Pulsed laser deposition of relaxor ferroelectric lead magnesium niobate-lead titanate thin films and heterostructures.

摘要

Since their discovery several decades ago, relaxor ferroelectrics (RFE) (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-PT), engineered near the morphotropic phase boundary (x = 0.3), have raised tremendous interest in the ferroelectric community, due to their broad ferroelectric transition, enormous dielectric constant, high piezoelectric coefficients and exceptional electromechanical coupling factors. However, these attractive qualities were demonstrated mainly by PMN-PT single crystals and ceramics and scarcely at all by PMN-PT thin films. This thesis addresses critical issues of the pulsed laser deposition growth of high quality epitaxial 0.70PMN-0.30PT thin films and heterostructures, for potential ferroelectric and piezoelectric thin film applications.; A review of relaxor ferroelectrics in terms of general characteristics, crystallographic structure, models and synthesis with a special emphasis on 0.70PMN-0.30PT solid solutions is presented, including current issues of the Pulsed Laser Deposition (PLD) method, highlighting particular problems of lead-compound synthesis.; The development of two pulsed laser deposition systems, two six target carousels, a target motion amplifier, an inverse position arm sputtering system, one He-Ne laser alignment system and one shadow mask set are described. These systems have been used for the growth of epitaxial RFE PMN-PT thin films and heterostructures.; The main issues regarding the growth of pure perovskite phase single crystal PMN-PT/(001)SrTiO₃ thin films and PMN-PT/SrRuO₃/(001)SrTiO ₃ heterostructures by PLD, the optimized deposition parameter space for in-situ growth, and the structural and chemical characterization of high quality PMN-PT thin films and heterostructures are addressed. Epitaxial PMN-PT thin films and heterostructures of unprecedented quality, resembling that of bulk single crystals are reported, and the thickness dependences of the strain state and crystalline quality are established. A film-growth, self-regulatory mechanism was found to adjust the non-stoichiometric atomic flux coming from the off-stoichiometric PMN-PT targets to form the perovskite phase when lower deposition rates were used. Laser instabilities and their impact upon the films reproducibility are discussed.; Dielectric and piezoelectric measurements performed on PMN-PT thin films, together with the factors that influence these measurements, are presented. The thickness dependences of dielectric and ferroelectric parameters is reported. Annealing treatments in high oxygen pressure are shown to improve film dielectric and ferroelectric properties. Thicker films demonstrate the highest room temperature dielectric constants (ϵ = 12000) and ferroelectric properties (remnant polarization Pr = 20 μC/cm2, coercive field E_C = 10 KV/cm, saturation polarization P_S = 38 μC/cm2, saturation field E_S = 30 KV/cm) ever reported for a PMN-PT film, regardless of the deposition method involved. The piezoelectric charge coefficient at high fields is found to be d₃₃ = 30 pm/V.