Large Concentration Of Defects Research Articles

Controlled growth of well-aligned hierarchical ZnO arrays by a wet chemical method

A simple synthesis route to hierarchical ZnO arrays on zinc substrate in aqueous solution containing zinc nitrate and ammonia water was reported, without the assistance of any seeds, catalysts and surfactants. The as-prepared product consisted of very thin nanosheets attached onto the top of well-aligned nanorod arrays. A possible growth mechanism was proposed. The room-temperature photoluminescence spectrum showed UV emission around 398 nm and diverse visible emission peaks, indicating there might be a large concentration of oxygen defects in the sample. This method is both controllable and reproducible.

Time-resolved spectroscopy of intrinsic luminescence of Y 3Ga 5O 12 and (LaLu) 3Lu 2Ga 3O 12 single crystals

The nature of intrinsic luminescence of Y 3Ga 5O 12 (YGG) and (LaLu) 3Lu 2Ga 3O 12 (LLGG) single crystals grown from a melt was determined. In the case of a YGG single crystal containing Y Ga antisite defects with a concentration of 0.25–0.275 at.% the intrinsic luminescence was considered as a superposition of luminescence of self-trapped excitons (STE), luminescence of excitons localized near antisite defects (LE(AD) centers) and luminescence caused by a recombination of an electron with a hole captured at Y Ga antisite defects. Due to a large (2–3%) concentration of Lu La antisite defects in LLGG single crystals the intrinsic luminescence was a superposition mainly of the LE(AD) center emission and the recombination luminescence of Lu La antisite defects. The energy structure of the mentioned centers in YGG and LGGG hosts was determined from the excitation spectra of their luminescence under excitation by synchrotron radiation in the range of the fundamental absorption edge of these garnets.

Practical applications of the chemical strain effect in ionic and mixed conductors

Abstract The chemical strain effect in solids is the deviation from linear elasticity due to the association and dissociation of point defects. Although to date this effect has been observed and studied only in Ce0.8Gd0.2O1,9, one may expect that it will be found in other ionic and mixed conductors containing a large concentration of point defects. In this work, some practical applications of materials exhibiting the chemical strain effect are discussed. Based on the example of Ce0.8Gd0.2O1,9, mechanical structures built from these materials should exhibit exceptional mechanical stability and are therefore very attractive for use as components of solid oxide fuel cells (SOFC) or other devices subjected to large and frequent temperature variations. The ability of these materials to withstand large strain without accumulating large stress also makes them potentially useful as flexible elements in micro-electromechanical systems (MEMS).

The Effect of Mo Crystallinity on Diffusion through the Si-on-Mo Interface in EUV Multilayer Systems

Thermally induced diffusion through the Si-on-Mo interface of multilayers with either amorphous or polycrystalline Mo layers has been investigated using grazing incidence and wide angle x-ray reflectometry. Diffusion through the Mo-on-Si interface was reduced by applying a diffusion barrier, allowing us to probe the diffusion at the opposite, Si-on-Mo interface.We found that diffusion through this interface is much slower for polycrystalline Mo than for amorphous Mo layers. The reason for this difference might be the larger defect concentration in amorphous Mo as compared to crystalline Mo.

Effect of tungsten and gallium on the structure and magnetic and mechanical properties of Fe-Cr-Co alloys

Effects of the alloying additions of W and Ga, cold deformation, and regimes of heat treatment on the magnetic and mechanical properties of precipitation-hardening Fe-Cr-Co alloys have been investigated. It has been shown that the alloying with tungsten leads to a strengthening of the α solid solution and an additional jumplike increase in strength after deformation and age hardening. The coercive force grows simultaneously with an increase in strength; the structure of the alloy in this case contains a large concentration of linear defects (dislocations), which indicates an important contribution of magnetoelastic energy to the formation of the high-coercivity state and of elastic strains to the increase in the strength properties. The character of fracture becomes brittle. The modifying of the alloy with gallium leads to an increase in plasticity due to the dispersion of the structure, formation of retained austenite in it, and formation of an interphase layer more uniform in composition. The character of fracture changes from brittle to brittle-ductile. A high-strength magnetically hard material has been obtained with Hc = 90–125 A/cm, σm = 120–130 G cm3/g, Br = 0.6–0.65 T, σ0.2 = 1300–1600 MPa, and δ = 2.0–4.6%.

