Impurity Clusters Research Articles

Distortion effects and clustering of isotopic impurities in solid molecularpara-hydrogen from variational Monte Carlo simulations with shadow wave functions

We employed a fully optimized Shadow Wave Function (SWF) in combination with Variational Monte Carlo techniques to investigate the properties of HD molecules and molecular ortho-deuterium (o-D_2) in bulk solid para-hydrogen (p-H_2). Calculations were performed for different concentrations of impurities ranging from about 1% to 25% at the equilibrium density for the para-hydrogen crystal. By computing the excess energy both for clustered and isolated impurities we tried to determine a limit for the solubility of HD and o-D_2 in p-H_2.

Effect of impurity clustering on ferromagnetism in dilute magnetic semiconductors

We establish a model to investigate the effect of clustering of impurities on the ferromagnetism in dilute magnetic semiconductors (DMS). The Curie temperature Tc is calculated by the mean-field theory on a lattice with randomly distributed clusters of magnetic impurities which are interacting with each other by carrier mediated RKKY exchange coupling together with the nearest-neighbor (NN) direct exchange interaction. We consider different types and sizes of the clusters and find that the clustering of impurities can either enhance or reduce Tc, depending on the type and strength of the NN exchange interaction. If the NN interaction is antiferromagnetic and strong compared with the RKKY interaction, the clustering will reduce Tc. On the other hand, if it is ferromagnetic interaction or weak antiferromagnetic one, the clustering can enhance Tc. The trend of enhancing Tc is magnified if the average size of clusters increases. The clustering also changes the distribution of polarizations of impurities. The obtained results provide natural explanations on the fact that the ferromagnetism of DMS samples depends on the preparing and annealing processes even though the density of the magnetic impurities is kept the same.

A vibronic model for the absorption spectra ofCr2+ ionsin CdSexS1−x

In the present paper we report a vibronic model for the absorption bands of theCr2+ ion inCdSe and CdSe0.6S0.4 crystals. For the most probable impurity clusters,CrSe4,CrSe3S,CrSeS3 and CrSe2S2, the crystal field splittings and the vibronic coupling parameters are estimated with the aid ofthe exchange charge model of the crystal field accounting for the exchange and covalence effects.On this basis the transitions responsible for the formation of the optical bands arising from theCrSe4,CrSe3S,CrSeS3 and CrSe2S2 species are determined and the profiles of the absorption bandscorresponding to these species are calculated. The total spectrum of theCdSe0.6S0.4:Cr2+ crystal is obtained by summation over partial spectra arising fromall mentioned species. The profiles of the absorption bands inCdSe:Cr2+ and CdSe0.6S0.4:Cr2+ calculated on the basis of the numerical solution of the dynamical vibronic problems proveto be in quite good agreement with the experimental data.

Dimension of an optimum impurity cluster and multiple-occupancy corrections in the theory of scattering of vibrational excitations in a solid solution

A relationship is derived for the correlation length L determining the size of the region in a solid solution in which excitations are scattered coherently. The correlation length depends on the fraction of impurity atoms x in the solid solution and the lattice dimension d. In the physical analysis of single-particle scattering processes in the solid solution and calculations, it is sufficient to take into account clusters with the number of cells n corresponding to the correlation volume Ld. A theoretical analysis is illustrated by calculations of the spectral functions of the solid solution at different values of x and n. The multiple-occupancy corrections (polynomials in powers of x) to scattering diagrams are calculated using the method of sequential breaking apart of the interaction lines in the diagrams for the self-energy part. The method used was previously applied to the case of scattering by a single impurity. In this paper, the efficiency of the method is checked for scattering by multi-impurity clusters. It is demonstrated that the method can be useful in analyzing and estimating the contributions of scattering diagrams.

Disorder-enhanced spin polarization in diluted magnetic semiconductors

We present a theoretical study of diluted magnetic semiconductors that includes spin-orbit coupling within a realistic host band structure and treats explicitly the effects of disorder due to randomly substituted Mn ions. While spin-orbit coupling reduces the spin polarization by mixing different spin states in the valence bands, we find that disorder from Mn ions enhances the spin polarization due to formation of ferromagnetic impurity clusters and impurity bound states. The disorder leads to large effects on the hole carriers which form impurity bands as well as hybridizing with the valence band. For Mn doping $0.01\ensuremath{\lesssim}x\ensuremath{\lesssim}0.04$, the system is metallic with a large effective mass and low mobility.

