Journal of the Korean Physical Society, Vol. 69, No. 10, November 2016, pp. 1571∼1574

Influence of Annealing Temperature on the Dielectric Properties of BaSrTiO3 Thin Films Deposited on Various Substrates Chil-Hyoung LEE and Young-Jei OH∗ Opto-electronics Materials Devices Research Center, Korea Institute of Science and Technology, Seoul 02792, Korea

Deuk Yong LEE Department of Biomedical Engineering, Daelim University, Anyang 13916, Korea

Doo-Jin Choi Department of Advanced Materials Science & Engineering, Yonsei University, Seoul 03722, Korea (Received 21 September 2016) (Ba0.5 Sr0.5 )TiO3 (BST) thin films were deposited on various substrates, such as LaAlO3 (100), MgO(100), R-plane sapphire[1012], and polycrystalline sapphire, by using RF magnetron sputtering to investigate the influence of annealing temperature on the dielectric properties and the tunability of the films. The BST thin films deposited on LaAlO3 (100) exhibited a high tunability of 42 % and a low dielectric loss of 0.004 due to the small differences in the lattice parameters and the thermal expansion coefficients between the BST films and the substrates. In contrast, the BST films deposited on a polycrystalline sapphire, exhibiting a relatively high mismatch factor, showed the tunability of ∼24 % and a dielectric loss of ∼0.007. The BST thin films on LaAlO3 (100), MgO(100), R-plane sapphire[1012], and polycrystalline sapphire were annealed. The optimized annealing temperatures were found to be 950 ◦ C, 1050 ◦ C, 1100 ◦ C, and 1150 ◦ C, respectively. The difference in annealing temperature is likely due to the differences in the lattice parameters and the thermal expansion coefficients between the films and the substrates. PACS numbers: 77.55.+f, 77.22.Ch Keywords: Ba0.5 Sr0.5 TiO3 , Substrates, Lattice mismatch, Thermal expansion coefficient, Tunable microwave device DOI: 10.3938/jkps.69.1571

I. INTRODUCTION

Oxide-based barium-strontium-titanate (Ba1−x Srx TiO3 , BST) ferroelectrics are polar materials that exhibit net spontaneous polarization without an external applied field [1–4]. The ferroelectric/paraelectric phase transition temperature (Tc ) can be tuned by varying the Ba/Sr ratio because the substitution of Sr for Ba in the BaSrTiO3 lattice may shift the Tc to lower temperature. The BST solutions with x = 0.2 − 0.5 are normally used to shift the Tc to room temperature [3]. Therefore, BaSrTiO3 is known to be a room-temperature paraelectric with a high dielectric constant, a low dc leakage current, a low loss tangent, and a large dielectric breakdown strength. It has been considered as the material of choice for tunable microwave devices such as microwave tunable phase shifters, tunable filters, and high-Q resonators for ∗ E-mail:

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pISSN:0374-4884/eISSN:1976-8524

radar and communication applications [1–9]. The realization of high-performance tunable microwave devices based on ferroelectric thin films with a combination of low dielectric losses and high tunability (% = [ε(0) − ε(E)]/ε(0), where ε(0) and ε(E) are the permittivities in the absence/presence of the electric field) will have a significant impact on wireless communications, including satellite applications [7]. Significant variations in the dielectric constant have been observed in epitaxial BST thin films for which the internal stress levels were systematically altered either by using different substrate materials or by adjusting the film’s thickness [8, 9]. These results are generally ascribed to the stress generated by the lattice mismatch, as well as the difference in thermal expansion coefficients (CTEs), between the film and the substrate [10]. In the present study, Ba0.5 Sr0.5 TiO3 (BST) thin films were sputtered on various substrates and then annealed at temperatures in the range from 950 ◦ C to 1150 ◦ C to relieve the stress generated by mismatches of the lat-

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c 2016 The Korean Physical Society

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Journal of the Korean Physical Society, Vol. 69, No. 10, November 2016

Table 1. Sputtering conditions for the BST thin films. Target material Base pressure Substrate temperature (◦ C) Sputtering gas (sccm) Target-substrate distance (mm) Plasma power (W) Working pressure

Ceramic BST 5 × 10−6 Torr 700 O2 : Ar = 50 : 38.4 60 100 1 × 10−3 Torr

Fig. 1. (Color online) Lattice parameter of the BST thin films as a function of the annealing temperature.

tice constant, as well as the CTEs. BST thin films with a thickness of 500 nm were deposited on various substrates, such as LaAlO3 (100), MgO(100), R-plane sapphire (1¯102), and polycrystalline sapphire, by using RFmagnetron sputtering to investigate the effect of post annealing on the microstructure and on the dielectric and tunability properties of the BST films. The properties of the BST films were evaluated by using an X-ray diffractometer (XRD), a scanning electron microscope (SEM), an atomic force microscope (AFM), a symmetrical stripline resonator, and an impedance/gain-phase analyzer (Hewlett Packard 4194A, USA).