Electrical characterization of defects introduced during metallization processes in n-type germanium

We have studied the defects introduced in n-type Ge during electron beam deposition (EBD) and sputter deposition (SD) by deep-level transient spectroscopy (DLTS) and evaluated their influence on the rectification quality of Schottky contacts by current–voltage ( I–V) measurements. I–V measurements demonstrated that the quality of sputter-deposited diodes are poorer than those of diodes formed by EBD. The highest quality Schottky diodes were formed by resistive evaporation that introduced no defects in Ge. In the case of EBD of metals the main defect introduced during metallization was the V–Sb complex, also introduced during by electron irradiation. The concentrations of the EBD-induced defects depend on the metal used: metals that required a higher electron beam intensity to evaporate, e.g. Ru, resulted in larger defect concentrations than metals requiring lower electron beam intensity, e.g. Au. All the EBD-induced defects can be removed by annealing at temperatures above 325 °C. Sputter deposition introduces several electrically active defects near the surface of Ge. All these defects have also been observed after high-energy electron irradiation. However, the V–Sb centre introduced by EBD was not observed after sputter deposition. Annealing at 250 °C in Ar removed all the defects introduced during sputter deposition.

Intrinsic and ${\rm Ce}^{3+}$- Related Luminescence of Single Crystals and Single Crystalline Films of YAP Perovskites: New Results

Intrinsic emission of undoped and Ce3+ doped single crystal (SC) and single crystalline films (SCF) of YALO3 (YAP) was studied using the time-resolved luminescence spectroscopy under excitation by pulsed X-ray and synchrotron radiation. In contrast to YAP SCF, the YAP bulk SC are characterized by the presence of Y-in-Al-positions antisite defects (AD) and large concentration of vacancy-type defects (VD) of different types, which contribute to the intrinsic emission of YAP SC. Such type defects are centers of the luminescence in the UV (6.0-4.75 eV) and near UV (4.75-2.2 eV) ranges, overlapping with the Ce absorption and emission bands. Furthermore, they can give rise to shallow trapping centers as well. For this reason the AD and VD can strongly influence the scintillation properties of YAP:Ce SC. Specifically, the decay kinetics of the Ce emission in YAP:Ce SC, besides the fast component (23.3 plusmn 3.5 ns) related to radiative 5d-4f transition of Ce ions, contains also two slow components in the hundred ns and mus ranges, which are due to the AD and VD-related traps. The advantages of scintillators based on YAP:Ce SCF, namely faster scintillation decay and lower contribution of slow components in the total light yield, can be achieved due to the absence of above mentioned bulk crystal defects.

Electrical characterization of defects introduced in Ge during electron beam deposition of different metals

AbstractWe have used deep level transient spectroscopy to characterize the defects introduced in n‐type, Sb‐doped, Ge during electron beam deposition of different metals for Schottky contact formation. We have found that the relative concentration of the electron beam induced defects depended on the metal used: metals that required a higher beam intensity to evaporate, e.g. Ru, resulted in larger defect concentrations than metals that required a lower beam intensity, e.g. Pd. The nature of some of these defects could be established by comparing their properties to those of well‐known defects introduced by electron irradiation of the same material. The most prominent defect thus identified was the E‐center, i.e. the Sb‐V center. This defect forms when vacancies, that are introduced during the interaction of energetic metal and other particles with the Ge surface, diffuse into the Ge where they bond to Sb dopant atoms. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth stress buildup in ion beam sputtered Mo thin films and comparative study of stress relaxation upon thermal annealing or ion irradiation

