SrRuO3 Bottom Electrode Research Articles

Direct observation of a fully strained dead layer at Ba0.7Sr0.3TiO3∕SrRuO3 interface

Epitaxial SrRuO3∕Ba0.7Sr0.3TiO3 (BST)/SrRuO3 thin film capacitors were prepared on SrTiO3 substrates by pulsed laser deposition. The structures of stacked BST films with different thicknesses were investigated by transmission electron microscopy. A distinctive layer of about 3nm of thickness was identified within BST films thicker than 9nm at the interface with the SrRuO3 bottom electrode. The distinctive layer is misfit dislocation-free showing pseudoconstant lattice parameters. Misfit dislocations are formed at the interface between the distinctive layer and the BST film bulk layer relaxing the latter as the film thickness increases. The effect of the distinctive layer on the system dielectric response is discussed within the framework of an interfacial dead-layer model.

Microstructure and ferroelectric properties of low-fatigue epitaxial, all (001)-oriented (Bi,La)4Ti3O12∕Pb(Zr0.4Ti0.6)O3∕(Bi,La)4Ti3O12 trilayered thin films on (001) SrTiO3 substrates

( Bi , La ) 4 Ti 3 O 12 ( BLT ) ∕ Pb ( Zr , Ti ) O 3 ( PZT ) ∕ ( Bi , La ) 4 Ti 3 O 12 trilayered ferroelectric thin films were epitaxially grown by pulsed laser deposition onto (001) SrTiO3 (STO) substrates with and without SrRuO3 (SRO) bottom electrodes. From x-ray pole figures and electron-diffraction patterns, the epitaxial relationships between BLT, PZT, SRO, and STO were identified to be BLT(001)‖PZT(001)‖SRO(001)‖SrTiO3(001); BLT[110]‖PZT[100]‖SRO[100]‖SrTiO3[100]. Cross-sectional transmission electron microscopy investigations revealed that 90° ferroelectric domain boundaries lying on {110} planes are present in the PZT layer, with an average domain width of 20nm and an average spacing of 120nm. These long 90° ferroelectric domains, as a rule having nucleated at the bottom of the PZT layer, extend to the top of the latter. The thin films have sharp BLT/PZT interfaces and a very flat surface. The remanent polarization and coercive field were determined as 13.9μC∕cm2 and 72.9kV∕cm, respectively, indicating that the epitaxial, all (001)-oriented trilayered thin films have rather good ferroelectric properties, although single epitaxial (001)-oriented BLT films are usually not favored for application due to their very poor ferroelectric properties. The thin films showed a high fatigue resistance at least up to 1010 switching pulse cycles, confirming that such a trilayered structure effectively shows the fatigue-free behavior of BLT.

Ferroelectricity and local currents in epitaxial 5- and 9-nm-thick Pb(Zr,Ti)O3 ultrathin films by scanning probe microscopy

Ferroelectricity and local currents in 5- and 9-nm-thick Pb(Zr,Ti)O3 ultrathin films epitaxially grown on SrRuO3∕SrTiO3(100) are investigated by current-sensitive atomic force microscopy (CS-AFM) and contact-resonance piezoresponse force microscopy (CR-PFM). CS-AFM observations reveal that large local currents flowed at the step edge of SrRuO3 bottom electrodes. Using CR-PFM, clear contrasts were observed for antiparallel domains, which were not observable by conventional PFM. The observed difference in the deflection signals of flexural vibrations of the cantilever between antiparallel domains is qualitatively explained by coupled contributions from piezoelectric vibrations of the PZT and electrostatic interaction between the cantilever and bottom electrodes. Based on this interpretation, both the 5- and 9-nm-ultrathin PZT films are concluded to exhibit a stable ferroelectric state.

Hysteretic current–voltage characteristics and resistance switching at a rectifying Ti∕Pr0.7Ca0.3MnO3 interface

We have characterized the vertical transport properties of epitaxial layered structures composed of Pr0.7Ca0.3MnO3(PCMO) sandwiched between SrRuO3(SRO) bottom electrode and several kinds of top electrodes such as SRO, Pt, Au, Ag, and Ti. Among the layered structures, Ti∕PCMO∕SRO is distinct due to a rectifying I–V characteristic with a large hysteresis. Corresponding to the hysteresis of the I–V characteristics, the contact resistance of the Ti∕PCMO interface reversibly switches between two stable states by applying pulsed voltage stress. We propose a model for the resistance switching at the Ti∕PCMO interface, in which the width and/or height of a Schottky-like barrier are altered by trapped charge carriers in the interface states.

