La0.5Sr0.5CoO3 Films Research Articles

Microstructural and electrical properties of lead-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–BCT) epitaxial films grown on Si (0 0 1) substrates

Abstract Lead-free 0.5Ba(Zr 0.2 Ti 0.8 )O 3 –0.5(Ba 0.7 Ca 0.3 )TiO 3 (BZT–BCT) and the La 0.5 Sr 0.5 CoO 3 (LSCO) bottom electrode films were epitaxially grown onto the CeO 2 /Y 0.15 Zr 0.85 O 1.93 (YSZ) buffered Si (0 0 1) substrates via pulsed laser deposition. The lattice alignments between CeO 2 /YSZ and BZT–BCT/LSCO showed a 45° twisted cube-on-cube epitaxial relationship, indicating a high crystallinity of the BZT–BCT and LSCO films. The constituent elements in the BZT–BCT/LSCO/CeO 2 /YSZ structure showed no distinct inter-diffusion between each layer. The BZT–BCT epitaxial films showed a well saturation of the polarization–electric field ( P–E ).

Bipolar resistive switching behavior of La0.5Sr0.5CoO3−σ films for nonvolatile memory applications

Polycrystalline La0.5Sr0.5CoO3 (LSCO) film is prepared on Pt/Ti/SiO2/Si substrates by pulsed laser deposition in order to explore the resistive switching behavior of the Ag/LSCO/Pt structure. It is found that the oxygen stoichiometric LSCO (LSCO-2) structure does not possess distinct rectifying behavior, while the oxygen deficient La0.5Sr0.5CoO3−σ (LSCO-1) structure exhibits very stable bipolar resistive switching behavior, which is attributed to the oxygen vacancies in the LSCO film. The resistance ratio between high resistance state and low resistance state is about 100, which can be maintained up to 200 cycles and 25 h with no observable degradation, indicating that the Ag/LSCO-1/Pt device possesses very good endurance and retention characteristics. Moreover, the current-voltage characteristic of Ag/LSCO-1/Pt heterostructure can be well explained by the space-charge-limited conduction mechanism. The present results provide essential evidence for the analysis of the switching mechanism and evaluation of the memory devices.

Tuning the ferromagnetic transition temperature in La0.5Sr0.5CoO3 thin films

Epitaxial La0.5Sr0.5CoO3 (LSCO) thin films are grown on LaAlO3 (100) and SrTiO3 (100) substrates by pulsed laser ablation. By tuning the growth parameters, we are able to enhance the ferromagnetic transition temperature (TC) up to 262 K as evident from ac susceptibility, dc magnetization, and resistivity measurements. The magnitude of TC is the same as that for the bulk stoichiometric LSCO illustrating the high quality of the grown films. Detailed structural analysis clearly reveals that the induced strain in the LSCO film has no role behind this enhancement; in fact, the determining factor is the oxygen stoichiometry. The films also exhibit ageing effect as the TC decreases with time. This is considered in terms of gradual change in the oxygen stoichiometry through diffusion process as the time progresses.

Tunable strain effect and ferroelectric field effect on the electronic transport properties of La0.5Sr0.5CoO3 thin films

Tensiled La0.5Sr0.5CoO3 (LSCO) thin films were epitaxially grown on piezoelectric 0.67Pb (Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-PT) single-crystal substrates. Due to the epitaxial nature of the interface, the lattice strain induced by ferroelectric poling or the converse piezoelectric effect in the PMN-PT substrate is effectively transferred to the LSCO film and thus reduces the tensile strain of the film, giving rise to a decrease in the resistivity of the LSCO film. We discuss these strain effects within the framework of the spin state transition of Co3+ ions and modification of the electronic bandwidth that is relevant to the induced strain. By simultaneously measuring the strain and the resistivity, quantitative relationship between the resistivity and the strain was established for the LSCO film. Both theoretical calculation and experimental results demonstrate that the ferroelectric field effect at room temperature in the LSCO/PMN-PT field-effect transistor is minor and could be neglected. Nevertheless, with decreasing temperature, the ferroelectric field effect competes with the strain effect and plays a more and more important role in influencing the electronic transport properties of the LSCO film, which we interpreted as due to the localization of charge carriers at low temperature.

