Nonstoichiometric Material Research Articles

Numerical studies of annealed non-stoichiometric low temperature grown GaAs

The means by which deep traps in annealed low temperature molecular beam epitaxy grown GaAs are responsible for its remarkable semi-insulating and short lifetime properties has been the subject of much discussion. For example, while low dc bias and high speed phototransient measurements can be explained as consequences of a homogeneous trap distribution in the non-stoichiometric material, experiments demonstrating the presence of electrically active precipitates must be dealt with. The study below, which is numerical, concludes that a consistent argument based on the presence of electrically active precipitates, coupled to a surrounding distribution of traps, will account for much of the observed experimental phenomena.

Smooth and low outgrowth-density Pr 1Ba 2Cu 3O 7− x/Y 1 Ba 2Cu 3O 7− x pulsed laser deposited thin films for ramp junctions

PBCO (150 nm) on YBCO (100 nm) thin films dedicated to Josephson junctions by ramp technology have been grown in situ on (100) SrTiO 3 substrates by pulsed laser deposition (KrF, 248 nm). For YBCO monolayers, the best resistive transition and crystalline quality were for target-to-substrate distance D TS=60 mm, and substrate temperature T s≈775 °C, but such films exhibited 10 7−10 8 outgrowths cm −2. Outgrowth density has been improved to 0.5−2×10 6 cm −2 by decreasing D TS to ≈45 mm. After outgrowth reduction, bottom roughness has been improved by lowering the substrate temperature T S to ≈730 °C. For PBCO, upper layers grown in these conditions often showed macroscopic defects, so T S≈750 °C and D TS=60 mm (PBCO at 60 mm showed no more outgrowths than YBCO at 45 mm) were found more appropriate. The bilayers were exclusively c-oriented with critical temperatures ( R=0) between 87 and 90 K; outgrowth densities in the range 0.5–3×10 6 cm −2 could be obtained with bottom roughness (averaged) of 2–3 nm. Moreover, with the substrate in front of the target and despite a shutter inserted between, preablation of the target has been shown to deposit about one monolayer of non-stoichiometric material on the substrate; care should then be taken against this layer which could probably affect quality and reproducibility.

A new synthesis of hydroxyapatite

An ethylene glycol solution of Ca(OAC) 2. xH 2O and a butanol solution of P 2O 5 were used as precursors to produce hydroxyapatite. Acetic acid (HOAC) and ammonium nitrate (NH 4NO 3) acted as a stabilizer and an oxidizer, respectively, during the process. A stable mixed solution of the two precursors could be obtained by adding acetic acid in the HOAC Ca ratio of 2. As-prepared powders were obtained by pouring the mixed solution into a hot plate to evaporate the solvents. A poorly crystallized hydroxyapatite phase was obtained from the as-prepared powders after calcination at 500 °C, where a dark powder was formed. Calcination at 1000 °C gave white powders having a well-crystallized hydroxyapatite phase with a small amount of CaO. When NH 4NO 3 was added to the stable mixed solution before pouring into the hot plate, the as-prepared powders calcined at 500 °C were white and composed of wellcrystallized hydroxyapatite with a small amount of β-tricalcium phosphate. The hydroxyapatite obtained in this work is a nonstoichiometric calcium-deficient material, Ca 10 − x (HPO 4) x (PO 4) 3 − x (OH). The described route to synthesize hydroxyapatite has potential applications for the preparation of hydroxyapatite coatings.

Surface analysis of films and film systems produced by pulsed laser deposition

Ceramic films and film systems (ZrO2 films, ZrO2/Ti multilayers, and BN films) are deposited by pulsed laser deposition (PLD) and analyzed using X-ray photoelectron (XPS), Auger electron (AES), and micro-Raman spectroscopies. The electron spectroscopies are used to determine the film stoichiometry, the nature of the bonding, and to specify contaminant species. The micro-Raman spectroscopy gives information on crystal structure, grain size, and mechanical stress within the films. In ZrO2 films a stoichiometry is achieved with typically 5%, with only weak dependencies on processing variables. The only contaminants are a small amount of water from the ambient gas and a carbonaceous surface layer. Multilayers consisting of alternating ZrO2 and Ti layers exhibit a TiC contamination within the Ti layers. Depending on the processing variables, BN films may be nearly stoichiometric or may have significant, even dominant contaminations throughout the film from elemental B, B2O3, and/or a boron-oxynitride species. The first component is due to the non-stoichiometric material removal from the target (N-depletion) at low laser fluences, as confirmed by XPS measurements on irradiated targets. The second and third arise from H2O in the ambient, and exhibit a complex dependence on processing variables. Micro-Raman spectra show only amorphous or hexagonalphase BN. Depending on the position on the substrate relative to the laser-induced vapour/plasma plume, there may be a particle deposition or mechanical stress within the films, as evidenced from large shifts (up to 15 cm−1) of the Raman spectral peaks.

