Impurity Pairs Research Articles

Chemical and strain effects on Boron-doped Si(100)

Theoretical investigations uncover complex strain-induced modes of boron ordering at the Si(100) surface, which are highly anomalous in comparison with other group III impurities. The fundamental units of the clustering process are subsurface pairs of B atoms. The structural relaxations around segregated B impurities are substantial and the induced strain fields couple with the $(2\ifmmode\times\else\texttimes\fi{}1)$ dimer reconstruction of the surface to stabilize complex, zigzag modes of ordering. Impurity configurations exist that are strongly bound with respect to isolated subsurface impurity pairs up to the critical doping level of 0.5 monolayer. Above this doping level, however, all modes of ordering of the impurities at the surface are repulsive. A number of experimental observations are explained and interesting structures are predicted.

Interaction of repulsive impurity pairs in silver

Probe-impurity interactions of In–Cd and In–Zn pairs in silver matrix have been studied using 111In-TDPAC measurements. The interactions are repulsive for both the pairs. For 3% alloys, the interaction energy of In–Cd pairs in Ag is 51± 2 meV and that of In–Zn pairs is 19± 1 meV. The Electric Field Gradient (EFG) at 111Cd due to a single Cd nn impurity to the In probe is found to be 0.83± 0.04× 1021 V/m2 while for a Zn impurity is 1.50± 0.05× 1021 V/m2. Cd–Cd contact interaction in silver is also found to be repulsive.

Hybridization model for a pair of intermediate valence manganese impurities

Manganese ions in a mixed-valent state of two magnetic configurations, and , play an important role in the magnetoresistance of -based systems. We describe each Mn impurity with the represented by a spin S = 3/2 (three localized d electrons in the orbitals with their spins ferromagnetically coupled) and the configuration having an additional localized d electron in one of the orbitals to form a total spin (S + 1/2). The electron hybridizes with the conduction electrons and the multiple occupancy of the level is excluded by a large Coulomb energy at each site. This gives rise to a quadrupolar Kondo effect, which compensates the orbital degrees of freedom into a quadrupolar singlet, and interferes with the usual spin Kondo effect. We consider a pair of such manganese ions and allow the electrons to hop between the two sites. Hence, bonding and antibonding levels are formed giving rise to the ferromagnetic double-exchange mechanism. We study the interaction between the impurities in the integer-valent and the mixed-valent regimes. In the integer-valent limit we renormalize the interactions using the vertex function in the leading logarithmic approximation. Two neighbouring impurities with the same integer valence interact ferromagnetically. ions have in addition a quadrupolar Kondo effect. In the intermediate valence regime we calculate the ground state energy, the valence, the population difference between the bonding and antibonding states, the charge susceptibility, the quadrupolar susceptibility and the response to a charge imbalance between the two sites as a function of the energy of the level in zero magnetic field and for the spin-polarized limit (ferromagnetic lattice) using a mean-field slave-boson formulation. The results indicate that the intersite hopping suppresses charge order and lattice distortions.

Magnetic properties of impurities and impurity pairs in magnetic multilayers

Abstract Spin-dependent scattering is considered to be the origin of giant magnetoresistance in magnetic multilayers, although the role of bulk or interface defects is still under discussion. We present calculations of spin-dependent impurity scattering potentials as a function of the impurity position in a Co/Cu-multilayer. The calculations are performed within spin density functional theory using a Green function method (the tight binding Korringa-Kohn-Rostoker method). It is shown that the impurity potential depends strongly on the impurity position, which is reflected in local densities of states and local impurity moments. Furthermore, calculations of magnetic properties of impurity pairs at the interface are presented and compared with results for single impurities.

Magnetic properties of impurities and impurity pairs in magnetic multilayers

Magnetic properties of impurities and impurity pairs in magnetic multilayers

Electronic properties and hyperfine parameters of gold–3d-transition-metal impurity pairs in silicon

