H2O在SrTiO3-(001)TiO2表面上吸附和解离的密度泛函理论研究

利用密度泛函理论研究了0.25单层(ML),0.5ML,0.75ML和1ML吸附率下H2O在SrTiO3-(001)TiO2表面上的吸附行为.比较了不同吸附率下分子吸附和解离吸附的稳定性,利用微动弹性带(nudged elastic band)方法计算了H2O的解离势垒.结果表明:在低吸附率(0.25ML和0.5ML)时,H2O表现为解离吸附;在0.75ML吸附率下,分子吸附和解离吸附同时存在;而在全吸附(吸附率为1ML)时,分子吸附更稳定.基于对H2O分子与表面之间以及H2O分子之间的电荷转移和相互作用的分析,讨论了吸附率对H2O吸附和解离的影响.     

Synthesis and electrical transport of SrHfO3 thin films grown on a SrTiO3 (001) substrate using a pulsed laser deposition

We report the deposition of epitaxial SrHfO₃ thin films on a SrTiO₃ (001) substrate in different substrate temperatures by using a pulsed laser deposition (PLD) method. We carried out X-ray diffraction (XRD), X-ray reflectivity (XRR), reciprocal space mapping (RSM), atomic force microscopy (AFM), resistivity, and Hall measurements to examine the crystallinity, morphology and electrical properties of these films. All films showed smooth and uniform morphology with small root mean square (RMS) roughness. While the SrHfO₃ sample grown at 750 °C is metallic, the films deposited at 600 °C, 650 °C, and 700 °C show an upturn at low temperatures. The temperature dependence of the metallic parts was analyzed based on the parallel resistor model that includes resistivity saturation. On the other hand, the low-temperature upturn was found to be well described by a weak localization mechanism. We also observed the possible emergence of non-Fermi liquid behavior when the upturn disappeared. All SrHfO₃ films have p-type charge carriers.     

In-situ imaging of the nucleation and growth of epitaxial anatase TiO2(001) films on SrTiO3(001)

The growth of TiO₂ anatase films on Nb‐doped SrTiO₃(001) molecular beam epitaxy has been studied in-situ by scanning tunneling microscopy. We show that the initial growth follows the Stranski–Krastanov mode, where islands form on top of a wetting layer consisting of two monolayers (ML) of TiO₂. The epitaxial islands subsequently nucleate and coalesce into large commonly oriented crystallites. The reconstruction observed by reflection high-energy electron diffraction (RHEED) is shown to result from the coexistence of individual (4 × 1) and (1 × 4) reconstructions present on different crystallite surfaces. The anatase grows in units of bilayers, resulting in a step height of 2 ML (~ 0.5 nm). This result explains the fact that the measured period of the RHEED specular-beam intensity oscillations corresponds to the time required for deposition of 2 ML. Ar ion sputtering and UHV annealing results in a transformation to coexisting (4 × 1) and (1 × 4) reconstructed terraces on individual crystallites, as commonly observed by ex-situ STM studies.     

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO₃(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh$\langle 110\rangle $ to FeRh$\langle 100\rangle $ directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.     

Electronic charge transfer between Au nano-particles and TiO2-terminated SrTiO3(0 0 1) substrate

The interaction between Au nano-particles and oxide supports is recently discussed in terms of the catalytic activities. This paper reports the electronic charge transfer between Au nano-particles and TiO₂-terminated SrTiO₃(0 0 1) substrate, which is compared with that for stoichiometric(S)-, pseudo-stoichiometric(S¹)- and reduced(R)-TiO₂(1 1 0) supports. We observed the photoelectron spectra of Au 4f, O 2s, Ti 3p, and Sr 4p lines and also measured the work functions for Au/oxides supports using synchrotron-radiation light. As the results, all the O 2s, Ti 3p, and Sr 4p lines for Au/SrTiO₃(0 0 1) show lower binding energy shifts in a quite same manner and abrupt increase in the work function is seen in an initial stage. This clearly evidences an electronic charge transfer from the substrate to Au probably due to a much larger work function of Au than SrTiO₃(0 0 1), which leads to an upward band bending (0.3 eV) just like a Schottky contact. Electronic charge transfers also take place at Au/S- and Au/S¹-TiO₂(1 1 0) and Au/R-TiO₂(1 1 0) interfaces, where electrons are transferred from Au to S- and S¹-TiO₂ and from R-TiO₂ to Au, as predicted by ab initio calculations.     

