SrRuO3 Bottom Electrode Research Articles

Is Ba3In2O6 a high-Tc superconductor?

It has been suggested that Ba3In2O6 might be a high-Tc superconductor. Experimental investigation of the properties of Ba3In2O6 was long inhibited by its instability in air. Recently epitaxial Ba3In2O6 with a protective capping layer was demonstrated, which finally allows its electronic characterization. The optical bandgap of Ba3In2O6 is determined to be 2.99 eV in-the (001) plane and 2.83 eV along the c-axis direction by spectroscopic ellipsometry. First-principles calculations were carried out, yielding a result in good agreement with the experimental value. Various dopants were explored to induce (super-)conductivity in this otherwise insulating material. Neither A- nor B-site doping proved successful. The underlying reason is predominately the formation of oxygen interstitials as revealed by scanning transmission electron microscopy and first-principles calculations. Additional efforts to induce superconductivity were investigated, including surface alkali doping, optical pumping, and hydrogen reduction. To probe liquid-ion gating, Ba3In2O6 was successfully grown epitaxially on an epitaxial SrRuO3 bottom electrode. So far none of these efforts induced superconductivity in Ba3In2O6, leaving the answer to the initial question of whether Ba3In2O6 is a high-Tc superconductor to be ‘no’ thus far.

Influence of Lattice Mismatch on Structural and Functional Properties of Epitaxial Ba0.7Sr0.3TiO3 Thin Films.

Substrate-induced strains can significantly influence the structural properties of epitaxial thin films. In ferroelectrics, this might lead to significant changes in the functional properties due to the strong electromechanical coupling in those materials. To study this in more detail, epitaxial Ba0.7Sr0.3TiO3 films, which have a perovskite structure and a structural phase transition close to room temperature, were grown with different thicknesses on REScO3 (RE-rare earth element) substrates having a smaller lattice mismatch compared to SrTiO3. A fully strained SrRuO3 bottom electrode and Pt top contacts were used to achieve a capacitor-like architecture. Different X-ray diffraction techniques were applied to study the microstructure of the films. Epitaxial films with a higher crystalline quality were obtained on scandates in comparison to SrTiO3, whereas the strain state of the functional layer was strongly dependent on the chosen substrate and the thickness. Differences in permittivity and a non-linear polarization behavior were observed at higher temperatures, suggesting that ferroelectricity is supressed under tensile strain conditions in contrast to compressive strain for our measurement configuration, while a similar reentrant relaxor-like behavior was found in all studied layers below 0°C.

Interface engineering for substantial performance enhancement in epitaxial all-perovskite oxide capacitors

Capacitors based on ABO3-type perovskite oxides show considerable promise for overcoming the limitations of nanoscale integration with dynamic random access memory (DRAM) devices. Among the thermodynamically stable perovskite oxides, titanates (ATiO3) exhibit high dielectric permittivity in metal–insulator–metal (MIM) configurations. However, their performance in mitigating the large leakage current caused by their narrow bandgap (3 eV) remain under scrutiny. Herein, substantially enhanced dielectric properties of an epitaxial SrRuO3/Ba0.5Sr0.5TiO3/SrRuO3 MIM capacitor with a thin dielectric layer (10 nm) are reported. The dielectric/electrode heterointerface was engineered to realize a capacitor with a low leakage current and high dielectric permittivity. A pit-free and stoichiometric SrRuO3 bottom electrode with an atomically smooth surface was exploited to suppress defect formation at the heterointerface. The critical roles of oxygen vacancies and substituted transition-metal atoms in determining the leakage current were assessed, and a strategy for reducing the leakage current via interface engineering was established. Consequently, a dielectric permittivity of 861 and a leakage current density of 5.15 × 10−6 A/cm2 at 1 V were obtained with the thinnest dielectric layer ever reported. Our work paves the way for the development of perovskite-oxide-based capacitors in next-generation DRAM memories.

Characterization of electro-optic coefficients r13, r23 and r33 in single crystalline BaTiO3 thin films

We report experimental characterization of DC electro-optic coefficients r13, r23 and r33 in thin film single crystalline BaTiO3 on DyScO3. The epitaxial BaTiO3 thin film is sandwiched between a SrRuO3 bottom electrode and an indium tin oxide (ITO) top electrode. We characterize the DC electro-optic coefficients r13, r23, and r33 by the prism coupling method at 636.6 nm for the BaTiO3 thin film. The results show the DC coefficients r13 = 4.2 pm/V, r23 = 4.6 pm/V and r33 = 9.0 pm/V.