Piezoresponse Force Microscopy Observations of Switching Behavior in Pb(Zr,Ti)O3 Capacitors

We report on the polarization switching behavior in Pt/Pb(Zr,Ti)O3 (PZT)/SrRuO3/SrTiO3(111) capacitors observed by piezoresponse force microscopy (PFM). Through PFM observations of capacitors partially switched by applying switching pulses shorter than the switching time, the nucleation behavior and domain growth were investigated. Polarization switching preferentially originated from preexisting latent nuclei and consequently, switched domains appeared at the same positions for various pulses with the same polarity. On the other hand, switched domains were observed at different positions for voltage pulses with different polarities. On the basis of a comparison of domain patterns with topography, it is suggested that switched domains are roughly associated with PZT grains and that each grain has preferred polarization directions. Switching current measurements revealed that a voltage drop across the series resistance of the SrRuO3 bottom electrode and PFM system occurred in our measurements because the switching currents were more than 10 mA. Due to this voltage drop, the effective voltage applied to PZT thin film (VPZT,eff) was almost constant at 2.8 V when external pulse voltages were changed from ±4 to ±10 V. For VPZT,eff of 2.8 V, the density of latent nuclei and the velocity of sideward domain growth were estimated to be ∼1 µm-2 and 15 m/s, respectively.

Ferroelectric properties of an epitaxial lead zirconate titanate thin film deposited by a hydrothermal method below the Curie temperature

Deposition of a hydrothermal method has a number of advantages; low deposition temperature, high-purity, deposition on a three-dimensional structure, and a large thickness. The present paper investigates the improvement of an epitaxial lead zirconate titanate (PZT) thin film deposited via hydrothermal synthesis. The critical parameter contributing to the film morphology was revealed to be the holding position of the reaction solution during deposition. In addition to this improvement, the SrRuO3 bottom electrode was confirmed to be able to withstand high alkali concentrations during the hydrothermal reaction. A hydrothermal synthesis time of 24 h yielded a PZT film with a thickness of 500 nm on SrRuO3/SrTiO3 (100) at 150 °C. The remanent polarization 2Pr and coercive electric field were 38.4 μC/cm2 and 21.8 kV/cm, respectively.

On a Novel Ferro Resistive Random Access Memory (FRRAM): Basic Model and First Experiments

We report on a novel non-volatile memory concept for resistive information storage. The particular device structure consists of a conductive ferroelectric/non-ferroelectric 2-layer sequence. Resistive switching is observed, by applying a voltage pulses. Our model predicts that the switching correlates with a change of the potential barrier height inside the structure. It may also explain resistive switching in systems consisting only of one ferroelectric layer and by assuming the presence of non-ferroelectric interface layers. The operation of the device is demonstrated for the PZT (48/52) system with a SrRuO3 bottom electrode and a Pt top electrode. The simulated and measured I-V curves are in good agreement.

Characteristics of the Interfacial Capacitance in Thin Film Ba0.5Sr0.5TiO3 Capacitors with SrRuO3 and (La, Sr)CoO3 Bottom Electrodes*

Thin film Ba0.5Sr0.5TiO3 (BST) capacitors of thickness ∼ 75 nm to ∼ 1200 nm, with Au top electrodes and SrRuO3 (SRO) or (La, Sr)CoO3 (LSCO) bottom electrodes were fabricated using Pulsed Laser Deposition. Implementing the “series capacitor model,” bulk and interfacial capacitance properties were extracted as a function of temperature and frequency. ‘Bulk’ properties demonstrated typical ceramic behaviour, displaying little frequency dependence and a permittivity and loss peak at 250 K and 150 K respectively. The interfacial component was found to be relatively temperature and frequency independent for the LSCO/BST capacitors, but for the SRO/BST configuration the interfacial capacitance demonstrated moderate frequency and little temperature dependence below T ∼ 300 K but a relatively strong frequency and temperature dependence above T ∼ 300 K. This was attributed to the thermal activation of a space charge component combined with a thermally independent background. The activation energy for the space charge was found to be E A ∼ 0.6 eV suggesting de-trapping of electrons from shallow level traps associated with a thin interfacial layer of oxygen vacancies, parallel to the electrodes. * Originally presented at 10th European meeting on Ferroelectricity, Cambridge, UK, August 3–8, 2003.

