Thickness effect in Pb(Zr0.2Ti0.8)O3 ferroelectric thin films grown by pulsed laser deposition

Epitaxial Pb(Zr_,Ti)O₃ (PZT) thin films with thicknesses in the range of 50-200 nm and with 0.2 Zr/(Zr + Ti) ratio, were grown by pulsed laser deposition (PLD).The substrates used for PLD deposition are single crystalline 0.5% Nb-doped (1 0 0) SrTiO₃ (STON). SrRuO₃ (SRO) thin films were deposited as bottom and top electrodes in order to have minimum structural misfit, to insure on one side high quality growth, and on the other side to minimize the influence of the extended structural defects. Structural and electrical characterization was performed. The epitaxial PZT films are c-axis oriented and have an average roughness of 0.4 nm. The ferroelectric behavior was proved in all investigated films by the presence of the hysteresis loops and by the butterfly shape of the capacitance-voltage (C-V) characteristics. The ferroelectricity was present even in the samples with relative high leakage currents, down to a thickness of 50 nm. These results are essential when small thickness is needed for miniaturization of ferroelectric devices using PZT.     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Effect of bilayer structure and a SrRuO3 buffer layer on ferroelectric properties of BiFeO3 thin films

Effects of the BiFe_0.95Mn_0.05O₃ thickness and a SrRuO₃ (SRO) buffer layer on the microstructure and electrical properties of BiFeO₃/BiFe_0.95Mn_0.05O₃ (BFO/BFMO) bilayered thin films were investigated, where BFO/BFMO bilayered thin films were fabricated on the SRO/Pt/Ti/SiO₂/Si(100) substrate by a radio frequency sputtering. All thin films are of a pure perovskite structure with a mixture of (110) and (111) orientations regardless of the BFMO layer thickness. Dense microstructure is demonstrated in all thin films because of the introduction of BFMO layers. The SRO buffer layer can also further improve the ferroelectric properties of BFO/BFMO bilayered thin films as compared with those of these thin films without a SRO buffer layer. The BFO/BFMO bilayered thin film with a thickness ratio of 220/120 has an enhanced ferroelectric behavior of 2P _r??165.23???C/cm² and 2E _c??518.56?kV/cm, together with a good fatigue endurance. Therefore, it is an effective way to enhance the ferroelectric and fatigue properties of bismuth ferrite thin films by constructing such a bilayered structure and using a SRO buffer layer.     

Ferroelectric domain structure of the BiFeO3 film grown on different substrates

Domain structure of BiFeO₃ (BFO) films grown on different substrates, with a conductive La_0.7Sr_0.3MnO₃ underlayer, has been experimentally studied. Two oppositely orientated polarizations, along the long body diagonal to the perovskite unit cell of BFO, are detected in the BFO films on the (0 0 1)-oriented NdGaO₃. Electric pulses applied in the [0 0 1] direction produce a polarization switching, resulting in the domain structure characterized by the 109° domain walls. Contrary to the BFO films on NdGaO₃, the BFO films on SrTiO₃ (0 0 1) exhibit a much complex domain structure. Both 71° and 109° domain walls are possible with a uniform polarization component pointing to the bottom electrode.     

Improvement of fatigue resistance on La modified BiFeO3 thin films

The effect of lanthanum (La) addition in BiFeO₃ (BFO) thin films deposited on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates prepared by soft chemical method was explained. Increasing La concentration promotes changes on structure, microstructure and dielectric/ferroelectric response of films. X-ray diffraction reveals that the films are free of preferred orientations and structural distortion. La addition promotes an increase in dielectric permittivity. The polarization switching and the fatigue behavior of the BFO films were significantly enhanced by the La concentration.     

Room temperature ferroelectricity of tetragonally strained SrTiO3 thin films on single crystal Rh substrates

Epitaxial SrTiO₃ thin films were deposited on single crystalline Rh substrates by pulsed laser deposition. The tetragonally stained structure of the SrTiO₃ thin films with a c/a ratio of 1.04 was confirmed by x-ray diffraction experiments. The SrTiO₃ thin films exhibited good ferroelectric properties with a high remanent polarization (2P_r) of 8 μC/cm² and a canonical ferroelectric piezoresponse hysteresis loop at room temperature. We estimated a high activation electric field of about 6.4 MV/cm for domain wall creeping. This activation electric field is higher than that of typical ferroelectric materials such as PbTiO₃.     

