Aluminum-doped zinc oxide (AZO) is an electrically conductive and optically transparent material with many applications in optoelectronics and photovoltaics as well as in the new field of plasmonic metamaterials. Most of its applications contemplate the use of complex and nanosized materials as substrates onto which the AZO forms the coating layer. Its morphological characteristics, especially the conformality and crystallographic structure, are crucial because they affect its optoelectrical response. Nevertheless, it was difficult to find literature data on AZO layers deposited on non-planar structures. We studied the AZO growth on silicon-nanowires (SiNWs) to understand its morphological evolution when it is formed on quasi one-dimensional nanostructures. We deposited by sputtering different AZO thicknesses, leading from nanoclusters until complete incorporation of the SiNWs array was achieved. At the early stages, AZO formed crystalline nano-islands. These small clusters unexpectedly contained detectable Al, even in these preliminary phases, and showed a wurtzite crystallographic structure. At higher thickness, they coalesced by forming a conformal polycrystalline shell over the nanostructured substrate. As the deposition time increased, the AZO conformal deposition led to a polycrystalline matrix growing between the SiNWs, until the complete array incorporation and planarization. After the early stages, an interesting phenomenon took place leading to the formation of hook-curved SiNWs covered by AZO. These nanostructures are potentially very promising for optical, electro-optical and plasmonic applications.

In this work, we present a comprehensive investigation of impurities contamination in silicon during UV Nanosecond Laser Annealing at high energy density. By investigating in detail the impact of the annealing ambient and of the surface preparation prior to UV-NLA (including the variation of the surface oxide thickness), we show that the observed oxygen penetration originates from the surface oxide layer. It is proposed that, at high energy UV-NLA, the prolonged contact of SiO2 with high temperature liquid Si induces a partial degradation of the SiO2/Si interface, leading to bond breaking and subsequent injection of O atoms into the substrate. A degradation involving less than 5% of the O atoms contained in the 1st SiO2 mono-layer is sufficient to account for the measured amount of in-diffused O in all of the analysed samples.

We investigated the complex interaction between a nickel layer and a 4H-SiC substrate under UV-laser irradiation since the early stages of the atomic inter-diffusion. An exhaustive description is still lacking in the literature. A multimethod approach based on Transmission Electron Microscopy, Energy Dispersive Spectroscopy and Diffraction (electron and X-ray) techniques has been implemented for a cross-correlated description of the final state of the contact after laser irradiation. They detailed the stoichiometry and the lattice structure of each phase formed as well as the Ni/Si alloy profile along the contact for laser fluences in the range 2.4-3.8 J/cm(2). To make a bridge between process conditions and post-process characterizations, time dependent ultra-fast phenomena (laser pulse approximate to 160 ns), such as intermixing driven melting and Ni-silicides reactions, have been simulated by a modified phase fields approach in the proper many-compounds formulation.

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.

Gas sensor, comprising: a substrate of semiconductor material; a first working electrode on the substrate; a second working electrode on the substrate, at a distance from the first working electrode; an interconnection layer extending in electrical contact with the first and the second working electrode, configured to change its conductivity when reacting with gas species to be detected. The interconnection layer is of titanium oxide, has a porosity between 40% and 60% in volume and is formed by a plurality of meso-pores having at least one dimension in the range 6-30 nm connected to nano-pores having at least one respective dimension in the range 1-5 nm.

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.

Device engineering with proper material integration into perovskite solar cells (PSCs) would extend their durability provided a special care is spent to retain interface integrity during use. In this paper, we propose a method to preserve the perovskite (PSK) surface from solvent-mediated modification and damage that can occur during the deposition of a top contact and furtherly during operation. Our scheme used a hole transporting layer-free top-contact made of Carbon (mostly graphite) to the side of hole extraction. We demonstrated that the PSK/graphite interface benefits from applying a vacuum-curing step after contact deposition that allowed mitigating the loss in efficiency of the solar devices, as well as a full recovery of the electrical performances after device storage in dry nitrogen and dark conditions. The device durability compared to reference devices was tested over 90 days. Conductive atomic force microscopy (CAFM) disclosed an improved surface capability to hole exchange under the graphite contact after vacuum curing treatment.

Carbon-based top electrodes for hole-transporting-layer-free perovskite solar cells (PSCs) were made by hot press (HP) transfer of a free-standing carbon-aluminum foil at 100 degrees C and at a pressure of 0.1 MPa on a methylammonium lead iodide (MAPbI(3)) layer. Under these conditions, the perovskite surface was preserved from interaction with the solvent. Over a timescale of 90 days, HP-PSCs were systematically compared to reference cells with carbon-based top electrodes deposited by doctor blading (DB). We found that all the photovoltaic parameters recorded in HP-PSCs during time under ambient conditions settled on values systematically higher than those measured in the reference DB-PSCs, with efficiency stabilized at around 6% within the first few measurements. On the other hand, in DB-PSCs, a long-lasting (similar to 14 days) degrading transient of the performances was observed, with a loss of efficiency from an initial similar to 8% to similar to 3%. Moreover, in HP-PSCs, a systematic day-by-day recovery of the efficiency after operation was observed (Delta similar to 2%) by leaving the cell under open circuit, a nitrogen environment, and dark conditions. Noteworthily, a full recovery of all the parameters was observed at the end of the experiment, while DB-PSCs showed only a partial recovery under the same conditions. Hence, the complete release of solvent from the carbon contact, before an interface is established with the perovskite layer, offers a definite advantage through the long period of operation in preventing irreversible degradation. Our findings indeed highlight the crucial role of the interfaces and their feasible preservation under nitrogen atmosphere.

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.

Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr(3), CsPbBr3 and for the first time FAPbBr(3)) by spectroscopic ellipsometry in the range of 1-5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic-tetragonal and tetragonal-cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.

