LEED crystallographic analysis for the structure formed by 2 ML of O at the Zr(0001) surface

A tensor LEED analysis is reported for the Zr(0001)-(1 × 1)-O surface which involves oxygen at a total coverage of 2 monolayers. The structure is indicated to have two layers of O: one forms an overlayer in which the O atoms bond to hollow sites of three-fold coordination on the regular metal surface, while the other layer has the O atoms in tetrahedral hole sites between the first and second metal layers. The stacking sequence, designated as (C)B(A)AB... corresponds to the first three layers of anion-terminated cubic ZrO₂, although some lateral compression is needed for superposition on the regular hcp Zr structure. The absorption of O in the tetrahedral holes results in a significant expansion in the first-to-second Zr---Zr interlayer spacing to about 3.44 Å from the bulk vaue of 2.57 Å. The O---Zr bond lengths are estimated to equal 2.07 Å for the overlayer O atoms, and 2.21 Å for the O atoms in tetrahedral hole sites. Comparisons are made with the structures of the corresponding 0.5 and 1 ML surfaces formed by the O/Zr(0001) system.     

Interactive effect of impurities on giant magnetoresistance of Co–Fe/Cu multilayers

Giant magnetoresistance of Co–Fe–B/Cu multilayers fabricated in the sputtering atmosphere, where the amount of oxygen impurity is varied, is discussed in connection with their interfacial roughness. The magnetoresistance (MR) ratio of Co–Fe/Cu multilayers is enhanced by up to 33% when the oxygen content is varied between 10 and 100 ppm of processing Ar gas. The enhancement of the MR ratio was due to the flattening effect of impurity oxygen on the multilayer interfaces: the root mean square roughness of the multilayer was decreased from 7.5 to 5 Å. With increasing boron content in Co–Fe layers, however, the enhancing effect of MR ratio by oxygen diminished and nearly vanished for 12 at%-B–(Co–Fe) case. The strong affinity of boron for oxygen is suggested as a probable mechanism.     

Magnetization temperature dependence in Fe/Cr superlattices

We report the temperature dependence of the saturation magnetization, M_s(T), of Fe(t)/Cr(40 Å) superlattices grown on MgO(1 0 0) and MgO(1 1 0) and its variation with Fe-layer thickness (5 Å<t<80 Å). The structure was characterized by X-ray diffraction. M_s(t) vs. T, as measured by DC magnetization and ferromagnetic resonance, show a large interface effect. The origins of this effect are discussed.     

Crystal structures of stage-n iodine-intercalated compounds IBi2nSr2nCanCu2nOx

The crystal structure of stage-3 iodine-intercalated superconducting IBi₆Sr₆Ca₃Cu₆O_x has been determined by transmission electron microscopy to belong to the space group Pma2 with lattice parameters a=5.4Å, B=5.4Å and C=49.4Å. Ioidine atoms intercalated between every three Bi---O bilayers expand the distance between the Bi---O layers by 3.6Å and alter the atomic stacking across Bi---O layers from the staggered configuration characteristic of host superconducting Bi₂Sr₂CaCu₂O_x to an aligned configuration characteristic of stage-1 iodine-intercalated superconducting IBi₂Sr₂CaCu₂O_x. Higher-stage intercalation has also been observed as stacking faults which predominantly contain both stage-2 and stage-3 phases. The space groups and c-axis dimensions of the higher-stage phases have been deduced to be Pma2 with c=3.6+15.3n Å when stage number n is odd, and Bbmb with c=2(3.6+15.3n) Å when n is even.     

Polarized neutron reflection study of an Er/Fe multilayer

Low angle polarized neutron scattering experiments have been performed on an Er91 Å/Fe39 Å multilayer. The evolution of the intensity of the first peak with the temperature and the external magnetic field shows that the average magnetizations of Fe and Er layers are in the applied field direction. The study of the intensities of the second and the third peaks indicates that there is an interface effect: some erbium atoms remain antiferromagnetically coupled to iron at the interface.     

