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.     

Enhanced memory window of Au/BaTiO3/SrTiO3/Si(001) MFIS structure with high c-axis orientation for non-volatile memory applications

Metal?Cferroelectric?Cinsulator?Csemiconductor (MFIS) structures with BaTiO₃ (BTO) as a ferroelectric film and SrTiO₃ (STO) as an insulating buffer layer were fabricated on p-type Si(001) substrates using an ion beam sputter deposition technique. The effect of out-of-plane orientation on the electrical properties of the MFIS structures, including leakage current density and memory window behavior, were studied using the growth of the BTO ferroelectric film on Si substrate buffered by highly c-axis-oriented and random-oriented STO buffer layers. The experimental results show that the out-of-plane orientations of the BTO films were almost identical to those of the STO buffer layers. The MFIS structure with a high c-axis orientation exhibited a maximum clockwise capacitance-voltage memory window of 1.17?V with a low leakage current of 1.05×10^?7?A/cm² at an applied voltage of 4?V, which is a significant improvement compared to the MFIS structure with a random orientation. The difference in the electrical properties of the MFIS structures with both types of orientation is discussed in detail. The results obtained from this study indicate that the Au/BTO/STO/p-Si MFIS structure with high c-axis orientation has good potential for use in non-volatile memory applications.     

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.     

Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO₃ (BFO) thin films on SrRuO₃ (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2P_r = 59.6 μC/cm² and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO₃ substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO₃ substrates.     

Effects of Mn doping on BaTiO3 thin films grown on highly oriented pyrolytic graphite substrates

We report multiferroelectric properties of Mn-doped BaTiO₃ (MBTO) thin films on highly oriented pyrolytic graphite (HOPG) substrates. The MBTO thin films were grown on the HOPG substrate by pulse laser deposition. For comparison purpose, undoped BaTiO₃ (BTO) thin films were also prepared under same experimental conditions. The BTO and MBTO thin films were polycrystalline, indicating that the MBTO thin film has better crystallinity than the BTO thin film. The leakage current of the MBTO thin film was reduced due to the Mn doping substitution. In addition, the MBTO thin film exhibited better than the BTO thin film in ferroelectric and magnetic behaviors. We suggest that the Mn doping bring about the improvements of ferroelectric and ferromagnetic properties of the BTO thin films. Based on atomic force microscopy (AFM) and conducting AFM (CAFM) studies, the grain size of MBTO thin film was much larger than that of BTO thin film.     

Fabrication and optical properties of InGaN/GaN multiple quantum well light emitting diodes with amorphous BaTiO3 ferroelectric film

BaTiO3 （BTO） ferroelectric thin films are prepared by the sol,el method. The fabrication and the optical properties of an InGaN/GaN multiple quantum well light emitting diode （LED） with amorphous BTO ferroelectric thin film are studied. The photolumineseence （PL） of the BTO ferroelectric film is attributed to the structure. The ferroeleetric film which annealed at 673 K for 8 h has the better PL property. The peak width is about 30 nm from 580 nm to 610 nm, towards the yellow region. The mixed electroluminescence （EL） spectrum of InGaN/GaN multiple quantum well LED with 150-nm thick amorphous BTO ferroelectric thin film displays the blue-white light. The Commission Internationale De L＇Eclairage （CIE） coordinate of EL is （0.2139, 0.1627）. EL wavelength and intensity depends on the composition, microstructure and thickness of the ferroelectric thin film. The transmittance of amorphous BTO thin film is about 93% at a wavelength of 450 nm-470 nm. This means the amorphous ferroelectrie thin films can output more blue-ray and emission lights. In addition, the amorphous ferroelectric thin films can be directly fabricated without a binder and used at higher temperatures （200 ℃-400 ℃）. It is very favourable to simplify the preparation process and reduce the heat dissipation requirements of an LED. This provides a new way to study LEDs.     

Fabrication and optical properties of InGaN/GaN multiple quantum well light emitting diodes with amorphous BaTiO₃ ferroelectric film

BaTiO3(BTO) ferroelectric thin films are prepared by the sol-gel method.The fabrication and the optical properties of an InGaN/GaN multiple quantum well light emitting diode(LED) with amorphous BTO ferroelectric thin film are studied.The photoluminescence(PL) of the BTO ferroelectric film is attributed to the structure.The ferroelectric film which annealed at 673 K for 8 h has the better PL property.The peak width is about 30 nm from 580 nm to 610 nm,towards the yellow region.The mixed electroluminescence(EL) spectrum of InGaN/GaN multiple quantum well LED with 150-nm thick amorphous BTO ferroelectric thin film displays the blue-white light.The Commission Internationale De L’Eclairage(CIE) coordinate of EL is(0.2139,0.1627).EL wavelength and intensity depends on the composition,microstructure and thickness of the ferroelectric thin film.The transmittance of amorphous BTO thin film is about 93% at a wavelength of 450 nm-470 nm.This means the amorphous ferroelectric thin films can output more blue-ray and emission lights.In addition,the amorphous ferroelectric thin films can be directly fabricated without a binder and used at higher temperatures(200℃-400℃).It is very favourable to simplify the preparation process and reduce the heat dissipation requirements of an LED.This provides a new way to study LEDs.     