II. EXPERIMENTS BST thin films with a thickness of 500 nm were prepared by using RF magnetron sputtering on 102) and LaAlO3 (100), MgO(100), R-plane sapphire (1¯ polycrystalline sapphire substrates. The BST target was prepared by using the mixing-oxides method starting with high-purity reagents of BaCO3 , SrCO3 , and TiO2 powders. A disc-type BST target with a diameter of 120 mm and a thickness of 5 mm was positioned 60 mm away from the substrate (on-axis sputtering). The thin films were synthesized in an oxygen and argon atmosphere under a fixed power of 100 W and a constant work pressure

of 1 × 10−3 Torr. The base pressure should be 5 × 10−6 Torr before introducing argon and oxygen (99.99 %). The substrates were heated to 700 ◦ C during sputtering. The sputtering parameters are list in Table 1. After deposition, the samples were annealed at temperatures from 900 ◦ C to 1200 ◦ C for 1 h in air. The substrates were then cleaned in acetone, ethanol, trichloroethylene, and distilled water. The film’s crystal structures were investigated by using an XRD. A scan speed of 0.5◦ /min was used in the 2θ range of 65 - 120◦ . After Kα2 peak stripping, peak positions were determined by profile refinement using the built-in PC-APD program. The details of the lattice-parameter refinement procedure have been described elsewhere [11,12]. The surface morphology of the BST films was analyzed by using an AFM under a noncontact mode. Single-gap planar capacitors with a gap of 7.8 μm were used for the microwave characterization. The capacitors were fabricated by using a DC magnetron sputtering and photolithograph lift-off process to deposit a 300 nm-thick Cu layers on top of the BST films. The microwave dielectric properties, the capacitance, and the dielectric Q (1/ tan δ) of the films were measured by using a symmetrical stripline resonator with shorted ends. The dielectric properties of the BST sample were measured at 1 ∼ 3 GHz by using an impedance/gain-phase analyzer.

III. RESULTS AND DISCUSSION The effect of annealing temperature on the lattice parameter of the BST thin films is shown in Fig. 1. XRD results revealed that the lattice parameter of the BST films deposited on R-plane sapphire (1¯ 102) and polycrystalline sapphire decreased dramatically with increasing annealing temperature from 900 ◦ C to 1200 ◦ C. However, the lattice parameter of the films deposited on the LaAlO3 (100) substrate rose slightly with increasing temperature to 1050 ◦ C and then decreased dramatically at temperatures above 1050 ◦ C. On the other hand, the lattice parameter of the films on MgO(100) decreased from 3.975 ˚ A to 3.96 ˚ A when the temperature was raised from A at temper900 ◦ C to 1050 ◦ C and then rose to 3.965 ˚ atures from 1050 ◦ C to 1200 ◦ C. The smallest deviation in the lattice parameter as a function of the annealing temperature was observed for the BST films sputtered on the MgO substrate, as displayed in Fig. 1. The variation in the lattice parameter may be due to the lattice size and the CTE mismatch between the film and the substrate, as depicted in Fig. 1 and Table 2. The mismatch factor (ξ) was determined by using the lattice parameters, ξ = 2(aS − aF )/(aS + aF ), where aS and aF are the lattice parameters of the substrate and the films, respectively. The mismatch factor and the CTEs are summarized in Table 2. The largest mismatch (18.6 %) was found for the cubic BST films deposited

Influence of Annealing Temperature on the Dielectric Properties of BaSrTiO3 · · · – Chil-Hyoung LEE et al.

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Fig. 2. (Color online) AFM images of the BST thin films annealed at different temperatures on various substrates: (a) 950 ◦ C on LaAlO3 , (b) 1050 ◦ C on MgO, (c) 1100 ◦ C on Rplane sapphire, and (d) 1150 ◦ C on polycrystalline sapphire. Table 2. Mismatch factor between the film and the substrates and their thermal expansion coefficients. Film and Substrate

Structure

Mismatch Thermal expansion factor (%) coefficient (∼ 10−6 /K) Cubic 10.5 Cubic −2.2 11 Cubic 6.4 13.8 Hexagonal 18.6 7.3

