BST thin films obtained by PLD for applications in electronics
Introduction
Ferroelectrics are very attractive materials for a large variety of applications. In informatics, they are used in DRAM memories [1], [2]. In high frequency range they have high applications potential, due to the nonlinear variation of their permittivity with the applied electric field. This property offers the opportunity to realize the electrical control of microwave devices [3], [4]. In this type of applications, materials are in the paraelectric phase in order to avoid high loss in microwave domain and thermal hysteresis.
The dielectric loss in ferroelectrics is not as small as that of common dielectric materials and the loss tangent (tan δ) is an important characteristic of the material, which should be taken into account in the device design. The temperature dependence of the dielectric permittivity over the operating temperature interval is another important issue: this is of particular concern in the vicinity of the ferroelectric transition temperature, where the material exhibits a strong tunability, which depends strongly on the temperature.
Barium strontium titanate (BST) ceramics and films have recently received much attention for its high electric-field tunability and low dielectric loss [5], [6], [7], [8], [9]. These materials are very promising for practical applications such as phase shifters, delay lines, tunable filters, steerable antennas. To be used in these devices, a BST system must possess the following characteristics: high dielectric constant, high tunability, low dissipation factor and low temperature dependence.
Section snippets
BST target preparation and characterization
Ba1−xSrxTiO3 ceramic compounds with the molar formula Ba0.5Sr0.5TiO3 (BST) were prepared by standard solid-state reaction. SrCO3, BaCO3, and TiO2 oxide powders of purity higher than 99.9% were used as starting materials. The BST samples were doped with 1.0 mol% MgO and 0.5 mol% MnO2 in order to improve the granular growth and to control the porous structure [10]. Mixing was carried out for 2 h in an agate bottle containing agate balls. The mixing–milling media were distilled water. The mixtures
Results and discussion
The X-ray diffraction pattern of the BST thin films shows narrow peaks, proving a good crystallization of the films. The X-ray diffraction patterns of the BST thin film deposited on alumina and sapphire substrate, are shown in Fig. 1, Fig. 2. Fig. 1 shows diffraction pattern for BST thin film deposited on sapphire substrate heated at 650 °C (BST 1). Diffraction pattern for BST thin film on alumina obtained in the same conditions and post-annealed at 800 °C for 6 h (BST 2) is presented in Fig. 2. A
Conclusions
BST thin films on alumina and sapphire substrates were obtained by PLD. The annealing improves the crystallographic properties.
The capacitance variation with the temperature indicates a diffuse ferroelectric–paraelectric phase transition with Tc = −72 °C for the BST thin film. Tunability measurements performed on a large temperature interval indicate the maximum tunability in the range of −113 to −53 °C for the interdigital structure. A 5% tunability was achieved in the transition region. The low
Acknowledgments
The work was partially supported by the CERES 4-99/2004 and 2-CEx. 06-11-44/2006 projects.
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