Dielectric and Antiferroelectric Properties of AgNbO3 Films Deposited on Different Electrodes

AgNbO3 antiferroelectric materials have become a hot topic due to their typical double polarization–electric field loops. AgNbO3 films usually exhibit superior properties to bulks. In this work, AgNbO3 films were fabricated via the pulsed laser deposition on (001) SrTiO3 substrate with (La0.5Sr0.5)CoO3, LaNiO3 and SrRuO3 bottom electrodes, in which the (La0.5Sr0.5)CoO3, LaNiO3 and SrRuO3 bottom electrodes were used to regulate the in-plane compressive stress of AgNbO3 films. It is found that AgNbO3 films deposited on (La0.5Sr0.5)CoO3, LaNiO3 and SrRuO3 bottom electrodes are epitaxial with dense microstructure. In changing the bottom electrodes from (La0.5Sr0.5)CoO3, LaNiO3 to SrRuO3, the in-plane compressive stress of AgNbO3 thin films becomes weaker, which leads to increased relative dielectric permittivity and reduced antiferroelectric–ferroelectric phase transition electric field EF from 272 kV/cm to 190 kV/cm. The reduced EF implies weakened antiferroelectric stability in AgNbO3 films. It can be seen that the antiferroelectric stability of AgNbO3 films could be regulated by changing the bottom electrodes.

Interface barrier-induced conversion of resistive switching mechanism in Mn-doped BiFeO3 memristor

Different from conductive filament (CF)-type counterparts, interface-type devices exhibit continuously gradual conductance changes, making them the potential for artificial synapses. In this paper, Mn-doped BiFeO3 (BFMO) devices with SrRuO3 and TiN bottom electrodes demonstrate the clear CF rather than the interface barrier type resistance-switching feature due to the high Schottky barrier. Considering the measured electron affinity of 3.52 eV and work function of 4.22 eV in the as-synthesized BFMO film (a weak n-type semiconductor, marked as n−), we fabricated a hetero-junction device with the Nb-doped SrTiO3 (NSTO) bottom electrode (a strong n-type semiconductor, marked as n+) exhibiting analog switch characteristics. The n−–n+ hetero-junction between BFMO and NSTO reverses the operation polarity and leads to a barrier transition-dominated conductive behavior in the BFMO-based memristor. The device shows a large ON/OFF ratio over 1200, favorable stability after 104 s, continual multi-value characteristics, symmetrical long-term potentiation and depression, and synaptic plasticity with about 80 ns time constant. The investigation of resistive switching features, band structure, and synapse performance in this work provides a reference for the application of BiFeO3 in the field of the memristor.

Stable retention in SrTiO3/SrRuO3 heterostructure-based memristive devices

The switching dynamics in memristive devices resembles biological synapses, which makes these devices promising candidates for the development of artificial neural networks. The retention of the synaptic weight is a key parameter in performing artificial intelligent tasks, particularly the inference process. However, many memristive devices show retention loss over time, especially the oxide device with switching behavior caused by oxygen migration. In this work, we report a memristive device based on the structure of Pt/SrTiO3/SrRuO3, which greatly improves the retention time of the device. Based on the investigation of the electrical transport mechanism and interface microstructure, the retention improvement in present devices is due to the restriction of oxygen vacancy migration through the SRO-rich interface layer formed on the surface of the SrRuO3 bottom electrode. Given the patternable bottom electrode, linear conductance modulation, and excellent performance in neural network simulation, the present device has shown great potential for hardware neuromorphic computing applications.

Synthesis and electrical behavior of VO2 thin films grown on SrRuO3 electrode layers

VO2 thin films were grown on conducting oxide underlayer SrRuO3 buffered SrTiO3 (111) and Si/SiO2 substrates, respectively, using sputtering. X-ray diffraction phi-scans revealed the epitaxial nature of the VO2 films grown on SrRuO3 buffered SrTiO3 and polycrystalline structure for films grown on SrRuO3 buffered Si/SiO2. X-ray photoelectron spectroscopy confirms a dominant presence of V4+ in both films and establishes a high-quality growth of single-phase VO2 films. Temperature and electric-field driven metal-insulator-transition in both the in-plane and out-of-plane configurations were investigated. Depending on the configuration, the resistance change across the metal-insulator-transition varies from a factor of 1.57–3. The measured resistance in each state as well as the magnitude of resistance change were similar during temperature and electric-field driven metal-insulator-transition. To shed light on the suppressed metal-insulator-transition characteristics due to the current shunting effect from conducting SrRuO3 bottom electrode, a distributed resistance network model is proposed and benchmarked against reports from the literature. The results demonstrate the growth of high-quality VO2 on conducting SrRuO3 layers and their electrical behavior, which is of particular interest for all-oxide electronic devices utilizing phase transitions such as resistive memory and neuromorphic oscillators.