Optimization of synthetic parameters for Tl-2201 superconducting phase by flash sintering process

Samples with nominal composition Tl₂Ba₂Cu_1.2 (copper-rich Tl-2201) were calcined at various temperature, i.e. from 865 °C to 910°C and for various durations (10–20 h). The samples were sintered for 31/2 min at various temperatures. The X-ray powder diffraction pattern indicates that most of the Tl-Ba-Cu samples (Tl-2201) are single-phase materials. All the lines in the X-ray pattern are indexable on a tetragonal unit cell showing tetragonal symmetry having space group 14/mmm with a=3.86 Å C=23.11 ± 0.06 Å as unit cell parameters. R−T and AC susceptibility measurements show T^on_c, from 90 to 104 K. There is an elongation of the c-axis (c=23.24 Å) for the sample showing the highest T_c. Wet chemical analysis confirms the deficiency of thallium. This suggests some substitution of Cu in place of thallium which leads to an increase in the average oxidation state of copper, responsible for the creation of holes (charge carriers).     

Study of the magnetism and the magnetic anisotropy of Fe layers in Ni/Fe/Ni(1 1 1) by Mössbauer spectroscopy

The hyperfine field and the magnetic anisotropy of a Fe layer as a function of thickness have been investigated in several Ni/⁵⁷Fe_x/Ni(1 1 1) trilayers with relatively thick Ni layers by Mössbauer spectroscopy. For Fe layers with thickness below 16 Å, the Mössbauer spectra show always the presence of two ferromagnetic phases with high-spin state. In the range between 6 and 8 Å, also a ferromagnetic phase with low-spin state and a paramagnetic phase have been found. The evolution of the mean hyperfine field of the ⁵⁷Fe nuclei is used to study the Fe growth. A structural FCCBCC phase transition is found to begin with an iron thickness of 8 Å. The easy direction of the magnetization is found out-of-plane for Fe interlayer with FCC structure, and perfectly in plane for Fe interlayer with BCC structure.     

LEED crystallographic analysis for the Rh(111)-(2 × 1)---O surface structure

A tensor LEED analysis is reported for the Rh(111)-(2 × 1)---O surface structure in which atoms in the O overlayer chemisorb close to the regular (fcc type) three-fold hollow sites for half-monolayer coverage. The structure shows significant relaxations: for example, a buckling of about 0.07 Å is indicated in the first metal layer and O appears to displace laterally by about 0.05 Å. The individual O---Rh bond lengths are around 2.01 and 1.92 Å to top layer Rh atoms, which bond to two and one O atoms, respectively, but the average value (1.98 Å) is close to that in bulk RhO₂ (1.96 Å). Comparison is also made with the previously determined O---Rh bond lengths in the Rh(110)-p2mg(2 × 1) surface structure.     

A 62 K superconductor with an original structure: Sr4−xBaxTlCu2CO3O7

A new superconductor with an original structure, Sr_4−xBa_xTlCu₂CO₃O₇ has been isolated for 1≤x≤2. It crystallizes in a A-type space group with a=3.84 Å≈a_p, b≈8 × a_p, c≈17.0 Å. The HREM study of this new curprate shows that it derives from the Tl_0.5Pb_0.5Sr₄Cu₂CO₃O₇ structure by a shearing mechanism. Indeed, it can be described as ribbons of the latter structure, four octahedra thick, shifted c/2 with respect to each other. It results in infinite single perovskite layers waving along the (001) plane and connected through flat mixed layers [(TIO)₄(CO)₄]∞. This oxycarbonate can also be described as a 1201-type structure in which the [TIO]∞ layers are replaced by mixed [(TIO)₄(CO)₄]∞ layers. The susceptibility measurements show that this cuprate exhibits a significantly higher critical temperature than the 1201-cuprates. The T_{c onset} of the synthesized phase is 56 K and it can be increased up to 62 K by hydrogen annealing; in the same way, one observes a large Meissner volume fraction of 35% at 4.2 K.     

