SrRuO3 Thin Films Research Articles

Magnetism of SrRuO3 thin films under high hydrostatic pressure

We investigated the magnetic properties of Co doped anatase TiO2 thin films. To incorporate Co into epitaxial anatase TiO2 thin films, we implanted them with Co+ ions. The temperature dependence of the magnetization of the implanted films, taken in zero-field-cooling (ZFC) and field-cooling conditions, exhibits the main features of superparamagnetic system: the ZFC curves are rounded at the blocking temperature (TB) and a paramagnetic-like behavior above TB. After annealing the films, TB increased with increasing annealing temperature. These phenomena can be explained by the formation of Co (or ferromagnetic Co compound) particles and their growth during annealing, as directly observed with transmission electron microscopy.

Metal oxide gate electrodes for advanced CMOS technology

We have used in situ synchrotron x-ray scattering to investigate PbTiO3 films grown epitaxially on SrTiO3 (001) substrates. When the films are cooled below a thickness-dependent Curie temperature, the ferroelectric phase forms as 180° stripe domains with a well-defined period. With further cooling, the period changes fairly abruptly from a lower to a higher value, suggesting a transition between two stripe domain phases. The periods of both stripe phases are observed to depend on the square root of film thickness, in agreement with theory.

Nanoscale Etching of Metallic Perovskites Using STM

ABSTRACTIn the present work we use a scanning tunneling microscope to modify the surface structure of epitaxial SrRuO3 thin films. Point and line etching experiments were carried out in ultra-highvacuum, using tungsten tips. The point etchings showed that pulses fired at small (< 4.5V) bias voltages did not bring about any physical modifications of the film surface, while voltages in excess of4.5 V led to etched holes accompanied by mounds. Moreover, well-defined line etching was achieved with atypical depth of approximately two unit cells and linewidths as small as 5 nm. The experiments demonstrate that a scanning tunneling microscope can be used for nanometer-scale patterning of SrRuO3 thin film surfaces.

SrRuO<sub>3</sub> 及び IrO<sub>2</sub> 薄膜上部電極を有する(111) 及び (100)/(001) 配向 Pb(Zr,Ti)O<sub>3</sub> 薄膜キャパシタの分極反転現象

(111)- and (100)/(001)-oriented Pb(Zr,Ti)O3 (PZT) thin films were deposited using a chemical-solution deposition (CSD) method. For the top electrodes, IrO2 and SrRuO3 (SRO) thin films were deposited by sputtering. The ferroelectric properties and crystal structure of the (111)- and (100)/(001)-oriented PZT thin-film capacitors were evaluated. The switchable polarization (Qsw) of a (100)/(001)-oriented PZT capacitor with an SRO top electrode was lower than that with an IrO2 top electrode. The opposite trend in Qsw was observed with IrO2 and SRO top electrodes in (111)-oriented PZT capacitors. With continuous switching cycles, the Qsw of a (100)/(001)-oriented PZT capacitor with IrO2 increased continuously with an increasing number of cycles. However, there was no increase in Qsw in a (100)/(001)-oriented PZT capacitor with SRO. Transmission electron microscopy (TEM) observation showed a local epitaxial-like interface between the SRO and PZT in both the (111) and (100)/(001) orientation. However, a heterostructure was observed in the IrO2/PZT interface in both orientations. TEM also showed the existence of several 90° domains in the (100)/(001)-oriented PZT films. At the SRO/PZT(100)/(001) interface, this local epitaxial-like interface seemed to block the movement of the 90° domain wall. As a result, the Qsw was lower in a (100)/(001)-oriented PZT capacitor with SRO than that with IrO2.

