Unusual Behavior of the Ferroelectric Polarization inPbTiO3/SrTiO3Superlattices

Abstract

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis RF magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cells SrTiO3 thickness used in the structure, the polarization is found to be reduced as the PbTiO3 thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.The construction of artificial ferroelectric oxide superlattices with fine periodicity presents exciting possibilities for the development of new materials with extraordinary properties and furthermore is an ideal probe for understanding the fundamental physics of ferroelectric materials.The most studied system at present is BaTiO 3 /SrTiO 3 [1,2,3,4,5,6,7,8] In BaTiO 3 /SrTiO 3 , first principles studies [5] suggest that both the SrTiO 3 and BaTiO 3 layers are polarized such that the polarization is approximately uniform throughout the superlattice. The driving force behind this is the large electrostatic energy penalty for a buildup of charge at the interface caused by discontinuous polarization in the normal direction. The electrostatic model proposed by Neaton and Rabe [5] to explain their first principles results for BaTiO 3 /SrTiO 3 superlattices is very similar to the electrostatic model applied to calculate the effect of the depolarization field in ultra-thin ferroelectric films with realistic electrodes [18,19,20]. Experimentally it was recently shown that the reduced polarization observed in monodomain thin PbTiO 3 can be explained by the presence of a depolarization field resulting from imperfect screening of the polarization [21]. Recent work also suggests that, under certain conditions, the electrostatic energy due to depolarization fields will drive the system to form domains as observed by Fong et al. [23] and Nagarajan et al. [24]. In this letter we use PbTiO 3 /SrTiO 3 superlattices to probe the effect of a reduced ferroelectric thickness in a dielectric environment. Our data show that the behaviour observed in PbTiO 3 thin films is reproduced for PbTiO 3 layers thicker than three unit cells. However, for thinner ferroelectric layers a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.The superlattices of PbTiO 3 /SrTiO 3 were prepared on conducting 0.5% Nb doped (001) SrTiO 3 substrates using off-axis RF magnetron sputtering with conditions similar to those used for growing high quality epitaxial c-axis PbTiO 3 thin films [21]. For all the samples discussed in this paper, the SrTiO 3 thickness was fixed at three unit cells (about 12Å). At room temperature the in-plane lattice parameters of tetragonal ferroelec...

Piezoelectricity in the Dielectric Component of Nanoscale Dielectric-Ferroelectric Superlattices

“…The average polarization can exceed the bulk polarization of the ferroelectric component [1], providing a new route for the enhancement of functional properties including piezoelectricity. This average polarization and structural evidence for a static polarization in the non-ferroelectric layers have been observed experimentally [5,6]. The desirable functional properties of unit-cell-scale superlattices, however, are defined by their responses to applied fields including mechanical stress and electric fields, and have yet to be fully exploited.…”

Polarization switching using single-walled carbon nanotubes grown on epitaxial ferroelectric thin films

“…The ferroelectrics were all tetragonal ͑001͒-oriented, epitaxial thin films grown by rf-magnetron sputtering on conducting Nb: SrTiO 3 substrates, allowing the polarization to be switched along the c-axis perpendicular to the plane of the film. Different materials, including PbTiO 3 − SrTiO 3 superlattices, 8 PbZr 0.2 Ti 0.8 O 3 , PbTiO 3 , and BaTiO 3 were investigated to test their response to the high temperatures ͑700-1040°C͒ and reducing atmosphere required for CNT growth. All Pb-containing oxides showed increased surface roughness and significant deterioration of ferroelectric properties.…”

Monodomain to polydomain transition in ferroelectric PbTiO3 thin films with La0.67Sr0.33MnO3 electrodes

“…8 The direct relation between tetragonality and polarization was also recently experimentally demonstrated in PbTiO 3 / SrTiO 3 superlattices. 12,13 In this letter, epitaxial c-axis PbTiO 3 thin films with thicknesses ranging from 480 Å down to 28 Å were grown on epitaxial La 0.67 Sr 0.33 MnO 3 electrodes ͑typically 200-300 Å thick͒ deposited onto ͑001͒ insulating SrTiO 3 substrates. It is found that the behavior of the tetragonality is dramatically different from what is observed for PbTiO 3 thin films prepared on metallic Nb-doped SrTiO 3 substrates, with an increase of c / a observed for the thinnest film studied.…”