Rare-earth Metal Films Research Articles

High-Performance Organic Field-Effect Transistors Using Rare Earth Metal Oxides as Dielectrics

Rare earth metal oxides (REOs) represent promising gate dielectric materials for field-effect transistors due to their high dielectric constants required for suppressing leakage currents in devices. In this paper, we present an approach to deposition of uniform REO gate dielectric coatings by oxidation of thin rare earth metal films in air at relatively low temperatures (100–250 °C). The proposed methodology was extensively explored in organic field-effect transistors (OFETs) using five benchmark organic semiconductors and six REOs as gate dielectrics. It was shown that OFETs with the REO gate dielectrics deliver on average considerably better characteristics compared to the reference devices using electrochemically grown aluminum oxide dielectric (AlOx). OFETs on a flexible plastic substrate were demonstrated, which makes this approach very attractive for developing flexible and wearable electronic devices with improved performance.

The Effects of Substrate Temperature on the Growth, Microstructural and Magnetic Properties of Gadolinium-Containing Films on Aluminum Nitride

To facilitate future novel devices incorporating rare earth metal films and III-V semiconductors on Si substrates, this study investigates the mechanisms of growth via molecular beam epitaxy of gadolinium (Gd) on aluminum nitride (AlN) by determining the impact of substrate temperature on microstructure. The Gd films underwent extensive surface analysis via in situ reflective high energy electron diffraction (RHEED) and ex-situ SEM and AFM. Characterization of the surface features of rare earth metal films is important, as surface geometry has been shown to strongly impact magnetic properties. SEM and AFM imaging determined that Gd films grown on AlN (0001) from 80 °C to 400 °C transition from wetting, nodular films to island–trench growth mode to reduce in-plane lattice strain. XRD and Raman spectroscopy of the films revealed that they were primarily comprised of GdN, Gd and Gd2O3. The samples were also analyzed by a vibrating sample magnetometer (VSM) at room temperature. From the room temperature magnetic studies, the thick films showed superparamagnetic behavior, with samples grown between 240 °C and 270 °C showing high magnetic susceptibility. Increasing GdN (111) 2θ peak position and single-crystal growth modes correlated with increasing peak magnetization of the thin films, suggesting that lattice strain in single-crystal films was the primary driver of enhanced magnetic susceptibility.

Enhanced oxygen reduction activity with rare earth metal alloy catalysts in proton exchange membrane fuel cells

Alloying platinum is an approach to increase the oxygen reduction reaction (ORR) activity and at the same time reduce the amount of precious platinum catalyst in proton exchange membrane fuel cells (PEMFC). In this work the cathode activity of thin films of rare earth metals (REM) alloys, Pt3Y, Pt5Gd and Pt5Tb, produced by sputter deposition onto gas diffusion layers, are evaluated in a fuel cell by means of polarization curves in O2/H2, and cyclic- and CO-stripping voltammetry in N2/5% H2. Prior to evaluation, the model electrodes were acid-treated to obtain a Pt skin covering the PtREM alloy bulk, as was revealed by energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The core shell alloys of Pt3Y and Pt5Gd catalysts show a specific activity enhancement at 0.9 V of 2.5 times compared to pure Pt. The slightly lower enhancement factor of 2.0 for Pt5Tb is concluded to be due to leaching of the REM, that resulted in a thicker, and subsequently less strained, Pt overlayer. The high activity, combined with the minor changes in surface composition, achieved in the fuel cell environment shows that PtREM core shell catalysts are promising for the cathode reaction in PEMFC.

Surface and bulk phenomena in the process of hydride formation in thin films of rare earth metals: Comparison between terbium and europium

Work function changes Δ Φ caused by H 2 interaction with thin terbium and europium films deposited on glass under UHV conditions were correlated with hydrogen uptake and electrical resistance R, measured in situ. For both metals, the course of Δ Φ(H/Me) at room temperature confirms the change in charge-transfer direction on the surface during hydride formation. As a result, the hydrogen adsorbate's nature is changed from positively polarized (precursor state) to negatively polarized adspecies. The hydrogen behavior is significantly different at low temperature due to the formation of the surface ordered low-temperature phase (α′) with positively polarized hydrogen adspecies. This phase, strongly inhibiting penetration of hydrogen into the bulk, is stable up to 100 K for terbium and 160 K for europium. Increasing temperature above these values resulted in additional large absorption of hydrogen. Moreover, differences in the course of R(H/Me) are clearly noticeable between the investigated metals. The resistance of thin TbH x ( x ∼ 3) films in our experiments did not exceed 1 kΩ, however transition of thin metallic europium film into EuH y ( y ∼ 2) increased the resistance up to 10 MΩ. This dissimilarity in electrical behavior can be explained by the coexistence of two factors which are different for the two metals in question: the phase relation of hydrides and the response of thin film to stress generated during hydride formation.

