First-principles Total-energy Calculations Research Articles

The role of carbon segregation in the electrical activity of dislocations in carbon doped GaN

Dislocations have been proposed to affect the performance and reliability of GaN power semiconductors by being conductive pathways for leakage current. However, no direct evidence of a link between their electrical behavior and physical nature in carbon-doped semi-insulating GaN buffer layers has been obtained. Therefore, we investigate the electrical activity of dislocations by conductive atomic force microscopy and electron beam induced current to distinguish electrically active dislocations from non-active ones. We investigated six electrically active dislocations and discovered distinct carbon enrichment in the vicinity of all six dislocations, based on cross-sectional scanning transmission electron microscopy using electron energy loss spectrometry. Electrically non-active dislocations, which are the vast majority, sometimes also showed carbon enrichment, however, in only two out of seven cases. Consequently, carbon segregation seems to be a requirement for electrical activity, but a carbon surplus is not sufficient for electrical activity. We also performed first-principles total-energy calculations for mixed type threading dislocations, which validates carbon accumulation in the dislocation vicinity. The electrical and physical characterization results, complemented by density functional theory simulations, support the previously hypothesized existence of a carbon defect band and add new details.

A first-principles study of the phase transitions in ultrahigh temperature shape memory alloy RuNb

Ultrahigh temperature shape memory alloys (UHT-SMAs) have transition temperatures above 600 °C. They are needed for sensing and actuating devices in aerospace applications. However, very few such UHT-SMAs have been found. Among them are Ru-based alloys such as RuNb and RuTa, whose martensite structures and phase transitions are totally different from those of NiTi-based SMAs and were poorly understood. In this work, we carried out a systematical study of RuNb using first-principles total energy calculations and molecular dynamics (MD) simulations. We revealed the transition paths and mechanisms in cubic → tetragonal → monoclinic transitions. We determined the transition sequence and martensitic transition temperatures (MTTs) by evaluating the Gibbs free energies using thermodynamic integration. The calculated MTTs are in very good agreement with the experimental data. We also found that the monoclinic phase at the second transition has the P21/m symmetry instead of experimentally identified P2/m. The insights gained by this study and the verified ab initio methods for accurate MTT calculations can be applied to fast screen and quantitatively design novel UHT-SMAs having similar properties with desirable MTTs and much reduced cost.

Modeling the manganese deposit on the BP (111)-(2×2) surface: Density functional theory studies

The structural, electronic, and magnetic properties of the manganese (Mn) adsorption and incorporation into the boron phosphide (BP) in surface (111) – periodicity (2 × 2) structure have been investigated using spin-polarized first-principles total-energy calculations. The exchange-correlation energies are treated within the generalized gradient approximation. The magnetic properties have been studied by considering different coverages and magnetic configurations. The Mn adsorption and incorporation range from ¼ to 1 monolayer (ML), with results displaying: Ferromagnetic (FM), antiferromagnetic (AFM), and non-magnetic (NM) behavior at low, intermediate, and high coverages, respectively. The electronic properties are explored considering the total density of states (DOS) and the projected density of states (PDOS). It is noted that the contribution of the Mn-d orbital modifies the metallicity of the surface. The results show that the most favorable structure is the 1 Mn ML adsorption, with an AFM alignment and a magnetic moment of 0 μB/atom. The material exhibits metallicity at the first monolayer induced by manganese, while the lower internal layers are semiconductors. Finally, the charge density distribution corroborates the surface metallic phase, which indicates that the proposed material can be helpful in the design of new spintronic devices.

Site-selective cobalt substitution in La–Co co-substituted magnetoplumbite-type ferrites: 59Co-NMR and DFT calculation study

The La–Co co-substituted magnetoplumbite-type (M-type) ferrites AFe12O19 (A = Ca, Sr and Ba, ion sizes Ca2+ < Sr2+ < Ba2+) with Co compositions around 0.2 have been subjected to 59Co-NMR. The results show that Co occupies the 4f1, 2a and 12k sites, and that the smaller the A ion, the more Co tends to occupy the 4f1 minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped AFe12O19 and a supercell ( 2×2×1 of the unit cell) in which 1/96 of Fe3+ is replaced by Co2+ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of A, Co is most stable when it occupies the 4f1 site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f2 sites. As the A ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f1 site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the c-axis associated with the difference in A. However, the influence of the A ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La–Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small A ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f1 site in tetrahedral coordination.