A mechanism for sensing reducing gases with vanadium pentoxide films

Vanadiium pentoxide is generally a non-stoichiometric material, which is known for its catalytic properties in oxidation reactions. As it is an electric conductor when oxygen vacancies are present, the purpose of this investigation is to determine the influence that reducing gases have on the conductance of vanadium pentoxide films. These films are prepared from crushed powder and by evaporation. It is found that a reducing gas such as hydrogen increases the conductance of the film in air, If a platinum catalyst is added. For the recovery process, the presence of oxygen is necessary, as demonstrated by the responses to hydrogen and oxygen in a nitogen flow. Additionally, vanadium pentoxide powder loses wieght when exposed to 5000 ppm hydrogen in air, and regains it when exposed to air alone. To explain the observed response, a model is proposed that involves a surface reaction of atmospheric reducing gas and atmospheric oxygen, and fifusion of lattice oxygen in response to changes at the surface.

A comparison of As and P-based semiconductors grown at low temperatures by MBE and GSMBE

Materials grown at low temperatures by molecular beam epitaxy techniques have exhibited high resistivity after annealing, and sub-picosecond carrier recombination times. Depending on growth parameters, the nonstoichiometric material can contain a high density of group V antisite defects and precipitates of group V and group III atoms. It is well known that a high density of As antisites and As precipitates formed in GaAs contribute to the high resistance. InP grown at low temperatures was also observed to contain precipitates consisting of both P and In. The latter material however does not exhibit the high resistivity of GaAs. This behavior may be due to the presence of indium or P antisite defects.

Surface studies on polycrystalline YBa2Cu3O7?? with Auger electron and low-energy electron energy-loss spectroscopy

Chemical and electronic properties of scrapecleaned YBa2Cu3O7−δ(YBCO) ceramics, which were also annealed in vacuum or exposed to H atoms or O2 molecules, were studied at room temperature by applying Auger electron (AES) and low-energy electron energy-loss spectroscopy (LEELS). Scanning electron micrographs showed such scraped surfaces to consist mainly of small, fractured YBCO crystallites, i.e. of clean YBCO surfaces. With only one exception, all low-energy AES lines were found to be shifted in energy compared to data recorded with surfaces of metallic Cu and Y. The analysis of the high-energy AES lines indicated the existence of non-stoichiometric material in grain boundaries. At least 25 different LEELS features were recorded. The 24.9-eV energy loss, which is due to the excitation of bulk valence-electron plasmons, indicates an enhanced oxygen deficit δs ≈ 0.8 at scrape-cleaned YBCO surfaces. Annealing of scraped YBCO samples at approximately 700 K in vacuum and also their exposure to hydrogen atoms resulted in an additional oxygen depletion within a few atomic layers near to the surfaces. These oxygen deficiencies were found to increase the intensity ratios of low-and high-energy AES lines of Cu, Ba, and Y and to intensify the energy-loss peak at 4.4 eV which has been attributed to O−Cu−O dumbbells.

Nano-probe microanalysis of grain boundary chemistry in YBa2Cu4O8 and its relationship to weak-link behavior

Weak-link behavior in high temperature superconductors limits bulk applications of polycrystalline high Tc compounds. Possible sources of the effect include grain boundary (GB) disorder, microcracks and nonstoichiometric material at grain boundaries. Recently, electron energy-loss spectroscopy (EELS) studies of YBa2Cu3O7-x (Y123) have shown that grain boundary oxygen deficiency is correlated with the crystallographic misorientations of between the Y123 grains. Studies of grain boundary chemistry in YBa2Cu4O8 (Y124) by nano-probe energy dispersive X-ray spectroscopy (EDS) and EELS are reported in this paper. Transmission electron microscopy (TEM) studies were performed at 100 kV in a Philips EM400 TEM/STEM equipped with a field emission gun (FEG), which generates an electron probe smaller than 2 nm in diameter. The cation composition was determined by EDS and the O:Ba composition was determined using EELS. Kikuchi patterns were recorded from both sides of the grain boundaries to determine the misorientation of the two adjacent grains.The observation of a pre-edge peak (Fig. 1) near the oxygen K ionization edge (O K) is related to the hole states produced by the O 2p state and Cu 3d states.

