SrRuO3-buffered SrTiO3 Substrates Research Articles

Growth of Samarium-Substituted Epitaxial Bismuth Ferrite Films by Chemical Vapor Deposition

There is great interest in BiFeO3 and various substituted heterostructures for ferroelectric random-access memory (RAM) and energy-efficient magnetoelectric logic and memory devices. High-quality heterostructures of these material systems with smooth topography are desirable to incorporate into functional devices. For this purpose, the direct liquid injection chemical vapor deposition (DLI-CVD) technique has been employed to grow BiFeO3 and Bi-site samarium-substituted epitaxial films on (001)-oriented SrTiO3 and SrRuO3-buffered SrTiO3 substrates. Structural and morphological characterizations of the films are carried out using X-ray diffraction, atomic force microscopy, and scanning electron microscopy techniques. The films show textured growth and exhibit smooth morphology with root mean square roughness below 2.5 nm. Ferroelectric characteristics and memory retention in thin films are demonstrated using piezoresponse force microscopy. The thickness scaling of the switching voltage has been analyzed to validate the Kay–Dunn law for Sm-substituted BiFeO3 and compared with that of pure BiFeO3.

Reduced Leakage Current and Enhanced Photovoltaic Effect in Zn-Doped BiFeO3 Thin Films

Zn-doped BiFeO3 (BFO) thin films with compositional formula BiFe1–xZnxO3 (x = 0, 0.1, and 0.2) have been epitaxially grown on SrRuO3 buffered SrTiO3 substrates by pulsed laser deposition. The high-concentration Zn doping does not suppress the ferroelectric polarization of the BFO films, and the Zn-doped BFO thin films also show reduced leakage current and an enhanced photovoltaic effect. By optical and photoelectron spectroscopy measurements, with Zn doping, the BFO thin films show more oxygen vacancies and a structural evolution, and moreover, a blue-shift of optical bandgap and an increase of work function are demonstrated. The reduction of leakage current and the enhancement of photovoltaic effect are related to the variation of interfacial Schottky junctions and oxygen vacancies with doping. This study reveals systematic insights into the effects of Zn doping on the physical properties of BFO thin films.

Strong modulation of magnetism in multiferroic CoFe2O4/0.7 Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 oxide composite films

Multiferroic magnetoelectric composite films have been becoming the research focus as the candidates for sensors, memories, spintronics and microwave devices. Compared to ferroelectric single crystal consisting of the heterostructure, the functional composite film realizes the magnetism control under only a few volts which brings the advantage of safety and low energy consumption in the memory devices. Here, the CoFe2O4/0.7PbMg1/3Nb2/3O3-0.3PbTiO3 oxide composite thin films were deposited on SrRuO3-buffered SrTiO3 substrate by pulsed laser deposition method. The structural, ferroelectric, magnetic and magnetoelectric properties have been studied respectively. The polarization reversibility of ferroelectric domain was investigated in the PMN-PT films by piezoresponse force microscopy. The coercive field of the CoFe2O4 film ranges from 900 Oe to 975 Oe under the applied direct current voltage. By performing the alternative current voltage modulated magneto-optical Kerr effect test, the dominating coupling mechanism in the composite films influenced by the CoFe2O4 film thickness was discussed. Our study indicates the potential application of electric field assisted magnetic data storage device.

Interfacial dielectric layer as an origin of polarization fatigue in ferroelectric capacitors

Origins of polarization fatigue in ferroelectric capacitors under electric field cycling still remain unclear. Here, we experimentally identify origins of polarization fatigue in ferroelectric PbZr0.52Ti0.48O3 (PZT) thin-film capacitors by investigating their fatigue behaviours and interface structures. The PZT layers are epitaxially grown on SrRuO3-buffered SrTiO3 substrates by a pulsed laser deposition (PLD), and the capacitor top-electrodes are various, including SrRuO3 (SRO) made by in-situ PLD, Pt by in-situ PLD (Pt-inPLD) and ex-situ sputtering (Pt-sputtered). We found that fatigue behaviour of the capacitor is directly related to the top-electrode/PZT interface structure. The Pt-sputtered/PZT/SRO capacitor has a thin defective layer at the top interface and shows early fatigue while the Pt-inPLD/PZT/SRO and SRO/PZT/SRO capacitor have clean top-interfaces and show much more fatigue resistance. The defective dielectric layer at the Pt-sputtered/PZT interface mainly contains carbon contaminants, which form during the capacitor ex-situ fabrication. Removal of this dielectric layer significantly delays the fatigue onset. Our results clearly indicate that dielectric layer at ferroelectric capacitor interfaces is the main origin of polarization fatigue, as previously proposed in the charge injection model.

Orientation dependent leakage current behaviors and ferroelectric polarizations of off-axis sputtered BiFeO3 thin films

Epitaxial BiFeO3 thin films with (100), (110) and (111) orientations were grown on the SrRuO3-buffered SrTiO3 substrates by using an off-axis magnetron sputtering. Unlike the BiFeO3(110) and BiFeO3(111) thin films that exhibited a single rhombohedral phase structure, a dominant rhombohedral phase accompanying with a small amount of tetragonal phase was identified in the BiFeO3(100) thin film. In particular, the leakage currents and ferroelectric polarizations of sputtered BiFeO3 thin films were focused on and these films showed utterly different current density-electric field (J-E) behaviors whether in the positive or negative electric field. Among the three films, the ferroelectric polarization of the BiFeO3(100) thin film presented a good frequency stability and had the maximum remnant polarization of Pr ∼ 78 μC/cm2 @ 10 kHz, which could be further demonstrated by pulsed polarizations of films. The distinct differences in electrical properties of orientation-engineered BiFeO3 thin films in present case can be attributed to their different crystallographic orientations and microstructures.