Determination of the composition, binding energy, and electron transition on the surface of intermetallides of transition metals with noble and ordinary metals

A method of the determination of the surface composition of intermetallic compounds (IMC) of transition metals with noble and ordinary ones, the determination of binding energy in IMC, and the direction of electron transitions upon IMC establishment is proposed. The method is based on the kinetic investigation of low-temperature homomolecular isotope hydrogen exchange: H2 + D2 = 2HD. This method was invented as a result of the investigation of the dependence of the specific catalytic activity (Ksp); films of rare-earth metals (REM), Hf, Zr, Ru, Rh, Pt, Pd; and IMC of Ln-(Cu, Au, Ag, Ar, In, Ga) and Hf-(Au, Ag, Cu) upon temperature in regards to the indicated reaction at intervals of 77-150-230 K.

Coadsorption of lanthanum with boron and gadolinium with boron on Mo(1 1 0)

Submonolayer to multilayer coadsorption of lanthanum (La) with boron (B) and gadolinium (Gd) with boron on the surface of Mo(1 1 0) has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and work function ( ϕ) measurements. The equilibrium state of double adsorbate systems achieved either by adsorption of rare-earth metal (REM) on boron precovered Mo(1 1 0) surface held at room temperature or after moderate annealing of the system with opposite order of adsorption (B on REM films) is the layer which is the inhomogeneous mixture of boron and REM atoms with preferential concentration of boron in the surface area of the mixed film. The work function of such films even at REM to boron concentration ratio much higher than 1/6 are very close to the values of corresponding bulk LaB 6 and GdB 6, favoring assumption of surface rearrangement as the dominant reason of high electron emission efficiency of hexaborides. Almost total similarity of the results for La–B and Gd–B systems can be viewed as the consequence of weak participation of Gd f-electrons in determining the thermionic properties of corresponding double layers.

Surface magnetism at the nanometer and atomic scale

During the past year, significant advances have been made in the characterization of low-dimensional magnetic structures down to the atomic length scale. Spin-polarized scanning tunneling spectroscopy and magnetic force microscopy under ultrahigh vacuum conditions have been applied to image the nanomagnetic domain structure of ultrathin transition and rare-earth metal films, whereas conventional scanning tunneling spectroscopy has been used to study Kondo scattering at single magnetic impurities. The fabrication of artificial magnetic nanostructures using atomic manipulation processes based on the scanning tunneling microscope will additionally allow fundamental investigations of magnetic interactions at the atomic level.

Electronic and magnetic structure of rare-earth materials studied by high-resolution photoemission

Recent experimental results concerning the surface and bulk electronic and magnetic structure of rare-earth (RE) metals are presented. Surface core-level shifts in photoemission (PE) and inverse photoemission (IPE) experiments are discussed to summarize the phenomena that occur due to the energy lowering of 4f states at the surface. In Ce metal, a highly correlated electronic system, surface effects are influenced by the strong interaction of the 4f electron with the valence band, leading to essentially no observable shift in PE. Resonant PE experiments with varying degree of surface sensitivity have been performed to investigate the surface electronic structure in these systems. With recent progress in the growth of well-ordered epitaxial films of RE metals, detailed band-structure studies have become possible. The existence of a localized d-state at the surface is demonstrated and consequences for the surface properties are discussed. Single-crystalline RE surfaces open up the possibility to study their magnetic properties as shown for the case of Gd, where a strong magnetic circular dichroism has recently been observed.

Rare-earth metal films modification by Ar ion irradiation during condensation

Using the method of X-ray analysis the influence of AR ion irradiation on the modification of lattices and their parameters and also types of formed solid solutions in Sm and Yb films including ion irradiation during condensation has been investigated.

Crystalline NdFe perpendicular magnetization film

New film media are attractively been studied for high density perpendicular magnetic recording (PMR). However, light rare earth metal films have not yet attracted a great attention. Some previous papers from Taylor et al. and Suzuki, showed that amorphous NdFe perpendicular magnetization film (PMF) can be obtained. We think that the magnetic anisotropy of the crystalline NdFe film will be stronger that that of amorphous film, thus, according to the theory of film perpendicular magnetization, if the content of Nd in NdFe film is adequately controlled, crystalline NdFe PMF might also be obtained. This is confirmed by the results reported in this paper, which we believe that for the crystalline NdFe perpendicular magnetization film, with its loops and ratio M rp/ M r larger than one been given, have yet not been reported.

