Epitaxial Rare Earth Research Articles

Tuning magnetization compensation and Curie temperatures in epitaxial rare earth iron garnet films

An ab initio effective Hamiltonian is developed and used to reveal the effect of misfit strain on magnetic properties of epitaxial (001) films made of a prototypical rare earth iron garnet: Gadolinium iron garnet. It is found that both the magnetization compensation temperature (${T}_{\mathrm{M}}$) and the Curie temperature (${T}_{\mathrm{C}}$) can be significantly and linearly enhanced when varying the strain from tensile to compressive values. Consequently, ${T}_{\mathrm{M}}$ can be designed to coincide with room temperature, and the magnitude of the magnetization can be optimized at around 300 K when choosing the appropriate substrate. The microscopic origins of these technologically promising effects are further revealed.

Effect of Ceramic-Target Crystallinity on Metal-to-Insulator Transition of Epitaxial Rare-Earth Nickelate Films Grown by Pulsed Laser Deposition

We demonstrate the effect of the crystallinity of ceramic targets on the electronic properties of LaNiO3 (001) thin films epitaxially grown by pulsed laser deposition (PLD). We prepared two kinds of LaNiO3 targets with different crystallinity by manipulating calcination temperature (i.e., 300 and 1000 °C) in the solid state reaction for ceramic synthesis. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) experiments of the as-sintered LaNiO3 ceramic targets clearly show that the LaNiO3 target sintered after high-temperature (1000 °C, high crystallinity) calcination is more oxidized to Ni3+ with better crystallinity than the LaNiO3 target sintered after low-temperature (300 °C, poor crystallinity) calcination. Using these two LaNiO3 ceramics as PLD targets, we fabricated epitaxial LaNiO3/LaAlO3 (001) thin-film heterostructures to examine how target crystallinity affects the physical properties of LaNiO3 films. Intriguingly, the electri...

Growth of Epitaxial Silicon-on-Insulator Substrates by Solid State Epitaxy

The growth of monolithically integrated silicon-on-insulator (SOI) substrates using rare earth oxide insulator layers is presented. Rare earth oxides may be grown epitaxially on Si(111) substrates due to their unique cubic crystal structure and lattice matching to twice the silicon lattice spacing. Growth of silicon on top of epitaxial rare earth oxides is achieved and several variations of growth schemes are investigated. The current process has been developed on 100mm silicon wafers and may be scaled up to 200mm wafer sizes in a production type cluster tool.

Structure and defect characterization of multiferroicReMnO3 films and multilayers by TEM

Epitaxial rare earth manganite thin films (ReMnO3;Re = Tb, Ho, Er, and Y) and multilayers were grown by liquid injection metal organicchemical vapor deposition (MOCVD) on YSZ(111) and the same systems were grownc-oriented on Pt(111) buffered Si substrates. They have been structurally investigated byelectron diffraction (ED) and high resolution transmission electron microscopy(HRTEM). Nanodomains of secondary orientation are observed in the hexagonalYMnO3 films. They are related to a YSZ(111) and Pt(111) misorientation.The epitaxial film thickness has an influence on the defect formation.TbO2 andEr2O3 inclusions areobserved in the TbMnO3 and ErMnO3 films respectively. The structure and orientation of these inclusions are correlated to theresembling symmetry and structure of film and substrate. The type of defect formed in theYMnO3/HoMnO3 andYMnO3/ErMnO3 multilayers is also influenced by the type of substrate they are grown on. In our work,atomic growth models for the interface between the film/substrate are proposed andverified by comparison with observed and computer simulated images.

Encapsulated solid phase epitaxy of a Ge quantum well embedded in an epitaxial rareearth oxide

An efficient method based on molecular beam epitaxy has beendeveloped to integrate an epitaxial Ge quantum well buried into a singlecrystalline rare earth oxide. The monolithic heterostructure comprised ofGd2O3–Ge–Gd2O3 grown on an Si substrate exhibits excellent crystalline quality with atomically sharpinterfaces. This heterostructure with unique structural quality could be used for novelnanoelectronic applications in quantum-effect devices such as nanoscale transistors with ahigh mobility channel, resonant tunneling diode/transistors, etc. A phenomenologicalmodel has been proposed to explain the epitaxial growth process of the Ge layerunder oxide encapsulation using a solid source molecular beam epitaxy technique.

Band offsets between Si and epitaxial rare earth sesquioxides (RE2O3, RE=La,Nd,Gd,Lu): Effect of 4f-shell occupancy

Internal photoemission of electrons and holes into cubic Nd2O3 epitaxially grown on (100)Si reveals a significant contribution of Nd 4f states to the spectrum of the oxide gap states. In contrast to oxides of other rare earth (RE) elements (Gd, Lu) epitaxially grown in the same cubic polymorph, to hexagonal LaLuO3, and to polycrystalline HfO2, the occupied Nd 4f states produce an additional filled band 0.8eV above the O 2p derived valence band. The unoccupied portion of the Nd 4f shell leads to empty electron states in the energy range of 1eV below the RE 5d derived oxide conduction band. The exposed Nd 4f states suggest the possibility to use this metal and, possibly, other REs with low f-shell occupancy to control the interface band offsets by selective interface doping.