Scattering of Cold Neutrons on Gel Samples Formed by Impurity Clusters in Superfluid He-II

The approach we have proposed earlier for a large increase in ultracold neutron density was based on equilibrium thermalization of cold neutrons during their diffusive motion in a large sample of an impurity-helium gel which consists of weakly bounded deuterium or heavy water nanoclusters in He-II cooled below a few mK. It is clear that the nanoparticles should provide a sufficiently large cross-section of coherent scattering of cold neutrons, but the scattering of neutrons on impurity gel samples in He-II has been never investigated earlier. In this paper we present results of the first observations of the scattering of a beam of cold neutrons with mean velocities from 30 to 160 m/s on heavy water and deuterium gel samples in He-II at temperatures T∼1.6 K. It has been observed that the beam transmission through the heavy water gel sample has been changed from ∼0% to ∼50% upon increasing the neutron velocity from 30 to 160 m/s, and that the neutron transmission trough the deuterium gel sample is less than 5% at all velocities. The angular distribution of the neutron scattering changes with variations in the neutron velocity: for both the gels at large neutron energies the scattering is directed in a front hemisphere mainly, and at low energies the angular distribution of scattered neutrons is close to uniform.

To the theory of rare gas alloys: heat capacity

The low-temperature heat capacity of cryocrystals containing impurity clusters is investigated theoretically and experimentally. Such defects might essentially enrich the low-frequency part of the phonon spectrum by introducing both localized and delocalized vibrations. The effect of both types of vibrations on the temperature dependence of the heat capacity is analyzed. The heat capacity of the disordered solid solution Kr–Ar (Ar concentration ∼25%) is studied as an example of the effect of the light, weakly coupled impurities on the low-temperature thermodynamic characteristics of a system. The mass defect of such an impurity induces “phonon pumping” from the low-frequency part of the spectrum into the high-frequency part and decreases the low-temperature heat capacity, while the weakened interaction between the impurity and the host atoms, combined with even weaker interaction between the impurities, leads to the formation of a low-temperature maximum on the heat capacity temperature dependence. The analysis performed shows that at rather high Ar concentrations, the nonmonotonic temperature dependence of the relative change in the heat capacity of Kr1−pArp solid solutions is a result of the excitation of delocalized high-dispersion low-frequency phonons.

Quantum Monte Carlo impurity solver for cluster dynamical mean-field theory and electronic structure calculations with adjustable cluster base

We generalized the recently introduced new impurity solver based on the diagrammatic expansion around the atomic limit and Quantum Monte Carlo summation of the diagrams. We present generalization to the cluster of impurities, which is at the heart of the cluster Dynamical Mean-Field methods, and to realistic multiplet structure of a correlated atom, which will allow a high precision study of actinide and lanthanide based compounds with the combination of the Dynamical Mean-Field theory and band structure methods. The approach is applied to both, the two dimensional Hubbard and t-J model within Cellular Dynamical Mean Field method. The efficient implementation of the new algorithm, which we describe in detail, allows us to study coherence of the system at low temperature from the underdoped to overdoped regime. We show that the point of maximal superconducting transition temperature coincides with the point of maximum scattering rate although this optimal doped point appears at different electron densities in the two models. The power of the method is further demonstrated on the example of the Kondo volume collapse transition in Cerium. The valence histogram of the DMFT solution is presented showing the importance of the multiplet splitting of the atomic states.

Effect of fine structure of Pb-In solid solutions on their deformation-induced strengthening at low temperatures

The specific features of low-temperature deformation-induced strengthening of Pb-In single crystals have been studied in a wide range of impurity concentrations. It was shown that a change in the strain-rate sensitivity of flow stress in the concentrated solid solution is connected with the inhomogeneous distribution of impurities. Based on the X-ray diffuse-scattering data, it was found that the short-range order of separation type was formed in these alloys; i.e., the lead atoms were randomly arranged and the indium atoms formed clusters whose characteristic size increased with concentration in a range of 1.0–1.9 nm. In the concentrated alloy, the appearance of coarse impurity clusters as strong obstacles for mobile dislocations in comparison with the impurity atoms and forest dislocations causes not only a sharp increase in the flow stress but also a change in the energy of dislocation-dislocation interaction.

Enhanced carrier scattering rates in dilute magnetic semiconductors with correlated impurities

In III-V dilute magnetic semiconductors such as ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$, the impurity positions tend to be correlated, which can drastically affect the electronic transport properties of these materials. Within the memory function formalism we have derived a general expression for the current relaxation kernel in spin and charge disordered media and have calculated spin and charge scattering rates in the weak-disorder limit. Using a simple model for impurity clustering, we find a significant enhancement of the charge scattering. The enhancement is sensitive to cluster parameters and may be controllable through postgrowth annealing.

