Investigation of the Behavior of Nickel Impurity Atoms in the Silicon Lattice Based on First Principles

This work presents a comprehensive theoretical and experimental study of the behavior of nickel impurity atoms in the silicon crystal lattice. The focus is on analyzing diffusion mechanisms, the energetic characteristics of interstitial nickel atoms, their interaction with defects and other impurities, as well as the formation of stable clusters within the crystal volume. First-principles quantum mechanical modeling was employed using the QuantumATK software, applying the Linear Combination of Atomic Orbitals (LCAO) method and the Local Density Approximation (LDA) exchange-correlation functional. Special attention was given to calculating the binding energy of nickel atoms in interstitial sites and estimating the activation energy for their migration along various crystallographic directions ([100] and [010]). The modeling results revealed that nickel atoms predominantly diffuse in interstitial positions with an activation energy around 0.31 eV, which aligns well with previously reported experimental data. It was found that interactions of nickel with oxygen, carbon impurities, and point defects have a minimal impact on diffusion processes. However, interactions with vacancies lead to the formation of stable nickel silicides and cause an increased concentration of nickel near the surface. The experimental part of the study confirmed the formation of nickel clusters under high-temperature treatments. Scanning Electron Microscopy (SEM), Secondary Ion Mass Spectrometry (SIMS), and Infrared (IR) microscopy revealed a high density and uniform distribution of clusters throughout the crystal volume. Cluster sizes ranged from 20 to 100 nm, with concentrations reaching approximately 1010 cm-3. These findings demonstrate that nickel acts as an effective gettering impurity, enhancing the electrophysical properties of silicon. The results provide valuable insights for optimizing the fabrication processes of high-efficiency silicon solar cells and microelectronic devices.

Cu3BiS3 (CBS) has emerged as a promising earth-abundant absorber for thin-film photovoltaics, offering a sustainable alternative to conventional technologies. However, ab initio studies on its optoelectronic properties remain scarce and often yield contradictory results. This study systematically examines the influence of two density functional theory (DFT) methodologies, linear combination of atomic orbitals (LCAO) and projector augmented wave (PAW), on the structural and electronic properties of CBS, aiming to establish a reliable computational framework for future research. With this in mind, we also assessed the impact of a wide range of exchange-correlation (XC) functionals within both methods, including 6 from the local density approximation (LDA) family (HL, PW, PZ, RPA, Wigner, XA), 10 from the generalized gradient approximation (GGA) family (BLYP, BP86, BPW91, GAM, KT2, PBE, PBEsol, PW91, RPBE, XLYP), 2 meta-GGA functionals (SCAN, R2SCAN), and the hybrid HSE06 functional. Both LCAO and PAW consistently predict an indirect bandgap for CBS across all XC functionals, aligning with most previous DFT studies but contradicting experimental reports of a direct transition. The LDA and meta-GGA functionals systematically underestimated the CBS bandgap (<1 eV), with further reductions upon structural relaxation. GGA functionals performed better, with BLYP and XLYP yielding the most experimentally consistent results. The hybrid HSE06 functional substantially overestimated the bandgap (1.9 eV), with minimal changes after relaxation. The calculated hole and electron effective masses reveal strong anisotropy along the X, Y, and Z crystallographic directions. Additionally, CBS exhibits an intrinsic p-type nature, as the Fermi level consistently lies closer to the valence band maximum across all methods and functionals. However, the PAW method generally predicted more accurate lattice parameters than LCAO; the best agreement with experimental values was achieved using the PW91 (1.2% deviation) and HSE06 (0.9% deviation) functionals within LCAO. Based on these findings, we recommend the PW91 functional with LCAO for structural optimizations in large supercell studies of CBS dopants and/or defects and BLYP/XLYP for electronic properties.

