the effect of bath temperature on the ZnO deposition and observed that the continuous film growth and surface passivation would not occur below the 34ºC temperature. They have reported that the oxide nucleation and thin film growth started above the 34°C temperature, whereas the optimum film transparency and crystallinity was obtained above the 40°C temperature. When the temperature of the bath reaches above the 40°C, the zinc hydroxide layer dehydrates by the reaction 3 to form ZnO thin film.

(3)

2.2. ZnO Thin Film Deposition from Zinc Nitrate Solutions

When the zinc nitrate as a precursor is dissolved in the water, two anionic groups such as NO₂^-and OH^- are formed in the solution. After that Zn²⁺ ions combined with the OH^- ions to form the zinc hydroxide. In the end, zinc hydroxide is finally oxidized into ZnO and water ^[27].

(4)

(5)

(6)

The Zn(OH)₂ can be decomposed into ZnO and water during the deposition or by the post annealing process. The one of the major advantages of using the zinc nitrate is to be the highly soluble in the aqueous solution.

3. Zinc Oxide Thin Film Prepared by Thermal Oxidation of Zinc Metal

Another method for the production of ZnO thin film is by the thermal oxidation of zinc thin films electrodeposited onto the conducting substrate. G. Zou et al. ^[28] reported that the ZnO thin film was prepared by the thermal oxidation of the zinc thin film. In this method, Zinc thin film was electrodeposited on to the indium tin oxide coated conductive glass slides from the zinc acetate dehydrate and ethylene glycol as a precursor. After that, the deposited Zn thin films were thermally oxidized in a muffle furnace at around 500°C to form the ZnO thin films.

4. Results & Discussion

4.1. Structural Characteristics of ZnO Thin Films

Various researchers up to date have used X-ray diffraction technique to examine the structural characteristics of the ZnO thin film electrodeposited on the several conducting substrates. M. Fahoume et al. ^[1] studied the structure of ZnO thin films by using the X-ray diffractometry (XRD) and transmission electron microscopy (TEM). They have reported that the ZnO thin films deposited on the ITO coated glass substrate were polycrystalline in nature with a hexagonal crystal structure (zincite) at pH = 4. At a temperature of 70ºC, the film had no preferred orientation and the most intense peak of XRD pattern was along the (101) plane. When the temperature of the bath was increased to 80ºC, the most intense peak of XRD pattern was changed and along the (002) plane. They have also reported that when the ZnO thin films were deposited on the copper substrate then it exhibits only one peak along the (002) plane and the degree of orientation along the (002) direction was increased with increasing the bath temperature. The grain size of the film deposited on the copper substrate was larger than the film deposited on the ITO coated glass substrate. Overall, they found that the preferred orientation of the grains was dependent on the type of substrates and bath temperature. N. H. Al-Hardan et al. ^[3] studied the XRD pattern of deposit ZnO thin film on the ITO coated glass substrate and reported that the film exhibited the polycrystalline nature and the resulted XRD pattern showed that ZnO thin film had hexagonal wurtzite crystal structure. They have reported that the most intense peak in the XRD pattern was along the (002) plane and the ZnO thin film was growing parallel to the c-axis of the hexagonal crystal structure. J.S. Wellings et al. ^[6] studied the various XRD patterns of the as deposited ZnO thin film on the FTO coated glass substrates at different potential from -0.9V to -1.025V. They have reported that ZnO thin films were polycrystalline in nature with hexagonal wurtzite crystal structure and (002) preferred orientation except a sample which was deposited at -0.925V vs. Ag/AgCl. They have also reported that improved crystallinity was observed at -0.975V as compared to the other potentials. They have also observed the effect of annealing on the XRD pattern of the ZnO thin film deposited at -0.975V and concluded that the XRD spectra of both the as deposited and annealed samples had no detectable or noticeable changes. The grain/crystallite sizes of the various ZnO films deposited at the different conditions are presented in the Table 2.

