DOMAIN FORMATION AND STRAIN RELAXATION IN EPITAXIAL FERROELECTRIC HETEROSTRUCTURES

The growth of PbTiO3 films by a metalorganic chemical-vapor-deposition technique has resulted in three-dimensionally epitaxial heterostructu res on various single-crystal substrates. These heterostructures consi st of PbTiO3 films on the (001) surface of the single crystals: potass ium tantalate (KTaO3), strontium titanate (SrTiO3), and magnesium oxid e (MgO). It was found that the presence of a structural (ferroelectric ) phase transition in PbTiO3 leads to a ''strain-accommodating'' mecha nism in which a domain pattern forms as the system cools through the C urie temperature and limits the extension of interfacial strain in the heterostructure-thus minimizing the total energy of the heterostructu re. For PbTiO3/KTaO3(001), the interfacial strain is accommodated by t he formation of a periodic domain pattern in the overlayer. In PbTiO3/ SrTiO3(001), which exhibits an excellent lattice match between respect ive a lattice parameters, the film exists as a single c domain. The Pb TiO3/MgO(001) system, having a poor lattice match for both the a and c axes, appears to find the energy minimum by locking into domains of t wo-dimensional superlattices with the greatest atomic coincidences. It is found that the nature of the domain pattern depends very strongly on both the film thickness and measuring temperature. A theoretical mo del of the domain-pattern formation has been developed by using linear -elasticity theory and a Landau-Ginzburg-Devonshire-type phenomenologi cal theory for the substrate and the overlayer, respectively. The theo retical predictions and the experimental measurements were in good agr eement in both the thickness and temperature dependence of the relativ e domain population and the spontaneous strains.