Improved Polarization Retention of BiFeO_3 Thin Films Using GdScO_3 (110) Substrates

Epitaxial ferroelectric thin films on single-crystal substrates generally show a preferred domain orientation in one direction over the other in demonstration of a poor polarization retention. This behavior will affect their application in nonvolatile ferroelectric random access memories where bipolar polarization states are used to store the logic 0 and 1 data. Here the retention characteristics of BiFeO_3 thin films with SrRuO_3 bottom electrodes on both GdScO_3(110) and SrTiO_3(100) substrates are studied and compared, and the results of piezoresponse force microscopy provide a long time retention property of the films on two substrates. It is found that bismuth ferrite thin films grown on GdScO_3 substrates show no preferred domain variants in comparison with the preferred downward polarization orientation toward bottom electrodes on SrTiO_3 substrates. The retention test from a positive-up domain to a negative-down domain using a signal generator and an oscilloscope coincidentally shows bistable polarization states on the GdScO_3 substrate over a measuring time of 500 s, unlike the preferred domain orientation on SrTiO_3, where more than 65% of upward domains disappear after 1 s. In addition, different sizes of domains have been written and read by using the scanning tip of piezoresponse force microscopy; where the polarization can stabilize over one month. This study paves one route to improve the polarization retention property through the optimization of the lattice-mismatched stresses between films and substrates.     

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.     

Photovoltaic effect in BiFeO3/BaTiO3 multilayer structure fabricated by chemical solution deposition technique

Photovoltaic (PV) properties of bismuth ferrite (BFO) and barium titanate (BTO) multilayered ferroelectric BFO/BTO/BFO/BTO thin film structure deposited on Pt/Ti/SiO₂/Si substrates using chemical solution deposition technique are presented. X-ray diffraction analysis confirms pure phase polycrystalline nature of deposited perovskite multilayered structures. Simultaneously both distorted rhombohedral (R3c) and tetragonal phases (P4mm) of the respective BFO and BTO components are also well retained. The ferroelectric sandwiched structures grown on fused quartz substrates exhibit high optical transmittance (~70%) with an energy band gap 2.62 eV. Current–voltage characteristics and PV response of multilayered structures is determined in metal-ferroelectric-metal (MFM) capacitor configuration. Considerably low magnitude of dark current density 1.53×10⁻⁷ A at applied bias of 5 V establish the resistive nature of semi-transparent multilayered structure. Enhanced PV response with 40 nm thin semitransparent Au as top electrode is observed under solid-state violet laser illumination (λ – 405 nm, 160 mW/cm²). The short circuit current density and open circuit voltage are measured to be 12.65 µA/cm² and 1.43 V respectively with a high retentivity. The results obtained are highly encouraging for employing artificial multilayered engineering to improve PV characteristics.     

Optically tuned dielectric characteristics of SrTiO_3/Si thin film in the terahertz range

Active control of the optical parameters in strontium titanate(SrTiO_3, STO) thin films is highly desirable for tunable terahertz(THz) integrated devices such as filters, phase modulators, and electro-optical devices. In this work, optically tuned dielectric parameters of a STO thin film epitaxially grown on a silicon wafer were characterized in the THz region with an 800 nm laser pump-THz detection system. The refractive index, extinction coefficient, and complex dielectric constant of the STO thin film were calculated using thin-film parameter extraction. Owing to carrier transportation and soft-mode oscillation, the above optical parameters changed notably with the pump power of the external laser. This study is of great significance for rapid and non-contact THz phase-modulation technology and may serve as a powerful tool to tune the dielectric properties of the STO thin films.     

Enhanced electrical and ferroelectric properties in a multiferroic (BiFeO3/Bi0.5Na0.5TiO3)3/LaNiO3 superlattices structure

Artificial multiferroic superlattices (SL), consisting of BiFeO₃ (16 nm)/Bi_0.5Na_0.5TiO₃ (5 nm) (BFO/BNT SL), were grown on (001) SrTiO₃ single crystal by pulsed laser deposition method. The cross-sectional, surface morphology, and crystallographic structure of BFO/BNT SL and BFO single layer were investigated. It was found that the electrical, ferroelectric, and magnetic properties of BFO/BNT SL exhibit a remarkably enhancement compared with BFO single layer. The influence of BNT buffering layer, lattice strain, and interfaces interplay of the SL structure are supposed to benefit their ferroelectric and ferromagnetic properties. Our works suggested the BFO/BNT SL with an improved multiferroic characteristics have a promising application for the future informational storage devices.     

PREPARATION OF a-AXIS ORIENTED YBa2Cu3Ox THIN FILMS

The a-axis oriented YBa₂Cu₃O_x(YBCO) thin films could be grown on (100) SrTiO₃(STO) substrates with STO buffer layers by dc and rf magnetron sputtering either by low-ering the deposition temperature, or by using a self-template technique. For the latter, the resistivity of the thin film at 290K along the substrate [001] direction is about four times larger than that in the [010] direction. The zero resistance temperatures T_c0 are 89 K in both directions. So high quality a-axis oriented YBCO thin films can be prepared by the self-template technique. Also the T_c0 increase monotonously with the reduction of the thickness of the YBCO seed layer.     