Probing of local ferroelectricity in BiFeO3 thin films and (BiFeO3)m(SrTiO3)m superlattices

Ferroelectric BiFeO₃ thin films and artificial superlattices of (BiFeO₃)_m(SrTiO₃)_m (m∼1-10 unit cells) were fabricated on (0 0 1)-oriented SrTiO₃ substrates by pulsed laser ablation. The variation of leakage current and macroscopic polarization with periodicity was studied. Piezo force microscopy studies revealed the presence of large ferroelectric domains in the case of BiFeO₃ thin films while a size reduction in ferroelectric domains was observed in the case of superlattice structures. The results show that the modification of ferroelectric domains through superlattice could provide an additional control on engineering the domain wall mediated functional properties.     

Piezoresponse force microscopy and magnetic force microscopy characterization of γ-Fe2O3-BiFeO3 nanocomposite/Bi3.25La0.75Ti3O12 multiferroic bilayers

The multiferroic behavior of epitaxial γ-Fe₂O₃-BiFeO₃ (composite)/Bi_3.25La_0.75Ti₃O₁₂ bi-layered heterostructures grown on SrRuO₃/SrTiO₃ (1 1 1) substrates has been studied using piezoresponse force microscopy, magnetic force microscopy and magnetometry. The ferroelectric domain structure is ascribed to the BiFeO₃ phase while the magnetism originates in the γ-Fe₂O₃ phase of the composite layer. Our studies demonstrate the presence and switching of magnetic and ferroelectric domains within the same area of the sample. This confirms the presence of multiferroic behavior at the nanoscale in our γ-Fe₂O₃-BiFeO₃ nanocomposite thin films.     

Effect of pore size on ferroelectric properties of multiferroic BiFeO3 films prepared on porous silicon

The ferroelectric properties of BiFeO₃ (BFO) films spray deposited on porous silicon have been studied. The analysis of XRD and FESEM investigations show that the crystalline strain in the BFO films increases with pore size. The BFO films on porous silicon substrate showed improvement in ferroelectric fatigue behavior, remanent polarization and ferroelectric switching time. A maximum memory window of 5.54 V at 1 MHz and a large remanent polarization (P_r) of 13.1 μC/cm² have been obtained at room temperature. The improvement in the ferroelectric properties of these films has been correlated to the crystalline strain.     

A comparative study of fibre-textured and epitaxial Nb-doped Pb(Zr0.53Ti0.47)O3 thin films on different substrates

Fibre-textured and epitaxial Nb-doped Pb(Zr_0.53Ti_0.47)O₃ (PNZT) thin films were grown on the different substrates by a sol-gel process. The [1 0 0]- and [1 1 1]-fibre-textured polycrystalline PNZT films were obtained on platinized silicon substrates by introducing PbO and TiO₂ seeding layers, while the [0 0 1]- and [1 1 1]-oriented epitaxial PNZT films were formed directly on Nb-doped SrTiO₃ (Nb:STO) single-crystal substrates with (1 0 0) and (1 1 1) surfaces, respectively. The preferential orientation and phase structure of the fibre-textured and epitaxial PNZT films, as well as their influences on the electrical properties were investigated. Higher remnant polarization (P_r) and piezoelectric coefficient (d₃₃) were obtained for the epitaxial PNZT films on Nb:STO substrates than that for the fibre-textured ones on platinized silicon substrates. For both fibre-textured and epitaxial cases, the PNZT films with [1 0 0]/[0 0 1] orientations show higher piezoelectric responses than [1 1 1]-oriented ones, whereas better ferroelectric properties can be obtained in the latter. The intrinsic and extrinsic contributions were discussed to explain the difference in electrical properties for differently oriented fibre-textured and epitaxial PNZT films on different substrates.     

