Abstract
Mg-doped zinc titanate (ZnTiO3) films were prepared using RF magnetron sputtering. Subsequent annealing of the as-deposited films was performed at a temperature ranging from 600 to C for 2 hours with a heating rate of C/min in air. It was found that the as-deposited films were amorphous and contained 2.77 at.% magnesium. This was further confirmed by the onset of crystallization that took place at annealing temperatures of C. The results showed that single Zn2Ti3O8 phase was existed at C. When annealing is at C, the results revealed that mainly a hexagonal ZnTiO3 phase accompanying a Zn2Ti3O8 minor phase was observed. When annealing is at C, the results showed that single hexagonal ZnTiO3 phase is stable at C. It means that ZnTiO3 phase containing no Mg is unstable at C and is decomposed from hexagonal ZnTiO3 to cubic Zn2TiO4 and rutile TiO2 at C. In addition, the effect of oxygen partial pressure for the films deposited on the phase transformations and microstructures of zinc titanites thin film was investigated.
1. Introduction
As the need for versatile electronic components with high reliability increases, the development of high-frequency electronic materials becomes imperative. Zinc titanate (ZnTiO3) has been reported to have specific electrical properties that are adequate for applications in microwave dielectrics [1–3]. The ZnO-TiO2 system exists in three forms: zinc metatitanate (ZnTiO3) with a hexagonal ilmenite structure; zinc orthotitanate (Zn2TiO4) with a cubic spinel crystal structure; zinc polytitanate (Zn2Ti3O8) with a cubic defect spinel structure [4]. Steinike and Wallis. [5] have reported on Zn2Ti3O8 materials, a low-temperature form of ZnTiO3 existing at temperature <820°C. The Zn2Ti3O8 compound was formed based on the Zn2TiO4 phase [6]. However, hexagonal ZnTiO3 decomposes into cubic Zn2TiO4 and rutile TiO2 at T >945°C [7]. Moreover, a ZnTiO3 single-phase compound can be prepared by zinc oxide and rutile hydrate at [4].
Pure ZnTiO3 shows good dielectric properties in the microwave range. It has a perovskite-type oxide structure and could be advantageous as a microwave resonator material [8]. Furthermore, ZnTiO3 can be sintered at 1100°C without the use of sintering aids [7, 8]. Moreover, when a sintering aid is added, it can be fired at temperatures below 900°C [9, 10]. ZnTiO3 has potential applications in gas sensors that detect ethanol or carbon monoxide. It is also a promising candidate for the use in nonlinear optics, as a luminescent material and in various photocatalytic roles [11, 12].
Pure zinc titanate thin films have been prepared by RF magnetron sputtering in previous studies [13]. It was shown that crystallization of the ZnTiO3 phase occurred at a substrate temperature of 400°C and annealing temperature of 700°C over 2 h. However, as the annealing temperature exceeded 900°C, the ilmenite ZnTiO3 decomposed into cubic Zn2TiO4 and rutile TiO2. In the present work, to suppress ZnTiO3 decomposition at 900°C, Mg-doped zinc titanate thin films were prepared by RF magnetron sputtering. The microstructure and phase transformation of Mg-doped zinc titanate thin films with different annealing temperatures were subsequently investigated. In addition, the impact of different atmosphere (Ar/O2 ratio) during sputtering was investigated.
2. Experimental Procedure
The Mg-doped zinc titanates thin films were prepared by RF magnetron sputtering with the deposition conditions listed below.
A bulk magnesium zinc titanate target was synthesized by conventional solid-state methods from high-purity oxide powders; MgO, ZnO, and TiO2 (>99.9%). The starting materials were mixed according to the stoichiometry of (Zn0.95Mg0.05)TiO3. The powder was then sintered and pressed into disks with a diameter of 76.2 mm and thickness of 3 mm. These were subsequently used as (Zn0.95Mg0.05)TiO3 targets. The Mg content in the zinc titanate thin films was analyzed using electron spectroscopy for chemical analysis (ESCA), showing that the as-deposited thin films contained 2.77 at.% Mg.
Magnesium zinc titanate thin films were fabricated onto SiO2/Si substrates using a 13.56 MHz, 150 W RF magnetron sputtering system. The sputtering chamber was evacuated by an oil diffusion pump to a base pressure of Pa. The sputtering gas Ar flow of 50 sccm with a purity of 99.999% was introduced into the chamber with mass flow controllers and a working pressure of 2.5 Pa. The target was cleaned at an RF power of 100 W for 5 min in an atmosphere of pure Ar, and the substrate was covered with the shelter. The films were deposited at 400°C of substrate temperature. The as-deposited films were annealed at 600 to 900°C for 2 hours with a heating rate of 5°C/min in air. The detailed deposition conditions of the zinc titanate films are listed in Table 1. For the different atmosphere, (Ar/O2 ratio) during sputtering was also carried out, the gas (Ar + O2) flow was fixed at 50 sccm. The O2 gas flow was changed from 0 to 10 sccm.
