(0 0 2)-oriented growth and morphologies of ZnO thin films prepared by sol-gel method

Zinc oxide (ZnO) is an n-type semiconductor with a wide direct band gap of 3.37 eV and a large exciton binding energy (60 meV). ZnO thin films have been widely applied in high technology such as optoelectronic devices, solar cells, piezoelectric transducers and gas sensors [1–7]. Many techniques have been utilized to prepare ZnO thin films, such as metal organic chemical vapor deposition, pulsed laser deposition, sputtering, hydrothermal, sol-gel method, etc. [8–18]. Due to the low cost and simple equipment, sol-gel method has been extensively applied to prepare ZnO thin films. During the sol-gel process, the (0 0 2)-oriented growth and morphology of ZnO thin films, which are important for their electrical and optical properties, are influenced by many factors.

Ohyama et al. [14, 15] prepared ZnO thin films by dip-coating method and studied the effects of heat-treatment conditions, ligands, withdrawal speed of the substrate and film thickness on crystallographic orientation and morphology of ZnO thin films. The highly (0 0 2)-oriented ZnO thin films were obtained on silica glass substrates by preheating at 300 °C and post-heating at 600 °C to 800 °C with a 2-methoxyethanol-monoethanolamine-Zn(CH₃COO)₂·2H₂O solution. They suggested that solvents, such as methanol, ethanol and propanol, with lower boiling temperature than that of 2-methoxyethanol, strongly hindered preferred (0 0 2) orientation of ZnO thin films. When the preheating temperature was too high (>300 °C), vaporization of the solvents and thermal decomposition of zinc acetate took place abruptly and simultaneously with the crystallization, disturbing the unidirectional crystal growth. On the contrary, when the preheating temperature was too low (<300 °C), complete vaporization and thermal decomposition of zinc acetate didn’t occur at the preheating step but occurred at the postheating step. The abrupt solvent vaporization and acetate decomposition occurred at the postheating step, which disturbed the unidirectional crystal growth. However, Santos et al. [19] dissolved Zn(CH₃COO)₂·2H₂O in methanol and obtained (0 0 2)-oriented ZnO thin films on glass substrates at a low pre-heating temperature of 120 °C and annealing temperature of 350 °C. Guo et al. [18] used the 2-methoxyethanol-monoethanolamine-Zn(CH₃COO)₂·2H₂O solution to prepare ZnO thin films on glass and Si substrates. As the ZnO thin films were baked at 210 °C and annealed at 400 °C, highly (0 0 2)-oriented ZnO thin film was obtained because of solid-state oriented aggregation of the adjacent ZnO nuclei. These results indicate that the further investigation should be done to clearly understand the (0 0 2)-oriented growth of ZnO thin films prepared by sol-gel method.

Recently, Segawa et al. [20] observed a porous wrinkled morphology of ZnO thin films prepared by sol-gel method. The formation of the wrinkled structure increased the roughness of the ZnO thin films, which deteriorated their electrical and optical properties. Maiti et al. [21] indicated that the formation of wrinkled surface was due to the presence of monoethanolamine in the precursor. Hou et al. [22] suggested that the formation of wrinkled structure was related to the release of mechanical stress, which was generated during the baking process. By adjusting the preheating conditions in the spin-coating process, the formation of wrinkled structure could be avoided. It implied that the boiling temperature of the solvents and ligands seriously affected the morphology of ZnO thin films. It is necessary to understand the relationship between the boiling temperature and morphologies of ZnO thin films.

In this study, solvents and ligands with different boiling temperatures were used to prepare Zn-solutions, and the ZnO thin films were deposited on Si substrates by spin-coating method. The effect of boiling temperature and pre-heating temperature on (0 0 2)-oriented growth and morphologies of ZnO thin films was investigated.

All the chemicals were analytic grade reagents used without further purification. Zn(CH₃COO)₂·2H₂O was used as the starting material. Methanol, ethanol, n-propanol and 2-methoxyethanol were used as the solvents, and 2-(methylamino)ethanol, monoethanolamine and diethanolamine were used as the ligands. Their atmospheric boiling temperatures are listed in Table 1. Fig. 1 shows the experimental procedure for preparation of ZnO thin films. A desired amount of Zn(CH₃COO)₂·2H₂O was dissolved in the solvent and ligand at 60 °C for 30 min to get the transparent and homogeneous solutions. The molar ratio of ligand to Zn(Ac)₂·2H₂O was kept at 1. The concentration of all Zn-solutions was 0.4 mol·L^-1. Table 2 lists the details of all solutions. The ZnO thin films were spin-coated on Si(1 0 0) substrates at 4000 rpm for 30 s. The wetting films were baked on a hot-plate at different temperatures for 10 min. The spin-coating and baking processes were repeated for 3 times. The final films were annealed in a furnace at 400 °C for 60 min in air ambient.

X-ray diffraction (XRD) patterns of the ZnO thin films were measured by a Rigaku D/MAX-IIIA X-ray diffractometer with CuKα radiation. Morphologies of the ZnO thin films were observed with a field emission scanning electron microscope (FESEM, JSM-7000FK, JEOL, Ltd.).