Development of Rare Earth Niobate Buffer Layer for YBCO Coated Conductor Using Chemical Solution Deposition Approach

AbstractWe have grown epitaxial rare earth niobate (RE3NbO7; RE – La, Ce, Nd, Sm, Eu, Gd, Ho, Y, and Yb) buffer layers on biaxially textured Ni-W substrates using chemical solution deposition approach. Precursor solutions of 0.4 M total cation concentration were spin coated on short samples of Ni-3 at.%W (Ni-W) substrates and heat-treated at 1050 to 1100°C in a gas mixture of Ar-4%H2 for 15 minutes. Effect of solution chemistry and processing atmosphere on texture and microstructure were studied. Detailed X-Ray studies indicated that all the rare earth niobates were grown epitaxially. There was no significant change observed in texture for different precursor chemistry, however, dramatic effect on surface morphology observed. SEM and AFM investigations of RE3NbO7 films reveal noticeable difference in surface morphology and roughness for various processing atmospheres. Processing under a 20% wet Ar-4% H2 gas mixture was found to be the optimum condition for growing Gd3NbO7 films with high quality microstructure.

Planar defects and double-domain epitaxy in epitaxial YSi2−x and ErSi2−x thin films on Si substrates

Interfacial reactions of Y and Er thin films on both (111)Si and (001)Si have been studied by transmission electron microscopy (TEM). Epitaxial rare-earth (RE) silicide films were grown on (111)Si. Planar defects, identified to be stacking faults on \(\left\{ {10\bar 10} \right\}\) planes with 1/6\(\left\langle {\bar 12\bar 13} \right\rangle \) displacement vectors, were formed as a result of the coalescence of epitaxial silicide islands. Double-domain epitaxy was found to form in RE silicides on (001)Si samples resulting from a large lattice mismatch along one direction and symmetry conditions at the silicide/(001)Si interfaces. The orientation relationships are [0001]RESi2−x//\(\left[ {1\bar 10} \right]\)Si, \((1\bar 100)\)RESi2−x//(001)Si and [0001]RESi2−x/\([\bar 1\bar 10]\)Si, \((1\bar 100)\)RESi2−x//(001)Si. The density of staking faults in (111) samples and the domain size in (001) samples were found to decrease and increase with annealing temperature, respectively.

Solid state amorphization in metal/Si systems

The formation of amorphous interlayer (a-interlayer) by solid-state diffusion in diffusion couples has been one of the most challenging problems in condensed matter physics in recent years. The a-interlayer has been found to occur in all refractory metal/Si and a number of rare-earth (RE) metal and platinum group metal and crystalline silicon systems. A systematic survey and review of extensive studies on the subject in the past years showed that (1) a negative heat of mixing provides the driving force for the reaction and fast diffusion of one component in the other preempts the formation of crystalline compounds, (2) the growth follows a linear law at the initial stage with activation energy around 1–1.5 eV for refractory metal/Si systems and 0.5 eV for RE metal/Si systems, (3) the dominant diffusing species is Si, (4) the stability of amorphous interlayer depends on the composition, (5) simultaneous presence of multiphases in the initial stage of metal/Si interaction, and (6) good correlations between physical parameters and kinetic data. From the investigation of amorphous interlayers, mechanisms of roughing of epitaxial RE silicide/(0 0 1)Si interface, formation of stacking faults and pinholes in RE silicides have gained in basic understanding. The insight led to successful growth of pinhole-free epitaxial RE silicide layer on (1 1 1)Si. Furthermore, the enhanced formation of technologically important C54-TiSi 2 by high temperature sputtering, a thin interposing Mo layer and tensile stress can all be explained involving some aspects of the amorphous interlayers.

Epitaxial rare earth fluoride insulating layers on semiconductors: Structure, phase transitions, and interface reactions

Layers of GdF3 were deposited on Ge(111) and GaAs(111)B surfaces, and the measurements compared with similar ones for HoF3 on Si(111) [C. L. Griffiths, J. E. Macdonald, and R. H. Williams, J. Appl. Phys. 70, 1858 (1991)]. It is shown that the trigonal structure normally only stable at higher temperatures is stable in all cases at room temperature, for thin layers. For thicker films a transition to the orthorhombic phase has been observed and studied by low-energy electron diffraction. Some interesting patterns are observed, and are interpreted in terms of a multidomain orthorhombic structure for HoF3 on Si(111) [C. L. Griffiths, J. E. Macdonald, and R. H. Williams, J. Appl. Phys. 70, 1858 (1991)], with the additional complexity of mirror planes for GdF3 on Ge(111). Interface reactions have also been studied in some detail, using x-ray photoelectron spectroscopy and soft x-ray photoemission spectroscopy. In all cases, except GdF3 on Ge, strong reactions are observed with rare earth silicide formation for HoF3 on Si and more complex products for GdF3 on GaAs. These reactions may be substantially reduced by the incorporation of a passivating interlayer. This is illustrated by the use of an ordered arsenic layer at the LaF3–Si interface.