Stoichiometric and Nonstoichiometric Surface Structures of Pyrochlore Y2Zr2O7 and Their Relative Stabilities: A First-Principles Investigation

First-principle total energy calculations were performed to investigate the atomic structures and relative stabilities of two low miller-index surfaces of pyrochlore Y2Zr2O7. The stoichiometric Y2Zr2O7 (110) and (100) surfaces were predicted, with lowest formation energies of 1.20 and 1.47 J/m2, respectively. Based on a thermodynamic defect model, non-stoichiometric Y2Zr2O7 surface energies were further evaluated as a function of environmental oxygen partial pressure (pO2) and temperature (T). With all of the results, we were able to construct the surface phase diagrams for T = 300 and 1400 K. The strong correlation between the structural stabilities and the surface stoichiometry was revealed as varying T and pO2. At a given T, the most stable termination of the (110) surfaces would change from a (Y,Zr)−rich (ns−2Y2Zr6O) to O−rich ones (ns−4O_2 and ns−4O_1) as increasing pO2, while that of the (100) surfaces would change from the stoichiometric (stoi−1Y1Zr_1) to the O−rich one (ns−5O). The critical pO2 value for termination transition moves to its higher end as increasing T.

Effects of transition metal segregation on the thermodynamic stability and strength of Ni Σ11 [110](113) symmetrical tilt grain boundary

Precisely controlling grain boundary (GB) thermodynamic stability and strength is important for processing nanograined alloys. The segregation-induced stabilization and strengthening GB mechanisms of 20 transition metal elements in Ni Σ11 [110](113) STGB with low interfacial excess energy were clarified from first-principles total-energy calculations in this work. The results indicate that the segregation energy and GB energy are inversely related to the radius difference between TM and matrix Ni atoms. The segregation of 11 elements were identified to enable the Ni Σ11 [110](113) STGB to be stable according to Schuh criterion for GB stability. The electronic mechanism of GB stabilization through TM segregation is mainly due to the less anti-bonding states of Ni-X pairs than those of Ni–Ni pairs in unsegregated GB, from the analysis of crystal orbital Hamiltonian populations. The GB embrittlement resulted from TM segregation is mainly from the mechanical contribution caused by atomic size mismatch between segregation atom and matrix atom. The segregation caused GB strengthening comes from the chemical contribution, which is due to the fact that the segregation of X leads to the formation of stronger Ni-X covalent-like bonds instead of Ni–Ni metallic bonds. This work could provide better understanding on designing high-stable Ni nanograined alloys.

Theoretical investigation of the MXene precursors MoxV4-xAlC3 (0 ≤ x ≤ 4)

By first-principles total-energy calculations, we investigated the thermodynamic stability of the MAX solid solution MoxV4-xAlC3 in the 0 ≤ x ≤ 4 range. Results evidence that lattice parameter a increases as a function of Mo content, while the c parameter reaches its maximum expansion at x = 2.5. After that, a contraction is noticed. Mo occupies VI sites randomly until the out-of-plane ordered Mo2V2AlC3 alloy is formed. We employed the Defect Formation Energy (DFE) formalism to evaluate the thermodynamic stability of the alloys. Calculations show five stable compounds. At V-rich conditions and from Mo-rich to Mo-moderated conditions, the pristine V4AlC3 MAX is stable. In the region of V-poor conditions, from Mo-rich to Mo-moderated growth conditions, the solid solutions with x = 0.5, 1, and 1.5 and the o-MAX Mo2V2AlC3 are thermodynamically stable. The line profiles of the Electron Localization Function and Bader charge analysis show that the V-C interaction is mainly ionic, while the Mo-C is covalent. Also, the exfoliation energy to obtain a MXene layer is ~ 0.4 eV/Å2. DFE also shows that MXenes exfoliated from the MAX phase with the same Mo content and atomic arrangement are thermodynamically stable. Our results get a deeper atomic scale understanding of the previously reported experimental evidence.