Atomic and electronic structure of both perfect and nanostructured Ni(111) surfaces: First-principles calculations

We present results from ab-initio, self consistent, local density approximation (LDA) calculations of electronic and related properties of cubic boron nitride (zb-BN). We employed the Ceperley and Alder LDA potential and the linear combination of atomic orbitals (LCAO) formalism in our non-relativistic computations. We solved the system of LDA equations self-consistently, through the implementation of the LCAO formalism, following the Bagayoko, Zhao, and Williams (BZW) method as enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method includes a methodical search for the optimal basis set that yields the absolute minima of the occupied energies. This search entails increasing the size of the basis set and the related modifications of angular symmetry and of radial orbitals. Our calculated, indirect band gap of 6.48 eV, from the Γ to the Χ points, and bulk modulus of 375 GPa are in excellent agreement with corresponding experimental values at room temperature (RT). The calculated widths of the lowest valance band and that of the entire valence bands of 5.65 eV and 20.26 eV are in excellent agreement with the measured values of 5.5 eV and 20 eV, respectively. We have also calculated electron and hole effective masses for zb-BN, and the total (DOS) and partial (pDOS) densities of states.  Â

The formation of stable clusters in colloidal systems is usually explained by the combination of a short ranged attractive force and a repulsive force with an interaction range in the order of the colloidal particle size. Using light scattering techniques we studied five different liposomic systems undergoing aggregation through five different mechanisms: aggregation by charge neutralization, DLCA and RLCA by charge screening, aggregation in a secondary minimum, depletion attraction and “patchy” electrostatic interactions. Surprisingly, three of these systems lead to the formation of stable clusters despite the Debye length of the electrostatic repulsion being less than 5% of the diameter of the liposomes. Alternative explanations for the stabilization of the clusters are discussed using non-DLVO forces. The understanding and controlling of the formation of stable clusters of liposomes may have an important impact in their application as a drug delivery system.

Single electron band structure techniques have been applied successfully to the interpretation of the near edge structures of metals and other materials. Among various band theories, the linear combination of atomic orbital (LCAO) method is especially simple and interpretable. The commonly used empirical LCAO method is mainly an interpolation method, where the energies and wave functions of atomic orbitals are adjusted in order to fit experimental or more accurately determined electron states. To achieve better accuracy, the size of calculation has to be expanded, for example, to include excited states and more-distant-neighboring atoms. This tends to sacrifice the simplicity and interpretability of the method.In this paper. we adopt an ab initio scheme which incorporates the conceptual advantage of the LCAO method with the accuracy of ab initio pseudopotential calculations. The so called pscudo-atomic-orbitals (PAO's), computed from a free atom within the local-density approximation and the pseudopotential approximation, are used as the basis of expansion, replacing the usually very large set of plane waves in the conventional pseudopotential method. These PAO's however, do not consist of a rigorously complete set of orthonormal states.

Formation of stable clusters in colloidal suspensions

Electronic structure of Ni2TiAl: Theoretical aspects and Compton scattering measurement

The first ever Compton profile of polycrystalline CdMoO4 has been measured using 137Cs spectrometer. The results are compared with theoretical Compton profiles deduced from free atom and linear combination of atomic orbitals (LCAO) methods. We have also computed the energy bands using density functional theory (DFT) within LCAO. The computed bands confirm the semiconducting behaviour of this compound. It is seen that the DFT theoretical profile (with local density approximation) gives a better agreement with the experimental Compton data than free atom Compton profile.

