50 research outputs found
Local Coordination Modulates the Reflectivity of Liquefied Si-Ge Alloys
The properties of liquid Si-Ge binary systems at melting conditions deviate
from those expected by the ideal alloy approximation. Particularly, a
non-linear dependence of the dielectric functions occurs with the reflectivity
of liquid Si-Ge being 10\% higher at intermediate Ge content than in pure Si or
Ge. Using \textit{ab initio} methodologies, we revealed a direct correlation
between reflectivity and atomic coordination, discovering that Si-Ge's higher
local coordination drives the aforementioned optical behavior. These findings
extend the physical understanding of liquefied semiconductors and hold the
promise of further generalization
Kinetic Monte Carlo simulations for transient thermal fields: Computational methodology and application to the submicrosecond laser processes in implanted silicon
Producción CientíficaPulsed 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
Solvent-aware Interfaces in Continuum Solvation
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. 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. Similarly, unphysical pockets of
continuum solvent may appear in systems featuring multiple molecular
components. 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, being it
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.
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
Surface reconstruction of fluorites in vacuum and aqueous environment
Surfaces and interfaces of bulk materials with liquids are of importance for a wide range of chemical processes. In this work, we systematically explore reconstructions on the (100) surface of calcium fluoride (CaF2) and other fluorites (MF2), M={Sr,Cd,Ba} by sampling the configurational space with the minima hopping structure prediction method in conjunction with density functional theory calculations. We find a large variety of structures that are energetically very close to each other and are connected by very low barriers, resulting in a high mobility of the topmost surface anions. This high density of configurational states makes the CaF2 (100) surface a very dynamic system. The majority of the surface reconstructions found in CaF2 are also present in SrF2,CdF2, and BaF2. Furthermore, we investigate in detail the influence of these reconstructions on the crystal growth of CaF2 in solvents by modeling the fluorite-water interface and its wetting properties. We perform a global structural search both by explicitly including water molecules and by employing a recently developed soft-sphere solvation model to simulate an implicit aqueous environment. The implicit approach correctly reproduces both our findings with the explicit-water model and the experimentally reported contact angles for the partial-hydrophobic (111) and hydrophilic (100) surfaces. Our simulations show that the high anion mobility and the low coordination of the (100) surface atoms strongly favors the adsorption of water molecules over the (111) surface. The aqueous environment makes terminations with low-coordination surface atoms more stable, promoting (100) growth instead of the (111)
Genesis and evolution of extended defects: The role of evolving interface instabilities in cubic SiC
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
Update on intensive motor training in spinocerebellar ataxia: time to move a step forward?:
Some evidence suggests that high-intensity motor training slows down the severity of spinocerebellar ataxia. However, whether all patients might benefit from these activities, and by which activity, and the underlying mechanisms remain unclear. We provide an update on the effect and limitations of different training programmes in patients with spinocerebellar ataxias. Overall, data converge of the finding that intensive training is still based either on conventional rehabilitation protocols or whole-body controlled videogames ("exergames"). Notwithstanding the limitations, short-term improvement is observed, which tends to be lost once the training is stopped. Exergames and virtual reality can ameliorate balance, coordination, and walking abilities, whereas the efficacy of adapted physical activity, gym, and postural exercises depends on the disease duration and severity. In conclusion, although a disease-modifying effect has not been demonstrated, constant, individually tailored, high-intensity motor training might be effective in patients with degenerative ataxia, even in those with severe disease. These approaches may enhance the remaining cerebellar circuitries or plastically induce compensatory networks. Further research is required to identify predictors of training success, such as the type and severity of ataxia and the level of residual functioning
A comprehensive review of transcranial magnetic stimulation in secondary dementia
Although primary degenerative diseases are the main cause of dementia, a non-negligible proportion of patients is affected by a secondary and potentially treatable cognitive disorder. Therefore, diagnostic tools able to early identify and monitor them and to predict the response to treatment are needed. Transcranial magnetic stimulation (TMS) is a non-invasive neurophysiological technique capable of evaluating in vivo and in "real time" the motor areas, the cortico-spinal tract, and the neurotransmission pathways in several neurological and neuropsychiatric disorders, including cognitive impairment and dementia. While consistent evidence has been accumulated for Alzheimer's disease, other degenerative cognitive disorders, and vascular dementia, to date a comprehensive review of TMS studies available in other secondary dementias is lacking. These conditions include, among others, normal-pressure hydrocephalus, multiple sclerosis, celiac disease and other immunologically mediated diseases, as well