Concentration Of Impurity Atoms Research Articles

Estimation of the sizes of disordered surface regions of Cu and Ag thin films using the ion transmission method

The dimensions of cluster phases that form on the surfaces of free Cu and Ag films in the case of different irradiation doses are estimated for the first time. It is shown that ion implantation leads to a significant decrease in the number of ions passed through a free film. Under the same conditions of the ion implantation, the concentration of impurity atoms on the surface and their depth-distribution profile for the free film differ noticeably from those for a bulk crystal.

Gold-embedded zigzag graphene nanoribbons as spin gapless semiconductors

Using density-functional theory calculations, we studied the electronic and magnetic properties of zigzag graphene nanoribbons (ZGNRs) with gold (Au) atoms embedded into different sites of the ZGNRs. Strong site dependence was found, and the system had the ferromagnetic or antiferromagnetic ground state depending on the Au atom position. Spin gapless semiconductor (SGS) behavior was observed when the Au atom was embedded into the center and edge sites of the ZGNRs. The simulations showed that the electronic structure of the ribbon strongly depends on ZGNR width, but the SGS behavior is always present when the Au atom is embedded into the center and edge sites. The SGS properties were also found to be dependent on impurity atom concentration, so that they can be tuned by either selecting the proper positions of Au atoms or changing their concentration. Our results suggest a flexible way of designing SGSs, which could be used in various spintronic, electronic, and optoelectronic applications.

Effect of the deposition process and substrate temperature on the microstructure defects and electrical conductivity of molybdenum thin films

The effect of point defects, dislocations and grain boundaries on the electron scattering in molybdenum thin films having a constant thickness of 500nm was described quantitatively in form of a dependence of the electrical resistivity on the concentration of impurity atoms, stress-free lattice parameter, microstrain and grain size. The concentration of impurity atoms and the dislocation density were modified by depositing the Mo thin films using different techniques (DC magnetron sputtering, pulsed DC magnetron sputtering and RF magnetron sputtering) and by varying the substrate temperature (25°C, 150°C, 250°C and 350°C). As expected, the electrical resistivity of the Mo films decreased with decreasing density of microstructure defects. For all deposition methods, the dislocation density decreased with increasing substrate temperature, which led to an overall decrease of the measured resistivity with increasing substrate temperature. Due to the deposition equipment constraints during the RF sputtering, up to 3at.% of Fe, Cr and Ni were incorporated at the regular lattice positions in the crystal structure of molybdenum, which increased the resistivity of the Mo films nearly two times as compared to the DC and pulsed DC sputtered films.

Physical mechanisms for the unique optical properties of chalcogen-hyperdoped silicon

A series of fundamental properties from atomic geometry, electronic band structure, optical absorption, to dynamics are systemically studied for the silicon doped with supersaturated chalcogens (S, Se, and Te). The atomic structures in a broad energy range are obtained and distinguished as three classes named the substitutional, interstitial, and quasi-substitutional. Their relative energies varying with the S, Se, and Te samples reveal that the concentration of impurity atoms occupying the substitutional position, which plays an important role in optical absorption, will be different and get progressively higher from S-, Se-, to Te-hyperdoped silicon. Electronic band structures show that for the most atomic geometries the defect-related states do appear into the gap of silicon, and the optical absorption calculations clarify that they are the very origin of the broadband absorption of chalcogen-hyperdoped silicon. Combining the optical absorption properties with the structural transformation from molecular-dynamics simulations, we disclose the micromechanism of annealing-induced reduction of infrared absorptance. Furthermore, we conclude that both the different concentrations of the substitutional doping and structural transformation will lead to the different annealing-induced reduction of infrared absorptance for S-, Se-, and Te-hyperdoped silicon as observed in experiments.

Superdiffusive nonequilibrium motion of an impurity in a Fermi sea

We treat the nonequilibrium motion of a single impurity atom in a low-temperature single-species Fermi sea, interacting via a contact interaction. In the nonequilibrium regime, the impurity does a superdiffusive geometric random walk where the typical distance traveled grows with time as $\sim t^{d/(d+1)}$ for the $d$-dimensional system with $d\geq 2$. For nonzero temperature $T$, this crosses over to diffusive motion at long times with diffusivity $D\sim T^{-(d-1)/2}$. These results apply also to a nonzero concentration of impurity atoms as long as they remain dilute and nondegenerate.

Structural changes during annealing of submicro- and nanocrystalline aluminum alloys

The annealing-induced evolution of the structure and microhardness of submicro-and nanocrystalline Al—3% Mg and Al 1570 alloys produced by torsional severe plastic deformation are studied. Annealing of the Al-3% Mg alloy at 373–423 K and annealing of the Al 1570 alloy at 373–473 K are shown to result in the relaxation of internal stresses and subsequent normal grain growth. As the annealing temperature increases, the microhardness decreases. At higher temperatures (473 K for the Al—3% Mg alloy and 573 K for the Al 1570 alloy), anomalous grain growth takes place. This growth is accompanied by the appearance of numerous grains with a high dislocation density, a high concentration of impurity atoms in grain boundaries, and an increase in the microhardness. These effects are explained.

