Antisite Point Defects Research Articles

Atomistic simulations of diffusion mechanisms in off-stoichiometric Al-richNi3Al

This paper presents dynamics simulation results of diffusion in off-stoichiometric Al-richNi3Al (Ni73Al27) attemperature ranging from 1300 to 1550 K. The interatomic forces are described by the Finnis–Sinclairtype N-body potentials. Particular attention is devoted to the effect of the extra2% of Al atoms sitting on the Ni sublattice as antisite point defects(AlNi) on diffusion. Simulation results show that Ni atoms mainly diffuse through theNi sublattice at the temperatures investigated. Al atoms diffuse via both theintrasublattice and antistructure bridge (ASB) mechanisms. The contributionto Al diffusion from the ASB mechanism decreases at the lower temperature(T<1400 K). The presence of antisite point defects(AlNi) enhances both Aland Ni diffusion in Ni73Al27. The Ni–Al coupled diffusion effect is observed and understood at the atomic level for thefirst time.

Deep-level transient spectroscopy of TiO2∕CuInS2 heterojunctions

Deep-level transient spectroscopy (DLTS) has been used to measure the concentration and energy position of deep electronic states in CuInS2. Flat TiO2∣CuInS2 heterojunctions as well as TiO2-CuInS2 nanocomposites have been investigated. Subband-gap electronic states in CuInS2 films are mostly due to antisite point defects and vacancies. Substitution of indium with copper, CuInII, leads to an acceptor state 0.15 eV above the valence band, while copper vacancies, VCuI, are acceptor states at 0.1 eV. Furthermore, indium on a copper position, InCu″, yields a donor state at 0.07 eV below the conduction band, while sulphur vacancies are donor states at 0.0=eV. With DLTS, these states are indeed found. For flat configurations, VCuI are the dominant acceptors with a concentration of 1.83×1017cm−3. In contrast for nanocomposites CuInII are the dominant acceptors having a concentration of 6.7×1017cm−3. We conclude that the concentration of antisite defects in nanocomposite CuInS2 is significantly higher than that in flat films of CuInS2.

Theoretical study of the group-IV antisite acceptor defects in CdGeAs2

Native and impurity antisite point defects in CdGeAs2 are studied here using an embedded quantum cluster model based on density functional theory. The calculated geometric relaxations and spin densities of the antisite defects considered here show a clear and distinct difference in the nature of native (i.e. [GeAs]) and impurity (i.e. [CAs] and [SiAs]) antisite defects in CdGeAs2. For the native antisite acceptor, the hole appears to be delocalized in contrast to impurity antisites where the hole is mainly localized at the acceptor site.

Growth and characterization of carbon-doped low-temperature GaAs

We present the results of double crystal X-ray diffraction and infrared absorption measurements on C-doped low-temperature GaAs (LT-GaAs:C) layers grown by solid source molecular beam epitaxy at different growth temperatures T G and C concentrations. Strain compensation is achieved at T G=220°C with a C concentration of 2.5×10 19 cm −3. The density of As Ga antisite point defects and the incorporation of C is studied by IRA. At T G=250°C and high C concentrations, we observe a drastic decrease in the intensity of the local vibrational mode of substitutionally incorporated C As. Simultaneously, new local vibrational modes appear whose intensity increases by a further lowering of T G. This result, which is neither observed at low C concentrations nor in the heavily doped GaAs:C grown at conventional T G, is ascribed to the formation of CC complexes in heavily doped LT-GaAs:C.

Effects of point defect distribution on arsenic precipitation in low-temperature grown III–V arsenides

The behavior of As precipitation in low-temperature grown III–V arsenides is investigated and correlated with the doping level, crystal bond strength, and dislocation density. Experimental results reveal that the doping level affects the concentration of charged defects, such as vacancy and antisite point defects, and hence leads to the selective precipitation of excess As in homojunctions. For heterostructures, As precipitates tend to condense in materials with a lower bond strength due to differences in point defect concentrations between the materials. In addition, dislocations are found to be a vacancy source that facilitates As precipitation around them. These results indicate that column III vacancies play an important role in As precipitation of low-temperature grown III–V arsenides.

