Influence of Thickness and Composition Dependence on Dielectric Tunability in Ba(Zr_xTi_1-x)O₃
Films Grown on MgO Substrate

Microwave tunable devices are necessary components for next-generation communication applications. Semiconductors, ferrite and ferroelectrics are typical tunable materials. Ferroelectric films have been studied because of their high dielectric tunability and low power consumption. Moreover, for microwave dielectric materials, they show advantages in terms of simple device structure, low loss, good temperature stability and low cost in producing communication devices. To date, (Ba_1-<i>x</i>, Sr<i>_x</i>) TiO₃[BST] is one of the most popular ferroelectric materials for tunable microwave devices. However, its dielectric loss is large and the temperature dependence of the dielectric constant is also insufficient for applications. Ba(Zr<i>_x</i>Ti_1-<i>x</i>)O₃[BZT] solid solutions are tunable materials with potential use in elements of phased array antenna, just above their Curie temperature in a paraelectric state. BZT is a possible alternative to BST in tunable microwave applications Zr⁴⁺ is chemically more stable than Ti⁴⁺ and maintains a low dielectric loss. BZT in the paraelectric phase also has higher temperature stability than BST. Therefore, we focused on BZT films as a tunable ferroelectric material.<br/>To evaluate electric properties with microwave region, BZT films are deposited on MgO substrate, which has low dielectric constant compare to films. The lattice mismatch strain between substrate and film, induces remarkable change in crystal structure. This is important parameter to use BZT film into microwave application but not to be reported about influence of thickness and composition dependence of BZT film grown on MgO substrate. Thus, we will discuss these dependence and their impact on microwave applications.<br/>(100)-oriented BZT (<i>x</i> = 0 ~ 1.0) thin films with various thickness by pulse laser deposition [PLD] technique on MgO substrates. The structural properties of the films were characterized by x-ray diffraction [XRD] (MRD, PANalytical). Wavelength-dispersive x-ray fluorescence spectrometry [WDX] (ZSX Primus4, Rigaku) was carried out to investigate the composition and thickness of BZT thin films. All the XRD patterns showed only (001)/(100) diffraction peaks and no secondary phase. Reciprocal space mappings (RSMs) of BZT thin films showed that BZT thin films epitaxially grew on MgO(100) substrates. The composition dependence of tetragonality, remanent polarization, and dielectric constant suggested that there may be a phase boundary between <i>x</i> = 0.2 and 0.3, and this tendency is similar to BZT ceramics. Therefore, our results suggest that BZT (<i>x</i> = 0 ~ 0.5) can be one of the significant candidates for microwave application which requires high tunability and low loss.