Ferroelectric domain structure of epitaxial (Pb,Sr)TiO 3 thin films
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Epitaxial (Pb1−xSrx)TiO3 (PST, x ⳱ 0.0–0.24) thin films were grown on MgO(001) single-crystal substrates by pulsed laser deposition. General x-ray diffraction techniques including –2 scan and rocking curve were used to determine lattice constants, degree of c-axis orientation, and crystal quality of the tetragonal thin films. The degree of c-axis orientation in the epitaxial PST films increased as Sr concentration (x) increased, which in turn induces the systematic change in the Curie temperature as well as the transformation strain at and below the Curie temperature. An inverse relation between the c-domain abundances and the transformation strains is established.
Studies on the domain structure and its formation in the epitaxial ferroelectric thin films have been investigated by many researchers.1–7 Kwak et al.4,5 studied epitaxial PbTiO3 thin films deposited on various cubic single-crystal substrates, such as KTaO 3 (001), SrTiO3(001), and MgO(001), and demonstrated that transformation strains and thermal strain are clearly related to the final domain structure. Kang et al.6 studied systematically the effects of transformation strains on the domain structure of epitaxial (Pb,La)TiO3 (PLT) thin films on various single-crystal substrates by changing La concentration in the films. The La substitution reduces the c/a ratio and lowers the Curie temperature (TC). They found enhancement in the degree of c-axis orientation (␣) of the epitaxial PLT films on MgO(001) with increasing La concentration. Fully c-axis-oriented films (␣ ⳱ 1) could be realized when La concentration exceeds 12 at.%. The results suggests that the transformation strain at and below TC is clearly one of the key factors that determine the final domain structures. In a recent paper, Lee et al.7 reported effects of the transformational strain on domain structures of epitaxial (Pb,La)(ZrTi)O 3 (PLZT) thin films on MgO(001). They changed the c/a ratios of PLZT by changing La and/or Zr concentrations. La substitution for Pb leads to steep contraction of c axis without significant change in a axis,8 while Zr substitution for Ti results in gradual expansion of the a axis with no significant change in c axis.9 They found an increase
a)
Current address: Material Design Lab., Electronic Material and Devices R.C., Korea Institute of Science and Technology, Seoul 136-791, Korea. b) Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 9, Sep 2001
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in the magnitude of ␣ in Pb(ZrxTi1−x)O3 (PZT) films on MgO(001) with increasing Zr substitution. However, it was not possible to achieve fully c-axis orientation even near 50 at.% of Zr concentration approaching the MPB composition. The change in lattice parameter due to Sr substitution for Pb in the (Pb,Sr)TiO3 system is analogous to the case of PLT system10 and accompanied by a continuous decrease in the Curie temperature. Such changes are expected to induce systematic changes in various strains involved during the film processing, thereby
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