AB-Initio Based Calculations of Vacancy Formation and Clustering Energies Including Lattice Relaxation in Fe3Al

AbstractVacancy formation and clustering significantly affect structural properties of transition-metal aluminides. Ab-initio quantum mechanical total-energy calculations using a full-potential linear combination of muffin-tin orbitals (LMTO) technique provide a convenient method of studying relevant characteristics such as changes in density of states, and charge redistribution around defects. Augmented with Hellmann-Feymann forces, LMTO allows calculations of relaxation geometries and relaxation energies. We have performed such calculations for vacancies and antisite substitutional point defects in Fe3Al with DO3 crystallographic structure. There are two limiting factors complicating calculations of defect formation energies directly from ab-inito calculations. The first is that a single defect, due to the lattice periodicity necessitated by the use of ab-inito total energy techniques, cannot be considered as an isolated defect, even in the maximum computable simulation cell. Unlike previous calculations [ I ], which did not find a dependency on the size of the simulation cell, our calculations have shown a significant difference in results for 32- and 16- atom cells. This difference provides information about vacancy clustering since it can be explained by a relatively small attractive interaction energy ~0.2 eV between two vacancies located in adjacent simulation cells and separated by the lattice constant distance (5.52Å) [2]. By comparing the internal energies for two configurations of 30 atom cells (32 atom - 2 vacancies) we were able to estimate that the attractive interaction between two vacancies could reach 1.2 eV. The second complication is the fact that chemical potentials of elements cannot be directly extracted from the total energy calculations for the compound. To deal with this problem, we considered two possible approximations and compared results, which were found to be quite similar for iron vacancies.

High energy As ion implantation to GaAs

Over the last few years there have been many studies of GaAs layers grown in the temperature range from 180°C to 300°C called LT-GaAs. These studies were motivated by the potential applications of that material in microwave and fast optoelectronic devices. Annealed LT-GaAs layers have interesting electrical properties including high resistivity and short photocarrier lifetime connected with the nonstoichiometry of that material. However, it is still not clear whether arsenic antisite point defects or arsenic precipitates formed during annealing cause these interesting properties of LT GaAs.A second method of creating highly nonstoichiometric GaAs beside low temperature MBE growth is implantation with a high dose of arsenic. This method first applied two years ago showed that 200 keV arsenic ion implantation also leads to semi-insulating properties and that successive post-implantation annealing results in arsenic precipitate formation, similar to that observed in LT GaAs. Recently, more elaborate studies concerning photocarrier lifetime, resistivity, mobility and some structural characteristics were performed on GaAs implanted with high energy arsenic ions.

Defect Structure of β NiAl Using the BFS Method for Alloys

AbstractThe semiempirical BFS method for alloys is generalized by replacing experimental input with firstprinciples results thus allowing for the study of complex systems. In order to examine trends and behavior of a system in the vicinity of a given point of the phase diagram a search procedure based on a sampling of selected configurations is employed. This new approach is applied to the study of the/3 phase of the Ni-Al system, which exists over a range of composition from 45–60 at. % Ni. This methodology results in a straightforward and economical way of reproducing and understanding the basic features of this system. At the stoichiometric composition, NiAl should exist in a perfectly ordered B2 structure. Ni-rich alloys are characterized by antisite point defects (with Ni atoms in the Al sites) with a decrease in lattice parameters. On the Al-rich side of stoichiometry there is a steep decrease in lattice parameter and density with increasing Al content. The presence of vacancies in Ni sites would explain such behavior. Recent X-ray diffraction experiments suggest a richer structure: the evidence, while strongly favoring the presence of vacancies in Ni sites, also suggests the possibility of some vacancies in Al sites in a 3:1 ratio. Moreover, local ordering of vacant sites may be preferred over a random distribution of individual point defects.

Planar Faults and Grain Boundary Precipitation in Non-Stoichiometric (Sr, Ca)TiO3 Ceramics

Microstructures of non-stoichiometric-composition (Sr0.85Ca0.15O)xTiO2 ceramics (with x=0.98 and 1.02) were observed using high-resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation of Magnéli-phase was found at the grain boundary in the specimen for x=0.98, and Ruddlesden-Popper-type insertion layers in that for x=1.02. The crystallographic site occupancy of impurity atoms determined by ALCHEMI (Atom Location using CHanneling-Enhanced MIcroanalysis), showed that the excess TiO2 was precipitated as Magnéli phase in the specimen for x=0.98, and similarly that excess SrO formed insertion planar faults of NaCl type in that for x=1.02. No anti-site point defects were formed in either of the specimens.