The preparation of ZnO thin film by sol-gel method mainly involved the following processes: (1) The Zn-solution was prepared and then the wetting film was spin-coated on Si substrate; (2) The wetting film was baked for evaporation of the solvent and ligand and decomposition of zincorganic; (3) The ZnO thin film was annealed to crystallize [18]. The boiling temperature of the solvents and ligands seriously influenced the quality of ZnO thin films in the processes (1) and (2). As methanol was used as a solvent, all prepared ZnO thin films showed porous wrinkled surfaces. Since its boiling temperature is only 64.7 °C, it has already volatilized during the spin-coating process. It was likely to cause the mechanical stress and form the porous wrinkled microstructure, as suggested by Hou et al. [22]. The boiling temperatures of ethanol and 2-(methylamino)ethanol are 78 °C and 159 °C, respectively. As they were used to prepare the Zn-solution, the prepared ZnO thin films also had wrinkled surfaces. When the 2-(methylamino)ethanol was replaced by monoethanolamine or diethanolamine with a higher boiling temperature, the ZnO thin films with smooth surface were obtained by adjusting the baking temperature. For the n-propanol solvent, the ZnO thin films with smooth surface could be formed. As the 2-methoxyethanol was used as a solvent, all prepared ZnO thin films had smooth surfaces. These results indicated that the formation of wrinkled surfaces was related to the evaporation rate of solvents and ligands. As the wetting film was baked, the evaporation rate of solvent and ligand with high boiling temperature was slower than that of the solvent and ligand with low boiling temperature, which was favorable for the relaxation of mechanical stress to obtain the ZnO thin film with smooth surface.

As methanol and ethanol were used as solvents to prepare Zn-solutions with 2-(methylamino)ethanol or monoethanolamine, respectively, the prepared ZnO thin films exhibited (0 0 2)-preferred orientation by adjusting the baking temperature. However, other peaks from (1 0 1) and (1 0 0) planes were also observed. Ohyama et al. [14, 15] suggested that the solvents with low boiling temperature disturbed the aligned grain growth in the film, which hindered the highly (0 0 2)-oriented growth of ZnO thin films. When the n-propanol and 2-methoxyethanol were used as solvents to prepare Zn-solutions with 2-(methylamino)ethanol, highly (0 0 2)-oriented ZnO thin films were formed at the baking temperature of 150 °C to 160 °C. The appropriate baking temperature was near the boiling temperature of 2-(methylamino)ethanol (159 °C). As 2-(methylamino)ethanol was replaced with monoethanolamine to prepare Zn-solutions with n-propanol or 2-(methylamino)ethanol, highly (0 0 2)-oriented ZnO thin films were prepared at the baking temperature of 160 °C to 170 °C, which was also near the boiling temperature of monoethanolamine (170 °C). However, as the diethanolamine was used as a ligand to prepare Zn-solutions, the prepared ZnO thin films didn’t show a preferred orientation at all, and the intensity of diffraction peaks was very weak. Due to the high boiling temperature of diethanolamine (268 °C), the carbonaceous materials resulting from incomplete combustion of diethanolamine remained in the films when ZnO crystallization occurred, which hindered the preferred grain growth along the (0 0 2) orientation. These results indicated that the boiling temperature of the solvents and ligands seriously affected the (0 0 2)-oriented growth of ZnO thin films.

Fig. 5

Fig. 6

Guo et al. [18] reported that the ZnO nuclei could be formed by the hydrolysis and/or thermal decomposition of zinc acetate. At low baking temperature, the hydrolysis predominated. At high baking temperature, the thermal decomposition became dominant. In our study, when 2-(methylamino)ethanol and monoethanolamine were used as ligands to prepare ZnO thin films, the baking temperature was low, which was suitable to form the ZnO nuclei by hydrolysis of zinc acetate. When diethanolamine was used as a ligand, the baking temperature was high, which favored the formation of ZnO nuclei by thermal decomposition of zinc acetate. These results indicated that the ZnO nuclei formed by hydrolysis of zinc acetate were favorable for the growth of (0 0 2)-oriented ZnO thin film.

The ZnO thin films were prepared by sol-gel method. Their morphologies and oriented growth were seriously affected by the boiling temperature of the solvents and ligands. The solvents and ligands with high boiling temperature were favorable for the relaxation of the mechanical stress to obtain smooth ZnO thin films. As the solvents and ligands with low boiling temperature were used to prepare the Zn-solutions, the prepared ZnO thin films showed (0 0 2) preferred orientation, which indicated that the ZnO nuclei formed by hydrolysis of zinc acetate were favorable for the growth of (0 0 2)-oriented ZnO thin film. As n-propanol, 2-methoxyethanol, 2-(methylamino)ethanol and monoethanolamine were used to prepare the Zn-solutions, highly (0 0 2)-oriented ZnO thin films were formed by adjusting the baking temperature.

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Fig. 8