Thomas J. Lenosky

Global Minimum Determination of the Born-Oppenheimer Surface within Density Functional Theory

We present a novel method, which we refer to as the dual minima hopping method, that allows us to find the global minimum of the potential energy surface (PES) within density functional theory for systems where a fast but less accurate calculation of the PES is possible. This method can rapidly find the ground state configuration of clusters and other complex systems with present day computer power by performing a systematic search. We apply the new method to silicon clusters. Even though these systems have already been extensively studied by other methods, we find new global minimum candidates for Si16 and Si19, as well as new low-lying isomers for Si16, Si17, and Si18.

Highly optimized empirical potential model of silicon

We fit an empirical potential for silicon using the modified embedded atom (MEAM) functional form, which contains a nonlinear function of a sum of pairwise and three-body terms. The three-body term is similar to the Stillinger-Weber form. We parametrized our model using five cubic splines, each with 10 fitting parameters, and fitted the parameters to a large database using the force-matching method. Our model provides a reasonable description of energetics for all atomic coordinations, Z, from the dimer (Z = 1) to fcc and hcp (Z = 12). It accurately reproduces phonons and elastic constants, as well as point defect energetics. It also provides a good description of reconstruction energetics for both the 30° and 90° partial dislocations. Unlike previous models, our model accurately predicts formation energies and geometries of interstitial complexes - small clusters, interstitial-chain and planar {311} defects.

Classical potential describes martensitic phase transformations between the α, β, and ω titanium phases

A description of the martensitic transformations between the , , and phases of titanium that includes nucleation and growth requires an accurate classical potential. Optimization of the parameters of a modified embedded atom potential to a database of density-functional calculations yields an accurate and transferable potential as verified by comparison to experimental and density-functional data for phonons, surface and stacking fault energies, and energy barriers for homogeneous martensitic transformations. Molecular-dynamics simulations map out the pressure-temperature phase diagram of titanium. For this potential the martensitic phase transformation between and appears at ambient pressure and 1200 K, between and at ambient conditions, between and at 1200 K and pressures above 8 GPa, and the triple point occurs at 8 GPa and 1200 K. Molecular-dynamics explorations of the kinetics of the martensitic - transformation show a fast moving interface with a low interfacial energy of 30 meV/A 2 . The potential is applicable to the study of

Ab initio energetics of boron-interstitial clusters in crystalline Si

We have performed an extensive first-principles study of the energetics of boron clustering in silicon in the presence of excess self-interstitial atoms (SIAs). We consider complexes with up to four B atoms and two SIAs. We have conducted an extensive search for the ground-state configurations and charge states of these clusters. We find the cluster containing three B atoms and one SIA(B3I) to be remarkably stable, while all our clusters with more than 80% boron content are unstable. Hence, we propose B3I to be a stable nucleus that can grow to larger clusters. The energetics presented here can be used as input to large-scale predictive models for B diffusion and activation during ion implantation and thermal annealing.

High-pressure structures of disilane and their superconducting properties.

A systematic ab initio search for low-enthalpy phases of disilane (Si2H6) at high pressures was performed based on the minima hopping method. We found a novel metallic phase of disilane with Cmcm symmetry, which is enthalpically more favorable than the recently proposed structures of disilane up to 280 GPa, but revealing compositional instability below 190 GPa. The Cmcm phase has a moderate electron-phonon coupling yielding a superconducting transition temperature T(c) of around 20 K at 100 GPa, decreasing to 13 K at 220 GPa. These values are significantly smaller than previously predicted T(c))s for disilane at equivalent pressure. This shows that similar but different crystalline structures of a material can result in dramatically different T(c)s and stresses the need for a systematic search for a crystalline ground state.Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp(3) bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band-gap semiconductor with a hardness comparable to diamond.