In an effort to address the understanding of the origin of growth stress in thin films deposited under very energetic conditions, the authors investigated the stress state and microstructure of Mo thin films grown by ion beam sputtering (IBS) as well as the stress relaxation processes taking place during subsequent thermal annealing or ion irradiation. Different sets of samples were grown by varying the IBS deposition parameters, namely, the energy E0 and the flux j of the primary ion beam, the target-to-sputtering gas mass ratio M1∕M2 as well as film thickness. The strain-stress state was determined by x-ray diffraction using the sin2ψ method and data analyzed using an original stress model which enabled them to correlate information at macroscopic (in terms of stress) and microscopic (in terms of defect concentration) levels. Results indicate that these refractory metallic thin films are characterized by a high compressive growth stress (−2.6to−3.8GPa), resulting from the creation of a large concentration (up to ∼1.4%) of point or cluster defects, due to the atomic peening mechanism. The M1∕M2 mass ratio enables tuning efficiently the mean deposited energy of the condensing atoms; thus, it appears to be the more relevant deposition parameter that allows modifying both the microstructure and the stress level in a significant way. The growth stress comes out to be highly unstable. It can be easily relaxed either by postgrowth thermal annealing or ion irradiation in the hundred keV range at very low dose [<0.1dpa (displacement per atom)]. It is shown that thermal annealing induces deleterious effects such as oxidation of the film surface, decrease of the film density, and in some cases adhesion loss at the film/substrate interface, while ion irradiation allows controlling the stress level without generating any macroscopic damage.

Elasticity of Solids with a Large Concentration of Point Defects II. The Chemical Strain Effect in Ce0.8Gd0.2O1.9

AbstractThe chemical strain effect describes a mechanism of stress relaxation in solids that can be attributed to the conversion of elastic energy into chemical energy of point defects. Experimental confirmation of this effect is presented here for the case of thin self‐supported films of the ionic conductor Ce0.8Gd0.2O1.9. If heated slowly, (< 5 °C min–1) these films remain flat within the temperature range of 25–180 °C. If heated more rapidly than ∼ 20 °C min–1, the films buckle above 53 °C, but after ∼ 3–30 min at elevated temperatures, they become flat again, demonstrating that stress‐relaxation has taken place. The degree of stress reduction observed is consistent with the value calculated for this system using Boltzmann statistics in the case of small strain. These findings confirm the concept of stress adaptability in solids that we introduced in Part I and suggest that a large class of such materials, which exhibit the chemical strain effect, is likely to be found.

Spin Dynamics in Narrow-Gap Semiconductor Epitaxial Layers

We report on investigation of the spin dynamics in InAs and InSb films grown on GaAs at a temperature range from 77 K to 290 K. For both materials, the large lattice mismatch with the GaAs substrate results in the formation of an interface accumulation layer with a large defect concentration, which strongly affects the spin relaxation in these areas. Moreover, the native surface defect in the InAs films resulted in an additional charge accumulation layer with high conductivity, but very short spin lifetime. In contrast, in InSb layers, the surface states introduce a depletion region. We have correlated the spin relaxation with a multi-layer analysis of the transport properties, and find that in a 1 μm thick InAs film, approximately 70% of the total current flows through the interface and surface accumulation layers, which have sub-picosecond lifetimes, whereas in InSb films of the same thickness, the semiconducting layer carries more than 90% of the total current, and the spin lifetime in the accumulation layer is only slightly less than that of the central semiconducting layer. We suggest that InSb could be a more attractive candidate for spintronic applications than InAs.

Dehydration Propensity of Order−Disorder Intermediate Regions in Soluble Proteins

Soluble folded proteins maintain their structural integrity by properly shielding most backbone amides and carbonyls from full hydration. This structure "wrapping" entails a proper packing of the intramolecular hydrogen bonds. Thus, a poorly wrapped hydrogen bond constitutes an identifiable packing defect. Such defects are promoters of protein associations since they favor the removal of hydrating molecules. In this work we show that large clusters of packing defects generate the most significant dehydration hot spots on the protein surface, inducing a strong dielectric modulation that is reflected by a local quenching of the dielectric permittivity. The PDB-reported proteins with the largest clusters of packing defects are found to be three cancer-related transcription factors, four highly interactive proteins related to cell signaling and cytoskeleton, and a cellular prion protein. A large concentration of packing defects in a soluble protein constitutes a structural singularity that is intermediate between order and disorder. The functional implications of this singularity are investigated to delineate diverse interrelated roles. The presence of these large clusters signals a structural vulnerability, a pronounced dehydration propensity, and a strong electrostatic enhancement.

