Effect of periodicity and composition in artificial BaTiO3/(Ba,Sr)TiO3 superlattices

We have prepared two series of BaTiO3/SrTiO3 (BT/ST) superlattices (SLs). In the first set, the modulation period (Lambda) was varied from 136 to 16 angstrom, whereas in the second set ST was replaced with Ba(1-x)SrxTiO3 (BST) (0 < x < 1) with constant periodicity of similar to 136 angstrom. A total stack height of 10 000 angstrom (1 mu m) was maintained for both systems. The compositional changes give large spectral changes at x = 40% where the lowest E mode in BT/BST SL becomes underdamped. X-ray-diffraction patterns and surface topography of the modulated period revealed a Stranski-Krastanov growth mechanism. The upward frequency shift of the E(1TO) in SLs with decrease in Lambda or Ba concentration in the BST layer was attributed to the in-plane compressive stress induced by the lattice mismatch of the constituent layers. [N.B., here the notation E(1TO) or A(1)(2TO) simply designates the vibrational mode of E or A(1) symmetry, with 1TO or 2TO designating the transverse optical branch that is lowest, 1, or second lowest, 2, in energy.] In the low-frequency region, the folded acoustic phonon doublets were observed. Since the low-frequency out-of-plane dielectric constant is directly related to the E(1TO) soft mode frequency, strain engineering in artificial SLs allowed us to modify their dielectric properties by varying either the Ba/Sr in BST sublayer concentration or the modulation periodicity in the superlattices.