Electrostatically Driven Polarization Flop and Strain-Induced Curvature in Free-Standing Ferroelectric Superlattices.

The combination of strain and electrostatic engineering in epitaxial heterostructures of ferroelectric oxides offers many possibilities for inducing new phases, complex polar topologies, and enhanced electrical properties. However, the dominant effect of substrate clamping can also limit the electromechanical response and often leaves electrostatics to play a secondary role. Releasing the mechanical constraint imposed by the substrate can not only dramatically alter the balance between elastic and electrostatic forces, enabling them to compete on par with each other, but also activates new mechanical degrees of freedom, such as the macroscopic curvature of the heterostructure. In this work, an electrostatically driven transition from a predominantly out-of-plane polarized to an in-plane polarized state is observed when a PbTiO3 /SrTiO3 superlattice with a SrRuO3 bottom electrode is released from its substrate. In turn, this polarization rotation modifies the lattice parameter mismatch between the superlattice and the thin SrRuO3 layer, causing the heterostructure to curl up into microtubes. Through a combination of synchrotron-based scanning X-ray diffraction imaging, Raman scattering, piezoresponse force microscopy, and scanning transmission electron microscopy, the crystalline structure and domain patterns of the curved superlattices are investigated, revealing a strong anisotropy in the domain structure and a complex mechanism for strain accommodation.

Selective Growth and Micropatterning Technique for Oxide Thin Films by Sacrificial a-CaO Layer

Selective growth and micropatterning through a water lift-off (WLO) process utilizing a sacrificial amorphous CaO (a-CaO) layer were demonstrated to fabricate hetero-epitaxial all-oxides film capacitors. Patterned Pb(Zr,Ti)O3 (PZT) and SrRuO3 (SRO) films were grown on (100) SrTiO3 (STO) substrate by pulsed laser deposition, and the subsequent lift-off process for target oxides was carried out using pure water. The results showed that selective growth and micropatterning of an all-oxides capacitor structure could be realized through WLO process using a-CaO as the sacrificial layer. Electron backscattered diffraction results revealed that SRO/PZT/SRO capacitor structures were grown hetero-epitaxially on the STO substrate. Furthermore, the electrical properties of patterned SRO/PZT/SRO film capacitors could be improved by suppressing the height of overgrown parts of the SRO bottom electrodes with a thick a-CaO sacrificial layer.

Optimization of SrRuO3 bottom electrodes fabricated by RF magnetron ion sputtering for new generation devices

Multiferroic materials show great academic and technological interest for their role in new generation devices due to the coexistence of magnetic, ferroelectric and elastic ordering. In this article the fabrication of SrRuO3 (SRO) thin films by the RF magnetron ion sputtering technique is presented where the deposition conditions, namely deposit time, substrate temperature and RF power were optimized. The growth of the SRO phase was evaluated using X-ray diffraction. The homogeneity of the films was analysed using energy dispersive X-ray spectroscopy. Film thickness and optical properties were determined by ellipsometry.

Evolution of structure and electrical properties of epitaxial BiFeO3 thin films through solution and annealing atmosphere

Abstract Solution derived high-quality epitaxial BiFeO3 (BFO) thin films, with robust spontaneous polarization, are still desired because of their advantages. Herein, epitaxial BFO thin films and SrRuO3 bottom electrodes are prepared on LaAlO3 substrates via chemical solution deposition (CSD) technique. The influences of precursor solution on the microstructure and electrical properties are investigated by using nitrate salt and acetate salt system. The XRD and P-E hysteresis loops results demonstrate that the BFO thin film prepared by acetate salt system as a relatively green CSD route shows good epitaxial characteristic and ferroelectric performances. In addition, the effect of the annealing atmosphere on the microstructure and electrical properties are studied for the acetate salt derived BFO thin films. The microstructure and defects chemistry are analyzed by SEM and XPS, the BFO thin film annealed in air shows better comprehensive ferroelectric performances with the lowest leakage current and enhanced ferroelectric polarization due to the improved microstructure and decreased defect concentrations. The results here thus provide an instructive approach to optimize the growth of solution-derived epitaxial BFO-based thin films.

