Structural Transitions and Ferroelectric Properties of Chemical Solution Deposition Derived Rare Earth Doped Bismuth Ferrite Films

Abstract

Over the past two decades, Bismuth ferrite (BiFeO3, BFO) thin films have attracted significant attention on account of their attractive multifunctional properties, ranging from room-temperature multiferroicity to robust high Curie temperature (Tc), large switchable spontaneous polarization (Ps) and enhanced electromechanical response, and ability to generate photovoltaic current etc. In particular, the past decade has seen huge efforts devoted to engineering the phase structure of BFO films to achieve a morphotropic phase boundary -(MPB)-like materials system with excellent ferroelectric properties and high electromechanical response. In this thesis, strain engineering and site engineering are applied to tailor the phase composition of epitaxial BFO films through a chemical solution deposition (CSD) method. The compressive epitaxial strain provided by the lattice misfit between the film and the lanthanum aluminate (LaAlO3, LAO) substrate can stabilize a tetragonal-like (T’) phase. The chemical pressure produced by the A-site substitution of smaller Sm element can induce a ferroelectric rhombohedral (R) to paraelectric orthorhombic (O) phase transition. The coexistence of these diverse polymorphs of BFO is expected to generate an MPB effect, where a maximum electromechanical property usually reports. The thesis first employs the misfit strain engineering (BFO films grown on LAO substrates) to tune the phase fraction in mixed-phase (rhombohedral-like (R’)-T’ coexistence) BFO films by altering the synthesis parameters via the CSD method. It shows that the T’-phase fraction ranging from 10% to 35% is achieved by decreasing the spin-coated layers from four to one layer. In a two-layer configuration, the T’-phase fraction is further found that can be varied from 8% to 38% by changing the precursor concentration and heating treatment parameters. The mixed-phase BFO films show a typical polydomain structure with a polarization-orientation dependent conduction behavior whereby poled-up (polarization pointing away from the lower electrode) domains have higher conductivity. An optimized film with a T’-phase fraction of ~ 28% shows the lowest leakage current of <0.1 A/cm2 up to field strengths of 500 kV/cm. Upon increasing electric field, the mixed-phase film shows an interface-limited Schottky emission to bulk-limited space-charge-limited-conduction (SCLC) mechanism predominate leakage current transition. The thesis then applies site engineering (A-site substitution with Sm) to tailor the phase structure of BFO films deposited on strontium titanate (SrTiO3, STO, the lattice parameter is similar to that of BFO) substrates. The role of the Sm on the precursor gelation chemistry is first studied. It is found that the electronegativity of the cation species in metal nitrates affects the reaction rate of the hydrolysis reaction and esterification reaction. The structural investigation of the crystalline films shows that the phase transition occurs at x = 0.10 with paraelectric O phase and antipolar phase appearance. The domain contrast of as-grown BFO/BSFO (BSFO: Bi1-xSmxFeO3) films reduces gradually with the increase of the Sm composition. More importantly, Sm introduction greatly improves the ferroelectric properties of BFO films. At an Sm ratio of 0.14, a fully developed polarization hysteresis loop is achieved. When the Sm ratio is increased to 0.15, an electric-field-induced distorted double hysteresis loop is observed. Then strain engineering and site engineering are combined to construct a complex phase configuration in BSFO films. A structural evolution from T’-R’ to T’-R’-O, and then to T’-O phases is demonstrated with the increase of the Sm ratio. The synergetic effects of misfit strain and chemical pressure drive the phase transition composition to a higher value of 0.14 compared to that of strain-free BSFO/LSMO//STO (LSMO: La0.67Sr0.33MnO3) films at 0.10. Likewise, Sm doping in the A-site leads to the decrease of the piezoresponse force microscopy (PFM) amplitude. While an enhanced domain switching behavior is attained at MPB. The ferroelectric properties show a transition from a single ferroelectric square-shaped to a slightly distorted double hysteresis loop with the Sm3+ doping content increasing from 0.14 to 0.16. The investigation of the resistive switching behavior shows an interesting transition of the current flow under the external bias from “high resistance state (HRS)-> low resistance state (LRS)->HRS->LRS” to “HRS->LRS->LRS->HRS”. This thesis provides a comprehensive understanding of the effects of epitaxial strain or/and chemical pressure on the phase composition of BFO films and their multifunctional properties. The appealing physical properties induced by the structure evolution promote the seeking of novel phase structure of perovskite oxides in thin-film form.