SrO Buffer Layer Research Articles

Complex exchange coupling mechanisms in SRO/BFO/Fe heterostructures

Temperature dependent interfacial coupling mechanisms in SRO/BFO/Fe layered structures were investigated. The BFO/Fe heterostructures were prepared by PLD and sputtering, respectively, on the STO(0 0 1) substrate with a 20 nm SRO buffer layer. An annealing treatment in external magnetic field was further applied. Complex characterizations with X-ray diffraction, atomic force microscopy, Transmission Electron Microscopy, Mössbauer spectroscopy, magneto-optic Kerr effect and SQUID magnetometry were performed. Before annealing, the films show good crystallization and epitaxy of the SRO and BFO layers with smooth interfaces. Two coupling mechanisms of the ferromagnetic layers (top Fe and bottom SRO, respectively) to the epitaxial BFO film with mainly antiferromagnetic structure were evidenced in the as deposited samples at low temperatures. Negative exchange bias fields of up to 67(10) Oe and 37(5) Oe at low temperatures were observed for the two ferromagnetic components, respectively, depending on the thickness of the Fe layer. The field annealing treatments induce a specific morphology and magnetic spin structure at both interfaces of the BFO spacer layer, giving rise to a long range magnetostatic coupling between the two ferromagnetic films, in addition to the interfacial couplings. Moreover, the experimentally evidenced Fe clusters penetrating the BFO/Fe interface toward the BFO layer give support for this interaction. As an additional consequence, a considerable enhancement of both uniaxial and unidirectional anisotropies as well as an increased blocking temperature of exchange bias were obtained. The involved exchange coupling mechanisms were discussed in detail.

Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides

We report the successful growth of high-quality SrO films on highly-ordered pyrolytic graphite (HOPG) and single-layer graphene by molecular beam epitaxy. The SrO layers have (001) orientation as confirmed by X-ray diffraction (XRD) while atomic force microscopy measurements show continuous pinhole-free films having rms surface roughness of <1.5Å. Transport measurements of exfoliated graphene after SrO deposition show a strong dependence between the Dirac point and Sr oxidation. Subsequently, the SrO is leveraged as a buffer layer for more complex oxide integration via the demonstration of (001) oriented SrTiO3 grown atop a SrO/HOPG stack.

Control of SrO buffer-layer formation on Si(001) using the pulsed-laser deposition technique

The overview of optimal parameters for deoxidation of the Si(001) surface using SrO and a pulsed-laser deposition method.

Fabrication of single crystalline EuO thin film with SrO buffer layer on SrTiO3 substrate

We report the new fabrication method of single crystalline EuO thin film along the (100) plane and its crystal structure and magnetic property. EuO thin film was fabricated on SrO buffer layer created on SrTiO3 substrate. The obtained EuO thin film was formed a single phase of the rock-salt structure with the lattice constant of 0.516 nm and showed ferromagnetism below the Curie temperature of 71 K. The examined physical properties are consistent with the bulk materials.

Epitaxial SrRuO3 thin films deposited on SrO buffered-Si(001) substrates for ferroelectric Pb(Zr0.2Ti0.8)O3 thin films

SrRuO3 thin film electrodes are epitaxially grown on SrO buffered-Si(001) substrates by pulsed laser deposition. The optimum conditions of the SrO buffer layers for epitaxial SrRuO3 films are a deposition temperature of 700 °C, deposition pressure of 1 × 10−6 Torr, and thickness of 6 nm. The 100 nm thick-SrRuO3 bottom electrodes deposited above 650 °C on SrO buffered-Si (001) substrates have a rms (root mean square) roughness of approximately 5.0 A and a resistivity of 1700 µω-cm, exhibiting an epitaxial relationship. The 100 nm thick-Pb(Zr0.2Ti0.8)O3 thin films deposited at 575 °C have a (00l) preferred orientation and exhibit 2Pr of 40 µC/cm2, Ec of 100 kV/cm, and leakage current of about 1 × 10−7 A/cm2 at 1 V. The silicon oxide phase which presents within PZT and SrRuO3 films, influences the crystallinity of the PZT films and the resistivity of the SrRuO3 electrodes.