Suppression of Spinel Formation to Induce Reversible Thermal Behavior in the Layered Double Hydroxides (LDHs) of Co with Al, Fe, Ga, and In

The layered double hydroxides (LDHs) of Co with trivalent cations decompose irreversibly to yield oxides with the spinel structure. Spinel formation is aided by the oxidation of Co(II) to Co(III) in the ambient atmosphere. When the decomposition is carried out under N2, the oxidation of Co(II) is suppressed, and the resulting oxide has the rock salt structure. Thus, the Co-Al-CO(3)(2-)/Cl- LDHs yield oxides of the type Co(1-x)Al(2x/3) square(x/3)O, which are highly metastable, given the large defect concentration. This defect oxide rapidly reverts back to the original hydroxide on soaking in a Na2CO3 solution. Interlayer NO(3)- anions, on the other hand, decompose generating a highly oxidizing atmosphere, whereby the Co-Al-NO(3)- LDH decomposes to form the spinel phase even in a N2 atmosphere. The oxide with the defect rock salt structure formed by the thermal decomposition of the Co-Fe-CO(3)(2-) LDH under N2, on soaking in a Na(2)CO(3) solution, follows a different kinetic pathway and undergoes a solution transformation into the inverse spinel Co(Co,Fe)(2)O(4). Fe3+ has a low octahedral crystal field stabilization energy and therefore prefers the tetrahedral coordination offered by the structure of the inverse spinel rather than the octahedral coordination of the parent LDH. Similar considerations do not hold in the case of Ga- and In-containing LDHs, given the considerable barriers to the diffusion of M3+ (M=Ga, In) from octahedral to tetrahedral sites owing to their large size. Consequently, the In-containing oxide residue reverts back to the parent hydroxide, whereas this reconstruction is partial in the case of the Ga-containing oxide. These studies show that the reversible thermal behavior offers a competing kinetic pathway to spinel formation. Suppression of the latter induces the reversible behavior in an LDH that otherwise decomposes irreversibly to the spinel.

Are the Materials Properties of Indiumnitride Dominated by Defects?

AbstractIndium nitride (InN) is a promising yet technologically challenging material with a high defect density and unusual material properties. Its high electron mobility may be utilized in high power electronic devices, and its high absorbance and low energy optical response make it a promising candidate for multi-junction, high-efficient solar cell technology. Studies of absorption and photoluminescence optical response of epitaxial InN resulted in a large correction of the fundamental bandgap from the originally proposed 1.9 eV to now below 0.7 eV. Yet, it is still debated if the commonly measured optical transitions below the original high bandgap values are actually caused by a large concentration of defects, on the order of 1020/cm3, rather than reflecting a low fundamental bandgap. Many applications of this material, e.g. in high-efficient solar cell technology, are primarily dependent on the successful production of a contacted p-n junction, which has not yet been achieved. This contribution addresses the controversy in the bandgap discussion of InN. Valence electron energy loss spectroscopy (VEELS) of InN allows spatially resolved analysis of the density of states in the transmission electron microscope (TEM). Standard optical characterization is compared with results from TEM characterization.

Thermo-chemical properties and electrical resistivity of Zr-based arsenide chalcogenides

abstractTernary phases in the systems Zr–As–Se and Zr–As–Te were studied using single crystals of ZrAs1.40(1)Se0.50(1) and ZrAs1.60(2)Te0.40(1) (PbFCl-type of structure, space group P4/nmm) as well as ZrAs0.70(1)Se1.30(1) and ZrAs0.75(1)Te1.25(1) (NbPS-type of structure, space group Immm). The characterization covers chemical compositions, crystal structures, homogeneity ranges and electrical resistivities. At 1223 K, the Te-containing phases can be described with the general formula ZrAsxTe2–x, with 1.53(1)≤x≤1.65(1) (As-rich) and 0.58(1)≤x≤0.75(1) (Te-rich). Both phases are located directly on the tie-line between ZrAs2 and ZrTe2, with no indication for any deviation. Similar is true for the Se-rich phase ZrAsx–ySe2–x with 0.70(1)≤x≤0.75(1). However, the compositional range of the respective As-rich phase ZrAsx–ySe2–x (0.03(1)≤y≤0.10(1); 1.42(1)≤x≤1.70(1)) is not located on the tie-line ZrAs2–ZrSe2, and exhibits a triangular region of existence with intrinsic deviation of the composition towards lower non-metal contents. Except for ZrAs0.75Se1.25, from the homogeneity range of the Se-rich phase, all compounds under investigation show metallic characteristics of electrical resistivity at temperatures >20 K. Related uranium and thorium arsenide selenides display a typical magnetic field-independent rise of the resistivity towards lower temperatures, which has been explained by a non-magnetic Kondo effect. However, a similar observation has been made for ZrAs1.40Se0.50, which, among the Zr-based arsenide chalcogenides, is the only system with a large concentration of intrinsic defects in the anionic substructure.