Sub-unit cell layer-by-layer growth of Fe3O4, MgO, and Sr2RuO4 thin films

The use of oxide materials in oxide electronics requires their controlled epitaxial growth. Recently, it was shown that Reflection High Energy Electron Diffraction (RHEED) allows to monitor the growth of oxide thin films even at high oxygen pressure. Here, we report the sub-unit cell molecular or block layer growth of the oxide materials Sr2RuO4, MgO, and magnetite using Pulsed Laser Deposition (PLD) from stoichiometric targets. Whereas for perovskites such as SrTiO3 or doped LaMnO3 a single RHEED intensity oscillation is found to correspond to the growth of a single unit cell, in materials where the unit cell is composed of several molecular layers or blocks with identical stoichiometry, a sub-unit cell molecular or block layer growth is established resulting in several RHEED intensity oscillations during the growth of a single unit-cell.

The Influence of Various Upper Electrodes on Fatigue Properties of Perovskite Pb(Zr,Ti)O3 Thin Films

Ferroelectric thin films of Pb(Zr,Ti)O3 (PZT) were prepared by chemical solution deposition (CSD). The top electrodes of platinum (Pt), SrRuO3 (SRO), and IrO2 thin films are deposited on the PZT thin films by RF magnetron sputtering (RF) and the pulsed laser deposition (PLD) methods, respectively. The polarization fatigue of ferroelectric thin-film capacitors is investigated. As a result, the remanent polarization value of the PZT thin films of SRO/PZT/PT/Pt/TiOx/SiO2/Si and IrO2/PZT/PT/Pt/TiOx/SiO2/Si remains more than 1011 switching cycles at constant value and the value of the PZT thin films of Pt/PZT/PT/Pt/TiOx/SiO2/Si decreases to approximately 50% of its initial value at 108 cycles. It is confirmed that polarization fatigue is improved using SRO and IrO2 thin-film electrodes as top electrodes.

Conducting Strontium Ruthenium Oxide Interface Layers for Application to PZT/SrRuO 3 /Metal Thin Film Capacitors

Ferroelectric lead zirconate titanate (PZT) thin films have been prepared by chemical solution deposition (CSD) and were spin-coated onto stainless steel substrates. In order to improve the ferroelectric properties, the influence of an additional layer of strontium ruthenium oxide (SrRuO3) at the interface between the perovskite type PZT and the metal substrate has been studied. The elaboration route of the SrRuO3 thin films by CSD and spin-coating is presented. The influence from the annealing temperature on the PZT crystallization behavior has been analyzed by X-ray diffraction, and the PZT coercive field and remanent polarization were investigated as a function of the SrRuO3 process parameters. The results are compared with those obtained for PZT/RuO2/metal thin film capacitors.

Conducting Strontium Ruthenium Oxide Interface Layers for Application to PZT/SrRuO3/Metal Thin Film Capacitors

Ferroelectric lead zirconate titanate (PZT) thin films have been prepared by chemical solution deposition (CSD) and were spin-coated onto stainless steel substrates. In order to improve the ferroelectric properties, the influence of an additional layer of strontium ruthenium oxide (SrRuO3) at the interface between the perovskite type PZT and the metal substrate has been studied. The elaboration route of the SrRuO3 thin films by CSD and spin-coating is presented. The influence from the annealing temperature on the PZT crystallization behavior has been analyzed by X-ray diffraction, and the PZT coercive field and remanent polarization were investigated as a function of the SrRuO3 process parameters. The results are compared with those obtained for PZT/RuO2/metal thin film capacitors.

Synthesis and properties of highly conductive thin films as buffer layer from sol-gel process