SrTiO3(110) thin films grown directly on different oriented silicon substrates

We have grown (110)-oriented SrTiO₃ (STO) thin films on silicon without any buffer layer, by means of pulsed laser deposition technique. The crystal structures of the grown films were examined by X-ray diffraction analysis including θ–2θ scan and rocking curve as well as Laue diffraction methods. STO films with single (110) out-of-plane orientation were formed on all (100), (110) and (111)-oriented Si substrates. The in-plane alignments for the epitaxial STO films grown directly on Si (100) were found as STO[001]//Si[001] and STO[11̄0]//Si[010]. The results should be of interest for better understanding of the growth of perovskite oxide thin films on silicon wafers. PACS 77.55.+f; 68.55.JK; 81.15Fg     

Multiferroic behaviour of epitaxial NiFe2O4--BaTiO3 heterostructures

Multiferroic NiFe2O4 （NFO）-BaTiO3 （BTO） bilayered thin films are epitaxially grown on （001） Nb-doped SrTiO3 （STO） substrates by pulsed-laser deposition （PLD）. Different growth sequences of NFO and BTO on the substrate yield two kinds of epitaxial heterostructures with （001）-orientation, i.e. （001）-NFO/（001）-BTO/substrate and （001）- BTO/（001）-NFO/substrate. Microstructure studies from x-ray diffraction （XRD） and electron microscopies show differences between these two heterostructures, which result in different multiferroic behaviours. The heterostructured composite films exhibit good coexistence of both ferroelectric and ferromagnetic properties, in particular, obvious magnetoelectric （ME） effect on coupling response.     

Off-axis electron holography of manganite-based heterojunctions:Interface potential and charge distribution

The interfacial electrical potentials and charge distributions of two manganite-based heterojunctions, i.e.,La_(0.67)Ca_(0.33)MnO_3/SrTiO_3:0.05 wt% Nb(LCMO/STON) and La_(0.67)Ca_(0.33)MnO_3/LaMnO_3/SrTiO_3:0.05 wt% Nb(simplified as LCMO/LMO/STON), are studied by means of off-axis electron holography in a transmission electron microscope.The influences of buffer layer on the microstructure and magnetic properties of the LCMO films are explored. The results show that when a buffer layer of LaMnO_3 is introduced, the tensile strain between the STON substrate and LCMO film reduces, misfit dislocation density decreases near the interfaces of the heterojunctions, and a positive magnetoresistance is observed. For the LCMO/STON junction, positive and negative charges accumulate near the interface between the substrate and the film. For the LCMO/LMO/STON junction, a complex charge distribution takes place across the interface, where notable negative charges accumulate. The difference between the charge distributions near the interface may shed light on the observed generation of positive magnetoresistance in the junction with a buffer layer.     

Interface-engineered resistive switching in Ag/SrTiO3/Nd0.7Ca0.3MnO3/YBa2Cu3O7 heterostructures

For Ag/Nd_0.7Ca_0.3MnO₃/YBa₂Cu₃O₇ (Ag/NCMO/YBCO) heterostructures, we investigate effects of an SrTiO₃ (STO) buffer layer inserted into the Ag/NCMO interface upon the room-temperature resistive switching. In comparison with the non-buffered (Ag/NCMO/YBCO) structure, the insertion of the STO buffer layer can greatly enhance the electric-field-induced-resistance (EPIR) effect. The STO-buffered (Ag/STO/NCMO/ YBCO) device can be switched on-and-off between the higher to lower resistance states at an EPIR ratio of 253% with pulsed voltage ±1.5 V and 405% with pulsed voltage ±3.0 V. The enhanced EPIR ratio is attributed to a combined effect of the migration of oxygen vacancies near the interface and ferroelectric polarization of the STO buffer.     

Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Enhanced ferroelectric property of (Pb0.95Ca0.05)(Nb0.02Zr0.80Ti0.20)O3 thin films prepared by RF magnetron sputtering

(Pb_0.95Ca_0.05)(Nb_0.02Zr_0.80Ti_0.20)O₃ [PCNZT] thin films were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering with and without a LaNiO₃ [LNO] buffer layer. Ca and Nb elements in PZT films enhance the ferroelectric property, LaNiO₃ buffer layer improves the crystal quality of the PCNZT thin films. PCNZT thin films possess better ferroelectric property than that of PZT films for Ca and Nb ion substitution, moreover, PCNZT thin films with a LNO buffer layer possess (1 0 0) orientation and good ferroelectric properties with high remnant polarization (P_r = 38.1 μC/cm²), and low coercive field (E_c = 65 kV/cm), which is also better than that of PCNZT thin films without a LNO buffer layer (P_r = 27.9 μC/cm², E_c = 74 kV/cm). The result shows that enhanced ferroelectric property of PZT films can be obtained by ion substitution and buffer layer.