This study presents a solution to an issue which became prominent due to the discovery of copper (Cu) oxides in 1986, namely, whether LDA (local density approximation) can describe antiferromagnetism. From an early stage, many LDA band structure calculations failed to reproduce the insulating antiferromagnetic state. The Hubbard model predicts antiferromagnetism in a system under appropriate conditions. The author’s LDA calculations were performed for elongated hydrogen molecules comprising multiple atoms using the discrete variational (DV) molecular orbital method. The LDA employed is the original Kohn–Sham formalism, since the magnetic properties by GGA (generalized gradient approximation) are closer to the original Kohn–Sham results than those obtained by VWN (Vosko–Wilk–Nusair) approximation. The DV method, with a basis set of numerically calculated atomic orbitals, derived the antiferromagnetic state for hydrogen molecules at long interatomic separations but, when used for Cu oxide molecules, was seemingly unable to describe antiferromagnetism, where a well potential with a usual depth of about −1 Eh within an ionic radius was added solely to the potential for generating basis atomic orbitals of O2−. However, the author finally achieved the antiferromagnetism description via a reduced well potential depth following long parameter surveys. The calculation was generalized to a periodic system CaCuO2 using a method employing Bloch-type linear combinations of atomic orbitals with all electrons. Furthermore, we determined a spherically averaged well potential depth having originated from the Coulomb potential by the nucleus and electron clouds around O2− in a solid. The system revealed antiferromagnetic ordering due to a shallow well depth, and since the well for the anionic basis set is induced by the Coulomb potential in general, this method is applied to molecular orbital calculations.KeywordsIonic RadiusHubbard ModelLocal Density ApproximationAtomic OrbitalAntiferromagnetic StateThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Recently there has been renewed interest in implementations of density-functional theory for solids using various types of localized basis sets, including atom-centered Gaussian-type functions. While such methods are clearly well adapted to most insulating and semiconducting systems, one might expect them to give a less-than-optimal description of metals relative to plane-wave-type methods. Nevertheless, several successful applications of local-basis methods to metals have recently been reported. Here, we report an application of our Gaussian linear combination of atomic orbitals (LCAO) code to some extremely free-electron-like metals, namely, the alkali metals Li, Na, and K. In agreement with other calculations (both local and plane wave) we find that the local-density approximation (LDA) lattice constants are relatively poor ({approximately}{minus}3{percent} from experiment for the alkali metals versus {plus_minus}1{percent} for many other solids) and that the LDA bulk moduli are {approximately}30{percent} too high. We find that the Perdew-Burke-Enzerhof (PBE) version of the generalized-gradient approximation (GGA) corrects most of this error, in agreement with earlier calculations using similar GGA functionals. The Becke-Lee-Yang-Parr GGA functional gives similar results for the alkali-metal equations of state but is found to overcorrect the errors of the LDA for the cohesive energies, for which the PBE functional is in better agreementmore » with experiment. Our results indicate that the Gaussian-LCAO method should be able to give accurate results for nearly any crystalline solid, since it succeeds even where it would be expected to have the most difficulty. {copyright} {ital 1998} {ital The American Physical Society}« less

8 - Electronic Structure

We report accurate, calculated electronic, transport, and bulk properties of zinc blende gallium arsenide (GaAs). Our ab-initio, non-relativistic, self-con-sistent calculations employed a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. We strictly followed the Bagayoko, Zhao, and William (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). Our calculated, direct band gap of 1.429 eV, at an experimental lattice constant of 5.65325 A, is in excellent agreement with the experimental values. The calculated, total density of states data reproduced several experimentally determined peaks. We have predicted an equilibrium lattice constant, a bulk modulus, and a low temperature band gap of 5.632 A, 75.49 GPa, and 1.520 eV, respectively. The latter two are in excellent agreement with corresponding, experimental values of 75.5 GPa (74.7 GPa) and 1.519 eV, respectively. This work underscores the capability of the local density approximation (LDA) to describe and to predict accurately properties of semiconductors, provided the calculations adhere to the conditions of validity of DFT.

We present measurements of electron momentum density (EMD) of V2O5 employing a lowest intensity 100 mCi 241Am Compton spectrometer. The experimental Compton profile is compared with those deduced using density functional theory with local density approximation (LDA) and revised functional of Perdew-Burke-Ernzerhof for solids (PBEsol) as facilitated in the linear combination of atomic orbitals (LCAO) method. On the basis of goodness of fitting, we observed that the theoretical EMDs derived using PBEsol scheme shows a close agreement with the experimental EMD than that of LDA. Going above EMDs, we have also computed the energy bands and density of states of V2O5 using PBEsol scheme. An analysis of energy bands shows the valence band maximum at T point and conduction band minimum at Γ point of the Brillouin zone. Calculated indirect energy band gaps of V2O5 are 1.63 and 1.58 eV using PBEsol and LDA schemes, respectively. Therefore, the band gap derived using PBEsol scheme is close to experimental band gap value 2.3 eV, confirming the use of applicability of PBEsol model for V2O5 type of materials.

Ab initio study of incommensurately modulated crystals