Comparison of Epitaxial and Textured Ferroelectric BaTiO<sub>3</sub> Thin Films

The properties of BaTiO3 (BTO) thin films deposited on different substrates by RF magnetron sputtering were investigated. Two representative substrates were selected and different heterostructures were studied. 1) SrTiO3 (STO) single crystals as a bulk oxide reference material, and 2) silicon as a semiconductor. SrRuO3 (SRO) and Pt bottom electrodes were deposited on the silicon substrate. The BTO structural characterizations show that all the films have (001) crystallographic orientation. We have compared the electrical properties of the different samples: the same dielectric constant and polarization values were obtained independently of the nature of the substrate.

Plan-view sample preparation of a buried nanodots array by FIB with accurate EDS positioning in thickness direction

Recently, there are growing demands on focus ion beam (FIB) sample preparation technique in plan-view geometry because it can provide the in-plane microstructure information of thin film and allows direct correlations of the atomic structure via transmission electron microscopy with micrometer-scale property measurements. However, one main technical difficulty is to position the buried thin film accurately in a sandwich structure. In this paper, an on-line positioning method based on the thickness monitoring by EDS is introduced, where the intensities of the characteristic X-ray peaks from different layers are proportional to the relative thickness of them at the same acquisition conditions. A high density array of ∼100 nm squares BiFeO3 nanodots with ∼ 25 nm thickness grown on a 20 nm thick SrRuO3 bottom electrode and (001)-oriented SrTiO3 substrate is selected for demonstration. By monitoring the intensities of Pt-M, Sr-L, Ti-K, Ru-L, Fe-K and Bi-M peaks, the relative thickness of Pt protection layer, the BiFeO3, SrRuO3 and SrTiO3 can be obtained, which provide accurate position of the BFO nanodots array in the thickness direction. With these information, the cutting parameters are optimized and a high quality plan-view specimen of BFO nanodots array is prepared, which is confirmed by high resolution transmission electron microscopy. This positioning method should have a wide application for material science.

All-Inorganic Flexible Ba0.67Sr0.33TiO3 Thin Films with Excellent Dielectric Properties over a Wide Range of Frequencies.

With rapid advances in flexible electronics and communication devices, flexible dielectric capacitors exhibiting high permittivity, low loss, and large electric-field tunability over a wide frequency range have attracted increasing attention. Here, a large-scale Ba0.67Sr0.33TiO3 (BST) dielectric thin film sandwiched between SrRuO3 (SRO) bottom electrode and Pt top electrode is fabricated on a flexible mica substrate. The mica/SRO/BST/Pt capacitor exhibits a dielectric constant (εr') of more than 1200, a loss tangent [tan(δ)] as low as 0.16, and a tunability of 67% at low frequencies around 10 kHz. Simultaneously, the capacitor can retain an εr' of 540 and a tan(δ) of 0.07 at microwave frequencies, e.g., 18.6 GHz. Moreover, even when the capacitor is bent to a small radius of 5 mm or undergoes 12 000 bending cycles (at 5 mm radius), almost no deterioration in εr', tan(δ), and tunability is observed. The excellent dielectricity and mechanical flexibility and durability endow the mica/SRO/BST/Pt capacitor with huge potential for flexible electronic and microwave applications.

Characteristics of MoS2 monolayer non-volatile memory field effect transistors affected by the ferroelectric properties of BiFeO3 thin films with Pt and SrRuO3 bottom electrodes grown on glass substrates

We investigated the characteristics of ferroelectric non-volatile random-access memory field effect transistors (MoS2-BFO NVRAM FETs) consisting of monolayer MoS2 channels, BiFeO3 (BFO) gate layers, and Pt and SrRuO3 bottom electrodes on glass substrates. As a result of the comparative experiments of the Pt and SrRuO3 bottom electrodes, it was confirmed that the BFO gate layer using the SrRuO3 bottom electrode showed a higher remanent polarization, faster switching time, and superior fatigue endurance than the BFO gate layer using the Pt bottom electrode. It is suggested that the good ferroelectric properties of the BFO gate dielectric layer on the SrRuO3 bottom electrode is resulting from its (001)-oriented growth. The MoS2 BFO NVRAM FET with a SrRuO3 bottom electrode exhibited a wider memory window than the Pt bottom electrode because the SrRuO3 bottom electrode had a higher remanent polarization of the BFO gate layer compared to the Pt bottom electrode.