Ba0.5 Sr0.5 TiO3 LaAlO3 MgO R-plane sapphire Polycrystalline Hexagonal sapphire

18.6

5.3

on R-plane hexagonal sapphire (1¯ 102) and on polycrystalline hexagonal sapphire due to the differences in the crystal structures and the CTEs. The smallest mismatch factor of −2.2 % was found for the BST films deposited on the LaAlO3 (100) substrate due to the difference in the CTEs (0.5 × 10−6 K−1 ) being the smallest, as listed in Table 2. The effect of annealing temperature on the surface morphology of the BST films was examined by using an AFM. The BST films deposited on various substrates were annealed at different temperatures. The films on LaAlO3 (100), MgO(100), R-plane sapphire, and polycrystalline sapphire were annealed at 950 ◦ C, 1050 ◦ C, 1100 ◦ C, and 1150 ◦ C, respectively, because annealing at those temperatures yielded the highest values of the permittivity and the tenability. The AFM images, as illustrated in Fig. 2, exhibited a dense microstructure without cracks or defects. The grain size of the BST films

Fig. 3. (Color online) (a) Permittivity and (b) dielectric loss of the BST thin films on various substrates as functions of the annealing temperature.

increased with increasing annealing temperature, regardless of the substrate type, because the amorphous phase was transformed to the crystalline phase during annealing. The crystalline phase was stabilized by decreasing the surface energy as a result of a reduction in the grainboundary area caused by large grains. The permittivity (dielectric constant) of a ferroelectric/paraelectric film is strongly affected by the grain size [13,14]. Higher permittivity is expected for the films having larger grain sizes because grain size scales with polarization. The value of dielectric polarization is also proportional to the grain size. The microwave dielectric properties of the BST films were examined by using a single-gap planar capacitorvaractor with a Cu electrode. The dielectric properties and the tunabilities of the BST thin films as functions of the annealing temperature are shown in Figs. 3 and 4. Higher dielectric constants in the range of 900 to 1100 ◦ C (Fig. 3) were observed for the BST films on LaAlO3 (100) and MgO substrates, respectively. In addition, relatively low dielectric losses were detected over the annealing temperature range (900 ◦ C to 1200 ◦ C), as displayed in Fig. 3. The tunabilities of the BST

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Journal of the Korean Physical Society, Vol. 69, No. 10, November 2016

sapphire substrates were prepared and annealed at various temperatures to investigate the effect of annealing temperature on the morphologies and the dielectric properties in the microwave frequency range from 1 to 3 GHz. Both the tunabilities and the dielectric constants increased in a certain range of temperatures. Optimized BST films possessing tailored tunabilities and dielectric constants were found for the films on LaAlO3 (100), MgO(100), R-plane sapphire, and polycrystalline sapphire that were annealed at 950 ◦ C, 1050 ◦ C, 1100 ◦ C, and 1150 ◦ C, respectively. The annealing temperature can be concluded to be crucial for the changes in the permittivity and the tunability. Fig. 4. (Color online) Tunability of the BST thin films on various substrates as a function of the annealing temperature.

films deposited on LaAlO3 (100) and MgO substrates, [ε(0) − ε(E)]/ε(0), were in the range of 26 to 42.5 % and 22.5 to 40 %, respectively. The permittivities of the films on R-plane sapphire and polycrystalline sapphire were 580 to 800 and 300 to 750, respectively. The tunabilities were 10 to 27 % and 7 to 22 %, respectively. Low dielectric loss is critical for a microwave tunable electronic device. The dielectric loss was reported to increase with an improvement in the tunability of the BST films [14]. For optimum device performance, the films should possess a high dielectric constant, low dielectric losses, and high tunability. Although the dielectric losses of the BST films on LaAlO3 and MgO increased slightly with increasing annealing temperature, the highest values of the dielectric constants and the tunabilities were observed for the BST films deposited on LaAlO3 (100), MgO(100), R-plane sapphire, and polycrystalline sapphire which were annealed at 950 ◦ C, 1050 ◦ C, 1100 ◦ C, and 1150 ◦ C, respectively, as shown in Figs. 3 and 4. The annealing process for the BST films was very effective for enhanced improvement in the permittivity and the tunability. The difference in the dielectric properties of the as-deposited and the annealed BST films is known to be due to the strains in the films [15]. The residual stress in the BST films induced by the lattice mismatch and the CTE mismatch between the BST film and the substrate was successfully relieved by annealing, resulting in enhanced dielectric properties and tunabilities [16].

IV. CONCLUSION In this study, high-quality BST films on LaAlO3 (100), MgO(100), R-plane sapphire (1¯ 102) and polycrystalline

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