Plastic relaxation anisotropy in epitaxial films grown on miscut Si(001) substrates

A way of calculating the structural parameters of semiconductor epitaxial layers grown on miscut (0 0 1) substrates is developed. It is shown that the difference between the tensor of film elastic strains and the spherical tensor of strains translating the film’s crystal lattice into the substrate’s lattice is a tensor whose components are proportional to X-ray strains. The technique is used to analyze GaAs and Ge_0.2Si_0.8 films grown on (1 1 13) Si substrates. The anisotropy of the degree of plastic relaxation is established for lateral directions perpendicular and parallel to the interfacial steps in the GeSi film. It is proposed that noninteger Miller indices be used to denote the direction and value of the rotation of the film’s crystal lattice with respect to the substrate lattice.     

Formation and thickness evolution of periodic twin domains in manganite films grown on SrTiO3(001) substrates

We present an extended synchrotron x-ray scattering study of the structure of thin manganite films grown on SrTiO3(001) substrates and reveal a new kind of misfit strain relaxation process which exploits twinning to adjust lattice mismatch. We show that this relaxation mechanism emerges in thin films as one-dimensional twinning waves which freeze out into a twin domain pattern as the manganite film continues to grow. A quantitative microscopic model which uses a matrix formalism is able to reproduce all x-ray features and provides a detailed insight into this novel relaxation mechanism. We further demonstrate how this twin angle pattern affects the transport properties in these functional films.     

Magnetic characterization of Bi2FeMnO6 film grown on (100) SrTiO3 substrate

Single phase Bi₂FeMnO₆ films on (100) SrTiO₃ substrate were fabricated using a pulsed laser deposition method through optimization of the preparation conditions. The magnetic moment is 0.30μ_B at 5 K in the magnetic field of 1 T, indicating that B site cations of Fe and Mn are disordered in the sample. The zero‐field‐cooling (ZFC) and field‐cooling (FC) magnetization curves measured from 2 K to 400 K coincide at 360 K. This is consistent with the observation that hysteresis disappears at 360 K, revealing the antiferromagnetic transition at this temperature. A spin‐glass‐like behaviour was observed at low temperature (～100 K) with a cusp of 25 K. Mn shows multiple valence states in the film. It is possibly because Mn²⁺ and Mn⁴⁺ could decrease the Jahn–Teller effect from Mn³⁺ in the film which results in less lattice distortion. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ab initio study of water adsorption on TiO2-terminated (100) surface of SrTiO3 with and without Cr doping

Water molecule adsorption on TiO₂-terminated (100) surface of SrTiO₃ with and without Cr doping is investigated by first principle calculation based on density functional theory. The band gap is shrunk compared with that of bulk due to the existence of defect states on the surface and 3d states of dopants. As a result the absorption energy edge is reduced and locates in the visible region. When adsorbed on the surface, energy levels of water molecules as a whole are lowered with respect to the Fermi energy, but the higher levels are split and electrons are transferred from low levels to high levels due to the decrease of the density of states in low energy region. Weak bonding is formed between water hydrogen atoms and surface oxygen atoms. This bonding causes the electron transferring from substrate to molecule and the occupation of the corresponding states.     