We prepared epitaxial growth SrRuO3 thin film on LaAlO3 (001) (LAO) single crystal substrate and highly oriented BaTiO3 ferroelectric thin film on the epitaxial SrRuO3 thin film. A homogeneous precursor solution for preparing SrRuO3 thin film was prepared with Sr(O—i—C3H7)2 and Ru(NO)(NO3)3 as starting materials, and 2-methoxy ethanol as solvents. The as-coated thin films were heat treated at temperatures from 723 to 1273 K for 1 h in air. SrRuO3 grew epitaxially on LAO(001) substrate, which were confirmed by XRD theta-2theta method and XRD pole figure analysis. The crystallographic relationship of the film and substrate was SrRuO3(001) parallel to LAO(001) and SrRuO3[110] parallel to LAO[100]. A homogeneous precursor solution for preparing BaTiO3 thin film was prepared with Ti[O—n—(CH2)3CH3]4 and Ba(OCOCH3)2 as starting materials, and acetic acid, 2-methoxy ethanol. SrRuO3 coated LAO substrates were coated by spin-coating method with the coating solution. The as-coated thin films were heat treated at temperatures from 973 to 1173 K in air. It was confirmed that the thin films were growing orientated for c-axis by measurement of XRD theta-2theta method.

Metalorganic chemical vapor deposition of SrRuO 3 thin film and its characterization

Synthesis of conducting oxide strontium ruthenate is carried out in a hot-wall tubular reactor, using Sr(C11H19O2)2/Ru(C5H5)2/O2 reaction system. Owing to a large difference in depositing efficiency between strontium and ruthenium precursors, the stoichiometric ratio of thin film is controlled in one cycle of two consecutive depositions at different temperatures. Thin films of SrRuO3 single phase are synthesized in the subsequent 700°C annealing. Thin films of SrRuO3 with extra ruthenium oxide can also be prepared by adjusting the molar ratio of RuO2 and SrO layers. The deposition sequence of ruthenium oxide first, strontium oxide later is preferred. If the deposition sequence is reverse, the thin film is plagued with unreacted oxides even when the annealing temperature is raised to 800°C. The relative ease of preparing SrRuO3 thin films, when RuO2 is under SrO, is attributed to evaporation of ruthenium oxide in O2 and diffusion in its open columnar microstructure. The sheet resistivity of thin film decreases with the ruthenium content. The room temperature resistivity of SrRuO3 film of Ru/(Sr + Ru) = 0.5 is around 910 μohm-cm. The room-temperature resistivity of Ru/(Ru + Sr) = 0.53 decreases to 470 μohm-cm. The root mean square surface roughness of 700°C synthesized SrRuO3 thin film is 22 nm, in a 2 × 2 μm2 area of film thickness 280 nm.

Ferroelectric property of an epitaxial lead zirconate titanate thin film deposited by a hydrothermal method

ABSTRACTHydrothermal method has various advantages; low deposition temperature, high-purity, deposition on a three-dimensional structure and a large thickness. Although epitaxial PZT thin film deposition has reported, ferroelectric measurement has not been conducted due to the peel-off morphology of the film. The present paper investigates the improvement of an epitaxial PZT thin film deposited via a hydrothermal method. By adjusting the position at which the substrate was suspended in the solution, smooth morphology surface was successfully obtained. As a bottom electrode, 200 nm SrRuO3 thin film was deposited on SrTiO3 single crystals, and the PZT thin was deposited on SrRuO3. The remanent polarization 2Pr for PZT on SrRuO3/SrTiO3 (001) was 19.5 μC/cm2 and that of PZT on SrRuO3/SrTiO3 (111) was 37.2 μC/cm2 respectively. The self alignment poling direction was confirmed via scanning nonlinear dielectric microscopy and is thought to have been related to the deposition mechanisms.

