Domain boundaries (DBs) generated during the growth of cubic silicon carbide (3C-SiC) on (001) Si and their interaction with stacking faults (SFs) were studied in this work. Direct scanning transmission electron microscopy (STEM) images show DBs are inverted domain boundaries (IDBs). The atomic arrangement of this IDB is different from the expected boundaries described in the literature; nevertheless, it has a highly coherent nature. The IDBs propagate in a complex way through the crystal forming "complex-IDBs" that interact strongly with SFs. In particular, we observed that IDBs can terminate and generate SFs. The presence of disconnections in the IDB could be responsible for this behavior. Some models are discussed in order to explain the interconnections between IDBs and SFs. Moreover, an ab initio Monte Carlo simulation was performed in order to shed light on the kinetics of the SFs-IDB interaction. We found that SF generation can be driven by surface instability during the growth of the crystal and that SFs can be terminated by IDBs.

Modeling thermal transport at the nanoscale is a difficult task, especially when external time-varying heating sources and the complexity of the studied systems make computational schemes that rely on accurate particlelike simulations nonaffordable. Alternative strategies based on corrections of the Fourier law could satisfy the trade-off between accuracy and computational efficiency, since they can be implemented in partial differential equation solvers. This continuum approach could also allow for the coupling between thermal transport and other evolving fields related to the generalized temperature field. Here we demonstrate that corrections due to the finite phonon mean free paths can be suitably included in annealing process simulations of three-dimensional nanosystems. Quantitative predictions can be obtained and readily compared with the experimental characterization of the processed samples.

Laser irradiation in liquid is a green, low cost, and tunable technique that can be used in order to improve solar photocatalytic activity of inorganic semiconductors. The interaction between the semiconductor and the dispersing medium is a key parameter to be investigated before and after the laser modification, since it can affect the final morphological and chemical-physical properties of the material. In this work, titania colloids were properly modified by UV laser irradiation process in different solvents. The interaction of solvents (water or ethanol) with titania surface was investigated by spectroscopic characterizations and ab initio calculations based on structure predictions at a density functional theory level. Both solvents interact with oxygen vacancies on titania surface, resulting in a partial or complete passivation of defects by water and ethanol molecules, respectively. The modified samples showed an increase of the photocatalytic hydrogen production under both UV and solar light irradiation. The use of ethanol as solvent during the laser process was found to be the best choice to improve the activity of TiO2 sample under visible light. The as-developed strategies may open up an interesting avenue for designing semiconductor materials with visible light absorption properties and enhanced photocatalytic performance.

Titanium dioxide exhibits superior photocatalytic properties, mainly occurring in liquid environments through molecular adsorptions and dissociations at the solid/liquid interface. The presence of these wet environments is often neglected when performing ab initio calculations for the interaction between the adsorbed molecules and the TiO2 surface. In this study, we consider two solvents, that is, water and ethanol, and show that the proper inclusion of the wet environment in the methodological scheme is fundamental for obtaining reliable results. Our calculations are based on structure predictions at a density functional theory level for molecules interacting with the perfect and defective anatase (101) surface under both vacuum and wet conditions. A soft-sphere implicit solvation model is used to describe the polar character of the two solvents. As a result, we find that surface oxygen vacancies become energetically favorable with respect to subsurface vacancies at the solid/liquid interface. This aspect is confirmed by ab initio molecular dynamics simulations with explicit water molecules. Ethanol molecules are able to strongly passivate these vacancies, whereas water molecules only weakly interact with the (101) surface, allowing the coexistence of surface vacancy defects and adsorbed species. The infrared and photoluminescence spectra of anatase nanoparticles predominantly exposing (101) surfaces dispersed in water and ethanol support the predicted molecule-surface interactions, validating the whole computational paradigm. The combined analysis allows for a better interpretation of TiO2 processes in wet environments based on improved computational models with implicit solvation features.