Analysis of diffusion in lithium niobate

Possible diffusion paths in lithium niobate are discussed. The preferred path for diffusion is strongly influenced by the distribution of ions between the lithium and niobium sites; site swapping is important both for creating non-stoichiometric material and for impurity incorporation. Various literature reports of diffusion rates are surveyed for comparison with the possible diffusion mechanisms. Titanium (which is thought to substitute primarily for niobium) appears to diffuse more rapidly on the lithium sublattice in lithium-deficient samples.

Structure and photoinduced structural changes in nonstoichiometric a-AsxS1-x: A study by x-ray-absorption fine structure.

X-ray-absorption fine-structure data show that chemical ordering plays an important role in nonstoichiometric amorphous (a-)${\mathrm{As}}_{\mathit{x}}$${\mathrm{S}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ films at 0.4\ensuremath{\le}x=0.5. The mixture of different chemical bonding behaviors of As-S and As-As in an a-${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ random covalent network and an a-${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$ disordered molecular solid lead the structure to a more disordered state for 0.4x0.5. In particular, the structure at a composition around x=0.43 in a-${\mathrm{As}}_{\mathit{x}}$${\mathrm{S}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ represents a maximum of flexibility. Photoillumination results in (1) a more disordered state as indicated by an increase in the static Debye-Waller factors (which are related to the creation of As-As homopolar bonds) and (2) modifications in the network structure, such as the increase of the As-As radial distances in the second shells. In nonstoichiometric material, the presence of ${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$ molecules in the ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ network tends to break up this network into small segments. This decoupling of pieces of the ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ network (composed of S-bridged ${\mathrm{AsS}}_{3}$ pyramids and As-As wrong bonds) may lead to increased steric freedom that is related to a more disordered state that appears to be associated with photoinduced structural changes in amorphous arsenic sulfide.

Raman study of low growth temperature GaAs

A Raman study has been made of GaAs grown by MBE at low temperature, which is known to produce nonstoichiometric (As-rich) material. The epilayer has a slightly larger lattice constant than the GaAs substrate, as measured by x-ray diffraction. Raman measurements show that the LO phonon of LT-GaAs is shifted down in frequency from that of bulk GaAs. The shift is related to the presence of AsGa antisite defects, which cause a reduction in the electric field associated with the LO phonon. After the sample was annealed, the epilayer had the same lattice constant as the substrate, and the LO phonon moved closer to that of bulk GaAs.

Post-composition control of hydroxyapatite in an aqueous medium

Composition control of hydroxyapatite by post-treatment in an alkaline solution was examined. Non-stoichiometric material having a Ca/P ratio of 1.60 was treated in distilled water (pH 6.5) and in a concentrated aqueous solution of ammonia (pH 11). The Ca/P ratio of the material increased with the latter treatment and the stoichiometric composition (Ca/P=1.67) was achieved after six treatments. In contrast, essentially no change in composition was found with the former treatment. The treated powder had high thermal stability of nominally a stoichiometric composition. Composition change by incongruent dissolution is discussed.

Stoichiometric silver beta alumina studied at 25, 300 and 500 degrees C by powder neutron diffraction

The structure of stoichiometric (s) silver beta alumina, Ag1-xAl11O17+x2/; x=0.0, prepared via hydrogen reduction of a non-stoichiometric (NS) material with x=0.45 has been determined at 25 degrees C, 300 degrees C and 500 degrees C by Rietveld analyses of powder neutron diffraction data. In contrast to the single Ag site observed previously at 4.2 K, the silver ion distribution becomes increasingly diffuse with increasing temperature. At 25 degrees C the mirror plane structure is similar to that seen at 4.2 K, but apparently with some anti-Beevers-Ross site occupancy, and still with an increased c-axis constant compared with the NS parent. At 500 degrees C no silver cation site is well defined, consistent with a two-dimensional quasi-liquid state suggested by earlier diffuse X-ray scattering experiments.

Granular and superconducting-glass properties of the high-temperature superconductors

The microstructure of bulk samples of the copper-oxide high-temperature superconductors commonly is describable in terms of anisotropic grains of stoichiometric material separated by layers of nonstoichiometric interface material. The granularity strongly influences the electromagnetic properties, especially the transport critical-current density and the magnetization. In this paper, a simple theoretical model for the granularity is introduced and then used to discuss a number of electrodynamic properties (hysteretic magnetization versus magnetic field, zero-field-cooled and field-cooled magnetization versus temperature, ac susceptibility, and flux creep with logarithmic time dependence). Special attention is drawn to the importance of distinguishing between intragranular and intergranular effects.