The effect of solute segregation on stability and strength of Cu symmetrical tilt grain boundaries from the first-principles study

Grain boundary (GB) stability and strength are two essential points for nanocrystalline materials. In this study, the effects and underlying mechanisms of segregation-induced GB stabilization and GB strengthening in various Cu GBs for selected 8 alloying elements (Mg, Ca, Cr, Ni, Zn, Zr, Ag and Sn) were investigated through first-principles total energy calculations. The segregation energy results show that all other elements excluding Ni can segregate to GB thus improving the GB stability. The segregation driving force is associated with the volume of Voronoi cell of solute, in the trend that increases along with the volume of Voronoi cell growth. The stabilizing mechanism should attribute to the decrease in the number of antibonding electrons. As to GB strength, the embrittling potency results show that Mg, Cr, Zr and Ag are enhancers in Σ5 [001](310) GB, while for Σ5 [001](210) and Σ11 [110](113) GBs, only Cr and Zr are enhancers. The embrittling potency is concerned with segregation energy and the volume of Voronoi cell of solute, in a trend that increases with the segregation energy decline and the volume of Voronoi cell growth. The enhancing mechanism lies in the formation of covalent-like bonds between solute and matrix atoms. Furthermore, by comparing the stabilizing and strengthening effectiveness of alloying elements in various Cu GBs. The trend that the worse the stability and strength of GBs, the better the stabilizing and strengthening effects of alloying elements were revealed.

Electronic and elastic properties of BN, AlN and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at r point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1 – D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones. © 1998 American Institute of Physics. [S0021-8979(98)07921-3]

Microscopic identification of stepped SiC(0001) and the reaction site of hydrogen-rich epitaxial growth

We report first-principles total-energy electronic-structure calculations, based on density-functional theory, that unveil detailed atomic structures of hydrogen-adsorbed step edges and their energetics of the silicon carbide (SiC) (0001) surface. The obtained adsorption energy of the hydrogen atom at each step reveals the microscopic reason for the step morphology on the Si-face SiC(0001) surfaces which are commonly inclined toward the $\ensuremath{\langle}11\overline{2}0\ensuremath{\rangle}$ direction in epitaxial growth. The calculated hydrogen coverages at each step and also on the surface terrace clearly identify the favorable reaction sites for the epitaxial growth, such as chemical vapor deposition (CVD) in which hydrogen is ubiquitous. The obtained results provide a firm theoretical framework to discuss the atom-scale mechanism of the epitaxial growth of SiC.

Electrostatic properties of two-dimensional C60 polymer thin films under an external electric field

Electrostatic properties of different C60 thin films under external electric fields have been investigated from first-principle total-energy calculations. Density functional theory calculations combined with the effective screening medium method reveal that the electrostatic properties of C60 thin films in an electric field strongly depend on the arrangement and conformation of the C60 molecules. The relative permittivity across the thin films exhibits clear a positional dependence resulting from the π electron distribution within the films. An electrostatic polarization is uniformly induced by weak electric fields, typically 0.1 V nm−1, because of the semiconducting electronic structure of the thin films, whereas the polarization is highly concentrated in the outermost C60 layer under strong electric fields of 0.5 V nm−1.

Phase diagrams and critical temperatures for coherent and incoherent mixtures of InAs1−xSbx alloys using first-principles calculations

Phase diagram calculations are performed for incoherent and coherent mixtures of an InAs1−xSbx (InAsSb) ternary alloy, which is an important material for the applications to infrared detector technology. Our calculations are based on the cluster expansion approach and Monte Carlo simulations combined with first-principles total energy calculations in the framework of density functional theory with Perdew–Burke–Ernzerhof (PBE) and Heyd–Scuseria–Ernzerhof (HSE) exchange-correlation functionals. Because of a lattice mismatch (∼7%) between InAs and InSb, coherency strain plays an important role for the phase stability of the InAsSb alloys. The alloys without the coherency strain (incoherent mixtures) show a miscibility gap with the critical temperature at ∼700 K with 42% (45%) Sb concentration in PBE (HSE), which is in good agreement with the experimentally determined equilibrium miscibility gap temperature. The alloys with the coherency strain (coherent mixtures) show several ground states whose structures are short period superlattices along the [201] direction. The critical temperature is ∼200 K with 50% Sb concentration in both PBE and HSE, which is reduced by ∼500 K compared to that of incoherent mixtures. This reduction of the critical temperature is consistent with the experimental observation where the homogeneous InAsSb alloy continues to grow inside the empirical miscibility gap.