Table 2. The grain/crystallite size of the various ZnO films deposited at the different conditions

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C. Gu et al. ^[19] studied the effect of different cathodic potentials and deposition time on the XRD patterns of the ZnO thin films electrodeposited on the ITO coated glass substrates. They have reported that the preferred orientation was changed with the cathodic potential and deposition time. At the potential of -1.3V for 300s, only the (002) preferred orientation was recognized. As the cathodic potentials were decreased from -1.3V to -2.0V, the (002) preferred orientation was weakened and at the -2.0V, the ZnO thin films had random orientations. They were also reported that the (002) preferred orientation of ZnO thin film was also retained at -1.3V after a long deposition time of 1200s. They have also reported that the initial deposited ZnO film on ITO coated glass substrate had (002) preferred orientation and it was independent of the applied cathodic potential. They have also studied the various XRD patterns of ZnO thin films electrodeposited on ITO coated glass substrates at -1.7V for different deposition times. As the deposition time was increased from 120s to 1200s, the (002) preferred orientation was weakened and after 1200s, the deposited films also had random orientation. C. Coskun et al. ^[20] studied the XRD patterns of annealed and non annealed ZnO thin films deposited on the ITO coated glass substrate. They have reported that the preferred orientation of the grains was along the (101) direction for both the annealed and non annealed ZnO thin films. They have also reported that the crystallite sizes of around 31nm and 63nm were obtained from the Scherrer equation for non annealed and annealed ZnO thin films respectively. Q. Wang et al. ^[21] also studied the effect of different deposition potentials and annealing on the XRD pattern of ZnO thin films deposited on the Zn substrate and reported that the films had hexagonal wurtzite crystal structure and the most intense peaks in all the patterns were corresponding to the (002) plane. They have also studied the annealing effect on the XRD pattern of ZnO thin film deposited at -1.0V and observed that the peak corresponding to (002) plane becomes broadened and debased after annealing at 380ºC in the horizontal tube furnace. LI Junwei et al. ^[22] studied the effect of potential and temperature on the XRD patterns of porous ZnO thin films. They have reported that the porous ZnO thin film prepared at the potential of -0.7V had a preferred orientation along the (002) direction, but at the lower potential of -0.9V, the (002) preferred orientation was weakened and the most intense peak corresponding to (101) plane was observed. At the potential of -1.1V and -1.4V, the other peaks corresponding to (100) plane also became stronger and films had random orientations. From the XRD patterns of the ZnO thin films, they have reported that the ZnO films showed the hexagonal wurtzite crystal structure. L. Zhang et al. ^[23] studied the XRD spectra of as prepared ZnO thin film deposited on the fluorine-doped SnO2 coated glass substrate. The XRD pattern of the prepared film revealed that the ZnO film had a hexagonal wurtzite crystal structure with preferred orientation along the (002) direction. J. Cembrero et al. ^[25] studied the XRD pattern of ZnO film deposited on the FTO coated substrate and reported that the film had a hexagonal wurtzite crystal structure with the preferred orientation along the (002) direction. The Lattice parameters and the band gap energy of the hexagonal wurtzite crystal structured ZnO thin films are presented in the Table 3.

Table 3. Lattice parameters of the hexagonal wurtzite crystal structure of ZnO thin films

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Table 4. The preferred orientation and thickness of various deposited ZnO thin films

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A. Goux et al. ^[26] studied the effect of different deposition temperature on the XRD patterns of ZnO thin films deposited on FTO coated glass substrate. At the temperature of 22ºC, the XRD pattern of film exhibited the peaks of substrate only and no peak of ZnO was observed. At the temperature of 34ºC, the XRD pattern exhibited the peaks of ZnO {(100) and (101)} along with the peaks of substrate and showed that the ZnO film had hexagonal wurtzite crystal structure. When the temperature was increased, then the peaks become narrower and (002) reflection started to emerge and becomes the preferred orientation at the higher temperature. T. Mahalingam et al. ^[27] studied the XRD patterns of ZnO thin films deposited at various potentials from -0.8V to -1.2V and reported that the films were polycrystalline in nature and showed the hexagonal wurtzite crystal structure. For all the films deposited at various potentials, the most intense peak in the XRD patterns was along the (002) plane. The intensity of the (002) plane was the maximum for the film deposited at -1.0V vs. SCE whereas the XRD patterns of other films deposited at -0.8V and -1.2V showed that the intensity of (002) plane decreased in comparison to the film deposited at the potential of -1.0V. The preferred orientation and thickness of various deposited ZnO thin films are presented in Table 4.