Low-temperature ortho-para conversion of hydrogen on films of rare-earth metals and their copper alloys

Kinetic data for the ortho-para conversion of H2 at 77 k rear earth metals (REM) and their Cu alloy films are given. Conversion on the surface of Sc, Y, La, Yb and Lu has been shown previously to follow a chemical mechanism, for the others it is magnetic. When alloyed with Cu, the specific catalytic activity (Ksp) of Sc, Y, La, Yb and Lu sharply increases. A sharp increase of Ksp is also observed on Cu alloys of Ce, Sm, Nd, Pr, Eu and Gd and the chemical conversion mechanism becomes predominant. On Tb, Dy, Er, Ho and Tm, Ksp rises only slightly and the magnetic mechanism preserves.

Structural and Magnetic Characterization of Rare Earth and Transition Metal Films Grown on Epitaxial Buffer Films on Semiconductor

ABSTRACTStructural and magnetic data are presented and discussed for epitaxial films of rare earth metals ( Dy, Ho, Er) on LaF3 films on the GaAs(TTT) surface and Fe on Ag films on the GaAs(001) surface. Both systems exhibit unusual structural characteristics which influence the magnetic properties of the metal films. In the case of rare earth epitaxy on LaF3 we present evidence for epitaxy across an incommensurate or discommensurate interface. Coherency strain is not transmitted into the metal which behaves much like bulk crystals of the rare earths. In the case of Fe films , tilted epitaxy and long-range coherency strain are confirmed by X-ray diffractometry. Methods of controlling some of these structural effects by modifying the epitaxial structures are presented.

X-Ray Diffraction Study of Rare Earth Epitaxial Structures Grown by MBE onto (111) GaAs

ABSTRACTWe report on the new epitaxial system LaF3/Er/Dy/Er/LaF3/GaAs (111) grown by molecular beam epitaxy. X-ray diffraction studies have been used to determine the epitaxial relationships between the rare earths, the LaF3 and the substrate. Further studies of symmetric and asymmetric reflections yielded the in-plane and perpendicular strain components of the rare earth layers. Such systems may be used to probe the effects of magnetoelastic interactions and dimensionality on magnetic ordering in rare earth metal films and multilayers.

Oxidation kinetics for thin rare-earth metal films

We measured the oxidation rates of thin samarium, gadolinium, ytterbium, and yttrium films at room temperature in air by a simple optical transmission technique to help understand how to produce flat 100 nm or so thick films on 150-nm freestanding polymer substrates for use as x-ray lasers. The validity of the method is established with aluminum, whose oxide layer reaches a limiting thickness of about 5 nm within 1 h. The oxidation kinetics of different rare earths follow different rate laws. Ytterbium and gadolinium are best described by an inverse logarithmic rate law, yttrium by a direct logarithmic rate law, and samarium by a parabolic rate law. After 2 months, the oxide thicknesses of Y, Yb, Gd, and Sm are about 5, 9, 15, and 17 nm, respectively. Only Sm continues to oxidize rapidly, reaching 35 nm in 6.5 months. Spectroscopic results on Eu and Gd are consistent with previous results that the oxidized layer on the rare earths is usually a carbonated hydroxide. The results of our measurements are compared to those from initial oxidation studies by surface analysis techniques and high-temperature gravimetric studies.

Epitaxial Growth of Rare Earths on Rare Earth Fluorides and Rare Earth Fluorides on Rare Earths: Two New Epitaxial Systems Accessed by MBE

ABSTRACTWe report two new epitaxial systems, prepared by MBE: basal plane epitaxy of the rare earth metal Dy onto LaF3 films and vice versa. SQUID magnetometry studies indicate that buried epitaxial Dy films, of ∼300Å thick, order ferromagnetically at similar temperatures to bulk Dy crystals.These epitaxial systems are one member of a new family of epitaxial systems of basal plane epitaxy of rare earth metals on rare earth fluorides and vice versa. Such systems may be used to probe the effects of strain and dimensionality on magnetic ordering in rare earth metal films and multilayers.