Influence of Metal Impurity Defects on the Electrical and Optical Properties of ITO Films on the PEN Substrates

AbstractIndium tin oxide (ITO) has drawn a great deal of attention due to its potential use as transparent electrodes in organic light emitting diode (OLED) and photovoltaic applications. This work focuses on understanding the role of impurity defects on the electrical conduction and transmittance of ITO. Thin films of ITO with high carrier concentration have been deposited onto polyethylene napthalate (PEN) substrates by electron-beam deposition without introduction of oxygen into the chamber. The influence of air anneals on the electrical and optical properties of ITO/PEN samples can is evaluated in terms of the oxygen content and is explained in terms of changes in the free electron concentrations. Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy analysis were used to determine the oxygen content in the film. A Hall effect measurement is used to determine the dependence of electrical properties on oxygen content. The electrical properties of the ITO films such as carrier concentration, electrical mobility, and resistivity abruptly changes after annealing in the air atmospheres. In addition, optical transmittance is improved from 7 to 71 % and optical band gap changes from 3.18 to 3.25 eV after heat treatment. The optical band gap narrowing behavior is because of impurity band and heavy carrier concentration. Metal impurity clusters form in the films as a result of oxygen deficiency and also generate defects and/or impurity states in the band gap and produces an optical band gap shift by merging of these impurity states and conduction band.

Modeling charged defects, dopant diffusion and activation mechanisms for TCAD simulations using kinetic Monte Carlo

This work will show how the kinetic Monte Carlo (KMC) technique is able to successfully model the defects and diffusion of dopants in Si-based materials for advanced microelectronic devices, especially for non-equilibrium conditions. Charge states of point defects and paired dopants are also simulated, including the dependency of the diffusivities on the Fermi level and charged particle drift coming from the electric field. The KMC method is used to simulate the diffusion of the point defects, and formation and dissolution of extended defects, whereas a quasi-atomistic approach is used to take into account the carrier densities. The simulated mechanisms include the kick-out diffusion mechanism, extended defect formation and the activation/deactivation of dopants through the formation of impurity clusters. Damage accumulation and amorphization are also taken into account. Solid phase epitaxy regrowth is included, and also the dopants redistribution during recrystallization of the amorphized regions. Regarding the charged defects, the model considers the dependencies of charge reactions, electric bias, pairing and break-up reactions according to the local Fermi level. Some aspects of the basic physical mechanisms have also been taken into consideration: how to smooth out the atomistic dopant point charge distribution, avoiding very abrupt and unphysical charge profiles and how to implement the drift of charged particles into the existing electric field. The work will also discuss the efficiency, accuracy and relevance of the method, together with its implementation in a technology computer aided design process simulator.

Modification of the steel microstructure and composition under the action of a fast microparticle flux

The changes in the microstructure and composition of steel subjected to a flux of microparticles with a density of ≈ 2 × 10−3 kg/m3 and a velocity of 0.8–1.5 km/s are studied. The resulting inhomogeneities of the steel microstructure are found to be independent of the particle material: they represent regions of local strain and clusters of steelmaking-related impurities. These regions expand with increasing shock-wave pressure.

Medium-ranged interactions of transition-metal (3d and 4d) impurity pairs in Al and atomic structures of Al-rich Al–transition-metal alloys

We give systematic ab initio calculations for the interaction energies (from first to eighth neighbors) of impurity pairs X–X (X = Sc–Zn, Y–Cd) in Al and discuss the micromechanism of the structural stability of Al-rich AlX alloys. The calculations are based on the generalized gradient approximation in the density-functional theory and employ the all-electron full-potential Korringa–Kohn–Rostoker Green's function method for point defects, which guarantees the correct embedding of the cluster of impurities and vacancies in an otherwise perfect crystal. We show: (1) the fundamental features of phase diagrams of these alloys, such as ordering and segregation, are understood by the sign of first-neighboring X–X interaction energies; (2) the structural stability of Al-rich AlX alloys such as L1 2 (Al 3Sc and Al 3Y), DO 22 (Al 3V and Al 3Nb), and Mackay icosahedron in AlMn quasicrystal, are understood qualitatively by using the medium-ranged and oscillating X–X interactions, due to the strong sp-d (Al–X) hybridization; (3) the strength and oscillating behavior of the medium-ranged X–X interaction energies are specified very well by the d-electron numbers of X impurities.

Control of metal impurities in “dirty” multicrystalline silicon for solar cells

The rapid growth of the global photovoltaics (PV) industry is increasingly limited by the availability of suitable Si feedstock material. Therefore, it is very important to explore new approaches that might allow processing of solar cells with satisfactory energy conversion efficiency based on inexpensive feedstock material with less stringent impurity control, i.e., “dirty” silicon. Our detailed studies of the distribution of metal impurity clusters in multicrystalline Si have demonstrated that cells with the same total impurity content can have widely different minority carrier diffusion lengths based on the distribution of the metals, i.e., whether they are dispersed throughout the material or concentrated in a few, large clusters. Possible approaches to defect engineering of metal clusters in silicon are discussed.