Effect of oxygen partial pressure on the electrical properties of the La0.5Sr0.5CoO3/Al-doped ZnO thin film heterojunctions

In this study, p–n heterojunction with La 0.5 Sr 0.5 CoO 3 (LSCO) and 3 wt.% Al-doped ZnO (AZO) thin films were prepared by radio frequency (r. f.) magnetron sputter. Heterojunction was made by stacking the p-type LSCO thin film on the pre-annealed n-type AZO thin film under a flow of different Ar:O 2 sputter gas ratio during deposition. The thickness of AZO and LSCO films is about 500 and 200 nm, respectively. The LSCO/AZO heterojunction shows somewhat lattice match between LSCO films with AZO films from XRD and TEM investigation. Rectifying behavior of the heterojunctions in the range of − 8 to 8 V for all of the samples are examined. The diode has the lowest resistance under Ar:O 2 sputter gas ratio of 10:5 (sccm). The turn-on voltage of the diodes is obtained from the I–V curve was around 1.5 V. ► The p-type LSCO with n-type AZO diodes were obtained by r. f. magnetron sputter. ► The LSCO/AZO diodes have good contact between LSCO films with AZO films. ► The diodes showed rectifying behavior in the backward mode for all of the samples. ► The diode has the lowest resistance under Ar:O 2 sputter gas ratio of 10:5 (sccm). ► The turn-on voltage is obtained around 1.5 V for all of the samples.

Orientation controlling of Pb(Zr0.53Ti0.47)O3 thin films prepared on silicon substrates with the thickness of La0.5Sr0.5CoO3 electrodes

Pb(Zr0.53Ti0.47)O3 (PZT) thin films were prepared on La0.5Sr0.5CoO3 (LSCO) coated Si substrates by a sol–gel route. The thickness of LSCO electrode was found to modify the preferential orientation of PZT thin films, which consequently affected the dielectric and ferroelectric properties. (100) textured PZT films with dense columnar structure could be obtained on the top of (110) textured LSCO with thickness of 230 nm. PZT thin films prepared on the optimized LSCO films exhibit the enhanced dielectric constant and remnant polarization of 980 and 20 μC/cm2, respectively.

Switching Properties of PST Ferroelectric Films for Memory Applications using Conductive Oxide LSCO Electrodes

ABSTRACTWe have investigated the switching properties of ferroelectric lead strontium titanate (Pbx Sr1-x)TiO3 (PST) capacitors using epitaxial and polycrystalline La0.5Sr0.5CoO3 (LSCO) electrodes to evaluate its potential for non-volatile memory applications (FRAM'S). The electrical performance of the PST based capacitors grown on epitaxial and polycrystalline LSCO substrates were evaluated through Polarization-Voltage (P-V) and Fatigue measurements. The [001] preferentially oriented PST ferroelectric capacitor did not show a decrease in polarization up to 108 switching cycles at an applied voltage of 4 volts and a frequency of 100 kHz. Polycrystalline PST films, on the other hand were severely fatigued. It is proposed that the good performance of the PST capacitors can be attributed to the high degree of orientation of the PST films in the [001] direction induced by the epitaxial LSCO film. However, after 1010 switching cycles in the fatigue tests, the decay of the non-volatile polarization of these films was about 18% of its initial value. We use a simple model to describe two major microscopic scenarios for the suppression of polarization at the electrodes interfaces. This model is supported by Auger Electron Spectroscopy to determine the bulk and interfacial characteristics. The depth distribution of the elements of the PST film is nearly uniform throughout the volume of the film; however the relative concentrations of Pb and O were abnormally distributed on the surface. It is concluded that, in spite of the good performance due to the favorable conditions for the growth of the PST films provided by the textured LSCO, higher quality interfaces and better control of composition is necessary to improve on fatigue.

