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In situ low-energy electron microscopy is used to investigate the near-equilibrium dynamics of surface-terminated dislocations during annealing of atomically smooth TiN(111) layers at temperatures between 1600 and 1735 K. At each temperature the nucleation and growth of spiral steps rotating around dislocation cores is observed.

We present an ab initio study of the diffusion profiles in CdTe of native, Te adatom and vacancy, and anionic non-native interstitial adatoms P, Sb, O, S, and Cl. A high symmetry Wyckoff position, 4(d) site, happens to be a global minimum energy location, only for O and Cl interstitials. Adatoms of P, Sb and S show an asymmetric shape of the energy diffusion barrier with two minima and two maxima in the pathway. The others, O, Cl, and Te interstitial and vacancy, show a symmetric diffusion barrier with a unique maximum and minimum. Diffusion for Te and S interstitials proceeds along the [1 1 0] channel in the crystal in a near straight line path. Diffusion for O and Cl proceeds along two nearly straight line paths along [1 1 1] and [1 1 –1]. Diffusion for P and Sb are along the [1 1 0] channel however they deviate from the straight line paths along [1 1 1] and [1 1 –1]. The rate-limiting diffusion barriers range from a low of 0.49 eV for the asymmetric diffusion path of an Sb interstitial to a high of 1.51 eV for the symmetric diffusion path of an O interstitial. The rate-limiting barriers for the others are 0.65 eV for S, 0.68 eV for both Cl and P, 1.37 eV for Te interstitial and 1.42 eV for the Te vacancy. These barriers are in agreement with the available experimental data for interstitials Te 1.40±0.02 eV, Cl 0.63±0.10 eV and S 0.64±0.02 eV. The symmetric or asymmetric nature of the diffusion path as well as the bond length and atomic coordination at the energetic-extrema positions influence the size of the diffusion energy barrier. In addition there exist two electronic signatures in the local density of states: one for the bond breaking in the symmetric diffusion barrier paths and the other in the difference in hybridization between the global minimum and global maximum energy positions for asymmetric diffusion barriers. This work should serve as a motivation for experimental verification of other barriers and elucidates the diffusion mechanisms very difficult to discover experimentally. [ABSTRACT FROM AUTHOR]

We investigate the impact of structure relaxation on the upper performance of a silicon nanowire metal-oxide-semiconductor field-effect-transistor (MOSFET) with small diameter employing a semiclassical transport model to calculate its ballistic I-V characteristics. For wires along the lang110rang axis and 1 nm diameter, structure relaxation induces large changes in the bond length of silicon atoms at the surface (~33%). Despite these bond length variations, the effect of reconstruction on the ballistic ON-current of Si-NW MOSFETs turns out negligible in the case considered, which can be attributed to an only slight variation of the electron effective mass after reconstruction [ABSTRACT FROM PUBLISHER]

We employ first principles methods based on density functional theory and beyond to study Zn x Cd 1-x Te, 0 ≤ x ≤ 1 , alloys in the zinc blende (B3) crystal structure. Cluster Expansion and Monte Carlo formalisms were deployed to provide a phase diagram determining consolute temperature of 387 K at 40% Zn concentration. Opto-electronic properties are computed with the hybrid HSE06 functional for disordered Zn x Cd 1-x Te alloys, which were simulated using special quasi-random structures. Bowing effects in the bandgap and effective carrier masses were observed in alloying which can be attributed to local geometrical distortions as portrayed by bond length distributions. Downward bowing in the electronic bandgap will be beneficial in photovoltaic applications through increased net photocurrent. Absorption coefficients show robust absorption in Zn x Cd 1-x Te as indicated by substantial optical absorption throughout all Zn concentrations, aided by low reflectivity that ensures a high absorption. Presence of short-range order in Zn 0.25 Cd 0.75 Te was observed through clustering and anti-clustering among different cationic species of Zn and Cd. It has a value of 0.11 at 1000 K compared to −0.79 at 400 K. The bandgap of Zn 0.25 Cd 0.75 Te at 1000 K was found to be slightly higher, 0.05 eV, than at 400 K, consistent with the value of short-range order. Thus, short-range order and possibly composition can be used in the design and synthesis of the appropriate solar cell, thereby improving the performance in this system. • T-x phase diagram calculation of Zn x Cd 1-x Te shows consolute temperature of 387 K. • Opto-electronic properties are computed with the hybrid HSE06 functional. • Downward bowing in the electronic bandgap and effective charge carrier masses. • Zn 0.25 Cd 0.75 Te has the best band gap of 1.49 eV for absorber layer in solar cells. • Short-range order observed in Zn 0.25 Cd 0.75 Te through clustering and anti-clustering. [ABSTRACT FROM AUTHOR]