Role of SrRuO&lt;sub&gt;3&lt;/sub&gt; Buffer Layers in Enhancing Resistance Changing of Pr&lt;sub&gt;0.7&lt;/sub&gt;Ca&lt;sub&gt;0.3&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; Films

Resistance-switching behaviors of the Pr0.7Ca0.3MnO3(PCMO) films based metalinsulator- metal (MIM) devices has been investigated. In this work, resistance change of PCMO films deposited with SRO buffer layers by using RF-magnetron sputtering system investigated at room temperature. The ratio of the resistance change of the PCMO films with SRO buffer layers in the high-resistance state to that in the low-resistance state turned out to be much lager than that of the PCMO films without SRO buffer layers. Moreover, The reproducible property of the fabricated samples were improved. Origin of resistance change is not clear, but PCMO films with SRO buffer layers have the possibility of application for nonvolatile memory device.

Multiferroic BiFeO3 thin films deposited on SrRuO3 buffer layer by rf sputtering

Sr Ru O 3 (SRO) acts as an effective buffer layer for growth of multiferroic BiFeO3 (BFO) thin films deposited on Pt∕TiO2∕SiO2∕Si substrates by radio frequency sputtering. Phase identification by using x-ray diffraction and texture studies by using atomic force microscopy show that the SRO buffer layer promotes crystallization and formation of the perovskite phase at lowered temperature. It significantly reduces the leakage current of multiferroic BFO films, giving rise to a much improved square ferroelectric hysteresis loop, in contrast to the poor loop for BFO on bare Pt∕TiO2∕SiO2∕Si substrate. A much enlarged remnant polarization (2Pr) of 144μC∕cm2 and a coercive field (Ec) of 386kV∕cm were obtained with the BFO thin film deposited on SRO∕Pt∕TiO2∕SiO2∕Si at 600°C. The BFO thin film with SRO buffer layer also shows a large nonvolatile polarization (ΔP=Psw−Pnsw) of 122μC∕cm2 at 20μs, which promises excellent performance for random access memories. Further interestingly, it exhibits little polarization fatigue up to 5×1010 switching cycles, at a relatively high voltage of 10V, although a notable degradation of polarization is shown at the low voltage of 6V, indicating weak domain pinning. The multiferroic thin film demonstrates a weak ferromagnetic loop with a saturation magnetization (Ms) of 1.92emu∕cm3 and coercivity (Hc) of 325Oe.

Crystal growth of SrTiO 3 films on H-terminated Si(1 1 1) with SrO buffer layers

Growth of epitaxial SrTiO 3 (STO) films has been examined on H-terminated Si(1 1 1) with SrO buffer layers. The epitaxial SrO buffer layers have reduced stress on H-terminated Si substrates. On the SrO buffer layers, the STO films grow epitaxially with triple domains at low temperature. Each STO domain has equivalent epitaxial relationship to SrO buffer layers, STO(1 1 0)∥SrO(1 1 1) and STO [ 0 0 1 ] ∥ SrO [ 1 1 ¯ 0 ] .

Strain Effects on the Dielectric Properties of Epitaxial SrTiO3 Thin Films Grown by Pulsed Laser Deposition

SrTiO 3 (STO) thin films 1 μm thick were deposited on conducting SrRuO 3 (SRO) bottom electrodes. The films were grown epitaxially on SrTiO 3 and LaAlO 3 substrates, respectively using a KrF-248 nm laser. The thickness of the SRO buffer (electrode) layer was varied between 150 nm–500 nm in order to study its effect on the dielectric properties of STO. Loss peaks were observed as a function of temperature at various frequencies and these are explained on the basis of the possible relation to the strain in the thin films. The possible effects of the lattice mismatch at the interfaces, thermal expansion coefficient differences and oxygen vacancies generated in the films during growth, on the loss factor are explained. The extremely low losses and reasonably high tunability in these samples are very encouraging, from the perspective of their possible application in microwave device applications.

Temperature effect on carrier transport characteristics in SrTiO3−δ/Si p-n heterojunction

A p-n junction has been fabricated by depositing an electron-doped (n-) SrTiO3−δ film on a hole-doped (p-) Si substrate with a two atomic-layers thickness epitaxial SrO buffer layer using laser molecular beam epitaxy technique. Good crystallinity and smooth surface of SrTiO3−δ were confirmed by reflection high-energy electron diffraction and x-ray diffraction. The junction shows good rectifying behavior at room temperature, and strong temperature dependence of current-voltage (I-V) properties in the range of 200–300 K. These results present potential applications in future microelectronic devices based on growing perovskite oxide thin films on conventional semiconductors.