Hot electrons in silicon dioxide: Ballistic to steady-state transport

Hot electron transport in silicon dioxide is examined with emphasis on current experimental and theoretical results. For oxide layers thicker than 100 Å, steady-state transport has been shown to control the carrier flow at all fields studied. The transition from a nearly thermal electron distribution at electric fields less than approximately 1.5 MV/cm to significantly hot distributions with average energies between 2 and 6 eV at higher fields of up to 16 MV/cm is discussed. The significance of nonpolar phonon scattering in controlling the dispersive transport at higher electric fields, thereby preventing runaway and avalanche breakdown, is reviewed. The transition from ballistic to steady-state transport on very thin oxides layers of less than 100 Å in thickness and the observation of single phonon scattering events are also discussed.     

Buffers for high temperature superconductor coatings. Low temperature growth of CeO2 films by metal–organic chemical vapor deposition and their implementation as buffers

Smooth, epitaxial cerium dioxide thin films have been grown in-situ in the 450–650°C temperature range on (001) yttria-stabilized zirconia (YSZ) substrates by metal–organic chemical vapor deposition (MOCVD) using a new fluorine-free liquid Ce precursor. As assessed by X-ray diffraction, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM), the epitaxial films exhibit a columnar microstructure with atomically abrupt film-substrate interfaces and with only minor bending of the crystal plane parallel to the substrate surface near the interface and at the column boundaries. With fixed precursor temperature and gas flow rate, the CeO₂ growth rate decreases from 10 Å/min at 450°C to 6.5 Å/min at 540°C. The root-mean-square roughness of the films also decreases from 15.5 Å at 450°C to 4.3 Å at 540°C. High-quality, epitaxial YBa₂C₃O_7−x films have been successfully deposited on these MOCVD-derived CeO₂ films grown at temperatures as low as 540°C. They exhibit T_c=86.5 K and J_c=1.08×10⁶ A/cm² at 77.4 K.     

Strain and defect densities in Si/Si1-xGex heterostructures investigated by ion scattering and X-ray diffraction

MBE-grown Si/Si_1-xGe_x heterostructures on (100) Si have been characterized by Rutherford backscattering spectroscopy (RBS), ion channeling and X-ray diffraction to investigate defect densities and tetragonal lattice distortion. Critical layer thickness and relaxation of strain by formation of misfit dislocations are strongly dependent on the growth temperature. A Si_0.67Ge_0.33 layer with a thickness of 2000 Å is found to be still fully strained at a growth temperature of 450°C, whereas the same layer grown at 550°C shows considerable strain relaxation by dislocations. To obtain better depth resolution than with conventional RBS, medium energy ion scattering (MEIS) experiments have been performed on Si/Ge superlattices with layer thicknesses of 10–40 Å. A position-sensitive toroidal electrostatic analyser was employed to detect the backscattered ions simultaneously over an angular range of 30° with an energy resolution of 1 keV FWHM for 250 keV He ions, corresponding to a depth resolution of about 10 Å.     

Relationship between electronic and crystal structure in Cu–Ni–Co–Mn–O spinels: Part A: Temperature-induced structural transformation

