Experiments have shown that superlattices that comprise alternating epitaxial layers of dissimilar paraelectric perovskites can exhibit large changes in permittivity with the application of electric fields. The superlattices are potentially useful as electrically tunable dielectric components of such microwave devices as filters and phase shifters.
The electrically tunable materials heretofore used in some microwave devices exhibit strong temperature dependences of dielectric properties and high microwave losses. Previous efforts to overcome these undesired effects have involved the addition of various dopants to SrTiO3, BaTiO3, and SrxBa1–xTiO3. Despite the amount of research in this area, results have been disappointing.
The present superlattice approach differs fundamentally from the prior use of homogeneous, isotropic mixtures of base materials and dopants. A superlattice can comprise layers of two or more perovskites in any suitable sequence (e.g., ABAB…, ABCDABCD…, ABACABACA…). Even though a single layer of one of the perovskites by itself is not tunable, the compositions and sequence of the layers can be chosen so that (1) the superlattice exhibits low microwave loss and (2) the interfacial interaction between at least two of the perovskites in the superlattice renders either the entire superlattice or else at least one of the perovskites tunable.
The perovskites investigated experimentally for use in superlattices include SrTiO3, SrCeO3, SrZrO3, BaTiO3, BaZrO3, CaZrO3, and LaAlO3. Superlattices for the experiments were fabricated by pulsed laser deposition onto mostly LaAlO3 substrates; a few specimens were prepared on SrTiO3 substrates. Microwave filters containing superlattices were also fabricated. The superlattices were evaluated with respect to structure, composition, and dielectric properties.
Analysis of the observations made in the experiments led to the following conclusions:
- Large tuning was observed in capacitors made from some superlattices (see figure). Ferroelectricity induced by lattice-mismatch strain in superlattices has been tentatively identified as the cause of tunability in the otherwise nontunable paraelectric perovskite constituents.
- Some of the superlattices exhibited positive dε/dΕ, where ε is permittivity and Ε is the magnitude of an applied electric field. This stands in contrast to the negative values of dε/dΕ typically observed in perovskites.
- Superlattices can be made to exhibit weak temperature dependences of tunability. For example, one SrTiO3/BaZrO3 superlattice was found to exhibit a tunability of 33 percent at both room temperature and at 77 K. In contrast, a typical ferroelectric material is tunable in only a narrow temperature range near a phase transition. Superlattices with weak temperature dependence of tunability could be attractive materials for situations in which precise control of temperature would be either impossible or too expensive.
- In tests on coplanar-waveguide microwave filters containing perovskite superlattices of a given total thickness, dielectric losses were found to be smaller than those in filters containing single SrTiO3 films of the same total thickness.
This work was done by H. M. Christen and K. S. Harshavardhan of Neocera, Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.
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