Protons and deuterons in stoichiometric and nonstoichiometric MgAl2O4

The OH− and OD− vibrational frequencies in stoichiometric and nonstoichiometric MgAl2O4 were measured. The nonstoichiometric material had a Al/Mg ratio of 7. The diffusion coefficient of deuterons in the nonstoichiometric material at 1600 K is about 6 ± 3 ⊠ 10−8 cm2/s. Deuterons can readily be swept out by applying an electric field at temperatures above 1000 K.

The Preparation and Properties of 4.7 uF 2220 Size 50 V Z5U Relaxor Multilayer Ceramic Capacitors

A new dielectric for multilayer ceramic capacitors is currently undergoing factory qualification. This dielectric is based on a nonstoichiometric lead magnesium niobate relaxor material. Dielectric constants in excess of 15000 have been achieved for bodies which sinter close to 900°C with 100 percent Ag internal electrodes. The use of very high Ag content electrode systems means that there is a considerable cost reduction so that multilayer ceramic chip capacitors become cost competitive with solid tantalum chip capacitors. The preparation and properties of the dielectric material are described and life test data are presented showing that capacitors of the new dielectric are extremely stable for periods up to two years at ambient and upper rated temperature. A direct comparison is made between the electrical properties of 4.7 uF 50 V 2220 size Z5U multilayer chip capacitors manufactured from the new relaxor dielectric and the equivalent value tantalum chip capacitors.

Aspects of the crystal growth of the intergrowth tungsten bronzes

The crystal growth of a few of the members of the family of the rubidium intergrowth tungsten bronzes has been investigated as part of a program to study the compositionally induced metal to insulator transition in this bronze system. Progress in the study of this nonstoichiometric material has been made possible by successful growth of large single crystal samples. Application of vapor-assisted seeded crystal growth has led to an increase of the product crystal size by two orders of magnitude, from 0.1 mm on an edge to about 2 cm on an edge. Scanning electron microscopy has been used to characterize cracks, voids and intergrowths in this material. Transmission electron microscopy has been used to image the lattice of this quasi-two dimensional material in order to observe the frequency and amplitude of stacking defects.

On the non-stoichiometry of the binding of Pt(II) anti-neoplastic agents to inosine 5′-monophosphate

A non-stoichiometric material [Na 4(5′-IMP) 2·15H 2O] 0.2[Na 2(Pt(5′-IMP) 2 (trimethylenediamine))·13.5H 2O] 0.8 has been prepared and investigated by single-crystal X-ray methods and 1H and 13C nmr spectroscopy. The compound is isomorphous with the monosodium and disodium salts of 5′-IMP and two Pt(II)-5′-IMP compounds previously reported to be non-stoichiometric. However, the structural changes in the packing motif of the 5′-IMP molecules induced on Pt(II) coordination are uniform only if the 5′-IMP complex containing (NH 3) 2Pt(II) is stoichiometric. Preliminary studies on the latter complex, synthesized in our laboratories, demonstrate that the complex is indeed stoichiometric.

Transport properties of impurity-doped VO2

The temperature dependence of the DC conductivity in the semiconducting phase of undoped and Al, Cr, and Ti-doped VO2 has been interpreted with the aid of AC conductivity and thermopower measurements down to 77 K. A hopping conductivity is shown to be dominant for T < 180 K and to make up about 10% of the total conductivity at the metal–semiconductor transition temperature. The frequency dependence of the AC conductivity, σ~fs, is shown to be sensitive to crystalline disorder through the parameter s; s ~0.5 for 'good' crystals and 0.7–0.9 for highly doped or nonstoichiometric material. For the best crystals, a phonon drag contribution is found to exist in the thermopower. The hopping component of the DC conductivity is analyzed in terms of variable range hopping within a nonuniform density of states. For a total density of states between 1018–1020 this leads to a localization radius of 3–11 Å and the conclusion that the position of the Fermi level within the gap strongly influences the relative contributions of hopping and band conduction. A model for the density of states within the energy gap for both pure and doped VO2 is presented on the basis of localized correlations between electrons on pairs of vanadium or vanadium–impurity sites.

Creep fracture processes in uranium dioxide

Creep fracture processes have been studied in UO 2. At high rates of strain fracture occurs by the propagation of the triple point cracks which extend large distances along grain boundaries. At lower rates of strain the fracture mode involves the growth of rounded cavities on boundaries parallel to the compressive stress axis. The growth of these cavities is attributed to a vacancy condensation mechanism driven by internally generated tensile components of stress. The magnitude of this tensile stress is calculated and the conditions for growth are estimated. It is shown that non-stoichiometric and large grained material is more susceptible to creep damage. This increased susceptibility is attributed to the increased magnitude in grain boundary sliding offsets in the large grained material and to increased ease of sliding in the non-stoichiometric material.