Interface-induced polarization and inhibition of ferroelectricity in epitaxial SrTiO₃/Si

We use SrTiO?/Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed (pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.     

Structural basis for the conducting interface between LaAlO3 and SrTiO3

The complete atomic structure of a five-monolayer film of LaAlO3 on SrTiO3 has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La(1-x)SrxTiO3. By invoking electrostatic potential minimization, the ratio of Ti{4+}/Ti{3+} across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.     

Photoemission from buried interfaces in SrTiO3/LaTiO3 superlattices

We have measured photoemission spectra of SrTiO3/LaTiO3 superlattices with a topmost SrTiO3 layer of variable thickness. A finite coherent spectral weight with a clear Fermi cutoff was observed at chemically abrupt SrTiO3/LaTiO3 interfaces, indicating that an "electronic reconstruction" occurs at the interface between the Mott insulator LaTiO3 and the band insulator SrTiO3. For SrTiO3/LaTiO3 interfaces annealed at high temperatures (approximately 1000 degrees C), which leads to Sr/La atomic interdiffusion and hence to the formation of La(1-x)Sr(x)TiO3-like material, the intensity of the incoherent part was found to be dramatically reduced whereas the coherent part with a sharp Fermi cutoff was enhanced due to the spread of charge. These important experimental features are well reproduced by layer dynamical-mean-field-theory calculation.     

高质量镍基超晶格薄膜的制备及表征

自发现镍基超导薄膜之后, 镍基材料体系已成为当前人们的研究热点. 而在相关报道中, 普遍认为高质量的镍基超导前驱体薄膜的制备对其拓扑还原后超导电性的实现具有重要影响. 前期研究表明高质量的前驱体薄膜只生长在SrTiO3 衬底附近, 这与我们的实验结果一致. 为了可控制备高质量的镍基超导前驱体薄膜, 本文利用脉冲激光沉积(PLD) 技术在SrTiO3 (001) 衬底上生长不同厚度的可层选择性还原的[SrTiO3 ]m/[ Nd0 .8Sr0 .2 NiO3 ]n[(STO)m/(NSNO)n ]超晶格薄膜. 采用反射高能电子衍射仪(RHEED) 及透射电子显微镜(STEM) 和 X 射线衍射(XRD) 技术对超晶格薄膜的结构进行原位检测及结构表征, 然后利用综合物性测量系统(PPMS) 测试薄膜的电磁输运性质. 结果表明, 超晶格薄膜的结构质量良好, 超晶格薄膜和高质量单层前驱体薄膜表现出类似的电阻和磁阻现象. 该研究为后续镍基超导薄膜的可重复制备提供了重要的参考.     

The structure of strained perovskite KTaO(3) thin films prepared by pulsed laser deposition

Epitaxial perovskite potassium tantalate (KTaO(3)) films with thicknesses of 7.4-36?nm are grown on SrTiO(3)(001) substrates by pulsed laser deposition. X-ray diffraction (XRD) analysis reveals evolution of lattice strain with increasing film thickness. A biaxial compressive in-plane strain as large as -?2.1% is obtained in the 7.4?nm-thick film. A bi-layer microstructure is detected in the 18?nm-thick film, suggesting the possibility for an abrupt strain relaxation.     

X-ray resonance in crystal cavities: realization of Fabry-Perot resonator for hard x rays

X-ray back diffraction from monolithic two silicon crystal plates of 25-150 microm thickness and a 40-150 microm gap using synchrotron radiation of energy resolution DeltaE = 0.36 meV at 14.4388 keV clearly show resonance fringes inside the energy gap and the total-reflection range for the (12 4 0) reflection. This cavity resonance results from the coherent interaction between the x-ray wave fields generated by the two plates with a gap smaller than the x-ray coherence length. This finding opens up new opportunities for high-resolution and phase-contrast x-ray studies, and may lead to new developments in x-ray optics.     

Structural comparison of n-type and p-type LaAlO3/SrTiO3 interfaces

Using a surface x-ray diffraction technique, we investigated the atomic structure of two types of interfaces between LaAlO3 and SrTiO3, that is, p-type (SrO/AlO2) and n-type (TiO2/LaO) interfaces. Our results demonstrate that the SrTiO3 in the sample with the n-type interface has a large polarized region, while that with the p-type interface has a limited polarized region. In addition, atomic intermixing was observed to extend deeper into the SrTiO3 substrate at the n-type interface compared to the p type. These differences result in distinct degrees of band bending, which likely contributes to the striking contrast in electrical conductivity between the two types of interfaces.     

Anharmonic lattice dynamics in germanium measured with ultrafast x-ray diffraction

Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T2) for 7-GHz longitudinal acoustic phonons in germanium.     