Strong impact of SrTiO3/TiO2 buffer layer on epitaxial growth and dielectric response of Ba0.7Sr0.3TiO3 thin films on MgO

We demonstrate a significant improvement in the epitaxial growth of perovskite dielectric films on MgO by introducing a SrTiO3 (STO)/TiO2 buffer layer, which enhances their dielectric response. 270-nm-thick (001)-epitaxial (Ba,Sr)TiO3 (BST) films were deposited by pulsed laser deposition on STO/TiO2-buffered MgO with a SrRuO3 (SRO) bottom electrode. The film directly deposited on SRO/MgO grew in a three-dimensional mode, resulting in a rough and poorly crystalline film with an almost relaxed strain. On the other hand, the film with a buffer layer grew in a two-dimensional mode, resulting in a flat and highly crystalline film with a large compressive strain (−0.80%). As a result, the paraelectric-to-ferroelectric phase transition temperature increased by 220 °C and an out-of-plane dielectric constant exceeding 1000 was achieved.

High-Performance Photovoltaic Readable Ferroelectric Nonvolatile Memory Based on La-Doped BiFeO3 Films.

Epitaxial La0.1Bi0.9FeO3 (LBFO) films with SrRuO3 (SRO) bottom electrodes were fabricated on SrTiO3(001) substrates by magnetron sputtering. The LBFO thin films exhibit strong ferroelectric properties. Nonvolatile reversible resistance switchings and switchable photovoltaic effects controlled by electric field have been observed in Pt/LBFO/SRO heterostructures. With the optimized LBFO film thickness, the observed room temperature pulsed-read resistance switching ratio can reach 105% magnitude by applying ±2.7 V pulse voltages. Besides, the observed ferroelectric switchable photovoltaic effect in the visible wavelength range shows a large tunable open-circuit photovoltage from -75 to -330 mV. The switching mechanisms in resistance and photovoltaic effects are demonstrated to be directly related to the ferroelectric reversal, which can be attributed to the polarization-modulated interfacial barriers and deep trap states.

Improved Polarization Retention of BiFeO3 Thin Films Using GdScO3 (110) Substrates**Supported by the National Basic Research Program of China under Grant No 2014CB921004, and the National Natural Science Foundation of China under Grant No 61225020.

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 BiFeO3 thin films with SrRuO3 bottom electrodes on both GdScO3 (110) and SrTiO3 (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 GdScO3 substrates show no preferred domain variants in comparison with the preferred downward polarization orientation toward bottom electrodes on SrTiO3 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 GdScO3 substrate over a measuring time of 500 s, unlike the preferred domain orientation on SrTiO3, 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.

Highly Oriented Growth of Piezoelectric Thin Films on Silicon Using Two-Dimensional Nanosheets as Growth Template Layer.

Ca2Nb3O10 (CNOns) and Ti0.87O2 (TiOns) metal oxide nanosheets (ns) are used as a buffer layer for epitaxial growth of piezoelectric capacitor stacks on Si and Pt/Ti/SiO2/Si (Pt/Si) substrates. Highly (001)- and (110)-oriented Pb(Zr0.52Ti0.48)O3 (PZT) films are achieved by utilizing CNOns and TiOns, respectively. The piezoelectric capacitors are characterized by polarization and piezoelectric hysteresis loops and by fatigue measurements. The devices fabricated with SrRuO3 top and bottom electrodes directly on nanosheets/Si have ferroelectric and piezoelectric properties well comparable with devices that use more conventional oxide buffer layers (stacks) such as YSZ, CeO2/YSZ, or SrTiO3 on Si. The devices grown on nanosheets/Pt/Si with Pt top electrodes show significantly improved polarization fatigue properties over those of similar devices grown directly on Pt/Si. The differences in properties are ascribed to differences in the crystalline structures and the density of the films. These results show a route toward the fabrication of single crystal piezoelectric thin films and devices with high quality, long-lifetime piezoelectric capacitor structures on nonperovskite and even noncrystalline substrates such as glass or polished metal surfaces.