Epitaxial Growth of Rare Earth Silicides on (111)Si

AbstractEpitaxial YSi2-x, TbSi2-x, and ErSi2-x. have been grown on (111)Si by solid phase epitaxy in ultrahigh vacuum deposited rare earth (RE) metal thin films on silicon. The evolution of vacancy ordering and defect structure in epitaxial RE silicide thin films on (111)Si have been studied by both conventional and high resolution transmission electron microscopy.Additional superlattice spots located at 1/3 <2110> in the diffraction pattern of RESi2-x, are attributed to the formation of ordered vacancy in the Si sublattice planes. The splitting of extra diffraction spots is correlated to the formation of an out-of--step structure. Streakings of the split diffraction spots in the diffraction pattern are attributed to the presence of an out-of-step structure with a range of M values. For YSi2-x and ErSi2-x, the M was found to settle down to a constant value after high temperature and/or long time annealing. For TbSi2-x, M is equal to 5 throughout the annealing.Planar defects in RESi2-x films were analyzed to be stacking faults on {1010} planes with 1/6<1213> displacement vectors. The size and density of stacking faults were found to increase and decrease, respectively, with annealing temperature and/or annealing time.

Surface electronic structure of erbium silicide epitaxially grown on Si(111)

The determination of the electronic structure of the rare-earth (RE) silicides epitaxially grown on Si(111) is of primary importance for the understanding of the unusually low Schottky barrier height of these silicides on n-type silicon (∼ 0.35 eV). We have performed angle-resolved UV photoemission (ARUPS) experiments on ErSi1.7 epitaxially grown “in-situ” on Si(111)7 × 7 with three photon energies (He I, He II and Ne I) in the two inequivalent directions of the surface Brillouin zone (SBZ). In both directions, some peaks are found at identical initial energies - within 0.1 eV or less - for those three photons energies. Some of these “stationary structures” might be due to surface states. We shall discuss the influence of gas exposure on the ARUPS spectra. These results will be compared with those obtained on other epitaxial RE (Tb, Y) silicides.

Magnetic ordering in strain-controlled epitaxial rare-earth films (abstract)

The magnetic properties of epitaxial rare-earth films and multilayers are known to be sensitive to lattice strain arising from the misfit between the substrate and the epitaxial rare earth overlayer. We find that rare-earth films grow epitaxially, with no coherency strain, onto rare-earth fluoride films on GaAs(111) substrates. This decoupling of the rare-earth film from substrate-imposed coherency strain permits controlled amounts of strain to be introduced into the rare earths by suitably designed sandwich structures. We report quantitative x-ray measurements of strain in films of Dy, Ho, and Er sandwiched by rare-earth fluorides and in Er/Dy/Er sandwich structures grown onto rare-earth fluoride buffer films on GaAs(111). SQUID magnetometry of these structures shows that the unstrained Dy, Ho, and Er films exhibit similar Curie temperatures to bulk crystals. On the other hand, in the case of Er/Dy/Er structures the Curie temperature of Dy is increased as a result of a designed, coherency-induced c-axis expansion of the Dy. The results of a study of strain in Y/Dy/Y structures will also be reported.

A High Resolution Cross-Section Transmission Electron Microscopy Study of Epitaxial rare Earth Fluoride / GaAs(111) Interfaces Prepared by Molecular Beam Epitaxy

AbstractWe report the first HRXTEM study of epitaxial rare earth fluoride/ GaAs(111) interfaces. Such interfaces are of interest because they are the starting point for growth of buried epitaxial rare earth / rare earth fluoride sandwich structures which exhibit interesting and non bulk-like magnetic properties. Also, the optical transitions in ultrathin epitaxial NdF3 films may be influenced by strain and defects in the NdF3 film and the nature of the interface to GaAs.We find that the rare earth fluoride / GaAs interfaces are semi- coherent but chemically abrupt with the transition taking place within 3Å. However, the interface is physically rough and multiple monolayer steps in the GaAs surface tend to lead to tilt boundaries in the fluoride. The origin of these steps is believed to be thermal etching of the GaAs during the heat-cleaning stage prior to epitaxy. The surface of the fluoride film is much smoother than the initial GaAs surface indicating planarization during epitaxy.

Chemical vapor deposition of rare earth iron garnets for bubble domain devices

Epitaxial rare earth iron garnet films have proven to have the most attractive properties for bubble domain devices. This paper describes chemical vapor deposition of rare earth iron garnet films with up to four metal ion components. Defect free regions up to 5mm square and thickness uniformity to within 4% have been achieved.