Interface charge behaviors of BaTiO3 film heterostructures with various crystal orientations

Heteroepitaxial BaTiO3 ferroelectric films with (001), (110), and (111) orientations were grown on SrRuO3-buffered SrTiO3 substrates by magnetron sputtering. The leakage current and interface charge behaviors were systematically investigated. Without a discernible orientation-dependence behavior, the leakage current behaviors were all well described by a modified Schottky-contact model. On the basis of this theory, the interface charge state parameters, including dynamic dielectric constant, potential barriers, depletion layer width, effective space-charge density and hole concentration, and their evolution behaviors were analyzed in detail. They all exhibited anisotropic characteristics and were proved to be essentially attributed to the macrophysical properties of BaTiO3 film heterostructures.

Crystallographic orientation dependent dielectric properties of epitaxial BaTiO3 thin films

Epitaxial BaTiO3 ferroelectric films with (001), (110), and (111) orientations were, grown on SrRuO3 buffered SrTiO3 substrates via a radio-frequency magnetron sputtering method. The effects of the orientation on dielectric response were systematically investigated. We found that the (110)-oriented film exhibited the largest dielectric permittivity, tunability and phase transition temperature, whereas the (111)-oriented film demonstrated the best figure of merits if the dielectric loss was taken into consideration. The large anisotropic dielectric behavior, which is predominantly attributed to the differences of polarization states in these orientation engineered BaTiO3/SrTiO3 heterostructures, was observed. These results suggest the (111)-oriented film may be an attractive candidate for applications in dielectric tunable devices.

Enhanced switching characteristics and piezoelectric response in epitaxial BiFeO3–TbMnO3 thin films

High-quality (001) oriented epitaxial 0.9BiFeO3–0.1TbMnO3 thin films were grown on La2/3Sr1/3MnO3 and SrRuO3 buffered SrTiO3 substrate using pulsed laser deposition. X-ray diffraction showed that the films are single-phase perovskite without secondary impurity phases. Domain structures and upward ferroelectric self-poling phenomenon were distinctly observed in both films with compressive epitaxial strains. Furthermore, the upward self-poling disappears in polycrystalline 0.9BiFeO3–0.1TbMnO3 thin film on Pt/TiO2/SiO2/Si substrates. Through local switching spectroscopy measurements, the evidence of enhanced ferroelectric switching and piezoelectric response characteristics have been provided.

Growth and local structure of BiFeO3 thin films with giant tetragonality on SrRuO3-buffered SrTiO3(001) substrate by ion beam sputtering

Rhombohedral (R-) and tetragonal-like (T-) BiFeO3 (BFO) mixed-phase thin films and single R-phase BFO thin films have been grown on SrRuO3-buffered SrTiO3 (STO) (001) substrate by ion beam sputtering. From X-ray diffraction study, it was found that T-BFO and R-BFO mixed-phase thin films can be grown at a low oxygen partial pressure of 1 mPa, and the lattice parameters of the T-BFO and R-BFO were a = 0.381 nm, c = 0.461 nm (c/a = 1.22), and a = 0.394 nm and c = 0.400 nm (c/a = 1.02), respectively. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation and energy dispersive X-ray analysis (EDXA) mappings have revealed a clear T-BFO lattice and a–c domain in the middle of the thin films. However, Bi-rich and Fe-rich secondary phases were also found with the T-BFO phase. Therefore, it can be concluded that secondary phases such as Bi2O3 or Bi-poor phase are needed for T-BFO formation on STO (001) substrate, and the a–c domain formation does not occur by stress relaxation but matching of the growth directions of different domains.

Influence of Lattice Distortion Induced by a Vicinal SrTiO3(001) Substrate in Single-Domain BiFeO3 Thin Films Prepared by Radio Frequency Planar Magnetron Sputtering

Single-domain BFO thin films are prepared on a SrRuO3-buffered SrTiO3 (STO) (001) substrate by RF planar magnetron sputtering. A domain structure is controlled by vicinal direction of the STO substrate. The BFO thin films on vicinal STO along <110> show single-domain structure without any domain walls. To confirm the influence of epitaxial strain on lattice distortion and ferroelectricity, single-domain BFO thin films with thicknesses ranging from 10–1000 nm are prepared. Synchrotron X-ray diffraction reveals that lattice relaxation and step bunching occur in the thickness range of 50–200 nm. The BFO films with thicknesses over 300 nm are almost free from the influence of the epitaxial strain induced by (001)-oriented substrates. The remanent polarization Pr is almost constant at about 60 µC/cm2. However, Pr slightly increases in the BFO films with thicknesses less than 200 nm. Even the 100-nm-thick BFO film show fully saturated D–E hysteresis at RT, and the Pr is 65 µC/cm2.