Crystallinity of the films was analyzed by X-ray diffraction (XRD, Bruker D8A Germany), with Cu Kα radiation for 2θ from 20° to 80° at a scan speed of 3° min−1 and a grazing angle of 0.5° under 40 kV and 40 mA. The DIFFRAC plus TOPAS version 3.0 program was used to determine the lattice parameters. Microstructural and thickness observations of the cross-section and plane-view morphology of the thin films grown on SiO2/Si (100) substrates were analyzed using field-emission scanning electron microscope (FE-SEM, Hitachi S-4700 Japan). Microstructure of the films and the ZnTiO3/SiO2 interfaces were investigated by filed-emission transmission electron microscopy (FE-TEM, FEI E.O. Tecnai F20) at an acceleration voltage of 200 kV, equipped with energy-dispersive spectroscopy (EDS). Atomic force microscopy (AFM, Veeco CP-II) was used to study the surface topography, with a scanned area of 5 μm × 5 μm.
3. Results and Discussion
3.1. The Effect of Mg Dopant on the Phase Transformation of ZnTiO3 Thin Film
Figure 1 (a) shows the X-ray diffraction (XRD) patterns of the 2.77 at.% Mg-doped zinc titanate thin films annealed at 600, 700, 800, and 900°C. It was observed that the as-deposited thin films were amorphous, indicating that no crystallization occurred in the as-deposited thin films. At 600°C, the Zn2Ti3O8 peaks appeared, which is a low-temperature form of ZnO-TiO2 system, as reported by Yamaguchi et al. [14]. Zn2Ti3O8 is a stable or metastable compound; its existence was first reported by Bartram and Slepetys [4], who found that it decomposes at temperatures above 700°C, and Zn2Ti3O8 can be existed stably between 600 to 700°C [4, 6]. However, the intensity of peaks increased rapidly up to 700°C. The majority crystalline phase was identified as hexagonal ZnTiO3, accompanied by Zn2Ti3O8 minor phases.
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As the annealing temperature was increased to 800°C, the hexagonal ZnTiO3 became a single crystalline phase. The intensity of the (104) peak was higher than the other peaks of the ZnTiO3 films, indicating that there is a high degree of (104)-oriented ZnTiO3 on the SiO2/Si(100) substrates. Chen and Huang. [15] have shown that (100)-oriented MgTiO3 films were obtained on the Si substrate. The preferred orientation tends to reduce the free energy to reach a stable state. When the temperature was further increased to 900°C, the ZnTiO3 single phase was remained. This result is unlike pure zinc titanate thin films where the hexagonal ZnTiO3 phase decomposes into TiO2 and Zn2TiO4 at 900°C, as shown in Figure 1 (b).
Figure 2 shows the SEM micrographs of the Mg-doped zinc titanate thin films deposited on SiO2/Si substrate annealed at different temperatures. The grain size increased with the annealing temperatures. The grain sizes of specimens for 700°, 800°, and 900°C are 25 nm, 139 nm, and 208 nm, respectively. To prove that pure ZnTiO3 phase exist in Mg-doped zinc titanate thin films at 900°C, high resolution TEM was used to analyze these Mg-doped ZnTiO3 thin films. Figure 3 shows the cross-section of Mg-doped ZnTiO3 thin films on a SiO2/Si substrate. The HRTEM of region 1 and 2 shows the d-spacing of h-ZnTiO3 phase, which is d003 = 0.441 nm and 0.449 nm, respectively. The XRD analysis revealed a similar trend. Comprehensively, the results confirmed that ZnTiO3 thin films were successfully prepared at 900°C. Interestingly, a twin is observed as shown in Figure 3 (a), this is a two-dimensional defect. A twin is defined as a region in which a mirror image of the structure exists across a plane or a boundary. This defect is often due to an atomic lattice defect forming a mirror image of undeformed lattice next to it. Based on the above results, it can be expected that the Zn atoms are substituted with Mg atoms in the ABO3 structure. This may be attributed to the ionic radius of Mg2+ (0.66 Å), which is smaller than that of Zn2+ (0.74 Å) [16, 17]. Hence, when Mg2+ substitutes on Zn2+ sites in the ABO3 structure, lattice strain will be created.
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3.2. The Effect of Oxygen Partial Pressure on the Phase Transformation of ZnTiO3 Thin Film
The effect of oxygen partial pressure on the phase transformation of zinc titanates thin films was also investigated. The different atmosphere (Ar/O2 ratio) during sputtering was carried out in this experiment. Table 2 lists the element analysis of zinc titanites thin films using ESCA equipment. It is found that the Zn/Ti ratio decreased significantly when O2 was used in the chamber. The compositions at different oxygen partial pressures are different from target composition, which results from the different striking coefficients and variation in sputtering yields of the constituent elements [18]. During sputtering, the target atoms are subject to collisions with gas atoms or molecules left in the chamber and other ejected atoms, resulting in a partial loss of energy and direction on their way to the substrate [19]. Because oxygen gas is biatom molecule and its radius is much larger than Ar, the sputtered particles suffer from more collision when more oxygen partial pressure is introduced. This changes the composition of thin film.