Perpendicular magnetic anisotropy induced by tetragonal distortion of FeCo alloy films grown on Pd(001)

We grew tetragonally distorted FexCo1-x alloy films on Pd(001). Theoretical first-principles calculations for such films predicted a high saturation magnetization and a high uniaxial magnetic anisotropy energy for specific values of the lattice distortion c/a and the alloy composition x. The magnetic anisotropy was investigated using the magneto-optical Kerr effect. An out-of-plane easy axis of magnetization was observed for Fe0.5Co0.5 films in the thickness range of 4 to 14 monolayers. The magnetic anisotropy energy induced by the tetragonal distortion is estimated to be almost 2 orders of magnitude larger than the value for bulk FeCo alloys. Using LEED Kikuchi patterns, a change of the easy axis of magnetization can be related to a decrease of the tetragonal distortion with thickness.     

SrTiO3(110) thin films grown directly on different oriented silicon substrates

We have grown (110)-oriented SrTiO₃ (STO) thin films on silicon without any buffer layer, by means of pulsed laser deposition technique. The crystal structures of the grown films were examined by X-ray diffraction analysis including θ–2θ scan and rocking curve as well as Laue diffraction methods. STO films with single (110) out-of-plane orientation were formed on all (100), (110) and (111)-oriented Si substrates. The in-plane alignments for the epitaxial STO films grown directly on Si (100) were found as STO[001]//Si[001] and STO[11̄0]//Si[010]. The results should be of interest for better understanding of the growth of perovskite oxide thin films on silicon wafers. PACS 77.55.+f; 68.55.JK; 81.15Fg     

Bimodal growth of Au on SrTiO3(001)

We have investigated the vapor phase growth of Au on SrTiO3(001)-(2 x 1) substrates by UHV scanning tunneling microscopy. Submonolayer (ML) coverages below 300 degrees C wet the surface as disordered metastable 2D islands. Beyond 0.75 ML fcc nanocrystals with a (111) interface are nucleated and ripen by dewetting the surrounding layer. Some multiply twinned fivefold symmetric clusters are also created. Above 400 degrees C dewetting occurs for all coverages and the surface is only populated by nanocrystals and fivefold clusters. A planar ground state configuration for small Au clusters and a higher interface energy for crystals than for wetted 2D ML films explains these results.     

Step-like growth of Bi2Sr2CaCu2O8 films on off axis oriented (001) SrTiO3

High-quality Bi₂Sr₂CaCu₂O₈ (BSCCO) films were grown by means of pulsed-laser deposition on 10° off-axis oriented (001) SrTiO₃ substrates. The films exhibit a step-like morphology and they are properly oriented along the (001) direction. Their normal-state resistivity measured along the tilt (off-axis (100) direction of the substrate) was, typically, 260 times higher than that along the (010) direction. Such films reveal a transverse Seebeck-effect which significantly exceeds that observed with YBa₂Cu₃O₇ (YBCO).     

Magnetoresistance anisotropy in La0.67Ba0.33MnO3 films laterally compressed by a neodymium gallate substrate

The mismatch in the crystal lattice parameters induces biaxial lateral compression of 35-nm La_0.67Ba_0.33MnO₃ films coherently grown on neodymium gallate substrates. Mechanical stresses emerging during the nucleation and growth of the manganite layer facilitate the depletion of this layer in the alkali-earth element. This results in an increase in the unit cell volume in the grown films and a decrease in temperature T _M at which the resistivity attains the maximal value. The extremal values of the negative magnetoresistance (MR ≈ 17% for μ₀ H = 1 T) of the grown films are observed at temperatures close to room temperature. At T < T _M , the response of the resistivity of the films to the magnetic field depends on the direction of this field relative to the normal to the substrate plane and to the direction of the measuring current. At T = 95 K, scattering of holes from 90°-domain walls leads to an increase in the resistivity of the manganite films by approximately 1.1%, while the negative anisotropic magnetoresistance reaches 1.5%.     