We report theoretical investigations of the chemical trends in the electronic properties of transition-metal impurity pair complexes in a semiconductor. Self-consistent spin-unrestricted electronic state calculations, with a scalar relativistic scheme, in the framework of the multiple-scattering X\ensuremath{\alpha} molecular cluster method, have been carried out for the substitutional gold-$3d$ interstitial transition-metal pairs in silicon in ${C}_{3v}$ symmetry. The role played by the 5$d$ and 3$d$ states of the transition metals in the formation of the impurity energy levels in the crystal band gap and resonances is established. The analysis of the one-electron energy spectra of the ${\mathrm{Au}}_{s}{\mathrm{Ti}}_{i},$ ${\mathrm{Au}}_{s}{\mathrm{V}}_{i},$ ${\mathrm{Au}}_{s}{\mathrm{Cr}}_{i},$ ${\mathrm{Au}}_{s}{\mathrm{Mn}}_{i},$ ${\mathrm{Au}}_{s}{\mathrm{Fe}}_{i},$ ${\mathrm{Au}}_{s}{\mathrm{Co}}_{i},$ and ${\mathrm{Au}}_{s}{\mathrm{Ni}}_{i}$ pair impurities leads to the conclusion that the electronic, magnetic, and optical properties of the series can be explained by a simple microscopic model. The calculations do not provide support for the ionic model, where these pairs are described as two point charges electrostatically bounded with a strong magnetic coupling between their spins. Instead, the results lead to a model in which the covalent effects are invoked to explain the chemical trends and the physical properties of the complexes. This model is substantiated by comparing the hyperfine parameters and transition energies with electron paramagnetic resonance and optical experimental data.

Tin-related double acceptors in gallium selenide single crystals

Gallium selenide single crystals doped with different amounts of tin are studied through resistivity and Hall effect measurements in the temperature range from 30 to 700 K. At low doping concentration tin is shown to behave as a double acceptor impurity in gallium selenide with ionization energies of 155 and 310 meV. At higher doping concentration tin also introduces deep donor levels, but the material remains p-type in the whole studied range of tin doping concentrations. The deep character of donors in gallium selenide is discussed by comparison of its conduction band structure to that of indium selenide under pressure. The double acceptor center is proposed to be a tin atom in interlayer position, with a local configuration that is similar to that of tin diselenide. The hole mobility exhibits an anomalous dependence on the tin content, attaining its maximum value in the ingot with 0.2% nominal tin content. This is proposed to be related to impurity pairing effects giving rise to thermal shallow acceptors with low ionization energy and low carrier scattering cross section, making the hole mobility to be controlled by phonon scattering mechanisms even for relatively high impurity content.

E-V energy transfer between F centers and distant [formula omitted] ions in CsBr crystals

The E-V energy transfer from electronic excited states of F centers in CsBr to the vibrational modes of OD − impurities is studied with Raman scattering. It was found that even in unaggregated samples, with statistically distributed defects separated by fairly long distances, a strong E-V transfer exists. For moderate OD −-concentrations (≈ 10 −4) its efficiency is almost 100%, similar to the one found for the next nearest 〈100〉-neighbor F H(OH −)-complex. Simultaneously, impurities or impurity pairs which have captured an extra electron are detected even at lowest temperatures. This is clear evidence that electron transfer occurs as well and suggests interrelation between the two transfer processes.

Interactions between impurity atoms of 3d transition metals dissolved in iron

We present results of our Mössbauer spectroscopy study on the distribution of impurity atoms in iron based solid solutions of iron with Ti, V, Mn, Co and Ni as a solute. The results are analysed in the terms of interaction energies of impurity pairs in iron. The energies obtained for the Ti, V and Mn pairs are positive (repulsive interaction) whereas for Co and Ni pairs they are negative (attractive interaction). The data are compared with those resulting from the Miedema–Królas model for the energy. From the comparison it follows that our experimental values are in agreement with the model for Ti and V pairs but that they are in disagreement with the model predictions for Mn, Ni and Co pairs.

Magnetic excitations in spin ladders with intrinsic impurities

We investigate antiferromagnetic spin ladders with nonmagnetic impurities and demonstrate that excitation energies and ferromagnetic correlations as found in recent neutron scattering experiments can be understood as resulting from pairs of impurities in decoupled ladders.

Competition between Kondo effect and RKKY interactions in heavy-fermion systems: Specific heat

The excess electronic specific heat for a Ce-based system ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ is calculated using the Kondo and Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction Hamiltonian within the framework of Abrikosov's pseudofermion representation in the presence of crystalline electric field (CEF) split levels. The spin-flip scattering of the electrons caused by the indirect exchange interaction of a pair of magnetic impurities has been considered. It is found that for Ce-based compounds in general, the competition between the Kondo and RKKY interactions, along with the effect of the CEF splitting, yields a behavior varying from a minimum at low temperatures to a monotonically decreasing behavior in $\ensuremath{\Delta}C/T$ vs ${T}^{2}$ towards higher $T.$ The overall behavior of $\ensuremath{\Delta}C/T$ is found to be quite sensitive to the RKKY coupling and CEF splitting \ensuremath{\delta} in the lowest doublet scheme. The results are found to be consistent with the experimental data.