Local atomic and crystal structures around Cu and Mn atoms in Mn_1.68Cu_0.6Ni_0.48Co_0.24O₄ spinel samples fabricated by metal–organic decomposition synthesis at different annealing temperatures were investigated by X-ray absorption fine structure analysis. There are two distinct copper cations, Cu¹⁺ and Cu²⁺, both of which maintain tetrahedral coordination. The bond-length distances are Cu¹⁺–O = 2.00 Å and Cu²⁺–O = 1.80 Å. The manganese cations are for the most part octahedral. The spinels prepared at low temperature (600 °C) contain smaller (Mn⁴⁺–O = 1.88 Å) undistorted MnO₆ octahedrons corresponding to Mn⁴⁺ valence, whereas the manganese octahedrons in high-temperature materials (800 °C and higher) were larger and had a pronounced tetragonal distortion pertaining to Mn³⁺ oxidation state (Mn³⁺–O = 1.93 Å and 2.11 Å). By rising the fabrication temperatures, relative concentration of the species of Mn⁴⁺ and Mn³⁺ varies as a result of the reaction represented by Cu¹⁺ + Mn⁴⁺  Cu²⁺ + Mn³⁺, implying irreversible temperature-induced structural transformation. Atomic coordinates in the low-temperature phase are similar to those found in the ideal cubic spinel with oxygen parameter u = 0.27, whereas local environments of the Cu and Mn atom correspond to the tetragonal CuMn₂O₄ phase (space group I4₁/amd). Unlike in CuMn₂O₄, orientation of the lattice distortions is random, however, the long-range cubic spinel structure is retained at all time.     

Surface and subsurface distortions of the Au{110}-(1×2) structure

The reconstruction of the Au{110}-(1×2) missing-row surface has been studied by means of the new scattering and recoiling imaging spectrometry (SARIS) technique. The three-dimensional focusing patterns observed for scattering of 4 keV He⁺, Ne⁺ and Ar⁺ ions are highly sensitive to the structure of both the surface and subsurface layers. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) were used to simulate the scattering patterns. Using an R-factor comparison of the experimental and simulated images, it is demonstrated that SARIS is sensitive to changes of the order of 0.02 Å in the structural parameters of this Au surface. These parameters involve interlayer spacings, row pairing and row buckling in the first-through fifth atomic layers. Results for the shallow surface layers are in general agreement with the those of previous studies. The new results include structural parameters for the deeper subsurface layers and the observation of an oscillatory behavior of the layer spacings which is damped towards deeper layers.     

Inhibition of the reduction of Cr(VI) at the magnetite–water interface by calcium carbonate coatings

The effect of calcium carbonate coatings on the reduction of aqueous chromate on the magnetite(1 1 1) surface has been investigated using a combination of synchrotron based X-ray photoemission spectroscopy (PES) and X-ray absorption near edge structure (XANES) spectroscopy, along with laboratory-based powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). CaCO₃ coatings (dominantly calcite with minor quantities of aragonite and vaterite) of thicknesses ranging from 10 Å to 20 m were grown on magnetite(1 1 1) surfaces by exposure to supersaturated aqueous solutions followed by evaporation of the solution—a process that mimics pore-water evaporation in vadose zones leading to the formation of caliche and calcium carbonate coatings on mineral grains. Coating thicknesses were determined from attenuation of the Fe 2p photoemission signal by the carbonate coating. For coatings less than 15 Å thick, Cr 2p photoemission and Cr L_II, L_III-edge XANES spectra show that chromate is reduced by the underlying magnetite surface; however, as the minimum coating thickness increases beyond 15 Å, the magnetite surface becomes passivated and further chromate reduction ceases. Our findings suggest that carbonate coatings on natural magnetite grains can significantly reduce or eliminate their ability to reduce Cr(VI), which is a toxic and highly mobile environmental contaminant.     

Thermally activated flux dissipation in c-axis-oriented YBa2Cu3O7−δ ultrathin films

The resistive transitions of ultrathin YBa₂Cu₃O_7−δ (YBCO) films with thicknesses 75 and 200 Å were studied under magnetic fields. For the 75 Å film under a 5 T parallel magnetic field (Hbab-plane), no broadening of the resistive transition occurred. In the perpendicular magnetic field (H ab-plane), the broadening of the resistive transition of the 75 Å film is larger than that of the 200 Å thick film. The flux activation energy U was found to be linearly dependent on the temperature and logarithmically dependent on the magnetic field for both 75 and 200 Å films, which means the two samples have a two-dimensional vortex lattice. Furthermore, the activation energy U also increased with the film thickness, indicating that the magnetic correlation length in the c-axis direction l_c is larger than the 200 Å for bulk YBCO.     