Coercive field of ultrathin Pb(Zr0.52Ti0.48)O3 epitaxial films

The polarization reversal in single-crystalline ferroelectric films has been investigated experimentally and theoretically. The hysteresis loops were measured for Pb(Zr0.52Ti0.48)O3 films with thicknesses ranging from 8 to 250 nm. These films were grown epitaxially on SrRuO3 bottom electrodes deposited on SrTiO3 substrates. The measurements using Pt top electrodes showed that the coercive field Ec increases drastically as the film becomes thinner, reaching values as high as Ec≈1200 kV/cm. To understand this observation, we calculated the thermodynamic coercive field Eth of a ferroelectric film as a function of the misfit strain Sm in an epitaxial system and showed that Eth strongly depends on Sm. However, the coercive field of ultrathin films, when measured at high frequencies, exceeds the calculated thermodynamic limit. Since this is impossible for an intrinsic coercive field Ec, we conclude that measurements give an apparent Ec rather than the intrinsic one. An enormous increase of apparent coercive field in ultrathin films may be explained by the presence of a conductive nonferroelectric interface layer.

Growth of uniformly a-axis-oriented ferroelectric lanthanum-substituted bismuth titanate films on silicon substrates

Uniformly a-axis-oriented, epitaxially twinned La-substituted Bi4Ti3O12 (BLT) thin films having the major spontaneous polarization entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates covered with very thin SrRuO3 bottom electrodes. Using SrRuO3 bottom electrodes of a specific low thickness in combination with a relatively high growth rate and a high oxygen pressure, the volume fraction of the BLT (100) orientation, which is competing with the BLT (118) orientation, was increased up to 99%. In this way the growth of fully a-axis-oriented BLT films was achieved, attaining a remanent polarization of 32 μC/cm2. Initial fatigue experiments indicated hardly any fatigue after 109 switching cycles.

Synchrotron X-ray Microdiffraction Images of Polarization Switching in Epitaxial PZT Capacitors with Pt and SrRuO3 Top Electrodes

ABSTRACTThe evolution of stored ferroelectric polarization in PZT thin film capacitors was imaged using synchrotron x-ray microdiffraction with a submicron-diameter focused incident x-ray beam. To form the capacitors, an epitaxial Pb(Zr,Ti)O3 (PZT) thin film was deposited on an epitaxially-grown conductive SrRuO3 (SRO) bottom electrode on a SrTiO3 (STO) (001) substrate. Polycrystalline SRO or Pt top electrodes were prepared by sputter deposition through a shadow mask and subsequent annealing. The intensity of x-ray reflections from the PZT film depended on the local ferroelectric polarization. With 10 keV x-rays, regions of opposite polarization differed in intensity by 26% in our PZT capacitor with an SRO top electrode. Devices with SRO electrodes showed just a 25% decrease in the remnant polarization after 107 switching cycles. In devices with Pt top electrodes, however, the switchable polarization decreased a by 70% after only 5×104 cycles.

Growth and piezoelectric properties of Pb(Yb1/2Nb1/2)O3−PbTiO3 epitaxial films

Epitaxial films of (1−x)Pb(Yb1/2Nb1/2)O3−xPbTiO3 (PYbN–PT, x=0.4,0.5) with SrRuO3 bottom electrodes were prepared on (100) LaAlO3, (100) SrTiO3, and (111) SrTiO3 substrates by pulsed laser deposition. It was found that vacuum annealing of the SrRuO3 before the deposition of PYbN–PT facilitated growth of perovskite PYbN–PT. With optimized growth conditions, (001) and (111) PYbN–PT epitaxial films with good phase purity were obtained in a range of 650–660 and 600–620 °C, respectively. The ferroelectric and transverse piezoelectric properties of these PYbN–PT films were investigated. In the (001) PYbN–PT (50/50) film, the highest remanent polarization (∼30 μC/cm2) and e31,f piezoelectric coefficient (−14 C/m2) were observed. The transition temperature of the (001) PYbN–PT (50/50) film was near 380 °C.