The possibility to realize three dimensional (3D) graphene based architectures, able to strongly increase the high specific surface areas of this challenging material, while maintaining strong mechanical strengths and fast mass and electron transport kinetics, make them very promising in fields like sensing or catalysis [1]. In this contest, dopants like N can be introduced to induce a charge polarization in the carbon lattice, modifying its electronic properties and surface wettability, or to create anchoring sites for chemical reactions often used in organic chemistry to functionalized surfaces, enabling specific reactions and catalysis processes [2,3]. In this work we realize N-doped 3D graphene foams (GF) by chemical vapor deposition on Ni foams used as templates. We used CH4 and H2 as gaseous precursors and NH3 as N-doping source. The synthesis process is widely investigated in order to study the doping process and optimize the N type bonding mechanism. We found that the state in which N rearranges into the graphene lattice strongly depends on the stage of the Chemical Vapour (CVD) Deposition synthesis in which the NH3 is introduced in the reaction chamber. Moreover, if the N doping level is very high, it can affect the lateral growth of the graphene clusters, thus leading to micrometric triangular graphene domains. Morphological characterizations as a function of the CVD process conditions are modelled and their scenario is validated by means of ab-initio calibrated kinetic Monte Carlo simulations. The obtained data are of fundamental importance for a successful application of N-doped GF structures in sensing or catalysis field. REFERENCES [1]C. Backets et al. 2D Mater. 7 (2020) 022001 [2]H. Xu et al.Journal of Energy Chemistry 27 (2018) 146-160 [3]H. Wang et al. ACS Catal. 2 (2012) 781-79.

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.

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.

Nanostructured materials represent a breakthrough in many fields of application. Above all for sensing, the use of nanostructures with a high surface/volume ratio is strategic to raise the sensitivity towards dangerous environmental gas species. A new Dc-Reactive sputtering Deposition method has been applied to grow highly porous p-type nitrogen-doped titanium oxide layers by modifying the previously developed reactive sputtering method called gig-lox. The doping of the films was achieved at room temperature by progressive incorporation of nitrogen species during the deposition process. Two different amounts of N were introduced into the deposition chamber at flow rates of 2 and 5 standard cubic centimeter per minutes (sccm) for doping. It has been found that the N uptake reduces the deposition rate of the TiO film whilst the porosity and the roughness of the grown layer are not penalized. Despite the low amount of N, using 2 sccm of gas resulted in proper doping of the TiO film as revealed by XPS Analyses. In this case, nitrogen atoms are mainly arranged in substitutional positions with respect to the oxygen atoms inside the lattice, and this defines the p-type character of the growing layer. Above this strategic structural modification, the multibranched spongy porosity, peculiar of the gig-lox growth, is still maintained. As proof of concept of the achievements, a sensing device was prepared by combining this modified gig-lox deposition method with state-of-the-art hot-plate technology to monitor the electrical response to ethanol gas species. The sensor exhibited a sensitivity of a factor of ?2 to 44 ppm of ethanol at ?200 °C as measured by a rise in the layer resistivity according to the p-type character of the material. At the higher temperature of ?350 °C, the sensor turned to n-type as without doping. This behavior was related to a loss of nitrogen content inside the film during the annealing. It was indeed proved that p-type doping of a gig-lox sponge during growth is feasible, even at room temperature, without losing the layer porosity and the capability to host and detect environmental species. Moreover, the material integration on a device is simply done as the last production step. Easy TiO doping procedures, combined with porosity, are of general purpose and interest for several applications even on flexible substrates.

Despite its structural and chemical stability, graphene is often subjected to n- or p-type doping when interacting with substrates, gate oxides, or environmental molecules. Such interaction shifts the Fermi level of the system away from the Dirac point and alters the intrinsic electronic and transport characteristics of the graphene sheet. The density functional theory is used herein to show that the Fermi level of a graphene/AlN or graphene/Al2O3 heterostructure can be extensively tuned through the polarity and surface reconstruction of either the nitride or the oxide layer. Hence, Fermi-level engineering through the manipulation of confining materials can become a viable route for enhancing the selectivity and optimizing the properties of graphene-based devices.

Studying defect formation and evolution in MethylAmmonium lead Iodide (MAPbI(3)) perovskite layers has a bottleneck in the softness of the matter and in its consequent sensitivity to external solicitations. Here we report that, in a polycrystalline MAPbI3 layer, Pb-related defects aggregate into nanoclusters preferentially at the triple grain boundaries as unveiled by Transmission Electron Microscopy (TEM) analyses at low total electron dose. Pb-clusters are killer against MAPbI3 integrity since they progressively feed up the hosting matrix. This progression is limited by the concomitant but slower transformation of the MAPbI3 core to fragmented and interconnected nano-grains of 6H-PbI2 that are structurally linked to the mother grain as in strain-relaxed heteroepitaxial coupling. The phenomenon occurs more frequently under TEM degradation whilst air degradation is more prone to leave uncorrelated [001]-oriented 2H-PbI2 grains as statistically found by X-Ray Diffraction. This path is kinetically costlier but thermodynamically favoured and is easily activated by catalytic species.

A stochastic simulation method designed to study at an atomic resolution the growth kinetics of compounds characterized by the sp(3)-type bonding symmetry is presented. Formalization and implementation details are discussed for the particular case of the 3C-SiC material. A key feature of this numerical tool is the ability to simulate the evolution of both point-like and extended defects, whereas atom kinetics depend critically on process-related parameters. In particular, the simulations can describe the surface state of the crystal and the generation/evolution of defects as a function of the initial substrate condition and the calibration of the simulation parameters. Quantitative predictions of the microstructural evolution of the studied systems can be readily compared with the structural characterization of actual processed samples is demonstrated.

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.