Structural and electrical properties of polycrystalline PbZr 0.5 Ti 0.5 O 3 films deposited on La 0.5 Sr 0.5 CoO 3 coated silicon by sol-gel process

La0.5Sr0.5CoO3 (LSCO) films have been grown on Si (100) by a metalorganic chemical liquid deposition (MOCLD) technique using lanthanum acetate, strontium acetate and cobalt acetate as the starting materials. Subsequent PbZr0.5Ti0.5O3 (PZT) films were deposited onto LSCO films by a modified sol-gel method. Field-emission scanning electron microscopy and X-ray diffraction analysis show that PZT and LSCO films are polycrystalline and entirely perovskite phase. At an applied electric field of 250 kV/cm, the Pt/PZT/LSCO capacitor shows no polarization fatigue after 3×109 switching cycles and an internal electric field; the remnant polarization Pr and the coercive field Ec are about 22 μC/cm2 and 73 kV/cm, respectively. The dielectric constant of PZT films is 650 at a frequency of 1 kHz.

Characterization of Highly Preferred Pb(Zr,Ti)O3 Thin Films on La0.5Sr0.5CoO3 and LaNi0.6Co0.4O3 Electrodes Prepared at Low Temperature

La0.5Sr0.5CoO3 (LSCO) and LaNi0.6Co0.4O3 (LNCO) thin films were deposited on Pt/Ti/SiO2/Si substrates by DC reactive sputtering at 450°C and were annealed at temperatures ranging from 550°C to 750°C for 30 min in an O2 ambient to improve the crystallinity of the films and to reduce their resistivity. LSCO and LNCO thin films were successfully prepared at temperatures as low as 450°C. Pb(Zr0.48Ti0.52)O3 (PZT) thin films of 150 nm thickness were deposited on the LSCO and LNCO electrodes by DC reactive sputtering at a substrate temperature of 550°C. PZT films grown on LSCO and LNCO electrodes showed a (001) preferred orientation and had a uniform matrix of densely packed round grains. The leakage current density remained on the order of 10-7–10-9 A/cm2 at an applied voltage below 5 V. PZT thin films grown on LSCO/Pt showed a remanent polarization (2Pr) of about 46–52 µC/cm2, and a coercive voltage of about 1 V. PZT thin films grown on LNCO/Pt electrodes showed a lower coercive voltage (<0.6 V) and a smaller remanent polarization (2Pr) of about 28.8 µC/cm2 than those of PZT films grown on LSCO/Pt. LSCO/Pt and LNCO/Pt electrodes were essential for lowering the crystallization temperature as well as for obtaining good electrical properties of PZT capacitors.

Preparation and properties of Pb0.35 Sr0.65 TiO3 films by magnetron sputtering method

Pb0.35Sr0.65TiO3 (PST) films were successfully deposited on La0.5Sr0.5CoO3 (LSCO) /ITO/Glass substrates at 200 °C using the magnetron sputtering method. The structure and phase of films were analyzed with XRD and SEM. The pure perovskite phase films were obtained by post-annealing at 550 °C. The dielectric and ferroelectric characteristics of films were measured. The Ps, Pr and Ec of PST films were 19 μ C/cm2, 6.6 μ C/cm2 and 16 kV/cm respectively. The typical value of dielectric constant was about 2200. LSCO/ITO double layer film as composite electrodes are with a good lattice matching between PST and LSCO films and the low surface resistivity, which resulted in PST films with superior electric properties.

Low temperature deposition of (Mn,Sb) co-doped pzt thin film on (La,Sr)CoO3 electrode films on amorphous silica substrates for pyroelectric apllication

(Mn,Sb) co-doped Pb(Zr, Ti)O3 (PMSZT) pyroelectric films were successfully deposited on polycrystalline La0.5Sr0.5CoO3 (LSCO) films on amorphous-glass-substrates in the low temperature range of 470°C to 550°C by pulsed laser deposition (PLD). Polycrystalline PMSZT films with preferred (110) surface normal orientation, but no in-plane order were grown on polycrystalline LSCO/silica substrates. For comparison, epitaxial PMSZT(001) films were grown under the same deposition conditions on epitaxial LSCO(001) films grown on LaAlO3 (LAO) (001) crystal substrates. Both polycrystalline and epitaxial PMSZT films on LSCO had leakage currents lower than 4×10−10 A/cm2 at 10V applied voltage. The polycrystalline PMSZT films on the LSCO/glass substrate unexpectedly showed superior ferroelectric and pyroelectric properties than the counterpart PMSZT(001) epitaxial films on LSCO(001)/LAO(001) substrates. Remnant polarization of 2Pr=62 μC/cm2 and IR photocurrent responsivity of Ri ∼2.0 mA/W at 50–60Hz were obtained for polycrystalline PMSZT films deposited on silica at 500°C, compared with 2Pr=37μC/cm2 and Ri ∼0.5 mA/W for the epitaxial PMSZT films.