Microstructure of epitaxial SrRuO3 thin films on MgO substrates

SrRuO₃ thin films have been grown on singular (1 0 0) MgO substrates using pulsed laser deposition (PLD) in 30 Pa oxygen ambient and at a temperature of 400-700 °C. Ex situ reflection high-energy electron diffraction (RHEED) as well as X-ray diffraction (XRD) θ/2θ scan indicated that the films deposited above 650 °C were well crystallized though they had a rough surface as shown by atom force microscopy (AFM). XRD Φ scans revealed that these films were composed of all three different types of orientation domains, which was further confirmed by the RHEED patterns. The heteroepitaxial relationship between SrRuO₃ and MgO was found to be [1 1 0] SRO//[1 0 0] MgO and 45°-rotated cube-on-cube [0 0 1] SRO//[1 0 0] MgO. These domain structures and surface morphology are similar to that of ever-reported SrRuO₃ thin films deposited on the (0 0 1) LaAlO₃ substrates, and different from those deposited on (0 0 1) SrTiO₃ substrates that have an atomically flat surface and are composed of only the [1 1 0]-type domains. The reason for this difference was ascribed to the effect of lattice mismatch across the film/substrate interface. The room temperature resistivity of SrRuO₃ films fabricated at 700 °C was 300 μΩ cm. Therefore, epitaxial SrRuO₃ films on MgO substrate could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.     

Interrelatedness of Fe composition on structural and magnetic properties in Fe-doped SrRuO3 thin films

Fe-doping (up to 11 mole%) into SrRuO₃ (SRO) thin films on SrTiO₃ substrates decreased correlation lengths of both surface and interface. It turned out that Fe was doped in the valence state of 3+ without formation of the Fe₂O₃ phase, which caused orthorhombic distortion. T _C values decreased from 145 K to 97 K with increasing Fe concentration (C _Fe). High magnetic switching fields were observed for all Fe-doped SRO thin films and their strengths showed a linear relationship with C _Fe. Detail structural characterization using synchrotron X-ray diffraction and X-ray photoemission spectroscopy were used to understand its unique magnetic switching field properties.     

Film thickness dependence of electro-optic effect in epitaxial BaTiO3 thin films

Based on the phenomenological Landau-Devonshire theory, we investigate the film thickness dependence of ferroelectric and electro-optic properties of epitaxial BaTiO₃ thin films grown on SrTiO₃ and MgO substrates. By using the effective substrate lattice parameter concept, the film thickness dependence of misfit strain is incorporated into the theory. Therefore, the film thickness dependence of ferroelectric and electro-optic properties in epitaxial BaTiO₃ thin films can be explained. Moreover, a large quadratic electro-optic effect was obtained in the BaTiO₃ thin films, which is in good agreement with the experimental result of BaTiO₃ thin films on the MgO substrate.     

Thickness-dependent of conduction mechanism and bias electric field modulation of transport properties for SrRuO3/PMN-PT heterostructures

Ultra-thin SrRuO₃ (SRO) films have been grown on ferroelectric and piezoelectric PMN-PT substrates. The structural properties of these films have been characterized by atomic force microscopy, x-ray diffraction and cross-sectional transmission electron microscopy. The nature of electric transport was analyzed in detail and the conduction mechanism of SRO films evolves through three regimes: from a three-dimensional (3D) metallic through a weakly localized to a strongly localized behavior as film thickness is reduced. The bias electric field modulations of transport properties and magnetic properties were explored for these films. We also demonstrate that ferroelectric (FE) domain switching induces a reversible tuning of the magnetic and electric properties in SRO/PMN-PT heterostructure. The FE domain switching in the substrate contributes to an in-plane strain that changes the spin exchange coupling in the SRO layer, and therefore results in a reversible resistance difference of up to 16%. This modulation effect on the electric properties by an electric field demonstrates great potential for the applications of all-oxides spintronics devices.     

Off-stoichiometry effects on BiFeO3 thin films

The effects of Bi and Fe-excess on the structure, ferroelectric, leakage current and magnetic properties of BiFeO₃ (BFO) thin films are reported. BFO with 5% excess exhibits no change in the structure with an improvement in leakage current properties in comparison to stoichiometric BFO. Raman spectroscopy of 10% Bi excess suggests a structural change from monoclinic to rhombohedral accompanied with an improvement of resistivity and ferroelectric polarization switching. A higher Fe-excess leads to the formation of pyrochlore Bi₂Fe₄O₉ and gamma-Fe₂O₃ that cause an increase in conductivity at the macroscopic scale. The results are discussed in terms of Fe and Bi-excess effects on the defect structure of BFO.     

