Search

Al 2 O 3 /AlGaN metal-oxide-semiconductor capacitors show a hysteretic behavior in their capacitance vs voltage characteristics, often attributed to near-interface traps deriving from defects within the oxide layer. The origin as well as the structural/electronic properties of such defects are still strongly debated in the literature. Here, we use ab initio molecular dynamics and the climbing-image nudged elastic band method to show that aluminum Frenkel defects give rise to bistable trap states in disordered and stoichiometric Al 2 O 3. Based on these results, we propose a calibrated polaron model representing a distribution of individually interacting energy levels with an internal reconfiguration mode and coupled to continuous bands of carriers to explain the hysteresis mechanism in Al 2 O 3 /AlGaN capacitors. [ABSTRACT FROM AUTHOR]

The computational study of chemical reactions in complex, wet environments is critical for applications in many fields. It is often essential to study chemical reactions in the presence of applied electrochemical potentials, taking into account the non-trivial electrostatic screening coming from the solvent and the electrolytes. As a consequence, the electrostatic potential has to be found by solving the generalized Poisson and the Poisson-Boltzmann equations for neutral and ionic solutions, respectively. In the present work, solvers for both problems have been developed. A preconditioned conjugate gradient method has been implemented for the solution of the generalized Poisson equation and the linear regime of the Poisson-Boltzmann, allowing to solve iteratively the minimization problem with some ten iterations of the ordinary Poisson equation solver. In addition, a self-consistent procedure enables us to solve the non-linear Poisson-Boltzmann problem. Both solvers exhibit very high accuracy and parallel efficiency and allow for the treatment of periodic, free, and slab boundary conditions. The solver has been integrated into the BigDFT and Quantum-ESPRESSO electronic-structure packages and will be released as an independent program, suitable for integration in other codes. [ABSTRACT FROM AUTHOR]

The diffusion and activation of arsenic implanted into germanium at 40 keV with maximum concentrations below and above the solid solubility (8 x 1019 cm-3been studied, both experimentally and theoretically, after excimer laser annealing (λ = 308 nm) in the melting regime with different laser energy densities and single or multiple pulses. Arsenic is observed to diffuse similarly for different fluences with no out-diffusion and no formation of pile-up at the maximum melt depth. The diffusion profiles have been satisfactorily simulated by assuming two diffusivity states of As in the molten Ge and a non-equilibrium segregation at the maximum melt depth. The electrical activation is partial and decreases with increasing the chemical concentration with a saturation of the active concentration at 1 x 1020 cm-3, which represents a new record for the As-doped Ge system. [ABSTRACT FROM AUTHOR]

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 × 1015, or 1 × 1016 B/cm2) and processed by ELA (λ = 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 × 1015 B/cm2 was correlated to an oxygen contamination, while the poor electrical activation of B in the samples implanted with 1 × 1016 B/cm2 was related to the precipitation of the dopant, in good agreement with the experimental and theoretical results. [ABSTRACT FROM AUTHOR]

The activation mechanism in phosphorous implanted silicon under excimer laser irradiation is investigated. The activation efficiency in the solid phase has been measured in a wide range of irradiation conditions, tuning the laser fluence in the sub-, partial, and total melting regime. Moreover, fixing the fluence, the activation as a function of the shot number has been analyzed. The total active fraction varies by several orders of magnitude and shows a complex trend depending on the process conditions. Our model, based on the interaction between defects and the active/inactive impurities, explains this scenario. In particular, it predicts experimental P active profiles, thus demonstrating that the status of the defect system rules the activation phenomenon, where the coupling between dopant and defect clusters at the early irradiation stage plays a crucial role. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this study, we explore the potential of a blended material comprising CsPbI3:EuCl3 perovskite and Gig-Lox TiO2, a unique transparent spongy material known for its multi-branched porous structure, for application in solar cells. The inclusion of EuCl3 in CsPbI3 serves to stabilize the photoactive γ-phase with a bandgap of 1.75 eV, making it suitable for solar energy conversion in tandem solar cells. Our study applies X-ray-based techniques to investigate the structural properties and interfacial behavior within this blended material, in comparison with a reference perovskite layer deposited on glass. In addition, Spectroscopic ellipsometry is complemented with density functional theory calculations and photoluminescence measurements to elucidate the absorption and radiative emission properties of the blend. Notably, our findings reveal a significant quenching of photoluminescence within the blended material, underscoring the pivotal role of the distributed interfaces in facilitating efficient carrier injection from the CsPbI3:EuCl3 perovskite into the Gig-Lox TiO2 sponge. These findings pave the way for the application of the blend as an Electron Transport Layer (ETL) in semi-transparent perovskite solar cells for tandem and building integrated photovoltaics.