XRD measurements were performed to examine the variation of structural properties with varying oxygen partial pressures . Figure 4 shows the XRD spectra of zinc titanate thin films grown under different Ar and O2 ratios, and these films were annealed at 800°C. For an Ar/O2 ratio of 9 : 1 (10% O2), the ZnTiO3 and Zn2Ti3O8 phases coexisted as shown in Figure 4 (b). This result indicates that 10% O2 in the sputtering atmosphere leads to the remaining Zn2Ti3O8 phase at 800°C, because the Zn2Ti3O8 phase is stable below 700°C without oxygen in the atmosphere, as seen in Figure 4 (a). However, when the O2 partial pressure was increased to 20% (Ar/O2 ratio of 8 : 2), the major and minor phases were ZnTiO3 and TiO2 phases, respectively. This result indicates that 20% O2 in the sputtering atmosphere leads to the decomposition of the Zn2Ti3O8 phase into ZnTiO3 and TiO2 at 800°C as shown in Figure 4 (c). Hence, there are two phases present at 800°C when O2 is used: Zn2Ti3O8 and TiO2. According to the phase diagram of ZnO-TiO2 [4], TiO2 appears at 945°C and Zn2Ti3O8 exists below 700°C. It is believed that the evolution of the phases is related to the variation of oxygen partial pressure. From Table 2, it is noticed that the oxygen partial pressure causes the compositional change of zinc titanates thin films. The variation in composition (especially Zn concentration) may result in the different phase transformations: lower Zn concentration leads to form Zn2Ti3O8 or TiO2. In addition, Zn concentration in zinc titanates thin film may be reduced again during annealing, because zinc is easy to vaporize at high temperature. Therefore, it cannot be formed a ZnTiO3 due to lower Zn concentration.
The film thickness with varying O2 partial pressure was measured using the cross-section of FE-SEM micrographs as shown in Figure 5. The cross-sectional views show that the thicknesses of the films significantly decrease when a small amount of oxygen is added to the sputtering ambient and subsequently decreases slowly as the oxygen partial pressure increases. The thicknesses of the films were 196, 98, and 85 nm for pure Ar and Ar: O2 flow ratios of 9 : 1 and 8 : 2, respectively. The gradual decrease in thickness with increase of the oxygen partial pressure can be explained by the smaller sputtering yields of oxygen ions than argon ions; the momentum transfer of oxygen is smaller than that of argon during ionic bombardment [20]. However, the thickness difference between the films that are grown with and without oxygen is too significant to be explained only by the momentum transfer process [21]. In addition, according to XRD analysis (Figure 4), increasing the oxygen partial pressure was found to degrade the crystallinity of the zinc titanites thin films due the formation of oxygen-induced defects [22, 23].
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Plane-view SEM micrographs of zinc titanate thin film deposited at 400°C substrate temperature and then annealed at 800°C with different Ar to O2 ratio are shown in Figure 6. As one can see, the grain size decreased with increasing O2 partial pressure. According to XRD analysis, the phases of the thin film also differ with O2 partial pressure. Moreover, it is found that there are two kinds of grains in the 10% samples as shown in Figure 6 (b). According to XRD analysis (Figure 4), ZnTiO3 grains and Zn2Ti3O8 grains are identified. The surface morphologies of zinc titanite films have been observed with AFM, and the results are shown in Figure 7, corresponding to the samples prepared at oxygen partial pressures of 0% and 10%. The left figures display the surface morphologies and the pictures on the right depicts typical three-dimensional representations (1000 nm by 1000 nm surface plots). All the films present a rough surface texture, consisted of particles fused together, building up high mountains and deep valleys. All the films can be described as a contiguous network of particles and aggregates with significant roughness. It is also shown that the TiO2 particles decrease in size with an increase in oxygen partial pressure, which may be ascribed to the deposition rate.
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4. Conclusion
The effects of Mg doping on zinc titanate thin films were investigated using a variety of analytical tools. The microstructure and phase transformation of zinc titanate thin films can be influenced by doping Mg. It is found that the as-deposited films were amorphous, as confirmed by the XRD results. The results showed that single Zn2Ti3O8 existed when the films were annealed at 600°C. When annealing was conducted at 700°C, the results revealed that the majority phase was hexagonal ZnTiO3, accompanied by minority amounts of Zn2Ti3O8. Unlike pure zinc titanate films, this result shows that the Zn2Ti3O8 phase can exist at temperatures above 700°C. However, there is no decomposition from hexagonal ZnTiO3 to cubic Zn2TiO4 and rutile TiO2 took place with a further increase in temperature to 900°C. It means that the addition of Mg to ZnTiO3 compound increases its stability up to 900°C. In addition, with increasing oxygen partial pressure (Ar-to-O2 ratio decreased from 10 : 0 to 8 : 2), the phase transformations versus temperatures changed. At an Ar-to-O2 ratio of 9 : 1, ZnTiO3 and Zn2Ti3O8 phases coexisted at 800°C. By increasing the partial pressure to 8 : 2, the ZnTiO3 phase remained as the main phase, accompanied by a TiO2 minor phase at 800°C
Acknowledgment
The authors would like to acknowledge the financial support of this research by the National Science Council of Taiwan under Contract no. NSC-98-2221-E-020-003.