Morphology of heteroepitaxial β-SiC films grown on Si(111) through high-vacuum chemical vapor deposition from hexane vapors

The characteristics of cubic silicon carbide films grown on silicon through high-vacuum chemical vapor deposition (HVCVD) from hexane vapors are investigated using scanning probe microscopy and x-ray diffraction. The surface morphology and structure of the films are analyzed as a function of the thickness of the deposited films and the nature of the substrate (silicon, sapphire). The role of different diffusion fluxes arising in the structure and the related possible mechanisms of growth of β-SiC layers on silicon substrates are discussed.     

Electronic and magnetic reconstructions in La0.7Sr0.3MnO3/SrTiO3 heterostructures: a case of enhanced interlayer coupling controlled by the interface

We report on the magnetic coupling of La0.7Sr0.3MnO3 layers through SrTiO3 spacers in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures. Combined aberration-corrected microscopy and electron-energy-loss spectroscopy evidence charge transfer to the empty conduction band of the titanate. Ti d electrons interact via superexchange with Mn, giving rise to a Ti magnetic moment as demonstrated by x-ray magnetic circular dichroism. This induced magnetic moment in the SrTiO3 controls the bulk magnetic and transport properties of the superlattices when the titanate layer thickness is below 1 nm.     

双层锰氧化物薄膜的制备及其物理性质

用脉冲激光沉积(PLD)方法成功地制备了双层钙钛矿结构的La2-2xSr1+2xMn2O7(x=032)单相薄膜.这种薄膜生长在具有不同晶格参数的两种衬底上.测量发现，两种衬底上生长的La2-2xSr1+2xMn2O7(x=032)薄膜具有迥然不同的金属-绝缘体转变温度TM-I及其他物性.界面应力的研究表明这是衬底晶格常数不同引起膜内应变的结果.在衬底的压应力下，薄膜的电阻-温度曲线的峰值(TM-I)向高温移动且电阻率(ρ)下降；相反，对于衬底张应力下的薄膜，TM-I下降ρ上升.这些结果可以用双交换模型做很好的解释. 关键词： CMR 双层锰氧化物薄膜 PLD 应力     

Critical adsorption and dimensional crossover in epitaxial FeCo films

The critical behavior of thin FeCo films grown on MgO has been studied using phase-sensitive synchrotron x-ray diffraction. These studies unravel several novel features of criticality in thin films, as the simultaneous appearance of the 3D-2D crossover and the truncation of the correlation length normal to the film at approximately 1/3 of the film thickness. Above the critical temperature of the film we observe a pronounced pinning of the order parameter at the MgO-FeCo interface, which indicates a novel critical adsorption behavior.     

Atomic imaging and direct phase retrieval using anomalous surface x-ray diffraction

The application of multi-wavelength anomalous diffraction to thin films, interfaces and surface structures is presented. The method directly determines the amplitudes and phases of the complex surface structure factors from surface x-ray diffraction data, measured at three different energies around the absorption edge of one of the elements present in the film. Thereby, one is able to directly Fourier transform the data, which immediately provides meaningful and unambiguous electron-density distributions. These serve as a starting point for subsequent structural refinement. The robustness of the algorithm was evaluated on simulated data as a proof of principle. The experimental limitations and their effect on the method will be discussed as well as stability tests for the algorithm, such as the positions of the anomalous scatterers and the interfacial roughness. It will be shown that the method can be applied to real structures. The algorithm was tested on real data from a thin film of SrTiO(3) grown on NdGaO(3)(110).     

Bulk dislocation core dissociation probed by coherent x rays in silicon

We report on a new approach to probe bulk dislocations by using coherent x-ray diffraction. Coherent x rays are particularly suited for bulk dislocation studies because lattice phase shifts in condensed matter induce typical diffraction patterns which strongly depend on the fine structure of the dislocation cores. The strength of the method is demonstrated by performing coherent diffraction of a single dislocation loop in silicon. A dissociation of a bulk dislocation is measured and proves to be unusually large compared to surface dislocation dissociations. This work opens a route for the study of dislocation cores in the bulk in a static or dynamical regime, and under various external constraints.     

Charge ordering modulations in a Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi 0.4 Ca 0.6 MnO 3 film with a thickness of 110 nm at 103 K.Six different types of superlattice structures are observed using the selected-area electron diffraction(SAED) technique,while three of them match well with the modulation stripes in high-resolution transmission electron microscopy(HRTEM) images.It is found that the modulation periodicity and direction are completely different in the region close to the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface from those in the region a little further from the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface,and the possible reasons for this are discussed.Based on the experimental results,structural models are proposed for these localized modulated structures.     