Dc and microwave resistivities of SrRuO3 films deposited on SrTiO3

Epitaxial SrRuO3 thin films on (001) SrTiO3 substrates grown by pulsed laser ablation at deposition temperatures ranging between 690° and 810 °C have been examined. The correlations between dc and microwave resistivity, microstructure, and surface morphology are obtained. According to the x-ray measurements the structure of all films is a mixture of highly oriented domains of strained SrRuO3 phases with different lattice parameters. The films grown at 780 °C consist mainly of orthorhombic phase I with a bulk lattice parameter close to the orthorhombic single crystal and reveal same resistivity values of 270 μΩ cm for both dc and microwave. Films obtained at other deposition temperatures contain a larger amount of orthorhombic phase II, with extended bulk lattice parameters and dc resistivity increases up to 1700 μΩ cm for the lowest deposition temperature. The corresponding microwave resistivity values are 50% higher than the dc ones. The dc resistivity of SrRuO3 films is in inverse proportion to the intensity of the (220) refection from orthorhombic phase I. The orthorhombic phase I domains reveal higher crystallinity (FWHM values are about 0.2°) and lower out-of-plane strain (0.4%) than corresponding values of the orthorhombic phase II domains (0.35° and 1.6%, respectively). According to AFM measurements the SrRuO3 films consist of a network of distinguishable islands separated by film areas with lower thickness. The islands consist mainly of orthorhombic phase II. At deposition temperatures more than 780 °C the film roughness is mainly due to holes formation caused, probably, by grain rotation and reevaporation of SrO chains. The difference between dc and microwave resistivity values is explained by influence of the film roughness.

Epitaxial Growth of SrRuO3 Thin Film Electrode on Si by Pulsed Laser Deposition

Pseudocubic SrRuO3 (100) epitaxial thin films were fabricated on Si (100) with a SrO buffer layer by pulsed laser deposition (PLD). Reflection high-energy electron diffraction (RHEED) revealed that the SrO layer is epitaxially grown on naturally oxidized Si substrates with an orientation relationship of SrO (110)/Si (100) and SrO <001>//Si <011>. Subsequent SrRuO3 deposition resulted in a (100) epitaxial thin film possessing good crystallinity with a full-width at half maximum (FWHM) of 1.9° in the SrRuO3 (200) rocking curve by X-ray diffraction (XRD). Based on the Gibbs free energy change in the reaction of silicon with alkaline earth metal oxides, deoxidization of SiO2 on Si by Sr is thought to play an important role in the epitaxial growth of SrO. Cross-sectional observation for the optimized SrRuO3/SrO/Si sample using transmission electron microscopy (TEM) revealed that the thickness of the SrO layer is less than 2 nm and that the SrRuO3 electrode forms an epitaxial thin film almost directly on Si.

Growth of SrRuO3 thin films on MgO substrates by pulsed laser ablation

SrRuO3 thin films have been grown on MgO substrates using pulsed laser ablation deposition. The orientation of the SrRuO3 films changes from the [001] or [110] direction normal to the substrate surface to become predominantly [100] as the thickness of the SrRuO3increases from 50 to 360 nm. This leads to a change in the magnetic response of the SrRuO3 films and to a dramatic improvement in the superconducting properties of YBa2Cu3O7−δ films grown on top of the SrRuO3 layer. The domain structures and the surface morphology of SrRuO3 films grown on MgO change as the thickness of the film increases. The 50 nm thick SrRuO3 film grown on an MgO substrate nucleates as rectangular islands, 0.5 to 1 μm in diameter. As the thickness increases to 185 nm, oriented grains ∼1 μm in size are observed. We have noted a different microstructure for the SrRuO3 thin films grown on SrTiO3 substrates. The 185 nm thick SrRuO3 film grown on a SrTiO3 substrate shows a step like growth pattern.

Effect of Strain in Epitaxially Grown SrRuO3 Thin Films on Crystal Structure and Electric Properties

SrRuO3 thin films were epitaxially grown on (001)SrTiO3 and (001) [(LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7] (LSAT) substrates by rf magnetron sputter deposition and metalorganic chemical vapor deposition (MOCVD), and their crystal structure, electric property and thermal stability were investigated. SrRuO3 films prepared by MOCVD (MOCVD-SrRuO3 films) had almost the same volume of unit cell as that of the single crystal, while those prepared by rf magnetron sputter deposition (sputter-SrRuO3 films) had a larger volume. This large volume of the unit cell of sputter-SrRuO3 films decreased by the post annealing up to 830°C but did not reach that of the single crystal one. Temperature dependence of the resistivity of MOCVD-SrRuO3 films was in good agreement with that of the single crystal, which corresponds to metallic behavior, while that of sputter-SrRuO3 films showed semiconductor-like behavior below 120 K.