New method to characterize mesoscopic range and very small strain with using multi-wave X-ray diffraction

A new technique to observe mesoscopic-range strain fields (up to several hundreds of nm) is proposed, using modulation of the crystal-truncation-rod (CTR) scattering caused by Bragg reflection. This technique is particularly sensitive to small, long-range strain fields near crystal surfaces and interfaces, which are usually difficult to be discriminated by using Bragg reflection. A simple interpretation can be made for the modulation profile: the technique is physically simple with a few parameters fitted to the data and, independently of any model, is able to determine the total displacement due to mesoscopic strain field for depths up to several hundreds of nanometers. We applied this method to a Si(0 0 1) wafer whose surface is covered with a thermal oxide layer 3 nm thick. On the basis of the expressions we obtained for the modulation profile a least-squares fitting was carried out to give a result that under the oxide layer there exists a total displacement of −0.16 Å. It was also revealed from the visibility of the modulation profile that the total displacement has a static fluctuation of at least ±0.13 Å in the lateral direction. The new method can be used for the correction of the errors of the X-ray standing wave (XSW) method produced by strained layers near crystal surfaces.     

Alloy disorder effects in III–V ternaries studied by modulation spectroscopy

Modulation spectroscopy has been applied to several III–V ternary alloys, grown by MBE. Thermal disorder caused by the exciton—LO phonon interaction at room temperature is treated along with alloy disorder in a universal way by assuming a Gaussian absorption profile. A linewidth analysis shows that, in some cases, the presence of inhomogeneous composition variations on a scale of about 7–15 Å dominate the spectra. The latter implies that non-random alloys can be grown by MBE.     

Synthesis and characterization of HgBa2Can−1CunO2n+2+δ (n = 1, 2, and 3)

We have successfully prepared the first three members of the mercury-based superconducting compounds Hg--- Ba₂Ca_n−1Cu_nO_2n+2+δ, namely Hg---1201, Hg---1212 and Hg---1223 with high purity and very good quality. T he influence of the synthesis parameters is studied in detail. Using the sealed quartz tube method, very simple procedures are found to ensure a 100% reproducibility of nearly 100% pure Hg---1201 and 85–90% Hg---1212 and Hg---1223. Oxygen annealing of the sample Hg---1201 at 300°C for 18 h results in an enhancement of its critical temperature up to 97 K. The symmetry of the first and second members is tetragonal with lattice parameters a = 3.8831 (1) Å, C = 9.5357 (2) Å, and A = 3.8624 (1) Å, C = 12.7045 (2) Å, respectively. X-ray diffraction lines of Hg---1223 can be indexed in a tetragonal cell with a = 3.8564 (1) Å and C = 15.8564 (9) Å as well as in an orthorhombic cell with lattice parameters a = 5.4537 (1) Å, B = 5.4247 (1) Å, and C = 15.8505 (7) Å.     

Planarization of CMOS ROIC dies for uncooled detectors

This paper presents a planarization procedure to achieve a flat CMOS surface of Readout Integrated Circuit (ROIC) for the integration between uncooled infrared detector arrays and ROIC. The CMOS fabrication process produces about 2 μm surface roughness on the silicon wafer, so the CMOS dies must be first planarized before integration with the detector arrays. To acquire a satisfying surface roughness in the small CMOS die, three commercially available polymers including bisbenzocyclobutene (BCB) and two types of polyimides are evaluated in our experiments. BCB shows the best results for our applications. A single layer of BCB coating successfully reduce the surface topology from 2 μm to less than 1500 Å and two layers of BCB coating reduce the surface topology to about 600 Å.