G. Zou et al. ^[28] studied the XRD patterns of the as deposited Zn thin film and the Zn thin films annealed at various temperatures from 100 to 500ºC. The XRD pattern of the as deposited Zn thin film revealed that the film had only one strong orientation along the (002) direction. When the Zn thin film was annealed at 300ºC for 6 hours, then the film was completely transformed to ZnO thin film and a weak peak {ZnO (100)} along with the two strong peaks {ZnO (002), (101)} was observed, which showed that the film possessed a hexagonal wurtzite crystal structure. When the Zn film was annealed at 500ºC for 6 hours, then the ZnO thin film exhibited polycrystalline hexagonal crystal structure with (002) preferred orientation. X. Qin et al. ^[29] studied the XRD patterns of the ZnO thin films deposited on the ITO substrates with 0.5% op-10 surfactant and without surfactant. They have reported that both the ZnO thin films showed the polycrystalline nature with the hexagonal wurtzite structure. The grain size of the ZnO thin film prepared without the surfactant and with the surfactant was calculated as 54.5nm and 38.8nm respectively. B. E. Prasad et al. ^[31] studied the effect of Zn concentration, deposition temperature, and time on the XRD patterns of the ZnO thin films deposited on the stainless steel flag. They have reported that at high concentrations, the crystals were growing along the (101) direction, but at the high deposition temperatures and at high deposition times, the crystals were growing along the (102) and (103) direction.

4.2. Optical Properties of ZnO Thin Films

M. Fahoume et al. ^[1] studied the effect of film thicknesses on the optical transmission spectra of the electrodeposited ZnO thin films and reported that the films showed the optical transmittance values above the 75% when the thickness of the films were below the 340nm. They have also reported that when the thicknesses of the films were increased, the transmission value was decreased. They also found that the optical band gap energy of the electrodeposited ZnO thin films was 3.26eV which was much closer to the band gap energy of ZnO single crystal (3.40eV). C. Gu et al. ^[19] studied the optical transmittance spectra of ZnO thin films deposited on the ITO coated glass substrate at different cathodic potentials and at the cathodic potential of -1.7V for different times. They have reported that the ZnO thin films were transparent to the visible light range and the films showed the optical transmittance values to about 80 to 90%. They have also found that the values of band gap energy (E_g) of the ZnO thin films deposited at different cathodic potential and at different deposition times were 3.5eV and it was independent of the applied potential and the deposition times. C. Coskun et al. ^[20] studied the optical absorption spectra of as-deposited ZnO thin film and annealed ZnO thin film at 300ºC. They have observed that the band gap energy of the as-deposited ZnO thin film was about 3.23eV corresponding to a wavelength of 385nm and the band gap energy of the annealed ZnO thin film was about 3.37eV corresponding to a wavelength of 368nm. At the wavelength of 600nm, they found that the reflectivity for as deposited ZnO thin film was 32.7% and for annealed ZnO thin film was 24.75%. They have also observed that the reflectivity of films decreased after annealing process which resulted in the improvement of crystal quality. Q. Wang et al. ^[21] reported that the band gap energy of the as-deposited ZnO thin film was high as 3.56eV but when the film was annealed at 380ºC; the band gap energy was decreased to about 3.29eV. They have also reported that the band gap energy of the as-prepared film was high due to the presence of Zn(OH)₂ with the ZnO in the film because Zn(OH)₂ is a wider band gap energy material than ZnO. LI Junwei et al. ^[22] studied the effect of different cathodic potential and deposition temperature on the optical transmittance spectra at 600nm wavelength of porous ZnO thin films of 1µm thickness. They have reported that the highest optical transmittance of about 63.6% was obtained from the film, which was deposited at -1.1V and 70ºC. They have found that the optical band gap energy of the films increases with increasing the absolute deposition potentials. They were also found that the optical transmittance values were increased with increasing the deposition temperatures up to 70ºC after which it was decreased with increasing the temperatures. The highest optical transmittance of about 63% was observed in the film, which was deposited at the temperature of 70ºC and the band gap energy of the films was also increased with increasing the temperature from 3.35 to 3.42eV. Figure 1 shows the variation of the band gap energy of various as-deposited ZnO thin films with the grain/crystallite size and indicates that the band gap energy of the ZnO thin films increases as the grain/crystallite size decreases.

Figure 1. Variations of the band gap energy with the grain/crystallite size of the various as-deposited ZnO thin films

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The relation between the crystallite size and band gap was derived using the non-linear curve fitting and is found to be:

Where d is the crystallite size of ZnO nano crystals in nm. The Band gap energy and transmittance values of ZnO thin films deposited at different conditions are presented in Table 5.

Table 5. Band gap energy and transmittance values of ZnO thin films deposited at different conditions

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