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 requires careful optimization together with the appropriate system design. We present a multiscale LA computational framework that can 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 superlattice 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., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The importance of lead analysis in environmental matrices becomes increasingly relevant due to the anthropogenic spread of toxic species in nature. Alongside the existing analytical methods to detect lead in a liquid environment, we propose a new dry approach for lead detection and measurement based on its capture from a liquid solution by a solid sponge and subsequent quantification based on X-ray analyses. The detection method exploits the relationship between the electronic density of the solid sponge, which depends on the captured lead, and the critical angle for total reflection of the X-rays. For this purpose, gig-lox TiO 2 layers, grown by modified sputtering physical deposition, were implemented for their branched multi-porosity spongy structure that is ideal for capturing lead atoms or other metallic ionic species in a liquid environment. The gig-lox TiO 2 layers grown on glass substrates were soaked into aqueous solutions containing different concentrations of Pb, dried after soaking, and finally probed through X-ray reflectivity analyses. It has been found that lead atoms are chemisorbed onto the many available surfaces within the gig-lox TiO 2 sponge by establishing stable oxygen bonding. The infiltration of lead into the structure causes an increase in the overall electronic density of the layer and, thus, an increment of its critical angle. Based on the established linear relationship between the amount of lead adsorbed and the augmented critical angle, a standardized quantitative procedure to detect Pb is proposed. The method can be, in principle, applied to other capturing spongy oxides and toxic species.

In this work we study isomers of several representative small clusters to find principles for their stability. Our conclusions about the principles underlying the structure of clusters are based on a huge database of 44 000 isomers generated for 58 different clusters on the density functional theory level by Minima Hopping. We explore the potential energy surface of small neutral, anionic and cationic isomers, moving left to right across the third period of the periodic table and varying the number of atoms n and the cluster charge state q (X q n , with X = {Na, Mg, Al, Si, Ge}, q = -1, 0, 1, 2). We use structural descriptors such as bond lengths and atomic coordination numbers, the surface to volume ratios and the shape factor as well as electronic descriptors such as shell filling and hardness to detect correlations with the stability of clusters. The isomers of metallic clusters are found to be structure seekers with a strong tendency to adopt compact shapes. However certain numbers of atoms can suppress the formation of nearly spherical metallic clusters. Small non-metallic clusters typically also do not adopt compact spherical shapes for their lowest energy structures. In both cases spherical jellium models are not any more applicable. Nevertheless for many structures, that frequently have a high degree of symmetry, the Kohn-Sham eigenvalues are bunched into shells and if the available electrons can completely fill such shells, a particularly stable structure can result. We call such a cluster whose shape gives rise to shells that can be completely filled by the number of available electrons an optimally matched cluster, since both the structure and the number of electrons must be special and match. In this way we can also explain the stability trends for covalent silicon and germanium cluster isomers, whose stability was previously explained by the presence of certain structural motifs. Thus we propose a unified framework to explain trends in the stability of isomers and to predict their structure for a wide range of small clusters., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Innovation is essential around the themes of climate change and sustainability. Commercial photovoltaics (PV) have noticeably contributed to getting to 22.1% share of the gross final energy consumption in Europe from renewable sources in 2020 but a steep further increase is urgent in the near future. Over the last few years, great success has been achieved by perovskites applied to PV, with mixed anions and cations in shared lattices that reached record efficiency values close to those of Si in laboratory-scale solar cells (∼26%). Their use has recently shed light on a medium/long-term compositional instability that arises from the partial miscibility of the species with similar role in the atomic lattice. The chemical route to prepare the materials for Perovskite Solar Cells (PSCs) also needs to be critically reviewed. Material waste and reuse are other concerns to be faced. This perspective paper indeed tackles some aspects for innovation and sustainability on the PSC field for production purposes. Some hints for technologically affordable processes based on in-vacuum deposition of Perovskites are provided in light of their sustainability and for the need to reduce production/maintenance costs. It is also discussed how to make in-vacuum production further competitive by boosting the material quality. Innovation is also projected into the theme of making sustainable choices for device architectures and materials. Carbon-based PSCs are highly focused since they allow avoiding the use of complex, unstable and costly HTLs. From the material side, pros and cons of using fully inorganic CsPbI 3 are commented, framed by the current revival of single-cation perovskites. CsPbI 3 , in particolar, enables recycling and reuse initiatives thanks to the overall mass preservation under degradation. Some closing remarks are provided on the safe use of Pb as its effective sequestration before release from the PSC into the environment is properly engineered. We lastly trust that initiatives bringing together academic and industrial know-how in complementary fields able to take up responsible innovation will contribute to accelerating the ecological transition and will enable the societal transformation to fulfil the 2050 EU agenda for a sustainable future.