Synthesis and Properties of Lead Zirconate Titanate Thin Films Via Metalorganic Chemical Vapor Deposition

Ferroelectric PZT thin films were deposited on Pt and SrRuO3 substrates in a cold-wall reactor, using the Pb(C2H5)4/Zr(OBu)4/Ti(OPri)4/O2 reaction system. In comparison with Pt substrate, the growth rate of lead zirconate titanate (PZT) thin film was higher on SrRuO3. Lead content of the thin film deposited on either substrate at low temperatures (723–863 K) was much more temperature dependent than the other two metal contents. The strong temperature dependence originated from the high activation energy in the initial decomposition of Pb(C2H5)4 vapor, which was 54 kcal/mol. The surface reaction constant of lead precursor had much lower temperature dependence. The activation energy of surface reaction for PbO, estimated from deposition in a mini-chamber, was 6 kcal/mol on Pt and 9 kcal/mol on the SrRuO3 substrate. Therefore, the incorporation path of component oxide PbO, whose apparent activation energy was 31 kcal/mol on Pt and 29 kcal/mol on SrRuO3, essentially involved considerable gas-phase reaction. The PZT (50/50) thin film on SrRuO3 bottom electrode possessed a lower coercive field and a smaller remnant polarization than that on Pt. The PZT capacitor on SrRuO3 was also less vulnerable to polarization fatigue.

Growth, Structure, and Properties of Uniformly a-Axis Oriented Ferroelectric Bi3.25La0.75Ti3O12 Thin Films on Si(100) Substrates

ABSTRACTUniformly a-axis-oriented, epitaxially twinned Bi3.25La0.75Ti3O12 (BLT) thin films having the major spontaneous polarization entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates covered with very thin SrRuO3 bottom electrodes. Using SrRuO3 bottom electrodes of a specific low thickness in combination with a relatively high growth rate and a high oxygen pressure, the volume fraction of the BLT (100) orientation, which is competing with the BLT (118) orientation, was increased up to 99%. In this way the growth of fully a-axis-oriented BLT epitaxial films was achieved, attaining a remanent polarization of 32 mC/cm2. Initial fatigue experiments indicated hardly any fatigue after 109 switching cycles.

Growth, Structure, and Properties of Uniformly a-Axis Oriented Ferroelectric Bi3.25La0.75Ti3O12 Thin Films on Si(100) Substrates

ABSTRACTUniformly a-axis-oriented, epitaxially twinned Bi3.25La0.75Ti3O12 (BLT) thin films having the major spontaneous polarization entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates covered with very thin SrRuO3 bottom electrodes. Using SrRuO3 bottom electrodes of a specific low thickness in combination with a relatively high growth rate and a high oxygen pressure, the volume fraction of the BLT (100) orientation, which is competing with the BLT (118) orientation, was increased up to 99%. In this way the growth of fully a-axis-oriented BLT epitaxial films was achieved, attaining a remanent polarization of 32 mC/cm2. Initial fatigue experiments indicated hardly any fatigue after 109 switching cycles.

Growth and characterization of epitaxial ferroelectric PbZrxTi1−xO3 thin film capacitors with SrRuO3 electrodes for non-volatile memory applications

Non-volatile FeRAMs based on thin film ferroelectric capacitors are emerging as an advantageous replacement for EEPROM and flash devices. In this work, ferroelectric Al/PbZrxTi1−xO3 (PZT)/SrRuO3 thin film capacitors were fabricated and its suitability for memory applications was tested. The PZT films, as well as the SrRuO3 bottom electrodes have been grown by pulsed laser deposition on LaAlO3(001) substrates. Epitaxial heterostructures with high crystalline quality were obtained. Top aluminum electrodes were thermally evaporated onto the PZT films. High dielectric constant of 700 and dielectric loss tangent of 0.03 were obtained. Good ferroelectric behavior with an activation field for switching γ=308 kV/cm and a low coercive voltage of 1.1 V at 40 Hz was observed. The reliability of the thin film capacitors has been tested by means of pulse switching measurements. The polarization shows very little fatigue with cumulative switching. Built-in imprint, arising from the use of asymmetric electrodes, along with induced imprint related to charge trapping at the interfaces were observed.