First-principles calculation of intrinsic defect formation volumes in silicon

We present an extensive first-principles study of the pressure dependence of the formation enthalpies of all the know vacancy and self-interstitial configurations in silicon, in each charge state from -2 through +2. The neutral vacancy is found to have a formation volume that varies markedly with pressure, leading to a remarkably large negative value (-0.68 atomic volumes) for the zero-pressure formation volume of a Frenkel pair (V + I). The interaction of volume and charge was examined, leading to pressure--Fermi level stability diagrams of the defects. Finally, we quantify the anisotropic nature of the lattice relaxation around the neutral defects.

Parameter-free modelling of dislocation motion: the case of silicon

Abstract In silicon and other materials with a high Peierls potential. dislocation motion takes place by nucleation and propagation of kink pairs. The rates of these unit processes are complex unknown functions of interatomic interactions in the dislocation core, stress and temperature. This work is an attempt to develop a quantitative physical description of dislocation motion in silicon based on understanding of the core structure and the energetics of core mechanisms of mobility. Atomistic simulations reveal multiple and complex kink mechanisms of dislocation translation; however, this complexity can be rationalized through the analysis of a straight kink-free dislocation, based on symmetry-breaking arguments. Further reduction is achieved by observing that the energetics of kink mechanisms is scaled by a single parameter, the energy required to break a bond in the core. To obtain accurate values of this energy we perform density functional calculations that lead us to conclude that the low mobility of the 30° dislocation results from its high bond-breaking energy. Armed with the knowledge of kink mechanisms, we develop a kinetic Monte Carlo model that makes direct use of the atomistic data as the material-defining input and predicts the dislocation velocity on the length and time scales accessible to experiments. This provides the connection between the atomistic aspects of the dislocation core and the mobility behaviour of single dislocations.

High power single mode vertical cavity surface emitting laser

Articles you may be interested in Optimal photonic-crystal parameters assuring single-mode operation of 1300 nm AlInGaAs vertical-cavity surface-emitting laser Single-mode 1.27 μ m InGaAs vertical cavity surface-emitting lasers with temperature-tolerant modulation characteristics Appl. High-power single-mode vertical-cavity surface-emitting lasers with triangular holey structure Appl. Theory of the mode stabilization mechanism in concave-micromirror-capped vertical-cavity surface-emitting lasers J.

Structure and stability of semiconductor tip apexes for atomic force microscopy

The short range force between the tip and the surface atoms, that is responsible for atomic-scale contrast in atomic force microscopy (AFM), is mainly controlled by the tip apex. Thus, the ability to image, manipulate and chemically identify single atoms in semiconductor surfaces is ultimately determined by the apex structure and its composition. Here we present a detailed and systematic study of the most common structures that can be expected at the apex of the Si tips used in experiments. We tackle the determination of the structure and stability of Si tips with three different approaches: (i) first principles simulations of small tip apexes; (ii) simulated annealing of a Si cluster; and (iii) a minima hopping study of large Si tips. We have probed the tip apexes by making atomic contacts between the tips and then compared force-distance curves with the experimental short range forces obtained with dynamic force spectroscopy. The main conclusion is that although there are multiple stable solutions for the atomically sharp tip apexes, they can be grouped into a few types with characteristic atomic structures and properties. We also show that the structure of the last atomic layers in a tip apex can be both crystalline and amorphous. We corroborate that the atomically sharp tips are thermodynamically stable and that the tip-surface interaction helps to produce the atomic protrusion needed to get atomic resolution.

Large enhancement of boron solubility in silicon due to biaxial stress

One of the important challenges to the semiconductor industry today is to enhance the solid solubility of several dopants, boron in particular, in silicon. We calculate the equilibrium solid solubility of boron in Si from first principles and examine the effect of biaxial stress. We find an unexpectedly large enhancement, on the order of 150%, for only 1% strain primarily due to the charge of the substitutional boron impurity in Si. We point out that this effect is an intrinsic property of Si and is expected to be important for other dopants as well.