Structural and electrical properties of La 0.5 Sr 0.5 CoO 3 films on SrTiO 3 and porous α-Al 2 O 3 substrates

Films of La0.5Sr0.5CoO3 (LSCO) have been deposited on specially treated TiO2-terminated (001) SrTiO3 substrate surfaces and on macroporous polycrystalline !-Al2O3 substrates, having a mean pore diameter of 80 nm, by pulsed laser deposition. The films deposited on SrTiO3 are good conducting, (001) textured, and exceptionally smooth (1–2 A for 100 nm thick films). LSCO films deposited on porous !-Al2O3 are polycrystalline and exhibit good crystallographic and electrical properties despite the large substrate roughness and the differences in lattice parameters and crystal structure between the film and the substrate. Different growth modes have been observed on the porous !-Al2O3 substrates depending on the oxygen pressure during film deposition. Films grown at an oxygen pressure of 10-1 mbar are macroporous, whereas films grown at 10-2 mbar completely cover the substrate pores. In the latter case, strain effects lead to film cracking.

Substrate Effect on Pb0.6Sr0.4TiO3 thin film growth by pulsed-laser-deposition

Pb0.6Sr0.4TiO3 (PSrT) thin films are grown on various substrates such as: (m00)-textured La0.5Sr0.5CoO3 (LSCO)/Si, In2Sn2O7-x (ITO)/glass and amorphous glass using Pulse-laser-deposition (PLD). X-ray diffraction results show that the structure and the lattice parameters of the substrate greatly influence the growth of PSrT films. For example, (001)-oriented PSrT films can be successfully deposited on the substrates with (m00)-textured LSCO films and ITO/glass substrates. (100)-like PSrT films tend to grow on the amorphous glass substrates. Both element diffusion and structure stability were found to play an important role on the ferroelectric properties of these deposited films. Microstructural characteristics as well as the electric and optical properties are compared and discussed.

Vacancy defects in (Pb, La)(Zr, Ti)O3 capacitors observed by positron annihilation

A study of vacancy-related defects in ferroelectric capacitors was performed using a variable energy positron beam (VEPB). Heterostructures of (Pb0.9La0.1)(Zr0.2Ti0.8)O3 (PLZT) ferroelectric with La0.5Sr0.5CoO3 (LSCO) electrodes were deposited by pulsed laser deposition and the effects of oxygen deficiency studied using structures grown with 760 and 1×10−5 Torr oxygen. The VEPB depth profile showed an increase in vacancy-related defects with increased oxygen nonstoichiometry. A study of LSCO and PLZT thin films was also performed. The formation of vacancy clusters in the LSCO top electrode, and VPb−VO defects in the PLZT layer, with increased oxygen deficiency is inferred.

Biaxially oriented conductive La0.5Sr0.5CoO3 thin films on SiO2/Si

We have grown biaxially textured La0.5Sr0.5CoO3 (LSCO) thin films on technically important SiO2/Si substrates by pulsed laser deposition, where a biaxially oriented yttria-stabilized zirconia (YSZ) produced by ion-beam-assisted deposition (IBAD) is used as a seed layer to enhance the texture of LSCO on SiO2/Si. The degree of in-plane texturing of LSCO is directly related to the quality of IBAD-YSZ. The smaller the value of full width at half maximum of the IBAD-YSZ (220) diffraction peak, the higher the crystallinity of the LSCO film. The biaxially oriented LSCO film deposited at 700 °C on SiO2/Si shows metallic resistivity versus temperature characteristics and has a room-temperature resistivity of around 110 μΩ cm.