as a number of inflammatory, infective, metabolic, toxic, nutritional, endocrine, sleep-related, and rare genetic disorders. Overall, we observed that, while in degenerative dementia neurophysiological alterations might mirror specific, and possibly primary, neuropathological changes (and hence be used as early biomarkers), this pathogenic link appears to be weaker for most secondary forms of dementia, in which neurotransmitter dysfunction is more likely related to a systemic or diffuse neural damage. In these cases, therefore, an effort toward the understanding of pathological mechanisms of cognitive impairment should be made, also by investigating the relationship between functional alterations of brain circuits and the specific mechanisms of neuronal damage triggered by the causative disease. Neurophysiologically, although no distinctive TMS pattern can be identified that might be used to predict the occurrence or progression of cognitive decline in a specific condition, some TMS-associated measures of cortical function and plasticity (such as the short-latency afferent inhibition, the short-interval intracortical inhibition, and the cortical silent period) might add useful information in most of secondary dementia, especially in combination with suggestive clinical features and other diagnostic tests. The possibility to detect dysfunctional cortical circuits, to monitor the disease course, to probe the response to treatment, and to design novel neuromodulatory interventions in secondary dementia still represents a gap in the literature that needs to be explored
Atomistic insights into ultrafast SiGe nanoprocessing
Controlling ultrafast material transformations with atomic precision is
essential for future nanotechnology. Pulsed laser annealing (LA), inducing
extremely rapid and localized phase transitions, is a powerful way to achieve
this, but it requires careful optimization together with the appropriate system
design. We present a multiscale LA computational framework able to simulate
atom-by-atom the highly out-of-equilibrium kinetics of a material as it
interacts with the laser, including effects of structural disorder. By
seamlessly coupling a macroscale continuum solver to a nanoscale super-lattice
Kinetic Monte Carlo code, this method overcomes the limits of state-of-the-art
continuum-based tools. We exploit it to investigate nontrivial changes in
composition, morphology and quality of laser-annealed SiGe alloys. Validations
against experiments and phase-field simulations, as well as advanced
applications to strained, defected, nanostructured and confined SiGe are
presented, highlighting the importance of a multiscale atomistic-continuum
approach. Current applicability and potential generalization routes are finally
discussed
Impact of surface reflectivity on the ultra-fast laser melting of silicon-germanium alloys
Ultraviolet nanosecond laser annealing (LA) is a powerful tool where strongly
confined heating and melting are desirable. In semiconductor technologies the
importance of LA increases with the increasing complexity of the proposed
integration schemes. Optimizing the LA process along with the experimental
design is challenging, especially when complex 3D nanostructured systems with
various shapes and phases are involved. Within this context, reliable
simulations of laser melting are required for optimizing the process parameters
while reducing the number of experimental tests. This gives rise to a virtual
Design of Experiments (DoE). SiGe alloys are nowadays used for their
compatibility with silicon devices enabling to engineer properties such as
strain, carrier mobilities and bandgap. In this work, the laser melting process
of relaxed and strained SiGe is simulated with a finite element method / phase
field approach. Particularly, we calibrated the dielectric functions of the
alloy for its crystal and liquid phase using experimental data. We highlighted
the importance of reproducing the exact reflectivity of the material in its
different aggregation states, to correctly mimic the process
Post-stroke aphasia at the time of COVID-19 pandemic: a telerehabilitation perspective
We report on our remote speech therapy experience in post-stroke aphasia. The aim was to test the feasibility and utility of telerehabilitation to support future randomized controlled trials. Post-stroke aphasia is a common and disabling speech disorder, which significantly affects patients' and caregivers' health and quality of life. Due to COVID-19 pandemic, most of the conventional speech therapy approaches had to stop or "switch" into telerehabilitation procedures to ensure the safety of patients and operators but, concomitantly, the best rehabilitation level possible. Here, we planned a 5-month telespeech therapy programme, twice per week, of a patient with non-fluent aphasia following an intracerebral haemorrhage. Overall, treatment adherence based on the operator's assessments was high, and incomplete adherence for technical problems occurred very rarely. In line with the patient's feedback, acceptability was also positive, since he was constantly motivated during the sessions and the exercises performed autonomously, as confirmed by the speech therapist and caregiver, respectively. Moreover, despite the sequelae from the cerebrovascular event, evident in some writing tests due to the motor deficits in his right arm and the disadvantages typical of all telepractices, more relevant results were achieved during the telerehabilitation period compared to those of the "face-to-face" therapy before the COVID-19 outbreak. The telespeech therapy performed can be considered successful and the patient was able to return to work. Concluding, we support it as a feasible approach offering patients and their families the opportunity to continue the speech and language rehabilitation pathway, even at the time of pandemic