Controlling functional properties of ferroelectric thin film by interface engineering

Abstract

Ferroelectric (FE) materials are defined as the materials that possess a spontaneous polarization, which can be switched by an external electric field. Due to the switchable spontaneous polarization, FE materials have attracted significant interests for potential applications in multifunctional electronic devices. The examples include nonvolatile FE random access memories, FE field-effect transistors, and FE tunnel junctions. In addition, the structural distortion and generation of electric field are accompanied by FE polarization switching. This enable the electric control of various physical properties, such as magnetism and superconductivity, which can be used for next-generation multi-functional devices. For such reasons, flurry of research on FEs has been performed in condensed matter physics as well as in material sciences. With recent trends on miniaturizing oxide-based devices, understandings of interface effects on FE properties become important. When the thickness of FE layer is reduced down to few unit cells (u.c.), the interfacial effects, which are negligible in thick films or in bulk form, significantly affect the various functional properties of FE heterostructures. For examples, the type and strength of chemical bonding at metal/FE interface was reported to be important in determining amount and stability of FE polarization of BaTiO3 and PbTiO3 thin films. The large electrostatic energy caused by polar discontinuity at insulator/FE interface causes unusual vortex- or flux-type FE domain configuration. In addition, interface structure, such as topography and atomic stacking sequence, also can affect the FE polarization switching properties of ultrathin FE layer. In the thesis, the novel experimental results on the effects of interfacial structures on functional properties in epitaxial FE thin films are presented. The polarization-electric field hysteresis (P – E hysteresis) loops is one of the remarkable features of FE capacitor devices. In epitaxial film, the P – E hysteresis loops shows the considerable dependence on frequency (f) of external ac field, which is caused by the microscopic FE domain wall motions. Under the external electric field, FE domain wall motion can be regarded as the growth of elastic objects in disordered medium, which exhibit strong nonlinear velocity (v) – E behavior. We were able to show that, by choosing proper electrode, we can significantly reduce the amount of disorder states at the ferroelectric BiFeO3 (BFO) interface. As a result, we can realize f-independent P – E hysteresis loops in high quality epitaxial BFO thin film. We also report the effect of background oxygen pressure (PO2) on interface atomic structure of SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructure made by pulsed laser deposition (PLD). For realizing ultrathin FE device with few nanometer thick thicknesses, atomic-scale control of interface/surface structure and realization of flat surface with uniform termination are highly required. However, a lack of understandings on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, we report that selective control of SrO-TiO2 or BaO-RuO2 termination sequence at top SRO/BTO interface via changing PO2. We found that, under high PO2 (150 mTorr), heterogeneous interface termination with both SrO-TiO2 and BaO-RuO2 was formed. On the other hand, a uniform SrO-TiO2 termination sequence at the SRO/BTO interface can be achieved by lowering the PO2 to 5 mTorr, regardless of the total background gas pressure, growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. The SRO/BTO/SRO capacitor with heterogeneous interface termination sequences and uniform SrO-TiO2 termination sequence shows the distinct FE properties. By using piezoelectric force microscopy (PFM), we investigated local FE polarization properties. In ultrathin limit of BTO layer, SRO/BTO/SRO heterostructure with heterogeneous interface termination sequence exhibits the significant local variation of FE polarization switching. In particular, more than half of measured area exhibits the pinned FE polarization state, caused by existence of BaO-RuO2 interface termination sequence. On the other hand, SRO/BTO/SRO capacitor with symmetric and uniform interfacial termination, whole measured region exhibit the clear FE polarization switching even with the 3.5 unit cells (~1.2 nm) of BTO thickness. We believe that our intensive researches in this thesis make important steps forward comprehensive understanding on deterministic role of interface structure in determining FE properties of ultrathin film.