Halide perovskites containing a mixture of formamidinium (FA(+)), methylammonium (MA(+)) and cesium (Cs+) cations are the actual standard for obtaining record-efficiency perovskite solar cells. Although the compositional tuning that brings to optimal performance of the devices has been largely established, little is understood on the role of even small quantities of MA(+) or Cs+ in stabilizing the black phase of FAPbI(3) while boosting its photovoltaic yield. In this paper, we use Car-Parrinello molecular dynamics in large supercells containing different ratios of FA(+) and either MA(+) or Cs+, in order to study the structural and kinetic features of mixed perovskites at room temperature. Our analysis shows that cation mixing relaxes the rotational disorder of FA(+) molecules by preferentially aligning their axis toward (100) cubic directions. The phenomenon stems from the introduction of additional local minima in the energetic landscape, which are absent in pure FAPbI(3) crystals. As a result, a higher structural order is achieved, characterized by a pronounced octahedral tilting and a lower vibrational activity for the inorganic framework. We show that both MA(+) and Cs+ are qualified for this enhancement, with Cs+ being particularly effective when diluted within the FAPbI(3) perovskite.

Emerging wide bandgap semiconductor devices such as the ones built with SiC have the potential to revolutionize the power electronics industry through faster switching speeds, lower losses, and higher blocking voltages, which are superior to standard silicon-based devices. The current epitaxial technology enables more controllable and less defective large area substrate growth for the hexagonal polymorph of SiC (4H-SiC) with respect to the cubic counterpart (3C-SiC). However, the cubic polymorph exhibits superior physical properties in comparison to its hexagonal counterpart, such as a narrower bandgap (2.3 eV), possibility to be grown on a silicon substrate, a reduced density of states at the SiC/SiO2 interface, and a higher channel mobility, characteristics that are ideal for its incorporation in metal oxide semiconductor field effect transistors. The most critical issue that hinders the use of 3C-SiC for electronic devices is the high number of defects in bulk and epilayers, respectively. Their origin and evolution are not understood in the literature to date. In this manuscript, we combine ab initio calibrated Kinetic Monte Carlo calculations with transmission electron microscopy characterization to evaluate the evolution of extended defects in 3C-SiC. Our study pinpoints the atomistic mechanisms responsible for extended defect generation and evolution, and establishes that the antiphase boundary is the critical source of other extended defects such as single stacking faults with different symmetries and sequences. This paper showcases that the eventual reduction of these antiphase boundaries is particularly important to achieve good quality crystals, which can then be incorporated in electronic devices.

Continuum models to handle solvent and electrolyte effects in an effective way have a long tradition in quantum-chemistry simulations and are nowadays also being introduced in computational condensed-matter and materials simulations. A key ingredient of continuum models is the choice of the solute cavity, i.e., the definition of the sharp or smooth boundary between the regions of space occupied by the quantum-mechanical (QM) system and the continuum embedding environment. The cavity, which should really reflect the region of space accessible to the degrees of freedom of the environmental components (the solvent), is usually defined by an exclusion approach in terms of the degrees of freedom of the system (the solute), typically, the atomic position of the QM system or its electronic density. Although most of the solute-based approaches developed lead to models with comparable and high accuracy when applied to small organic molecules, they can introduce significant artifacts when complex systems are considered. As an example, condensed-matter simulations often deal with supports that present open structures, i.e., low-density materials that have regions of space in which a continuum environment could penetrate, while a real solvent would not be able to. Similarly, unphysical pockets of continuum solvent may appear in systems featuring multiple molecular components, e.g., when dealing with hybrid QM/continuum approaches to solvation that involve introducing explicit solvent molecules around the solvated system. Here, we introduce a solvent-aware approach to eliminate the unphysical effects where regions of space smaller than the size of a single solvent molecule could still be filled with a continuum environment. We do this by defining a smoothly varying solute cavity that overcomes several of the limitations of straightforward solute-based definitions. This new approach applies to any smooth local definition of the continuum interface, it being based on the electronic density or the atomic positions of the QM system. It produces boundaries that are continuously differentiable with respect to the QM degrees of freedom, leading to accurate forces and/or Kohn Sham potentials. The additional parameters involved in the solvent-aware interfaces can be set according to geometrical principles or can be converged to improve accuracy in complex multicomponent systems. Benchmarks on semiconductor substrates and on explicit water substrates confirm the flexibility and the accuracy of the approach and provide a general set of parameters for condensed-matter systems featuring open structures and/or explicit liquid components.