Heteroepitaxial growth of LaAlO3 films on Si (1 0 0) by laser molecular beam epitaxy

The LaAlO3films were grown epitaxially on Si (100) substrates by inserting SrO or SrTiO3 buffer layer using a computer-controlled laser molecular beam epitaxy system. Structural characterization indicated that the LaAlO3 films were two-dimensional layer-by-layer growth. The atomic force microscopy observations showed that the surfaces of the epitaxial LaAlO3 films were atomically smooth. The crystallinity of the LaAlO3 films determined by X-ray diffraction and high-resolution transmission electron microscopy was a single-crystalline structure. After being annealed at 1050°C in N2 for 5min, the crystallinity of the LaAlO3 film improved obviously. The successful LaAlO3/SrO/Si and LaAlO3/SrTiO3/Si epitaxial growth predicted that the possibility of the development of 3D heterostructrues on Si in a new generation of microelectronics devices.

Integration of MgO on Si(001) Using SrO and SrTiO3 Buffer Layers by Molecular Beam Epitaxy

Epitaxial MgO was deposited onto Si(001) substrates by molecular beam epitaxy using elemental metallic sources and molecular oxygen at temperatures from 150 to 400∘C. To facilitate epitaxy through misfit strain relaxation, epitaxial MgO layers were grown on SrO and SrTiO3 buffer layers deposited on Si(001) substrates. The structure of the epitaxial layers was determined by X-ray diffraction, reflection high-energy electron diffraction and transmission electron microscopy. The observed orientation for the MgO/SrO/Si multilayer is cube-on-cube. The X-ray rocking curve full width half maximum of the MgO on SrO buffer layers was 2.2∘. SrTiO3 buffer layers grown by recrystallization were epitaxial and exhibited improved morphology relative to those grown at a fixed growth temperature. X-ray analysis of a 5.2 nm recrystallized SrTiO3 film indicates a fully relaxed and phase pure film. The observed orientation of MgO using SrTiO3 buffer layers is MgO[100]‖SrTiO3[100]‖Si[110].

Epitaxial growth and magnetic properties of EuO on (001) Si by molecular-beam epitaxy

Epitaxial (001) EuO thin films have been grown on (001) Si utilizing an intermediate, epitaxial SrO buffer layer by molecular-beam epitaxy. Four-circle x-ray diffraction reveals nearly phase-pure samples. Magnetic measurements indicate that the EuO layer is ferromagnetic, with a transition temperature (68 K) close to the bulk value and a saturation magnetic moment of 4.7 Bohr magnetons per Eu atom. The magneto-optic Kerr effect observed is also comparable to bulk EuO. Such heterostructures have potential as a means to inject spin-polarized electrons into silicon for use in spintronics applications.

Study of epitaxial SrTiO 3 (STO) thin films grown on Si(0 0 1)–2 × 1 substrates by molecular beam epitaxy

Perovskite-type oxide SrTiO 3 (STO) thin films with thicknesses of 30, 100, 300 and 1000 Å have been grown epitaxially on Si(0 0 1) substrates using co-evaporation of Sr and Ti in an O 2 atmosphere of 9.0×10 −8 Torr at 500 °C by molecular beam epitaxy (MBE). SrO buffer layers of 100 Å have been grown at 350 °C on Si substrates. The SrTiO 3 films have been studied using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and atomic force microscopy (AFM) as a function of the thickness of STO films. By heating the Si(0 0 1) substrates at 800 °C under Sr exposure for 2 min after the chemical cleaning and formation of protective SiO 2 layer, RHEED patterns of the atomically clean Si(0 0 1)–2 × 1 surface have been observed. RHEED pattern becomes unclear after the growth of 30 and 100 Å-thick STO films. On the contrary, spotty and streaky patterns, and spotty patterns are observed clearly for 300 and 1000 Å-thick films, respectively. The surface crystallinity improves for all the samples after annealing at 800 °C. The STO films with thicknesses of 30 and 1000 Å consist of grains in the images of atomic force microscopy. The flatness and crystallinity of STO films with the thickness of 100–300 Å are better than the other films.

