Journal of NanomaterialsVolume 2012 (2012), Article ID 539657, 8 pagesdoi:10.1155/2012/539657

Research Article

The Effect of Mg Dopant and Oxygen Partial Pressure on Microstructure and Phase Transformation of Thin Films

Lay Gaik Teoh,¹ Wei-Hau Lu,² Ting Hsiang Lin,² and Ying-Chieh Lee²

¹Department of Mechanical Engineering, National Pingtung University of Technology & Science, Pingtung 91201, Taiwan²Department of Materials Engineering, National Pingtung University of Technology & Science, Pingtung 91201, Taiwan

Received 13 October 2011; Revised 2 December 2011; Accepted 3 December 2011

Academic Editor: Fathallah Karimzadeh

Copyright © 2012 Lay Gaik Teoh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

Table 1: RF sputter conditions for the magnesium zinc titanate films.

Target
(Zn0.95Mg0.05)TiO₃

Substrates
SiO₂/Si

Target size in diameter (mm)
76.2

Target to substrate distance (mm)
90

RF power (W)
150

Chamber pressure (Pa)
6.6 × 10⁻⁴

Working pressure (Pa)
2.5

Sputtering gas
Ar

Ar flow (sccm)
50

Substrate temperature (°C)
400

Deposition time (min)
120

Deposition rate (nm/min)
1.4

Substrate bias
Grounded

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.

(a)

(b)

Figure 1: X-ray diffraction patterns of thin films annealed at different temperatures, (a) Mg-doped zinc titanates thin film and (b) pure zinc titanates thin film.

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.

(a)

(b)

(c)

(d)

Figure 2: The SEM micrographs of Mg-doped zinc titanates thin films annealed at (a) as-deposited, (b) 700°C, (c) 800°C, and (d) 900°C.

(a)

(b)

(c)

Figure 3: TEM micrographs of the zinc titanates thin films with Mg dopant annealed at 800°C. (a) Twin, (b) region 1 of ZnTiO₃ grain, and (c) region 2 of ZnTiO₃ grain.

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.

Table 2: The elements analysis of zinc titanate thin films at different oxygen partial pressure was measured using ESCA equipment.

Substrate
Oxygen partial pressure
Surface element composition, X%
Zn/Ti

temperature (°C)
(Ar to O₂ ratio)
Zn2p
Ti2p
O1s

400
50/0 (10 : 0)
13.23
20.55
63.19
0.64

400
45/5 (9 : 1)
11.07
22.22
65.57
0.50

400
40/10 (8 : 2)
12.23
20.92
65.41
0.58

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.

Figure 4: X-ray diffraction patterns of the zinc titanates thin films deposited at RF power: 200 W, substrate temperature of 400°C, and then annealed at 800°C with different Ar to O₂ ratio (a) 10 : 0, (b) 9 : 1 and (c) 8 : 2.

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].

(a)

(b)

(c)

Figure 5: The film thickness of the zinc titanate deposited at 400°C of substrate temperature and then annealed at 800°C with different Ar to O₂ ratio (a) 10 : 0, (b) 9 : 1, and (c) 8 : 2.

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.

(a)

(b)

(c)

Figure 6: Plane-view SEM micrographs of zinc titanate thin film deposited at 400°C of substrate temperature and then annealed at 800°C with different Ar to O₂ ratio (a) 10 : 0, (b) 9 : 1, and (c) 8 : 2.

(a)

(b)

(c)

(d)

Figure 7: AFM surface morphologies and the right pictures depict typical three-dimensional representations (1000 nm × 1000 nm surface plots) of the zinc titanate thin films deposited at 400°C of substrate temperature and then annealed at 800°C: (a) and (b) Ar to O₂ ratio 10 : 0, and (c) and (d) Ar to O₂ 9 : 1.

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.

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