Coherent tunneling of lithium defect pairs in KCl crystals

AbstractThe tunneling of two lithium ion impurities on next‐nearest neighbor sites in potassium chloride are investigated both experimentally and theoretically. The strong dipolar interaction leads to coherent tunneling motion of the two defect ions between degenerate off‐center positions. Comparing data of rotary echo experiments for impurity pairs 7Li—7Li, 6Li—6Li, and 7Li—6Li with theory permits a thorough investigation of the isotope effect and of the effect of the interaction on the tunnel states. Our findings confirm the tunneling model with <111> off‐center states to be valid even for strongly interacting impurities. Using degenerate perturbation theory in terms of two‐particle states, we obtain essentially exact expressions for the tunneling spectrum and the dynamical susceptibility which agree well with the measured data.

Chemical Trends in Electronic Properties of Gold-3D Transition Metal Impurity Pairs in Silicon

ABSTRACTWe report theoretical investigations of the chemical trends in the electronic properties of substitutional gold-interstitial transition-metal complexes in silicon. The results show that the stable pairs in trigonal symmetry are formed by a covalent mechanism which includes, besides Au and TM impurities, also the Si neighbors, rather than being derived from interactions between two electrostatically bound point charges.

The dissipative dynamics of a pair of paraelectric tunnelling impurities

Paraelectric impurities contribute significantly to the low-temperature properties of alkali halide crystals. Even at very low density the dipolar interaction of adjacent defect ions may lead to deviations from the behaviour expected for isolated impurities; in a certain range of concentration it is sufficient to consider pairs of coupled defects. Applying a projection operator method, previous work on this pair model is extended to the case of finite asymmetry and weak coupling to acoustic phonons. After performing the ensemble averaging, the specific heat and the zero-frequency susceptibility are calculated and compared with experimental data on KCl:Li and KCl:CN. The isotope effect on the Rabi frequency and relaxation rate is discussed.

High-resolution infrared spectroscopy of isotopic impurity Q1(0) transitions in solid parahydrogen

We have made a high-resolution infrared spectroscopic study of the Q1(0) (v=1←0, J=0←0) vibrational transitions of the isotopic impurities D2 and HD in solid parahydrogen. Each impurity has a spectrum composed of ∼100 sharp lines spread over ∼0.4 cm−1. The linewidths vary, but are on the order of 10 MHz. These spectra make clear: (1) the infrared Q1(0) transitions of J=0 isotopic impurities are induced by the quadrupolar fields of nearby impurity J=1 molecules; and (2) the spectral pattern of strong Q1(0) lines is due to the splitting of the M-orientational levels of J=1/J=0 o-D2 or J=1/J=0 HD nearest-neighbor (nn) impurity pairs. With the aid of several theoretical works, the strong lines in the D2 and HD spectra can be individually and unambiguously assigned as specific quantum state Q1(0) transitions of nn impurity pairs containing p-D2/o-D2 or o-H2/o-D2, and o-H2/HD, respectively. The assigned transitions of nn impurity pairs containing o-H2 are confirmed by combination differences which agree to within 5×10−4 cm−1, the instrumental precision. These assignments yield complete Q1(0) energy level diagrams for the nn impurity pairs o-H2/o-D2 and o-H2/HD embedded in solid parahydrogen. The experimental energy level splittings are fit to a two parameter model which describes anisotropic interactions in the parahydrogen crystal. These experimental parameters appear to have significant contributions from the changes in renormalization and lattice constant around the heavier isotopic impurity. We have also assigned a few of the weaker spectral features as Q1(0) transitions of more distant impurity pairs, but the bulk of these transitions are yet to be assigned. They do form a distinctive pattern and are thought to be the Q1(0) transitions of impurity triples and larger clusters. This study is one of the few cases for which high-resolution laser spectroscopy has been successfully applied to the condensed phase and for which many of the transitions have rigorous quantum state assignments.

Annealing of radiation-induced defects in vanadium and vanadium-titanium alloys

The annealing of defects induced by electron irradiation up to a dose of 6 × 10 21 m −2 at T < 293K has been investigated in single-crystals of pure vanadium and in vanadium-titanium alloys with compositions 0.3, 1 and 5 at.% Ti using positron annihilation spectroscopy. The recovery of the positron annihilation parameters in V single-crystals indicates that the defect annealing takes place in the temperature range 410–470 K without formation of microvoids for the present irradiation conditions. For the alloys the recovery onset is shifted to 460 K, the width of the annealing stage is gradually broadened with increasing Ti content, and microvoids are formed for annealing temperatures at the end of the recovery stage. The results show that the vacancy release from vacancy-interstitial impurity pairs and subsequent recombination with interstitial loops is the mechanism of the recovery in pure V. For V-Ti alloys, vacancy-Ti-interstitial impurity complexes and vacancy-Ti pairs appear to be the defects responsible for the positron trapping. The broadening of teh recovery stage with increasing Ti content indicates that solute Ti is a very effective trap for vacancies in V.