Electronic and elastic properties of BN, AlN and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at r point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1 – D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones. © 1998 American Institute of Physics. [S0021-8979(98)07921-3]

Equilibrium phase diagrams of isostructural and heterostructural two-dimensional alloys from first principles

SummaryAlloying is a successful strategy for tuning the phases and properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). To accelerate the synthesis of TMDC alloys, we present a method for generating temperature-composition equilibrium phase diagrams by combining first-principles total-energy calculations with thermodynamic solution models. This method is applied to three representative 2D TMDC alloys: an isostructural alloy, MoS2(1-x)Te2x, and two heterostructural alloys, Mo1-xWxTe2 and WS2(1-x)Te2x. Using density-functional theory and special quasi-random structures, we show that the mixing enthalpy of these binary alloys can be reliably represented using a sub-regular solution model fitted to the total energies of relatively few compositions. The cubic sub-regular solution model captures 3-body effects that are important in TMDC alloys. By comparing phase diagrams generated with this method to those calculated with previous methods, we demonstrate that this method can be used to rapidly design phase diagrams of TMDC alloys and related 2D materials.

Modeling the cobalt deposit on the AlN (0001)-(2 × 2) surface: Density functional theory studies

Structural, electronic and magnetic properties of the cobalt (Co) adsorption and incorporation on the AlN (0001)-(2 × 2) surface have been investigated using spin-polarized first-principle total-energy calculations. The exchange–correlation energies are treated within the generalized gradient approximation. Provided that Co is a transition metal with highly correlated electrons, the Hubbard correction is considered with U = 1 eV. The magnetic properties have been investigated considering different magnetic configurations and different coverages. In the Co adsorption, a ferromagnetic (FM) behavior is obtained for coverages from ¼ monolayer (ML) to 1 ML. Concerning the Co incorporation, we observed: FM, antiferromagnetic (AFM) and non-magnetic (NM) behavior at low, intermediate and high coverages, respectively. In the Co substitution, the ¼ an ¾ ML of coverage are FM and the ¼ and ¾ ML of coverage are AFM. We employed the surface formation energy formalism to investigate the thermodynamic stability of the different surfaces. According to the formalism, Co deposit takes place at Co-rich and Al-poor conditions. The DOS and PDOS indicate that the 1st ML of the surfaces is metallic and the lower layers are semiconductor.

Investigating the magnetic and atomic interface configuration for a model Fe/CrN bilayer system

A bilayer of iron on chromium nitride (Fe/CrN) is an interesting system for exchange biasing and sensing applications as the Néel temperature of CrN is 280 K and the Curie temperature of Fe is 1043 K. In this paper, we study the crystal and magnetic structures of the Fe/CrN interface at the atomic level. High quality epitaxial Fe/CrN bilayers prepared by molecular beam epitaxy grow in 001 orientation on MgO(001) substrates with uniform layer thicknesses and sharp interfaces. Our data reveal the epitaxial correlation between Fe and CrN crystals and their magnetic structures at the interface. The magnetic anisotropy directions of Fe and CrN are found parallel to [110]MgO. We studied the electronic and magnetic properties of the interface by performing the first-principles total-energy calculations. We present a model that combines the crystal and magnetic structures of the Fe/CrN bilayer and fully explains all results.