Tuning of the electronic properties of self-assembled InAs∕InP(001) quantum dots using grown-in defect mediated intermixing

This paper examines the influence of rapid thermal annealing on the photoluminescence spectra of self-assembled InAs∕InP(001) quantum dots capped with 760nm InP deposited at a reduced temperature. The capping layer contained a large concentration of point defects that can promote interdiffusion upon annealing. The onset temperature for measurable blueshift in the emission spectra was found to be ∼600°C whereas shifts of 270meV were obtained after annealing at 750°C for 300s. Gradual etching of the InP capping layer enabled to progressively quench energy shifts upon annealing, a promising result for spatially selective emission tuning.

Defects in ZnO Nanorods Prepared by a Hydrothermal Method

ZnO nanorod arrays were fabricated using a hydrothermal method. The nanorods were studied by scanning electron microscopy, photoluminescence (PL), time-resolved PL, X-ray photoelectron spectroscopy, and positron annihilation spectroscopy before and after annealing in different environments and at different temperatures. Annealing atmosphere and temperature had significant effects on the PL spectrum, while in all cases the positron diffusion length and PL decay times were increased. We found that, while the defect emission can be significantly reduced by annealing at 200 degrees C, the rods still have large defect concentrations as confirmed by their low positron diffusion length and short PL decay time constants.

Mechanical properties and defect chemistry

A brief overview of changes in mechanical properties of solids driven by the chemistry of point defects is given. Two major types of effects are considered: direct effects caused by non-interacting point defects and collective effects induced by interacting point defects. The first group includes (1) changes in the linear dimensions of a solid in response to a change in defect concentration and (2) stress induced due to an inhomogeneous distribution of point defects, a so-called chemical stress. The second group includes (1) defect order–disorder transitions accompanied by self-strain and (2) deviations from linear elastic behavior due to the dissociation/association of point defects. All of the above become important if the concentration of point defects is very high (above 10 21 cm − 3 ). These effects may lead to significant anomalies in mechanical properties: spontaneous stress and strain and changes in elastic constants. These effects may significantly affect the application of materials with a large concentration of point defects.

Elasticity of Solids with a Large Concentration of Point Defects

AbstractThe elastic behavior of solids with a large concentration of interacting point defects has been analyzed. The analysis predicts that, in such solids, mechanical stress may be partially relieved by a shift in the association/dissociation equilibrium of the point defects. Association/dissociation of the point defects in response to an external stress will proceed until the decrease in elastic energy is balanced by the increased chemical energy of the defect distribution. The resulting change in the linear dimensions may be called “chemical strain”, in analogy to the previously studied “chemical stress”. A solid in which chemical strain may develop in response to external stress should exhibit two distinct Young's moduli: relaxed, on a time scale which allows the defects to reach equilibrium; and unrelaxed, on a time scale which is too short for the defect equilibrium to be established. Our analysis suggests that materials exhibiting the chemical‐strain effect are capable of reversible adaptation to external mechanical constraints. Measurements on a self‐supported film of Ce0.8Gd0.2O1.9 strongly support the theoretical predictions.

Atomic mechanisms of grain boundary diffusion: Low versus high temperatures

We analyze recent results of atomistic computer simulations of grain boundary (GB) diffusion in metals. At temperatures well below the bulk melting point T m GB diffusion occurs by random walk of individual vacancies and self-interstitials. Both defects are equal participants in the diffusion process and can move by a large variety of diffusion mechanisms, many of which are collective transitions. GB diffusion coefficients can be computed by kinetic Monte Carlo simulations. At high temperatures, the presence of large concentrations of point defects is likely to alter the diffusion mechanisms. Molecular dynamics simulations of GB structure and diffusion in copper reveal a continuous GB premelting in close vicinity of T m . However, diffusion in high-energy GBs becomes almost independent of the GB structure (“universal”) at temperatures well below T m . This behavior can be tentatively explained in terms of heterophase fluctuations from the solid to the liquid phase. The exact diffusion mechanisms in the presence of heterophase fluctuations are yet to be established.