Impurity states in the narrow band-gap semiconductor n-type InSb

We investigate impurity pair and triad formations in the narrow band-gap semiconductor n-doped InSb. It is found that in the region of the band-gap energy, E g = 0.23 eV, at 0 K, the effects of such clusters play a relevant role in the optical properties around and below the ionization energy of an isolated impurity (i.e., E i = 0.64 meV). The transition for the uncompensated system is obtained as the concentration of impurities exceeds N c = 4.7 × 10 13 cm −3.

Stability of small iridium clusters near the Ir(111) surface

Iridium atoms deposited on the Ir(111) surface associate to form different cluster configurations (isolated adatoms, dimers, trimers…). In order to understand the various arrangements observed at the surface, we have computed the effective interactions of small clusters adsorbed on Ir(111). Our model is an extension of the DCA (direct configurational averaging) methods. We work in a tight binding framework using the recursion method and the “orbital peeling” scheme to compute directly the energies from the electronic structure. Both “bulk” sites (i.e. which continue the fcc lattice) and “surface” sites (i.e. which sit in an hcp arrangement and create a fault plane) are considered. Following our results, the effective interaction energy of first neighbour pairs is the most attractive, favouring compact clusters formation at the surface. Concerning the trimers we show that the compact triangle (2D) is more stable than the linear chain (1D) and than the triangle formed by second neighbor impurity pairs. Our conclusions are in good agreement with the available experimental data.

Point defect-based modeling of diffusion and electrical activation of ion implanted boron in crystalline silicon

The time evolution of the transient enhanced diffusion and of the electrical activation of boron in crystalline silicon during thermal annealing subsequent to boron ion implantation is modeled by a system of diffusion-reaction equations for the dopant species and the silicon point defects. The concept of point defect impurity pair diffusion under equilibrium conditions is used to describe the diffusion process. Outdiffusion of implantation-induced silicon self-interstitials and the kick-out reaction Bi■Bs+I are assumed to be the leading mechanisms for boron activation. In the case of low-dose boron ion implantation, we start from a defect distribution of Gaussian shape with one interstitial per implanted boron atom. For higher boron doses, the area density of this interstitial distribution is constant, but the depth position of its peak depends on boron dose. Local equilibrium for the reactions between the point defects and the boron species is postulated to be realized before the onset of diffusion. The computed boron depth profiles are compared to data from the literature. Implantation doses from 2×1014 cm−2 up to 5×1015 cm−2 are analyzed, annealing temperatures and times are considered over the ranges 800–1000 °C and 10 s–8 h, respectively. Although this approach is characterized by a number of simple assumptions, essential deficiencies are only found in certain cases of annealing subsequent to high-dose boron implantation. Trapping of free interstitials by extended defects seems to become important at low temperatures and for long annealing times. If the depth region with maximum boron concentration is in its as-implanted state close to amorphization, a boron overactivation which is beyond the present model can be found. For all other cases it is possible to achieve a reasonable modeling of transient enhanced diffusion and of electrical activation.

Three-dimensional effects on extended states in disordered models of polymers.

We study electronic transport properties of disordered polymers in a quasi-one-dimensional model with fully three-dimensional interaction potentials. We consider such qausi-one-dimensional lattices in the presence of both uncorrelated and short-range correlated impurities. In our procedure, the actual physical potential acting upon the electrons is replaced by a set of nonlocal separable potentials, leading to a Schr\"odinger equation that is exactly solvable in the momentum representation. By choosing an appropriate potential with the same spectral structure as the physical one, we obtain a discrete set of algebraic equations that can be mapped onto a tight-binding-like equation. We then show that the reflection coefficient of a pair of impurities placed at neighboring sites (dimer defect) vanishes for a particular resonant energy. When there is a finite number of such defects randomly distributed over the whole lattice, we find that the transmission coefficient is almost unity for states close to the resonant energy, and that those states present a very large localization length. Multifractal analysis techniques applied to very long systems demonstrate that these states are truly extended in the thermodynamic limit. These results are obtained with parameters taken from actual physical systems such as polyacetylene, and thus reinforce the possibility of verifying experimentally theoretical predictions about the absence of localization in quasi-one-dimensional disordered systems.