This work reports on the properties of cubic silicon carbide (3C-SiC) grown epitaxially on a patterned silicon substrate composed of squared inverted silicon pyramids (ISP). This compliant substrate prevents stacking faults, usually found at the SiC/Si interface, from reaching the surface. We investigated the effect of the size of the inverted pyramid on the epilayer quality. We noted that anti-phase boundaries (APBs) develop between adjacent faces of the pyramid and that the SiC/Si interfaces have the same polarity on both pyramid faces. The structure of the heterointerface was investigated. Moreover, due to the emergence of APB at the vertex of the pyramid, voids buried on the epilayer form. We demonstrated that careful control of the growth parameters allows modification of the height of the void and the density of APBs, improving SiC epitaxy quality.

In this paper we present a computational tool for the simulation of laser annealing processes in FinFET structures. This is a complex self-consistent problem, where heating is evaluated by means of the time harmonic solution of the Maxwell equations. The main features of our computational code include: A versatile graphical user interface for the structure design; The assignment of materials and the simulation analysis; An interface with the finite element method solver for the automatic generation of the mesh and the runtime control; Parameters for numerous materials (optical/thermal properties and mass transport) as a function of temperature and phases; An efficient coupling with electromagnetic simulations for the self-consistent source estimate (i.e. the power dissipation) in nanostructured topographies; Experimental validation of nanostructured samples; Multiple-do-pant models simulating dopant redistribution, including diffusion solubility and segregation; Alloy model, e.g. SiGe (where the melting point depends on the alloy fraction); Multiple phases (e.g. amorphous, liquid, crystal). As a particular application of the tool we present a study of the laser process design by varying the laser fluence, polarization of the electromagnetic field and the pitch of the devices. Results are in excellent agreement with the experiment and could serve as guidelines for the realization of targeted laser annealing processes.

Silicon nanowires inspire since decades a great interest for their fundamental scientifc importance and their potential in new technologies. When decorated with organic molecules they form hybrid composites with applications in various felds, from sensors to life science. Specifcally the diethyl 1-propylphosphonate/Si combination is considered as a promising alternative to the conventional semiconductor n-type doping methods, thanks to its solution-based processing, which is damage-free and intrinsically conformal. For these characteristics, it is a valid doping process for patterned materials and nanostructures such as the nanowires. Our joined experimental and theoretical study provides insights at atomistic level on the molecular activation, grafting and self-assembling mechanisms during the deposition process. For the frst time to the best of our knowledge, by using scanning transmission electron microscopy the direct visualization of the single molecules arranged over the Si nanowire surface is reported. The results demonstrate that the molecules undergo to a sequential decomposition and self-assembling mechanism, fnally forming a chemical bond with the silicon atoms. The ability to prepare well-defned molecule decorated Si nanowires opens up new opportunities for fundamental studies and nanodevice applications in diverse felds like physics, chemistry, engineering and life sciences.

Laser annealing of semiconductor materials is a processing technique offering interesting application features when intense, transient and localized heat sources are needed for electronic device manufacturing or other nano-technological applications. The space-time localization of the induced thermal field (in the nanoseconds/nanometers scale) promotes interesting non-equilibrium phenomena in the processed material which only recently have been systematically investigated and modelled. In this review paper we discuss the current knowledge on anomalous kinetics occurring in implanted silicon and germanium (i.e. thin layers of disorder diluted alloys of Si and Ge, with variable initial disorder status according to the implantation conditions) during the pulsed laser irradiation. In particular, we focus our attention on the anomalous impurity redistribution in the transient melting stage and on the formation of non conventional and metastable extended defects.

