Eero Holmström

A new parametrization of the Stillinger–Weber potential for an improved description of defects and plasticity of silicon

A new parametrization of the widely used Stillinger-Weber potential is proposed for silicon, allowing for an improved modelling of defects and plasticity-related properties. The performance of the new potential is compared to the original version, as well as to another parametrization (Vink et al 2001 J. Non-Cryst. Solids, 282 248), in the case of several situations: point defects and dislocation core stability, threshold displacement energies, bulk shear, generalized stacking fault energy surfaces, fracture, melting temperature, amorphous structure, and crystalline phase stability. A significant improvement is obtained in the case of dislocation cores, bulk behaviour under high shear stress, the amorphous structure, and computation of threshold displacement energies, while most of the features of the original version (elastic constants, point defects) are retained. However, despite a slight improvement, a complex process like fracture remains difficult to model.

Threshold defect production in germanium determined by density functional theory molecular dynamics simulations

We studied threshold displacement energies for creating stable Frenkel pairs in silicon using density functional theory molecular dynamics simulations. The average threshold energy over all lattice directions was found to be 36 2ST AT 2SY ST eV, and thresholds in the directions 100 and 111 were found to be 20 2SY ST eV and 12.5 1.5SY ST eV, respectively. Moreover, we found that in most studied lattice directions, a bond defect complex is formed with a lower threshold than a Frenkel pair. The average threshold energy for producing either a bond defect or a Frenkel pair was found to be 24 1ST AT 2SY ST eV.

Spin Crossover in Ferropericlase from First-Principles Molecular Dynamics

Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of Earths lower mantle. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to nonmagnetic spin state. We present a finite-temperature first-principles phase diagram of this spin crossover, finding a broad pressure range with coexisting magnetic and nonmagnetic ions due to favorable enthalpy of mixing of the two. Furthermore, we find the electrical conductivity of the mineral to reach semimetallic values inside Earth.

Enhancement of irradiation-induced defect production in Si nanowires

We performed classical molecular dynamics simulations of defect production in small-diameter hexagonal Si nanowires under Ar ion irradiation. Using irradiation energies of 30 eV to 10 keV, we find that for low energies the defect production in the nanowires may be enhanced by as much as a factor of 3 in comparison to bulk Si due to the large surface-to-volume ratio of the systems. Conversely, at higher energies the increased transmission of ions causes a significant decrease in defect production.

Atomic-scale effects behind structural instabilities in Si lamellae during ion beam thinning

The rise of nanotechnology has created an ever-increasing need to probe structures on the atomic scale, to which transmission electron microscopy has largely been the answer. Currently, the only way to efficiently thin arbitrary bulk samples into thin lamellae in preparation for this technique is to use a focused ion beam (FIB). Unfortunately, the established FIB thinning method is limited to producing samples of thickness above ∼20 nm. Using atomistic simulations alongside experiments, we show that this is due to effects from finite ion beam sharpness at low milling energies combined with atomic-scale effects at high energies which lead to shrinkage of the lamella. Specifically, we show that attaining thickness below 26 nm using a milling energy of 30 keV is fundamentally prevented by atomistic effects at the top edge of the lamella. Our results also explain the success of a recently proposed alternative FIB thinning method, which is free of the limitations of the conventional approach due to the absence...

Atomistic simulations of fracture in silica glass through hypervelocity impact

Fracture of silica glass through hypervelocity impact was studied using large-scale classical molecular-dynamics simulations. The fracture is found to proceed through the coalescence of nanopores created upon straining of the glass by the pressure wave originating in the impact. Cratering of the glass substrate according to a two-phase compression and expansion mechanism accompanies the fracture.

Local changes of work function near rough features on Cu surfaces operated under high external electric field

Metal surfaces operated under high electric fields produce sparks even if they are held in ultra high vacuum. In spite of extensive research on the topic of vacuum arcs, the mystery of vacuum arc origin still remains unresolved. The indications that the sparking rates depend on the material motivate the research on surface response to extremely high external electric fields. In this work by means of density-functional theory calculations we analyze the redistribution of electron density on {100} Cu surfaces due to self-adatoms and in presence of high electric fields from −1 V/nm up to −2 V/nm (−1 to −2 GV/m, respectively). We also calculate the partial charge induced by the external field on a single adatom and a cluster of two adatoms in order to obtain reliable information on charge redistribution on surface atoms, which can serve as a benchmarking quantity for the assessment of the electric field effects on metal surfaces by means of molecular dynamics simulations. Furthermore, we investigate the modif...

Amorphous defect clusters of pure Si and type inversion in Si detectors

Si detectors subjected to energetic particle bombardment are known to undergo a deleterious type inversion from n type to p type. The effect is due to defects that trap electrons but the identity of the main responsible traps remains unknown. Using a combination of classical molecular-dynamics simulations and large-scale density-functional theory calculations, we show that amorphous defect clusters formed under particle bombardment are strong acceptors of electrons and may as such well explain the phenomenon.