Microstructural investigation of nickel silicide thin films and the silicide-silicon interface using transmission electron microscopy

This article discusses the results of transmission electron microscopy (TEM)-based investigation of nickel silicide (NiSi) thin films grown on silicon. Nickel silicide is currently used as the CMOS technology standard for local interconnects and in electrical contacts. Films were characterized with a range of TEM-based techniques along with glancing angle X-ray diffraction. The nickel silicide thin films were formed by vacuum annealing thin films of nickel (50 nm) deposited on (100) silicon. The cross-sectional samples indicated a final silicide thickness of about 110 nm. This investigation studied and reports on three aspects of the thermally formed thin films: the uniformity in composition of the film using jump ratio maps; the nature of the interface using high resolution imaging; and the crystalline orientation of the thin films using selected-area electron diffraction (SAED). The analysis highlighted uniform composition in the thin films, which was also substantiated by spectroscopy techniques; an interface exhibiting the desired abrupt transition from silicide to silicon; and desired and preferential crystalline orientation corresponding to stoichiometric NiSi, supported by glancing angle X-ray diffraction results.     

Effect of microstructure of MgO buffer layer on BaTiO3 grown on silicon substrates

MgO ultrathin films were grown on Si(1 0 0) substrates as buffer layers for the growth of ferroelectric BaTiO₃ thin films by laser molecular beam epitaxy (L-MBE). The deposition process of MgO buffer layers grown on silicon was in situ monitored by reflection high-energy electron diffraction (RHEED). The structure of BaTiO₃ films fabricated on MgO buffers was investigated by X-ray diffraction. Biaxially textured MgO was obtained at high laser energy density, but when the laser energy was lowered, MgO buffer was transformed to the form of texture with angular dispersion with the increase of the film thickness. BaTiO₃ films grown on the former buffer were completely (0 0 1) textured, while those on the latter were (0 0 1) preferred orientated. Furthermore, the fabricated MgO buffers and BaTiO₃ films had atomically smooth surface and interface. All these can reveal that the quality of textured MgO buffer is a key factor for the growth of BaTiO₃ films on silicon.     

Ti-Si-N复合膜的界面相研究

为了揭示Ti_Si_N复合膜中Si₃N₄界面相的存在方式及其对薄膜力学 性能的影响 ，采用x射线衍射仪、高分辨透射电子显微镜、俄歇电子能谱仪和显微硬度仪对比研究了磁 控溅射Ti_Si_N复合膜和TiN/Si₃N₄多层膜的微结构和力学性能. 实 验结果表明 ，Ti_Si_N复合膜均形成了Si₃N₄界面相包裹TiN纳米晶粒的微结构. 其中低Si 含量的Ti_Si_N复合膜中Si₃N₄界面相的厚度小于1nm，且以晶体态 存在，薄膜 呈现高硬度. 而高Si含量的Ti_Si_N复合膜中的Si₃N₄界面相以非晶 态存在，薄 膜的硬度也相应降低. 显然，Ti_Si_N复合膜中Si₃N₄界面相以晶体 态形式存在 是薄膜获得高硬度的重要微结构特征，其强化机制可能与多层膜的超硬效应是相同的. 关键词： Ti－Si－N复合膜 界面相 微结构 超硬效应     

Interface effects in ferroelectric PbTiO3 ultrathin films on a paraelectric substrate

Interface effects on the ferroelectric behavior of PbTiO3 ultrathin films deposited on a SrTiO3 substrate are investigated using an interatomic potential approach with parameters fitted to first-principles calculations. We find that the correlation of atomic displacements across the film-substrate interface is crucial for the stabilization of the ferroelectric state in films a few unit cells thick. We show that the minimum film thickness for the appearance of a spontaneous polarized domain state is not an intrinsic property of the ferroelectric film but depends on the polarizability of the paraelectric substrate. We also observe that the substrate displays an induced polarization with an unusual oscillatory behavior.     

不同晶向SrTiO3上外延GaAs薄膜的光谱研究

利用MBE生长技术,成功地在不同晶向SrTiO3(100)(111)(110)衬底上生长了GaAs薄膜,利用显微Raman和荧光光谱（PL）对此进行了研究。实验结果表明,在不同晶向SrTiO3上生长的GaAs薄膜有不同的晶向和应力状态。荧光光谱（PL）研究表明在SrTiO3(100)(111)晶面上生长的GaAs薄膜的PL峰发生明显的蓝移。研究表明在SrTiO3(110)面上生长的GaAs薄膜和体单晶基本上一致,有更好的光学质量。