BACKGROUND: We aimed to investigate the effect of cell-cell dipole interactions in the equilibrium distributions in dielectrophoretic devices. METHODS: We used a three dimensional coupled Monte Carlo-Poisson method to theoretically study the final distribution of a system of uncharged polarizable particles suspended in a static liquid medium under the action of an oscillating non-uniform electric field generated by polynomial electrodes. The simulated distributions have been compared with experimental ones observed in the case of MDA-MB-231 cells in the same operating conditions. RESULTS: The real and simulated distributions are consistent. In both cases the cells distribution near the electrodes is dominated by cell-cell dipole interactions which generate long chains. CONCLUSIONS: The agreement between real and simulated cells' distributions demonstrate the method's reliability. The distribution are dominated by cell-cell dipole interactions even at low density regimes (105 cell/ml). An improved estimate for the density threshold governing the interaction free regime is suggested.

Anomalous impurity redistribution after a laser irradiation process in group-IV elements has been reported in numerous papers. In this Letter, we correlate this still unexplained behavior with the peculiar bonding character of the liquid state of group-IV semiconductors. Analyzing the B-Si system in a wide range of experimental conditions we demonstrate that this phenomenon derives from the non-Fickian diffusion transport of B in l-Si. The proposed diffusion model relies on the balance between two impurity states in different bonding configurations: one migrating at higher diffusivity than the other. This microscopic mechanism explains the anomalous B segregation, whereas accurate comparisons between experimental chemical profiles and simulation results validate the model.

An experimental and theoretical study of the effect of excimer laser annealing (ELA) on B redistribution and electrical activation in Ge is reported. We performed detailed structural, chemical, and electrical characterizations of Ge samples implanted with B (20 keV, 1 x 10 15, or 1 x 10(16) B/cm(2)) and processed by ELA (lambda = 308 nm) with multiple pulses (1, 3, or 10). We also developed a diffusion model, in order to simulate the B redistribution induced by the ELA process. We found an anomalous impurity redistribution in the molten phase, which causes a dopant incorporation during the melt-growth at the maximum melt depth. The investigated samples showed a partial electrical activation of the B dopant. The inactivation of B in the samples implanted with 1 x 10(15) B/cm(2) was correlated to an oxygen contamination, while the poor electrical activation of B in the samples implanted with 1 x 10(16) B/cm(2) was related to the precipitation of the dopant, in good agreement with the experimental and theoretical results.

Pulsed laser irradiation of damaged solids promotes ultrafast nonequilibrium kinetics, on the submicrosecond scale, leading to microscopic modifications of the material state. Reliable theoretical predictions of this evolution can be achieved only by simulating particle interactions in the presence of large and transient gradients of the thermal field. We propose a kinetic Monte Carlo (KMC) method for the simulation of damaged systems in the extremely far-from-equilibrium conditions caused by the laser irradiation. The reference systems are nonideal crystals containing point defect excesses, an order of magnitude larger than the equilibrium density, due to a preirradiation ion implantation process. The thermal and, eventual, melting problem is solved within the phase-field methodology, and the numerical solutions for the space- and time-dependent thermal field were then dynamically coupled to the KMC code. The formalism, implementation, and related tests of our computational code are discussed in detail. As an application example we analyze the evolution of the defect system caused by P ion implantation in Si under nanosecond pulsed irradiation. The simulation results suggest a significant annihilation of the implantation damage which can be well controlled by the laser fluence.

An innovative Kinetic Monte Carlo (KMC) code has been developed, which rules the post-implant kinetics of the defects system in the extremely far-from-the equilibrium conditions caused by the laser irradiation close to the liquid-solid interface. It considers defect diffusion, annihilation and clustering. The code properly implements, consistently to the stochastic formalism, the fast varying local event rates related to the thermal field T(r,t) evolution. This feature of our numerical method represents an important advancement with respect to current state of the art KMC codes. The reduction of the implantation damage and its reorganization in defect aggregates are studied as a function of the process conditions. Phosphorus activation efficiency, experimentally determined in similar conditions, has been related to the emerging damage scenario.