Hydrogen-induced surface metallization of SrTiO3(001)

Surface metallization of SrTiO3(001) by hydrogen adsorption is experimentally confirmed for the first time by photoemission spectroscopy and surface conductivity measurements. The metallic state is assigned to a quantized state in the space-charge layer induced by electron doping from hydrogen atoms. The measured two-dimensional (2D) conductivity is well above the 2D Ioffe-Regel limit indicating that the system is in a metallic conduction regime. The mean free path of the surface electron is estimated to be several nanometers at room temperature.     

Epitaxial Fe films on ZnSe(001): effect of the substrate surface reconstruction on the magnetic anisotropy

It is well known that Fe films deposited on a c(2 × 2)-reconstructed ZnSe(001) surface show a strong in-plane uniaxial magnetic anisotropy. Here, the effect of the substrate reconstruction on the magnetic anisotropy of Fe has been studied by in situ Brillouin light scattering. We found that the in-plane uniaxial anisotropy is strongly reduced for Fe films grown on a (1 × 1)-unreconstructed ZnSe substrate while the in-plane biaxial one is nearly unaffected by the substrate reconstruction. Calculations of magnetic anisotropy energies within the framework of ab initio density functional theory reveal that the strong suppression of anisotropy at the (1 × 1) interface occurs due to complex atomic relaxations as well as the competing effects originating from magnetocrystalline anisotropy and dipole-dipole interactions. For both sharp and intermixed c(2 × 2) interfaces, the magnetic anisotropy is enhanced compared to the (1 × 1) case due to the further lowering of symmetry. The theoretical results are in agreement with the experimental findings.     

Energy storage properties of antiferroelectric 0.92NaNbO3-0.08SrZrO3 film on (001)SrTiO3 substrate

The present study demonstrates the fabrication of an antiferroelectric 0.92NaNbO₃-0.08SrZrO₃ film deposited on a SrRuO₃ coated (001)SrTiO₃ single crystal substrate by pulsed laser deposition. In the 0.92NaNbO₃-0.08SrZrO₃ film, the domain with its c-axis aligned with the out-of-plane direction contributed to the stabilization of an antiferroelectric phase under the high electric field. The film had an energy storage density of 2.9 J cm⁻³ and storage efficiency of 67% at room temperature, which kept at 2.5 J cm⁻³ and 50% at high temperature of 150 °C.     

In situ RHEED analysis of epitaxial Gd2O3 thin films grown on Si (001)

Epitaxial Gd₂O₃ thin films were successfully grown on Si (001) substrates using a two-step approach by laser molecular-beam epitaxy. At the first step, a ～0.8 nm thin layer was deposited at the temperature of 200 ^°C as the buffer layer. Then the substrate temperature was increased to 650 ^°C and in situ annealing for 5 min, and a second Gd₂O₃ layer with a desired thickness was deposited. The whole growth process is monitored by in situ reflection high-energy electron diffraction (RHEED). In situ RHEED analysis of the growing film has revealed that the first Gd₂O₃ layer deposition and in situ annealing are the critical processes for the epitaxial growth of Gd₂O₃ film. The Gd₂O₃ film has a monoclinic phase characterized by X-ray diffraction. The high-resolution transmission electron microscopy image showed all the Gd₂O₃ layers have a little bending because of the stress. In addition, a 5–6 nm amorphous interfacial layer between the Gd₂O₃ film and Si substrate is due to the in situ high temperature annealing for a long time. The successful Gd₂O₃/Si epitaxial growth predicted a possibility to develop the new functional microelectronics devices.     

Atomic structures of reduced SrTiO3(001) surfaces

The scanning tunneling microscope has been employed to study the reduced SrTiO₃(001) surface in ultrahigh vacuum. Topographic images show a change of structural ordering for the surface annealed under three different conditions. In addition, our STM images reveal row-like structures with spacings of 12 and 20 Å. The observations are discussed in terms of the formation of intergrowths of lamellae of Sr_{n + 1} Ti_nO_{3n + 1}; a result of accommodation of oxygen and titanium deficiencies.