NbC x N 1− x layers were deposited on MgO(001) by reactive magnetron co-sputtering from Nb and graphite targets in 5 mTorr pure N 2 at T s = 600–1000 °C. The anion-to-Nb ratio of 1.09 ± 0.05 is independent of T s and indicates a nearly stoichiometric rock–salt structure Nb(N,C) solid solution. In contrast, the C-to-N ratio increases from 0.20–0.59 for T s = 600–1000 °C, which is attributed to a low C sticking probability at high N surface coverage at low T s . Layers grown at T s ≥ 700 °C are epitaxial single-crystals with a cube-on-cube relationship to the substrate, (001) NbCN ||(001) MgO and [100] NbCN ||[100] MgO , as determined from X-ray diffraction θ –2 θ and ϕ -scans. Reciprocal space mapping on a NbC 0.37 N 0.63 layer deposited at T s = 1000 °C indicates an in-plane compressive strain of − 0.4% and a relaxed lattice constant of 4.409 ± 0.009 Å. The lattice constant of NbC x N 1− x increases with x , consistent with a linear increase predicted by first-principles density functional calculations. The calculated bulk modulus, 307 GPa for NbN and 300 GPa for NbC, is nearly independent of x . Similarly, c 11 increases slightly from 641 to 666 GPa, but c 12 decreases considerably from 140 to 117 GPa, and c 44 more than doubles from 78 to 171 GPa as x increases from 0 to 1, indicating a transition from ductile NbN to brittle NbC. This also results in an increase in the predicted isotropic elastic modulus from 335 to 504 GPa, which is in good agreement with the measured 350 ± 12 GPa for NbC x N 1− x (001) with x = 0.19–0.31. The hardness H = 22 ± 2 GPa of epitaxial NbC x N 1− x layers is nearly independent of x = 0.19–0.37 and T s = 700–1000 °C, but is reduced to H = 18.2 ± 0.8 GPa for the nanocrystalline layer deposited at T s = 600 °C. The electrical resistivity decreases strongly with increasing T s < 800 °C, due to increasing crystalline quality, and is 262 ± 21 μΩ-cm at room temperature and 299 ± 22 μΩ-cm at 77 K for T s ≥ 800 °C, indicating weak carrier localization due to the random distribution of C atoms on anion sites. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ab initio modeling of kesterite phase for full range of Cu2ZnSn(S x Se1−x)4 was done. [•] Lattice constants increase with the decrease of S/Se ratio. [•] All elastic constants and moduli except C16 decrease with the decrease of S/Se ratio. [•] Poisson’s ratio stays the same with the decrease of S/Se ratio. [Copyright &y& Elsevier]

Highlights: [•] We did ab initio calculations on 29 transition metal nitrides (TMN) in NbO structure. [•] Five of 10 3d TMNs have similar energetic stability as their rocksalt counterparts. [•] Large variations in mechanical properties are found between the two structures. [•] CrN, MoN and WN in NbO structure show values of hardness (HV ) larger than 20GPa. [•] HV is anti-correlated with density of electronic states at Fermi energy. [ABSTRACT FROM AUTHOR]

Abstract: Several III-VI body-centered tetragonal layered compounds belonging to space group I41/amd have been a subject of interest recently because of their potential applications in high efficiency and environmentally friendly copper–indium–gallium–selenide solar cells and molecules. Here we have studied the structural, energetic, and electronic properties of four compounds β-In2X3 (X=O, S, Se, Te), in this space group. Using first principles computations, we have fully determined the lattice constants a and c, as well as 10 internal parameters that define this unique structure of primitive unit cells of 40 atoms. For β-In2S3 our computed values are found to be consistent with experimental measurements. The bulk modulus B, local electronic density of states, total density of states, and band gap Eg of these phases have been investigated. [Copyright &y& Elsevier]

We study the phase stability, mechanical properties, and electronic structure of two quasi-binary ceramic systems, Ti 1-x Sc x N and Ti 1-x Y x N (0 ≤ x ≤ 1), using first principles methods based on density functional theory, cluster expansion formalism, and Monte Carlo techniques. Owing to the similarity in ionic radii and electronegativities of their respective transition metals, strong exothermic mixing of TiN and ScN is predicted, with four ordered intermetallic phases lying on the convex hull: TiScN 2 , TiSc 8 N 9 , TiSc 9 N 10 , and Ti 3 Sc 2 N 5. These structures form layered rocksalt-type configurations to minimize strain energy while maximizing occupation of bonding states. The fully-detailed phase diagram including these predicted ground states and known end members is constructed, revealing an upper consolute temperature of 660 K. In contrast to Ti 1-x Sc x N, the mismatched properties of TiN and YN lead to large structural distortions and positive strain energies. As a result, endothermic mixing with significant upward bowing in the formation energy is observed at intermediate concentrations, with the consolute temperature of 7225 K being predicted from the phase diagram of Ti 1-x Y x N. TiN, ScN, and YN are found to display hardness values of 23.4, 25.1, and 20.6 GPa respectively, in good agreement with experimental data. The intermetallic phase Ti 3 Sc 2 N 5 is predicted to exhibit an exceptionally high hardness of 27.3 GPa. From analysis of projected electronic density of states and Crystal Orbital Hamiltonian Populations, we attribute enhanced hardness to strong nitrogen p and metal d hybridization, being related to 3 d e g occupation, and decreased tendency towards shearing, being related to minimal 3 d t 2g occupation. These features extend to the case of random solutions, which we model using special quasirandom structures, showing a maximum hardness at a valence electron concentration of 8.4. Based on these findings, we suggest Ti 1-x Sc x N alloys for implementation in hard coating applications. • Phase stability of Ti 1-x Sc x N and Ti 1-x Y x N (0 ≤ x ≤ 1) alloy systems were investigated. • Novel intermetallics in Ti 1-x Sc x N exhibited strong stability and high hardness up to H V = 27.3 GPa. • Hardness in Ti 1-x Sc x N is maximized at intermediate Sc concentration at VEC 8.4–8.6. • In contrast to Ti 1-x Sc x N, solubility in Ti 1-x Y x N is achieved only at very high temp (> 7225 K). • Enhanced hardness of Ti 1-x Sc x N is attributed to strong metal d and nitrogen p hybridization. [ABSTRACT FROM AUTHOR]