Dielectric tunability of coherently strained LaAlO 3/SrTiO 3 superlattices

Abstract

Dielectric LaAlO 3 and SrTiO 3 thin films and LaAlO 3/SrTiO 3 multilayers were grown epitaxially by pulsed laser deposition on (001) oriented (LaAlO 3) 0.3(Sr 2AlTaO 6) 0.7 substrates. Their structural characterization was carried out by x-ray diffraction and cross section transmission electron microscopy, which allowed us to determine the degree of strain in the dielectric material. For a film thickness of 200 nm we observed significant structural relaxation of the LaAlO 3 and SrTiO 3 single layers toward their single crystal lattice parameters in contrast to LaAlO 3/SrTiO 3 multilayer structures, where the dielectric material remained coherently strained. The influence of strain on the dielectric properties was studied by impedance spectroscopy in the frequency range of 40 Hz-10 MHz at room temperature. The measurements were performed on parallel plate capacitors, using epitaxial La 0.4Sr 0.6CoO 3 films as bottom and top electrodes. The dielectric constant ε of partially relaxed and coherently strained material was nearly the same. However, the dielectric tunability, i.e., the influence of a direct current bias voltage on ε, was found to be significantly larger for coherently strained dielectrics. For [LaAlO 3(30Å)/SrTiO 3(60Å)] 20 multilayers we observed a tunability of nearly 20% at room temperature for a bias voltage of only 1 V, corresponding to an electric field strength of 50 kV/cm. The total dielectric loss of the multilayer capacitors is below 1% for frequencies above 1 MHz and depends only slightly on the bias voltage. © 2002 American Institute of Physics.