10.1007/s11453-008-2016-7

The effect of low concentrations of two groups of doping impurities (one group includes Sn, Pb, Dy, Ho, Y and the other group includes In, Cs, and Al) on photoelectric properties of As2Se3 and (As2S3)0.3(As2Se3)0.7 was investigated. Studies of spectral distributions of photoconductivity and optical absorption showed that there is a distinct increase in photoconductivity if the level of doping with elements of the first group is as high as 0.015%. Elements of the second group of impurities did not exert any significant effect on photoconductivity. The effect of impurities of the first group is attributed to occupation of vacant sites (formed as a result of removal of highly volatile Se and S atoms) by low concentrations of impurity atoms. The observed effect is related to the fact that substitutional atoms can preserve covalent bonds and to the small difference between atomic sizes and electron affinity for doping impurities and S and Se. Atoms of the second group of impurities lack these features.

Boron: a hunt for superhard polymorphs

Boron is a unique element, being the only element, all known polymorphs of which are superhard, and all of its crystal structures are distinct from any other element. The electron-deficient bonding in boron explains its remarkable sensitivity to even small concentrations of impurity atoms and allows boron to form peculiar chemical compounds with very different elements. These complications made the study of boron a great challenge, creating also a unique and instructive chapter in the history of science. Strange though it may sound, the discovery of boron in 1808 was ambiguous, with pure boron polymorphs established only starting from the 1950s-1970s, and only in 2007 was the stable phase at ambient conditions determined. The history of boron research from its discovery to the latest discoveries pertaining to the phase diagram of this element, the structure and stability of beta-boron, and establishment of a new high-pressure polymorph, gamma-boron, is reviewed.

Some aspects of selection of impurities improving photoelectric characteristics of chalcogenide vitreous semiconductors

The effect of low concentrations of two groups of doping impurities (one group includes Sn, Pb, Dy, Ho, Y and the other group includes In, Cs, and Al) on photoelectric properties of As2Se3 and (As2S3)0.3(As2Se3)0.7 was investigated. Studies of spectral distributions of photoconductivity and optical absorption showed that there is a distinct increase in photoconductivity if the level of doping with elements of the first group is as high as 0.015%. Elements of the second group of impurities did not exert any significant effect on photoconductivity. The effect of impurities of the first group is attributed to occupation of vacant sites (formed as a result of removal of highly volatile Se and S atoms) by low concentrations of impurity atoms. The observed effect is related to the fact that substitutional atoms can preserve covalent bonds and to the small difference between atomic sizes and electron affinity for doping impurities and S and Se. Atoms of the second group of impurities lack these features.

Elaboration of physical fundamentals of nanosized structures on the basis of S++Mn−− and Se++Mn−− molecule formation in silicon lattice

One possible way to create nanosized structures on the basis of the formation of molecules (S++Mn−−) and (Se++Mn−−) among impurity atoms S, Se, Mn in the silicon lattice is described in the work. Relationships between the concentration of molecules (S++Mn−−) and (Se++Mn−−) and the concentration of impurity atoms are established.

The problem of doping of non-stoichiometric phases

An overview and analysis of our experimental data on the crystal structure, mechanical, thermal, galvanomagnetic and electrical properties of the non-stoichiometric semiconducting IV–VI phases under doping are presented. The peculiarities and general regularities of the doping of these phases by introduction of elementary impurities and by cation substitution are considered. The presence of non-stoichiometric vacancies in the cation sublattice of the host compound leads to the appearance of a new, in comparison with a crystal free of defects, mechanism of impurity dissolution consisting in occupation of vacancies by impurity atoms. This process is accompanied by a decrease in microhardness and charge carrier concentration, and an increase in charge carrier mobility and lattice thermal conductivity. It is shown that there is correlation between the value of impurity solubility limit and relative difference in ionic radii of IV atoms and those of the impurity. Under cation substitution, the existence of a large number of non-stoichiometric defects in the host compound determines the specificity of the concentration dependences of the properties in the range of small impurity concentrations. The introduction of cations with variable valence leads to a specific effect caused by the presence of non-stoichiometric vacancies and consisting in the recharging of cations under changing impurity atom concentration or deviation from stoichiometry.

Specific features of the transformation of local levels into an impurity band of Ag1−p Al p disordered solid solutions

The phonon densities of states in disordered solid solutions of aluminum in silver are determined using the Jacobi matrix method. The transformation of discrete vibrational levels (local vibrational modes) into an impurity band with increasing concentration of impurity atoms is investigated. The formation of discrete vibrational levels is associated with the presence of individual aluminum impurity atoms in the lattice. It is demonstrated that, at relatively low (approximately 5–15%) aluminum concentrations, the broadening of the main local level is accompanied by the appearance of additional resonance peaks, which contain important information on the interatomic interactions in these solid solutions.