Structural Properties of Single Crystal Rare-Earth Thin Films Y and Gd Grown by Molecular Beam Epitaxy

AbstractBy means of metal MBE technique with in-situ RHEED characterization, high-quality single crystal rare earth metal films of yttrium and gadolinium were grown as a necessary requirement for studying Gd/Y superlattices. The key step of this successful growth is the employment of the single-crystal Nb film as a buffer layer to eliminate the interaction of rare earth metals with most substrates. Structural analyses by X-ray diffraction and ion channeling show that these crystals exhibit not only complete texture of [00.1], but also narrow rocking curves both perpendicular (00.2) and parallel (10.0) to the film.

Surface reactions on rare earth metals monitored by work function measurements

Surface reactions on clean, oxidized, and partially hydrided rare earth metal films with O 2, H 2, H 2O, CO, and CO 2 were monitored by dynamic work function measurements under ultrahigh vacuum conditions. The work function data are discussed in the light of electron spectroscopic evidence, where available. The main results of this study are: Initial oxidation of Yb seems to follow a different oxidation mechanism as compared to other rare earth metals. Reaction of H 2O with clean, oxidized, and hydrided rare earth metal surfaces yields surface hydroxyl species. H 2 uptake on divalent Yb metal is much slower than on trivalent Er metal, but occurs at a comparable rate on the oxidized systems. On oxidized surfaces, surface OH formation is substantiated for room-temperature reaction with H 2. The Δφ data indicate high reactivity of CO and CO 2 toward clean rare earth metal surfaces and are consistent with dissociative adsorption of both molecules.

Microstructural Studies of Laser-Melted Titanium-Rare-Earth and Titanium-Boron Alloys

The feasibility of producing super-saturated solid solutions with small grain sizes and fine, stable, incoherent dispersoids was determined in laser-surface-melted Ti-rare-earth (RE) (RE = Er, Y, Gd, Nd, or Sc) and Ti-B alloys. Titanium specimens, coated with 0.02 to 1-μm thick electron-beam-evaporated rare-earth metal films or with a 10 to 25-μm thick boron slurry, were surface melted using a 1.5 kW continuous-wave CO2 laser beam at specimen traversing speeds of 0.5 to 40 cm/s. The microstructures of the laser-melted regions, determined by scanning and transmission electron microscopy, were correlated with the laser-processing parameters.Figures 1a-1c are scanning electron micrographs of transverse sections of the laser-melted Ti-Y alloy at traversing speeds of 0.5, 10, and 30 cm/s respectively. The positions of the solid-liquid interfaces match the theoretically predicted melting isotherms for a specimen-beam coupling efficiency of 25%. Constant cooling-rate curves, calculated from the heat-transfer model for a rapidly moving, high-power, Gaussian source, are included in Figures 1a-1c. In the laser-melted Ti-RE alloys with the 0.2-μm thick rare-earth films, the microstructure consists of fine 10-20 nm dispersoids. The dispersoid size decreases with increasing specimen traversing speed as well as with increasing distance from the specimen surface. The volume fraction of the dispersoids increases with increasing film thickness and with melt depth. The equilibrium, coarse, constituent particles, which are generally observed in conventionally cast Ti alloys containing corresponding amounts of rare-earth elements, were completely absent in the laser-melted alloy.

The effect of oxygen and hydrogen contamination on the electrical resistivity of rare earth metal films

The effect of air and hydrogen contamination on the electrical resistivity of Er, Ho, Dy, Tb, Gd and Yb films was studied. The resistivity of the films increases almost discontinuously as soon as air is admitted into the vacuum chamber. After this sudden increase, the change becomes slow and the resistivity attains a constant value for thicker films (> 200 Å). For very thin films (<100 Å) the resistivity increases rapidly after the initial jump and very soon attains a very high value owing to the complete oxidation of the film. When hydrogen is admitted into the system, the resistivity of the films increases as for air contamination, but after a few minutes the resistivity starts to decrease except for Yb films. The increase in the resistivity for Er, Ho, Dy, Tb and Gd films is due to the interstitial solution of hydrogen in these metals (α phase); however, the decrease in ϱ after the initial increase is due to the formation of high conductivity f.c.c. dihydride. In the case of Yb the resistivity does not decrease because YbH2 has a higher resistivity than pure Yb. For thin erbium films (about 200 Å) at a hydrogen pressure of several torrs the resistivity rises again after reaching a minimum value. This increase is thought to be due to the interstitial solution of hydrogen in the metallic conductor ErH2. The investigation supports an earlier suggestion of the authors that the dihydride phase present in rare earth metal films (except Yb and Eu) made in ordinary vacuum may decrease the resistivity of these films below their bulk value.

Density and electrical conductivity of an abnormal fcc phase observed in rare earth metal films

Density and electrical conductivity of an abnormal fcc phase observed in rare earth metal films