Influence of Asymmetric Oxide Electrode Structures on the Interface Capacity and the Failure Mechanisms in PZT Thin Films

The influence of symmetric and asymmetric electrodes including SRO and IrO2 within the electrode structure on the imprint and fatigue behavior in sputtered and CSD derived PbZrxTi1 − xO2 (PZT) thin films is investigated. It is found that SRO buffer layers are needed within top and bottom electrode to improve the fatigue behavior. However, for improvement of the imprint behavior, only one SRO buffer layer within either top or bottom electrode is sufficient, whereas IrO2 reveals no improvement of the imprint behavior. Furthermore the direction dependence of the imprint behavior is examined. To examine the influence of the interface capacity on the fatigue mechanism in more detail, the capacity is measured for different PZT film thicknesses during fatigue treatment. It is shown that there is hardly any difference in interface capacity during fatigue.

Microstructure of epitaxial SrRuO3 films on Si(001) substrates

Abstract The microstructure of pulsed-laser-deposited SrRuO3 films on Si(001) with SrO buffer layers has been studied by means of high-resolution electron microscopy and energy-dispersive X-ray spectroscopy. It was found that good epitaxial growths of the SrO buffer layers and the subsequent SrRuO3 films were achieved on the Si(001) substrates. Multiple domains were formed in the SrRuO3 films with domain boundaries nearly perpendicular to the interface between the SrRuO3 films and the SrO buffer layers. A high density of planar defects, such as twins, stacking faults and antiphase boundaries, were formed along the (022) lattice plane of the SrRuO3 films. Lattice strains in the constituting layers were investigated by means of fast Fourier transformation of local regions in the high-resolution images. It was found that, close to the interface between the SrRuO3 films and the SrO buffer layers, both the layers were elastically strained in the opposite sense. The structure of coherent twin boundary in the Sr...

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.

Stability of Two-Step-Growth Bi2Sr2CuOx Films on Si(001) using SrO Buffer Layer

We have studied the growth conditions of high-quality crystalline Bi2Sr2CuOx films on a Si(001)-2×1 substrate using molecular beam expitaxy. A buffer layer of SrO, heteroepitaxially grown at 350°C, is used between Bi2Sr2CuOx films and the Si substrate. The Bi2Sr2CuOx films are grown at different temperatures. Among them, the film grown at 600°C shows a good crystal structure, but degradation of the crystal structure is observed in an X-ray diffraction pattern when the film is left in air. On the other hand, the film grown at 650°C shows a bad crystalline structure, but the film is not degraded in air. The films grown by two-step growth with two different growth temperatures, however, are found to have good crystal structures and not to be degraded in air. The cause of the film degradation is discussed based on the results of the X-ray diffraction pattern.

In-situ Annealing of Thin SrO Films Grown on Si(001)-2×1 by Molecular Beam Epitaxy

Thin strontium oxide films were grown at 350°C in an oxygen atmosphere of 5 ×10-8 Torr by Sr-deposition using molecular beam epitaxy (MBE). The evolution of 300-Å-thick SrO films on a Si(001)-2 ×1 substrate is studied by reflection high-energy electron diffraction (RHEED) as a function of annealing temperature in an oxygen atmosphere of around 1 ×10-6 Torr. The RHEED patterns of the SrO crystalline films show streaks with spots up to an annealing temperature of 625°C, and then Debye-Scherrer rings appear with broad spots at 650°C. Above this temperature, the patterns consist only of the rings. From the results, we find that the growth temperature for any epitaxial film on Si with a MBE-grown SrO buffer layer is restricted to be under 625°C.

Characterization of Ferroelectric BaSrTiO3 Thin Films Using a Flip-Chip Technique at Microwave Frequency Ranges

Microwave properties of ferroelectric Ba0.5Sr0.5TiO3 (BST) thin films were investigated utilizing a gold resonator combined with a planar flip-chip structure. The resonator was designed to have bias circuits at the zero field points in the microstrip line. The dielectric properties, capacitance (C) and dielectric loss (tan δ) of the BST film are characterized under the influence of an applied dc voltage, Vb, at room temperature. The BST ferroelectric capacitor with a capacitance of about 0.6 pF successfully controlled the tunability of the resonance frequency in the resonator with a factor of ∼2. In addition, dependence of the loss tangent and the capacitance on the electrode were determined for both of the SrRuO3/BST/LaAlO3 (SRO/BST/LAO) and BST/LAO structures. The use of a SRO buffer layer in the multilayer provided a low loss tangent of about 3×10-3 at about 2 GHz and 80 V bias voltage. This low loss seems to be related to the reduction of defect density in the interfacial layer between the electrode and the ferroelectric thin film.