Mechanisms of arsenic clustering in silicon

A model of arsenic clustering in silicon is proposed and analyzed. The main feature of the proposed model is the assumption that negatively charged arsenic complexes play a dominant role in the clustering process. To confirm this assumption, electron density and concentration of impurity atoms incorporated into the clusters are calculated as functions of the total arsenic concentration. A number of the negatively charged clusters incorporating a point defect and one or more arsenic atoms are investigated. It is shown that for the doubly negatively charged clusters or for clusters incorporating more than one arsenic atom the electron density reaches a maximum value and then monotonically and slowly decreases as total arsenic concentration increases. In the case of doubly negatively charged cluster incorporating two arsenic atoms, the calculated electron density agrees well with the experimental data. Agreement with the experiment confirms the conclusion that two arsenic atoms participate in the cluster formation. Among all present models, the proposed model of clustering by formation of doubly negatively charged cluster incorporating two arsenic atoms gives the best fit to the experimental data and can be used in simulation of high concentration arsenic diffusion.

Defect structural evolution in high purity tungsten irradiated with electrons using high voltage electron microscope

Four types of high purity tungsten were irradiated with 2 MeV electrons to 5 dpa using a high voltage electron microscope, and defect structural evolutions were examined as a function of the irradiation temperature and the concentration of impurity atoms. Three of materials were made by sintering of tungsten powder with purity of 99.999% (5N-W), 99.99% (PF-W) and 99.95% (N-W), and one was a chemical vapor deposited tungsten of 99.9999% (CVD-W) purity. The formation of interstitial type dislocation loops is observed above room temperature by electron irradiation. In sintered tungsten, the number density of loops increases with increasing density of impurity atoms, i.e., N-W > PF-W > 5N-W. The density of loops in CVD-W is relatively high, contrary to its purity. In CVD-W, a heterogeneous formation of loops is observed at above 573 K. Loops are aligned on layers, and no loops are formed between the layers. All four types of specimens have a change in slop of the temperature dependence of loop number density at around 500 K which is caused by impurity atoms. Results of radioactivation analysis and hardness testing are also presented.

Metastructural systems of activated nanospheres and optical near-field resonances

The near-field interaction of two spherical nanoparticles containing dense ensembles of two-or multilevel atoms in an external field of optical low-intensity radiation is shown to result in the formation of resonances whose frequencies differ considerably from the transition frequencies in the spectrum of the interacting atoms. Optical near-field resonances are shown to play an important role in metastructural systems composed of activated nanospheres. The reflectance of a metastructural system of activated nanospheres oriented along a certain direction depends strongly on the polarization and the frequency of external radiation, as well as on the concentration of impurity atoms inside the nanospheres and on their sizes.

Structural engineering of ferromagnetism in III–V digital ferromagnetic heterostructures

We investigate the possibility of modulating the magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures (DFHs) via strain engineering. We p-dope DFHs below the compensation threshold of residual As antisites to achieve variations in strain without introducing free carriers and with relatively modest concentrations of impurity atoms. X-ray diffraction and superconducting quantum interference device measurements reveal a trend toward higher TC as the out-of-plane strain is increased. Additionally, we demonstrate a second method for strain engineering wherein DFHs are grown on anisotropically relaxed (Ga,In)As stressor layers. We show that the ferromagnetic properties are independent of strain in this regime and conclude that the structure-dependent modulation of magnetic properties in DFHs cannot be explained by simple strain effects alone.

A theory of the effect of impurities on the yield stress of silicon crystals

A theoretical model that attributes the effect of mobile neutral impurities on the special features of strain in semiconductor crystals to the phenomenon of dynamic aging of dislocations is suggested. Dependences of the upper yield stress on the concentration of impurity atoms, the strain rate, and other parameters are calculated. The results of calculations are consistent with experimental data for silicon.

Phonons in conductors with magnetoimpurity electron states

A study is made of the changes in on the phonon spectrum and damping in a conductor in a quantizing magnetic field under the joint influence of quasilocal vibrations of impurity atoms in the lattice and magnetoimpurity electron states at those atoms. A system of equations for the electron and ion Green’s function is obtained on the basis an electron–ion model of a nonideal conductor in a magnetic field with the quasilocal vibrations and the magnetoimpurity states taken into account. At a low concentration of impurity atoms the magnetoimpurity states give resonance contributions to the polarization operator. They have a substantial influence on the phonon spectrum and damping. The magnetoimpurity states cause a crossover splitting of the phonon dispersion curves and give rise to new branches in the spectrum of elastic vibrations.

Model of an Irregular Structure

A model of forming a disordered structure on a two-dimensional lattice with allowance for particle interactions is suggested. Noninteracting particles and two types of particle interaction are investigated. Based on the method of ensemble averaging, fractal dimensions, number of clusters, their total volume, and other parameters are calculated. Dependences of these parameters on the concentration of impurity atoms are examined.

Thermal conductivity of liquid phase sintered silicon carbide

This study investigates the thermal diffusivity and conductivity of YAG/AlN-alloyed LPS-SiC as a function of composition and temperature using the laser-flash technique. Maxwell's model for the thermal conductivity of composites with spherical inclusions is adapted to typical features of the LPS-microstructure and its predictions are compared with experimental data. The results indicate that the thermal conductivity of LPS-SiC is controlled by (i) concentration of impurity atoms in the SiC-phase, (ii) fraction of bulk oxide phase and (iii) amorphous interphases which act as thermal resistance barriers.