Fabrication of atomically smooth SrRuO3 thin films by laser molecular beam epitaxy

High-quality SrRuO₃ (SRO) thin films and SrTiO₃/SRO bilayer were grown epitaxially on SrTiO₃ (STO)(001) substrates by laser molecular beam epitaxy. The results of in situ observation of reflection high-energy electron diffraction and ex situ X-ray diffraction ϑ-2ϑ scan indicate that the SRO thin films have good crystallinity. The measurements of atomic force microscopy and scan tunneling microscopy reveal that the surface of the SRO thin film is atomically smooth. The resistivity of the SRO thin film is 300 μΘ·cm at room temperature. Furthermore, the transmission electron microscopy study shows that the interfaces of STO/SRO and SRO/STO are very clear and no interfacial reaction layer was observed. The experimental results show that the SRO thin film is an excellent electrode material for devices based on perovskite oxide materials. Supported by the National Natural Science Foundation of China (Grant No. 10334070)     

Oxygen deficiency effect on resistive switching characteristics of copper oxide thin films

In this Letter, bilayered Cu₂O/CuO thin films were grown on Nb doped SrTiO₃ (Nb:STO) substrates by plasma assisted molecular beam epitaxy. The current-voltage characteristics of Pt/Cu₂O/CuO/Nb:STO devices show reproducible and pronounced current-voltage hysteresis which was induced by the CuO/Nb:STO junctions. By comparing the current-voltage curves of the bilayered and single-layered CuO thin films, we attribute the prominent switching behavior to the oxygen-vacancies-mediated-carriers-trapped-detrapped process with the aid of the applied forward (reversed) bias voltage.     

Thickness dependent magnetic and ferroelectric properties of LaNiO3 buffered BiFeO3 thin films

BiFeO₃ (BFO) thin films with thickness increasing from 40 to 480 nm were successfully grown on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si(100) substrate and the effects of thickness evolution on magnetic and ferroelectric properties are investigated. The LNO buffer layer promotes the growth and crystallization of BFO thin films. Highly (100) orientation is induced for all BFO films regardless of the film thickness together with the dense microstructure. All BFO films exhibited weak ferromagnetic response at room temperature and saturation magnetization is found to decrease with increase in film thickness. Well saturated ferroelectric hysteresis loops were obtained for thicker films; however, the leakage current dominated the ferroelectric properties in thinner films. The leakage current density decreased by three orders of magnitude for 335 nm film compared to 40 nm film, giving rise to enhanced ferroelectric properties for thicker films. The mechanisms for the evolution of ferromagnetic and ferroelectric characteristics are discussed.     

Tunable magnetic anisotropy of CoFe2O4 nanopillar arrays released from BiFeO3 matrix

We deposited epitaxial BiFeO₃–CoFe₂O₄ (BFO–CFO) self‐assembled thin films on (001) SrTiO₃ (STO) substrates. We find that a combined annealing and etching process could remove the BFO matrix, thereby resulting in free‐standing CFO nanopillar arrays. Scanning electron and atomic force microscopies showed well separated CFO nanopillars, which were very similar to the original CFO ones in the self‐assembled structure. Finally, comparison of the magnetic hysteresis loops before and after removal of the BFO matrix showed a significant decrease of the coercive field and a dramatic decrease in the strain dominated magnetic anisotropy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on ferroelectric polarization induced resistive switching characteristics of neodymium-doped bismuth ferrite thin films for random access memory applications

The present work reports on resistive switching (RS) characteristics of Neodymium (Nd)-doped bismuth ferrite (BFO) layers. The Nd (2–10 at%) doped BFO thin film layers were deposited using a spray pyrolysis method. The structural analysis reveals that a higher Nd doping concentration in BFO leads to significant distortion of the prepared Nd:BFO thin films from rhombohedral to tetragonal characteristics. The morphological analysis shows that all the deposited Nd:BFO thin films have regularly arranged grains. The X-ray photoelectron spectroscopy (XPS) analysis reveals that the prepared Nd:BFO thin films have a higher Fe ³⁺/Fe ²⁺ratio and less oxygen vacancy (V_O) defects which enriches the ferroelectric characteristics in Nd:BFO layers. The polarization-electric field (P-E) and RS characteristics of the fabricated Nd:BFO-based RS device were examined. It was observed that the Nd (7 at%) doped BFO RS device shows large remnant polarization (P _r) of 0.21 μC/cm² and stable RS characteristics.