First Principles Study of High-Pressure Phases of ScN

We report the results of first-principles total-energy calculations for structural properties of scandium nitride (ScN) semiconductor compound in NaCl-type (B1), CsCl-type (B2), zincblende-type (B3), wurtzite-type (B4), NiAs-type (B81), CaSi-type (Bc), B-Sn-type (A5), and CuAu-type (L10) structures. Calculations have been performed with the use of the all-electron full-potential linearized augmented plane wave FP-LAPW method based on density-functional theory (DFT) in the generalized gradient approximation (GGA) for the exchange correlation energy functional. We predict a new phase transition from the most stable cubic NaCl-type structure (B1) to the B-Sn-type one (A5) at 286.82 GPa with a direct band-gap energy of about 1.975 eV. Our calculations show that ScN transforms from the orthorhombic CaSi-type structure (Bc) to A5 at 315 GPa. In agreement with earlier ab initio works, we find that B1 phase transforms to Bc, L10, and B2 structures at 256.27 GPa, 302.08 GPa, and 325.97 GPa, respectively. The electronic structure of A5 phase shows that ScN exhibits a direct band-gap at X point, with Eg of about 1.975 eV.

Site preference and atomic ordering in the ternary Rh5Ga2As: first-principles calculations

Abstract The site preference and atomic ordering of the ternary Rh5Ga2As have been investigated using first-principles density functional theory (DFT). An interesting atomic ordering of two neighboring elements Ga and As reported in the structure of Rh5Ga2As by X-ray diffraction data only is confirmed by first-principles total-energy calculations. The previously reported experimental model with Ga/As ordering is indeed the most stable in the structure of Rh5Ga2As. The calculation detected that there is an obvious trend concerning the influence of the heteroatomic Rh–Ga/As contacts on the calculated total energy. Interestingly, the orderly distribution of As and Ga that is found in the binary GaAs (Zinc-blende structure type), retained to ternary Rh5Ga2As. The density of states (DOS) and Crystal Orbital Hamiltonian Population (COHP) are calculated to enlighten the stability and bonding characteristics in the structure of Rh5Ga2As. The bonding analysis also confirms that Rh–Ga/As short contacts are the major driving force towards the overall stability of the compound.

Microscopic mechanism of adatom diffusion on stepped SiC surfaces revealed by first-principles calculations

We report first-principles total-energy calculations based on real-space density-functional theory that unveil the atom-scale mechanisms of surface diffusion of adatoms on the Si-faced 3C-SiC(111) stepped surface. The quality of the epitaxial layer of SiC affects the device performance. Therefore, fundamental knowledge of the microscopic mechanisms of epitaxial growth is crucial for the improvement of the quality of SiC power devices. However, adatom diffusion on the growing stepped SiC surfaces is still unknown. We identify diffusion pathways for three important adatom species of SiC chemical vapor deposition (CVD), Si, C, and H, and obtain the corresponding energy profiles for both on the surface terraces and near the surface steps, providing a complete picture of the adatom diffusion. We find that the Si adatom is most mobile on the terrace and shows prominent Ehrlich-Schwoebel (ES) effect in the inter-terrace diffusion, whereas the C and H adatoms show less ES effect and in an ascending diffusion even the inverse ES effect appears for the C adatom. The results obtained are fundamentals to explore the microscopic mechanism of the epitaxial growth on 3C-SiC(111) surfaces and equivalently hexagonal SiC(0001) surfaces.

Microstructure and properties of NbVZr refractory complex concentrated alloys

The microstructure and mechanical properties of the equiatomic NbVZr alloy are reported. The laser-processed and as-cast samples both exhibit a dendritic BCC solid solution with hexagonal C14 and cubic C15 Laves phases in the interdendritic regions. The stability of the Laves phases is discussed based on first-principles total energy calculations. Nb is found to increase the stability of C14 and C15 phases. The Laves phases strengthen the material by acting as obstacles to dislocation motion, as evidenced by nanoindentation and TEM observations of deformed samples. A high concentration of stacking faults was observed in the Laves region, which confirms the low stacking fault energy predicted by first-principles calculations, and are expected to result in additional plasticity. This work serves as an example of a systematic study of the stability of intermetallic phases in refractory complex concentrated alloys using a high-throughput synthesis technique. Such phases play an important role in the overall mechanical properties of the alloy.