Near-electrode model of transient currents in (Ba,Sr)TiO3 thin film capacitor structures

Transient currents in SrRuO3//Ba0.8Sr0.2TiO3//SrRuO3 thin film capacitors, in the time interval 0.05–2.5 s after the step voltage switching on and off, have been investigated. The origin of these currents is proposed, including two components: charging (discharging) of near-electrode potential barriers and oxygen vacancy migration. It has been established that transient current of capacitor charging is mainly determined by the barrier component, and transient current of capacitor discharging is due to both components. The parameters of potential barriers were determined giving a good agreement of the calculated time dependences of transient currents with the experimental data.

Magnetic behavior of epitaxial SrRuO3 thin films under pressure up to 23 GPa

We report on measurements of electrical resistivity and determination of the Curie temperature of single-crystal epitaxial ferromagnetic SrRuO3 thin films under hydrostatic pressures up to 23 GPa. The SrRuO3 thin film was grown on vicinal (001) SrTiO3 substrate using 90° off-axis magnetron sputtering. At atmospheric pressure, the thin-film Curie temperature is about 150 K. To generate the high pressure, the Bridgman anvil technique was used. Up to 13 GPa, a linear decrease of the Curie temperature was observed at a rate of 5.9 K GPa−1. Above 13 GPa, a striking saturation of the magnetic transition temperature at 77 K was observed.

Strain effect on transport properties of SrRuO 3 films grown by laser MBE

Transport properties of SrRuO3 thin films were studied as a function of the epitaxial strain. SrRuO3 films were grown on (100) SrTiO3 substrates by the Pulsed Laser Deposition technique equipped with Reflection High Energy Electrons Diffraction (RHEED). Samples thickness has been varied from a few unit cells to above 1000 A while monitoring RHEED intensity oscillations. In thicker films epitaxial strain was found to be progressively relaxed. SrRuO3 relaxed films (thickness 1000 A) show metallic behavior for the whole temperature range with a ferromagnetic ordering at about 150 K. For thinner films, ferromagnetic ordering occurs at progressively lower temperatures, until in films thinner than 400 A it disappears. Films thinner than 80 A show a semiconducting behavior at low temperatures. Our results provide direct evidence of the crucial role of the strain effect for conducting and magnetic properties of SrRuO3.

Correlation between microstructure, DC resistivity and magnetoresistance of SrRuO3 films.

ABSTRACTWe have investigated the correlation between microstructure, DC resistivity and magnetoresistance of SrRuO3 thin films. The films were epitaxially grown by pulsed laser deposition on (001) SrTiO3 substrates in a temperature range of 690–810°C. According to x-ray measurements, the structure of all films is a mixture of highly oriented domains of strained orthorhombic phases (ortho-I and ortho-II) with different lattice parameters. Films deposited at 780°C show a minimum resistivity (270 μΩcm at 300 K) and a maximum magnetoresistance (8% at 5 K). These films consist mainly of ortho-I phase (a=0.393 nm). Films deposited at 690°C (predominantly ortho-II) have the highest resistivity (up to 1700 μΩcm at 300 K) and lowest magnetoresistance (3% at 5K).

Structural evolution of epitaxial SrRuO3 thin films grown on SrTiO3(001)

Microstructural changes in epitaxial SrRuO3 thin films grown on SrTiO3(001) during the early stages of growth by pulsed laser deposition were studied using synchrotron x ray scattering. We find that at a very early stage of growth (⩽110 Å), the film is composed of a single (110) domain only, in sharp contrast to double (110) domains reported previously for thick (⩾1000 Å) films. As the film grows further (&amp;gt;110 Å), another, 90° rotated, (110) domain starts to form, coexisting with the original (110) domain. This domain evolution is attributed to strain relaxation, eventually resulting in significant atomic ordering in the in-plane direction.