Structure control of pulsed-laser-deposited Pb0.6Sr0.4TiO3/La0.5Sr0.5CoO3 thin films on various substrates

Structure development is an important issue for the growth of ferroelectric thin film. In the present experiment, Pb0.6Sr0.4TiO3(PSrT)/La0.5Sr0.5CoO3 (LSCO) thin films have been grown on various substrates, such as: single crystal Si with (100) and (111) orientations, Au/Si, Pt/Si and glass, using XeCl excimer laser (wavelength = 308 nm) under various processing conditions. X-ray diffraction results show that highly (100)-oriented LSCO films can be deposited successfully under appropriate processing conditions regardless the type of the substrate. PSrT films can be deposited upon LSCO layer with the same structure arrangements if LSCO can develop desired structures. It appears that appropriate processing parameters are more important than lattice parameter consideration for growing LSCO films using pulsed laser deposition. Microstructure investigations indicate that the substrate with an amorphous surface layer may help the development of the desired structure.

Preparation of PZT thin films on conductive perovskite LSCO electrodes films by pulsed laser deposition

High quality La0.5Sr0.5CoO3 (LSCO) thin films have been prepared by pulsed laser deposition(PLD). The target was prepared by using citrate processing method. The electrical resistivity versus temperature has been measured. The LSCO thin films were grown epitaxially on LaAlO3(100) substrates, whereas the LSCO films on Si(100) or Si(111) show to be polycrystalline. The resistivity of the films is slightly larger than that of the targets. The PZT films were grown on LSCO/LAO(100) substrates forming heterostructures and their ferroelectric hysterisis has been measured

Preparation of PLZT/LSCO/ITO/Si multilayer films by RF-Magnetron sputtering

PLZT/LSCO/ITO/Si textured multilayer films were prepared using the rf-magnetron sputtering method. La0.5Sr0.5CoO3 (LSCO) films as the bottom electrode have been grownin situ on Si with a conductive ITO buffer layer with RF sputtering. The LSCO film orientated in the (100) direction deposited at 600°C. The sheet resistance of LSCO/ITO compsite electode is about 100 Ω□. The PLZT(8/65/35) films have been deposited on t he substrates textured LSCO/ITO/Sistructure by RFsputtering with the PLZT powdertargets. The PLZT films with a pure perovskite structure and highly prefer orientation along (100) direction can be obtained when the deposition temperature is 580°C. The films obtained in this study show a high resistivity of over 1010 Ω cm and low dielectric loss tan δ in the range of 0.01-0.03

Pulsed laser ablation-deposition of La0.5Sr0.5CoO3 for use as electrodes in nonvolatile ferroelectric memories

La0.5Sr0.5CoO3 (LSCO) thin films have been deposited on (100) MgO substrates using pulsed laser ablation-deposition (PLAD). The crystallographic orientation of LSCO was found to be dependent on the surface treatment of (100) MgO prior to deposition. PLAD deposition parameters were optimized to yield LSCO films with an RMS surface roughness of 40–50 Å. A smooth surface morphology was reproduced as long as the oxygen content of the LSCO target was preserved. Otherwise, “splashing” occurred which resulted in the transfer of condensed particles from molten spherical globules of LSCO from the target to the substrate. Splashing was subsequently eliminated and smooth surface quality was restored after annealing the LSCO target at 550 °C in oxygen for 3 h. Optical emission spectroscopy (OES) of the LSCO's plume identified excited atomic cobalt neutrals, excited singly ionized strontium and lanthanum, and excited molecular LaO species. Oxygen interaction with the plume produced no new species. Furthermore, the OES data suggest that the observed LaO molecules were not created by the chemical reaction between La and O2 during ablation, but were ejected directly from the target during the PLAD process.

Synthesis of low resistivity complex oxides on MgO using Pt as buffer layer

Highly crystalline SrRuO3 (SRO) and La0.5Sr0.5CoO3 (LSCO) thin films were deposited on (100) Pt/ MgO by pulsed laser deposition. The films were mainly (001) textured normal to the substrate surface with a high degree of in-plane orientation with respect to the substrate’s major axes. These films were characterized using x-ray diffraction, Rutherford backscattering, four-point probe resistivity measurement, and transmission electron microscopy. The room temperature resistivity for LSCO and SRO films on Pt/MgO was found to be ∼35 and ∼40 μΩ-cm, respectively. An ion beam minimum channeling yield of ∼43% and ∼33% was obtained for LSCO and SRO films, respectively. The interface between Pt and oxide was found to be smooth and free from any interfacial diffusion. This result showed that high-quality low resistivity oxide thin films can be deposited on Pt.