Igor A. Karateev

Europium Silicide – a Prospective Material for Contacts with Silicon

Metal-silicon junctions are crucial to the operation of semiconductor devices: aggressive scaling demands low-resistive metallic terminals to replace high-doped silicon in transistors. It suggests an efficient charge injection through a low Schottky barrier between a metal and Si. Tremendous efforts invested into engineering metal-silicon junctions reveal the major role of chemical bonding at the interface: premier contacts entail epitaxial integration of metal silicides with Si. Here we present epitaxially grown EuSi2/Si junction characterized by RHEED, XRD, transmission electron microscopy, magnetization and transport measurements. Structural perfection leads to superb conductivity and a record-low Schottky barrier with n-Si while an antiferromagnetic phase invites spin-related applications. This development opens brand-new opportunities in electronics.

Atomic-Scale Engineering of Abrupt Interface for Direct Spin Contact of Ferromagnetic Semiconductor with Silicon

Control and manipulation of the spin of conduction electrons in industrial semiconductors such as silicon are suggested as an operating principle for a new generation of spintronic devices. Coherent injection of spin-polarized carriers into Si is a key to this novel technology. It is contingent on our ability to engineer flawless interfaces of Si with a spin injector to prevent spin-flip scattering. The unique properties of the ferromagnetic semiconductor EuO make it a prospective spin injector into silicon. Recent advances in the epitaxial integration of EuO with Si bring the manufacturing of a direct spin contact within reach. Here we employ transmission electron microscopy to study the interface EuO/Si with atomic-scale resolution. We report techniques for interface control on a submonolayer scale through surface reconstruction. Thus we prevent formation of alien phases and imperfections detrimental to spin injection. This development opens a new avenue for semiconductor spintronics.

Emerging two-dimensional ferromagnetism in silicene materials

The appeal of ultra-compact spintronics drives intense research on magnetism in low-dimensional materials. Recent years have witnessed remarkable progress in engineering two-dimensional (2D) magnetism via defects, edges, adatoms, and magnetic proximity. However, intrinsic 2D ferromagnetism remained elusive until recent discovery of out-of-plane magneto-optical response in Cr-based layers, stimulating the search for 2D magnets with tunable and diverse properties. Here we employ a bottom-up approach to produce layered structures of silicene (a Si counterpart of graphene) functionalized by rare-earth atoms, ranging from the bulk down to one monolayer. We track the evolution from the antiferromagnetism of the bulk to intrinsic 2D in-plane ferromagnetism of ultrathin layers, with its characteristic dependence of the transition temperature on low magnetic fields. The emerging ferromagnetism manifests itself in the electron transport. The discovery of a class of robust 2D magnets, compatible with the mature Si technology, is instrumental for engineering new devices and understanding spin phenomena.Exploring the magnetism in 2D materials paves the way to low-dimensional spintronics. Here the authors report evolution of bulk antiferromagnetism to intrinsic 2D in-plane ferromagnetism in layered structures of silicene functionalized by rare-earth atoms as they are scaled down to one monolayer.

High-Temperature Magnetism in Graphene Induced by Proximity to EuO

Addition of magnetism to spectacular properties of graphene may lead to novel topological states and design of spin logic devices enjoying low power consumption. A significant progress is made in defect-induced magnetism in graphene-selective elimination of p z orbitals (by vacancies or adatoms) at triangular sublattices tailors graphene magnetism. Proximity to a magnetic insulator is a less invasive way, which is being actively explored now. Integration of graphene with the ferromagnetic semiconductor EuO has much to offer, especially in terms of proximity-induced spin-orbit interactions. Here, we synthesize films of EuO on graphene using reactive molecular beam epitaxy. Their quality is attested by electron and X-ray diffraction, cross-sectional electron microscopy, and Raman and magnetization measurements. Studies of electron transport reveal a magnetic transition at TC* ≈ 220 K, well above the Curie temperature 69 K of EuO. Up to TC*, the dependence R xy( B) is strongly nonlinear, suggesting the presence of the anomalous Hall effect. The role of synthesis conditions is highlighted by studies of an overdoped structure. The results justify the use of the EuO/graphene system in spintronics.

Fine structure of metal–insulator transition in EuO resolved by doping engineering

Metal-insulator transitions (MITs) offer new functionalities for nanoelectronics. However, ongoing attempts to control the resistivity by external stimuli are hindered by strong coupling of spin, charge, orbital and lattice degrees of freedom. This difficulty presents a quest for materials which exhibit MIT caused by a single degree of freedom. In the archetypal ferromagnetic semiconductor EuO, magnetic orders dominate the MIT. Here we report a new approach to take doping under control in this material on the nanoscale: formation of oxygen vacancies is strongly suppressed to exhibit the highest MIT resistivity jump and magnetoresistance among thin films. The nature of the MIT is revealed in Gd doped films. The critical doping is determined to be more than an order of magnitude lower than in all previous studies. In lightly doped films, a remarkable thermal hysteresis in resistivity is discovered. It extends over 100 K in the paramagnetic phase reaching 3 orders of magnitude. In the warming mode, the MIT is shown to be a two-step process. The resistivity patterns are consistent with an active role of magnetic polarons-formation of a narrow band and its thermal destruction. High-temperature magnetic polaron effects include large negative magnetoresistance and ferromagnetic droplets revealed by x-ray magnetic circular dichroism. Our findings have wide-range implications for the understanding of strongly correlated oxides and establish fundamental benchmarks to guide theoretical models of the MIT.