Revealing of dislocations in diamond crystals by the selective etching method

Etching of synthetic and natural diamond single crystals has been carried out in KNO 3 or NaNO 3 melts at 600–900 °C. A correlation between the etch pits and dislocations emerging on the {1 1 1} faces has been established by the etching of oppositely matched cleavages, thin plates, natural diamond crystals with trigons on the octahedral faces and repeated etching. The etching results were correlated with X-ray topography data. It has been revealed that pyramidal etch pits on the {1 1 1} diamond faces with the inclination angles of 2–8° are formed at the emergence points of dislocations. In this case, the etch pits with the inclinations of 5°, 6°, 5° and 8° originate at the emergence points of edge dislocations, more shallow pits are probably related to screw and partial dislocations. The etch pits with the inclination of 11–15° are not associated with dislocations and can be related to impurity clusters, microcracks and other surface defects.

Green function study of quantum transport in ultra-small devices with embedded atomistic clusters

Transport in limiting scale MOSFET transistors will be strongly influenced by quantum effects and the presence of atomistic scattering centres either intentionally or un-intentionally present in the channel and the device environs. The scattering in such systems is non-asymptotic and the selfaveraging conditions of the Kohn-Luttinger theorem fail so that a self-energy for impurity scattering does not exist. Atomistic scattering must therefore be treated non-perturbatively. Previously it has been shown that quantized micro-vortices may occur at definite energies in the current flow contributing to both the blocking effect and to effective mobility. The present study uses the Glasgow and NASA NEGF simulators to study vortex formation and tunnelling through small clusters of atomistic impurities arranged with various configurations within the 5 nm wide by 12 nm long channel of a Double Gate MOSFET. The I-V characteristics and the threshold voltage are severely affected by the distribution of the charges in the channel. A variety of different geometry atomistic clusters have been studied. Examination of the energy dependent current density allows an evaluation of the admixture of strong quantum flows such as micro-vortices to the net current. It is found that the threshold voltage and conductance are strongly dependent on the impurity configuration. The I-V characteristics are monotonic in most cases due to the strong thermal smoothing that prevents resolution of the mode structure.

Nonstoichiometry and low-temperature magnetic properties of FeSi crystals

The results of experimental and theoretical studies of the low-temperature properties of FeSi crystals are presented. The specific features of the magnetic susceptibility are shown to be related to the superparamagnetic behavior of impurity clusters. The thermomagnetic hysteresis phenomena observed are explained using the model of exchange-coupled clusters.

Molecular dynamics simulation of crack propagation in fcc materials containing clusters of impurities

Molecular dynamics simulation method was employed to model mode I crack propagation in plates containing nanoscale clusters of impurities located in the vicinity of the crack tip. The plates and the clusters were considered to have an fcc structure. The interactions between the dissimilar atoms in the plates and clusters were modeled via the many-body Sutton–Chen inter-atomic potentials [Sutton, A.P., Chen, J., 1990. Long-range Finnis–Sinclair potentials. Philos. Mag. Lett. 61, 139–164], and their extended versions developed for fcc alloys by [Rafii-Tabar, H., Sutton, A.P., 1991. Long-range Finnis–Sinclair potentials for fcc metallic alloys. Philos. Mag. Lett. 63, 217–224]. In this paper, two different geometries for the clusters, in the form of a long strip and a cylinder, were considered, and their effects on the critical load and crack propagation were examined. Before reaching the critical load, the temperature of the system was held constant at 0 K by applying both a simple temperature scaling method and the Nose–Hoover thermostat [Rafii-Tabar, H., 2000. Modelling the nano-scale phenomena in condensed matter physics via computer-based numerical simulations. Phys. Rep. 325, 239–310]. The temperature was allowed to evolve, however, during all the subsequent stages of crack propagation. Velocity Verlet algorithm [Allen, M.P., Tildesley, D.J., 1987. Computer Simulation of Liquids, Clarendon Press] was employed to integrate the equations of motion.

Study of the stabilization and recombination of nitrogen atoms in impurity–helium condensates

The stabilization and recombination of nitrogen atoms N(4S) in nitrogen-helium and nitrogen–neon-helium condensates obtained by the injection of impurity particles from a gas discharge into bulk superfluid helium are investigated by the EPR method. It is established that the stabilized nitrogen atoms reside inside and on the surface of impurity clusters forming a porous structure in the bulk superfluid helium. The possibility of increasing the specific energy of impurity–helium condensates by increasing their density through mechanical pressing is investigated. For nitrogen-helium condensates an eightfold increase in the specific energy is achieved. The recombination loss of N(4S) upon heating of impurity–helium condensates in the temperature range 1.7–7 K is detected. This permits verification of the mechanism of thermoluminescence of impurity–helium condensates.