In this review paper, several new approaches about the 3C-SiC growth are been presented. In fact, despite the long research activity on 3C-SiC, no devices with good electrical characteristics have been obtained due to the high defect density and high level of stress. To overcome these problems, two different approaches have been used in the last years. From one side, several compliance substrates have been used to try to reduce both the defects and stress, while from another side, the first bulk growth has been performed to try to improve the quality of this material with respect to the heteroepitaxial one. From all these studies, a new understanding of the material defects has been obtained, as well as regarding all the interactions between defects and several growth parameters. This new knowledge will be the basis to solve the main issue of the 3C-SiC growth and reach the goal to obtain a material with low defects and low stress that would allow for realizing devices with extremely interesting characteristics.

We present a method for the simulation of the kinetic evolution in the sub µs timescale for composite materials containing regions occupied by alloys, compounds, and mixtures belonging to the Ni-Si-C ternary system. Pulsed laser irradiation (pulses of the order of 100 ns) promotes this evolution. The simulation approach is formulated in the framework of the phase-field theory and it consists of a system of coupled non-linear partial differential equations (PDEs), which considers as variables the following fields: the laser electro-magnetic field, the temperature, the phase-field and the material (Ni, Si, C, C clusters and Ni-silicides) densities. The model integrates a large set of materials and reaction parameters which could also self-consistently depend on the model variables. A parameter calibration is also proposed, specifically suited for the wavelength of a widely used class of excimer lasers ( λ = 308 nm). The model is implemented on a proprietary laser annealing technology computer-aided design (TCAD) tool based on the finite element method (FEM). This integration allows, in principle, numerical solutions in systems of any dimension. Here we discuss the complex simulation trend in the one-dimensional case, considering as a starting state, thin films on 4H-SiC substrates, i.e., a configuration reproducing a technologically relevant case study. Simulations as a function of the laser energy density show an articulated scenario, also induced by the variables' dependency of the materials' parameters, for the non-melting, partial-melting and full-melting process conditions. The simulation results are validated by post-process experimental analyses of the microstructure and composition of the irradiated samples.

The realization of stable inorganic perovskites is crucial to enable low-cost solution-processed photovoltaics. However, the main candidate material, CsPbI 3 , suffers from a spontaneous phase transition at room temperature towards a photo-inactive orthorhombic δ-phase (yellow phase). Here we used theoretical and experimental methods to study the structural and electronic features that determine the stability of the CsPbI 3 perovskite. We argued that the two physical characteristics that favor the black perovskite phase at low temperatures are the strong spatial confinement in nanocrystalline structures and the level of electron doping in the material. Within this context, we discussed practical procedures for the realization of long-lasting inorganic lead halide perovskites.

The BigDFT project was started in 2005 with the aim of testing the advantages of using a Daubechies wavelet basis set for Kohn-Sham (KS) density functional theory (DFT) with pseudopotentials. This project led to the creation of the BigDFT code, which employs a computational approach with optimal features of flexibility, performance, and precision of the results. In particular, the employed formalism has enabled the implementation of an algorithm able to tackle DFT calculations of large systems, up to many thousands of atoms, with a computational effort that scales linearly with the number of atoms. In this work, we recall some of the features that have been made possible by the peculiar properties of Daubechies wavelets. In particular, we focus our attention on the usage of DFT for large-scale systems. We show how the localized description of the KS problem, emerging from the features of the basis set, is helpful in providing a simplified description of large-scale electronic structure calculations. We provide some examples on how such a simplified description can be employed, and we consider, among the case-studies, the SARS-CoV-2 main protease.