Formation of BiFeO3 from a Binary Oxide Superlattice Grown by Atomic Layer Deposition

We report on the growth of polycrystalline BiFeO3 thin films on SiO2 /Si(001) and Pt(111) substrates by atomic layer deposition using the precursors ferrocene, triphenyl-bismuth, and ozone. By growing alternating layers of Fe2 O3 and Bi2 O3 , we employ a superlattice approach and demonstrate an efficient control of the cation stoichiometry. The superlattice decay and the resulting formation of polycrystalline BiFeO3 films are studied by in situ X-ray diffraction, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. No intermediate ternary phases are formed and BiFeO3 crystallization is initiated in the Bi2 O3 layers at 450 °C following the diffusion-driven intermixing of the cations. Our study of the BiFeO3 formation provides an insight into the complex interplay between microstructural evolution, grain growth, and bismuth oxide evaporation, with implications for optimization of ferroelectric properties.

Effects of Kr and Xe ion irradiation on the structure of Y2O3 nanoprecipitates in YBCO thin film conductors

ABSTRACT The response of Y2O3 nanoprecipitates in a 1-µm YBa2Cu3O7-x layer from a superconducting wire Ag/YBCO/buffer metal oxides/Hastelloy to 107 MeV Kr and 167 MeV Xe ion irradiation was investigated using a combination of transmission electron microscopy, diffraction and X-ray energy-dispersive spectrometry. The direct observation of the radiation-induced tracks in Y2O3 nanocrystals is reported for the first time to the authors’ best knowledge. Structureless damaged regions of 5–9 nm (average 8 nm) in diameter were observed in Y2O3 nanocrystals when the electronic stopping power Se was about or higher than 4.7 keV/nm. This value of Se is the upper estimate of the minimum electronic stopping power to create damage in yttria nanocrystals. The electron diffraction patterns, high-resolution transmission electron microscopy, high-resolution scanning transmission electron microscopy, Fourier transform patterns from areas extending a few nanometres around the tracks show that yttria and YBCO keep their respective cubic and orthorhombic pristine structures.

Evidence of extended cation solubility in atomic layer deposited nanocrystalline BaTiO3 thin films and its strong impact on the electrical properties

Thin films of ≈50 nm thickness with Ba/Ti-ratios ranging from 0.8 to 1.06 were prepared by depositing alternating layers of Ba(OH)2 and TiO2. Annealing at 750 °C promoted the solid-solid transformation into polycrystalline BaTiO3 films containing a mixture of the perovskite and the hexagonal polymorphs with average crystallite sizes smaller than 14 nm and without impurity phases. This, together with an increase of the cubic lattice parameters for Ba-rich films, suggests an extended metastable solubility range for the perovskite-phase in these nanocrystalline thin films on both sides of the stoichiometric composition. Mapping of the cation distribution utilizing energy-filtered transmission electron microscopy corroborates defect accommodation within the BaTiO3 grains. While the cation off-stoichiometry in thermodynamic equilibrium is negligible for BaTiO3, the metastable extended solubility range in the thin films can be directly correlated to the low annealing temperature and nanocrystalline nature. The leakage current behavior can be explained by the formation of Schottky defects for nonstoichiometric films, and the cation ratio has a distinct impact on the dielectric properties: while excess-BaO has a marginal detrimental effect on the permittivity, the dielectric constant declines rapidly by more than 50% towards the Ti-rich side. The present findings highlight the importance of compositional control for the synthesis of nanocrystalline BaTiO3 thin films, in particular for low annealing and/or deposition temperatures. Our synthesis approach using alternating layers of Ba(OH)2 and TiO2 provides a route to precisely control the cation stoichiometry.

Coupling of magnetic orders in a 4f metal/oxide system

Antiferromagnetic spintronics is actively explored for use in data storage to enhance robustness, switching speed and packing. A number of archetypal antiferromagnets formed by d-electron (transition) metals are traditionally employed in studies of spin dynamics and control. In contrast, rare-earth antiferromagnets exhibiting 4f-electron magnetism form an emerging class of prospective spintronic materials. In particular, little is known about the potential of elemental rare-earth metals in spintronics. Here, we synthesize films of metallic Eu, an antiferromagnet with a helical magnetic structure, on YSZ and an underlayer of the ferromagnetic semiconductor EuO using molecular beam epitaxy. Their structural quality is established with electron and X-ray diffraction, as well as analytical electron microscopy. Magnetization measurements reveal a peculiar coupling between Eu and EuO magnetic systems leading to negative exchange bias. We demonstrate a strong influence of EuO magnetic state on the electron transport in metallic Eu – a feature appears in the temperature dependence of Eu resistivity around the Curie temperature of EuO. Moreover, magnetic field dependence of the Hall resistivity becomes essentially non-linear; not only the shape of magnetoresistance changes qualitatively but also its sign becomes negative as soon as an ultrathin layer of Eu is in contact with EuO. The results also suggest a magnetic transition in pristine Eu in high fields. The rich physics of Eu and the Eu/EuO structure justifies their exploration with the aim of new spintronic functionalities.

Interface-Induced Anomalous Hall Conductivity in a Confined Metal

The mature silicon technological platform is actively explored for spintronic applications. Metal silicides are an integral part of the Si technology used as interconnects, gate electrodes, and diffusion barriers; their epitaxial integration with Si results in premier contacts. Recent studies highlight the exceptional role of electronic discontinuities at interfaces in the spin-dependent transport properties. Here, we report a new type of Hall conductivity driven by sharp interfaces of Eu silicide, an antiferromagnetic metal, sandwiched between two insulators - Si and SiO x. Quasi-ballistic transport probes spin-orbit coupling at the interfaces, in particular, charge-spin interconversion. Transverse magnetic field results in anomalous Hall effect signals of an unusual line shape. The interplay between opposite-sign signals from the two interfaces allows efficient control over the magnitude and sign of the overall effect. Selective engineering of interfaces singles out a particular spin signal. The two-channel nature of the effect and its high tunability offer new functional possibilities for future spintronic devices.