Epitaxial growth of (103)-oriented ferroelectric SrBi2Ta2O9 thin films on Si(100)

Non-c-axis-oriented ferroelectric SrBi2Ta2O9 (SBT) epitaxial thin films with (103) orientation have been grown by pulsed laser deposition on buffered Si(100) substrates. For the buffer layers, a heterostructure consisting of MgO(111)/YSZ(100)/Si(100) was applied to induce the growth of a (111)-oriented SrRuO3 (SRO) bottom electrode. X-ray diffraction θ–2θ and φ scans revealed well-defined orientation relationships, viz. SBT(103)∥SRO(111)∥MgO(111)∥YSZ(100)∥Si(100); SBT[010]∥SRO[01̄1]∥MgO[01̄1]∥YSZ〈001〉∥Si〈001〉. The ferroelectric measurements of the (103)-oriented SBT films showed a remanent polarization (Pr) of 5.2 μC/cm2 and a coercive field (Ec) of 76 kV/cm for a maximum applied electric field of 440 kV/cm.

Transmission electron microscopy study of (103)-oriented epitaxial SrBi2Nb2O9 films grown on (111) SrTiO3 and (111) SrRuO3/(111) SrTiO3

Portions of the same epitaxial (103)-oriented SrBi2Nb2O9 film grown on (111) SrTiO3 for which we recently reported the highest remanent polarization (Pr) ever achieved in SrBi2Nb2O9 (or SrBi2Ta2O9) films, i.e., Pr = 15.7 μC/cm2, have been characterized microstructurally by plan-view and cross-sectional transmission electron microscopy (TEM) along three orthogonal viewing directions. SrBi2Nb2O9 grows with its c axis tilted 57° from the substrate surface normal in a three-fold twin structure about the substrate [111], with the growth twins' c axes nominally aligned with the three 〈100〉 SrTiO3 directions. (103) SrBi2Nb2O9 films with and without an underlying epitaxial SrRuO3 bottom electrode have been studied. Dark-field TEM imaging over a 12 μm2 area shows no evidence of second phases (crystalline or amorphous). A high density of out-of-phase boundaries exists in the films.

Growth and characterization of non-c-axis-oriented SrBi2Ta2O9 epitaxial thin films on Si(100) substrates with SrRuO3 bottom electrodes

We have successfully grown non-c-axis-oriented epitaxial ferroelectric SrBi2Ta2O9 (SBT) films with (116) and (103) orientations on Si(100) substrates using epitaxial (110)- and (111)-oriented SrRuO3 (SRO) bottom electrodes, respectively. The SRO orientations have been induced by coating the Si(100) substrates with epitaxial YSZ(100) and MgO(111)/ YSZ(100) buffer layers, respectively. All films were sequentially grown by pulsed laser deposition. Specific in-plane orientations of the epitaxial SBT films were found, which are in turn determined by specific in-plane orientations of the epitaxial SRO bottom electrodes. These include a diagonal rectangle-on-cube epitaxy of SRO(110) on YSZ(100) and a triangle-on-triangle epitaxy of SRO(111) on MgO(111).

Effects of annealing of (Ba,Sr)RuO3 bottom electrodes on the electrical properties of (Ba,Sr)TiO3 thin films

The annealing of (Ba,Sr)RuO3 films which are structurally and chemically matched with (Ba,Sr)TiO3 films was performed in oxygen and nitrogen atmospheres in the temperature range of 600–750 °C for 30 min. The effects of annealing the (Ba,Sr)RuO3 on the physical and electrical properties of the (Ba,Sr)TiO3 films were investigated. The x-ray diffraction peak of the annealed (Ba,Sr)RuO3 film at high temperatures revealed that the (Ba,Sr)TiO3 film is thermally more stable than RuO2. The (Ba,Sr)RuO3 film under N2 annealed showed lower electrical resistivity and larger surface roughness compared with those under O2. In addition, from the sequential two-step annealing process using O2 and N2, and by reversing the annealing sequence, the electrical resistivity and the surface roughness of the (Ba,Sr)RuO3 film turned out to be reversible. The (Ba,Sr)TiO3 film on the N2 annealed (Ba,Sr)RuO3 showed a higher leakage current than that on the O2 annealed bottom electrode because rough surface can cause a high local electric field. The dielectric constant of the (Ba,Sr)TiO3 thin film increased with the annealing temperature of the (Ba,Sr)RuO3 bottom electrode regardless of the annealing atmosphere.