The R(T) dependences of thin superconducting aluminum films deposited on leucosapphire and gallium arsenide substrates by electron beam sputtering and molecular beam epitaxy have been experimentally studied. Regardless of morphology, a noticeable increase in the critical temperature of the superconducting transition with a decrease in the film thickness is found. The effect is interpreted as a manifestation of the quantum size effect.

The impact of film thickness to the electron–phonon coupling and the critical temperature of the superconducting transition, as well as to the structural, phonon and electronic properties of Al films in the ultrathin regime, was studied by means of ab initio calculations. The presence of the inevitable quantum fluctuations in the regime of 5 to 9 atomic layers was found not to be detrimental for the transition critical temperature, as quantum confinement was found to have a monotonically positive impact. As calculations were performed on perfect crystalline films, results of this work give further indirect evidence of the impact of the experimentally observed defects on the superconducting transition. The reduction of the dimensions of materials in the critically low regime of quantum effects can unlock new prospects for high Tc superconducting devices, such as cryogenic light detectors and superconducting single-electron transistors; hence, the understanding of the underlying mechanisms is required for the deterministic tailoring of their properties.

Here we present results on the first atomic simulation of the threading $$\left( {\vec a + \vec c} \right)$$ –mixed dislocation cores in wurtzite GaN. These calculations are based on a modified Stillinger-Weber potential. For this dislocation two core configurations are shown to be stable, one with a complex double 5/6-atoms rings and the other with a 5/7-atom rings structures. These two cores do not contain wrong bonds.

InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of 33%, narrowing down the potential range of applications. Here, it is demonstrated that quasi two-dimensional (quasi-2D) QWs with thickness of one atomic monolayer can be deposited with indium contents far exceeding this limit, under certain growth conditions. Multi-QW heterostructures were grown by plasma-assisted molecular beam epitaxy, and their composition and strain were determined with monolayer-scale spatial resolution using quantitative scanning transmission electron microscopy in combination with atomistic calculations. Key findings such as the self-limited QW thickness and the non-monotonic dependence of the QW composition on the growth temperature under metal-rich growth conditions suggest the existence of a substitutional synthesis mechanism, involving the exchange between indium and gallium atoms at surface sites. The highest indium content in this work approached 50%, in agreement with photoluminescence measurements, surpassing by far the previously regarded compositional limit. The proposed synthesis mechanism can guide growth efforts towards binary InN/GaN quasi-2D QWs.

Following the general strategy of ‘strain engineering’, ab initio calculations based on density functional theory (DFT) were performed to investigate the behavior of wurtzite indium nitride under an equibiaxial strain, applied in the basal plane. The evolution of structural, electronic, elastic and piezoelectric properties was investigated over both compressive and tensile strains up to 10%. To overcome the inherent shortcoming of DFT in reproducing band gaps, pseudo-potentials modified à la Christensen were used. A wurtzite to graphitic-like phase transition was found to take place at a tensile strain of +6% and identified to be of the first order. This transformation was found to be accompanied with a direct–indirect band gap transition. In the wurtzite structure, a non-linear enhancement of the out-of-plane piezoelectric constant was evidenced in the expansion regime where its value, near the phase transition, attains several times that of the strain free structure. This large piezoelectric response is very attractive for practical applications, such as piezoelectric sensors and resonators. Substrate candidates for growing pseudomorphic wurtzite InN with a suitable strain and appreciable critical thickness are discussed.

The effective stabilization of tannery sludge wastes is explored using X-Ray Absorption Fine Structure (XAFS) spectroscopies. Solidification of the Cr-rich waste was realized via vitrification of the incinerated sludge with silica and flux agents. It is demonstrated that the effective reduction of Cr(VI) and the structural role of Cr are strongly modulated by the chemical composition of the waste. Eskolaite microcrystallites are embedded in the silica matrix of all vitrified samples and the extent of microcrystalline formation is strongly related to the glass basicity. Both Cr(VI) and Cr(III) species are identified, corresponding to Cr(VI)O4 glass formers and Cr(III)O6 network modifiers. The toxic Cr(VI) prevails only in the glasses with the highest basicity index and lowest waste content, nevertheless it is safely incorporated and immobilized in the silica matrix. However, the detected abundance of Cr(VI) increases glass basicity and as a result, glass polymerization is hindered. Thermal treatment, a process that leads to glass-ceramics transforms almost all Cr(VI) to Cr(III), while eskolaite formation is promoted concurrently. Nevertheless, microcrystalline growth proceeds mainly via depletion of Cr(III) from the silica matrix and not from the reduced Cr(VI); yet, Cr-removal from the glass matrix does not impair the chemical stability of the devitrified products.

III-nitride compound semiconductors are breakthrough materials regarding device applications. However, their heterostructures suffer from very high threading dislocation (TD) densities that impair several aspects of their performance. The physical mechanisms leading to TD nucleation in these materials are still not fully elucidated. An overlooked but apparently important mechanism is their heterogeneous nucleation on domains of basal stacking faults (BSFs). Based on experimental observations by transmission electron microscopy, we present a concise model of this phenomenon occurring in III-nitride alloy heterostructures. Such domains comprise overlapping intrinsic I1 BSFs with parallel translation vectors. Overlapping of two BSFs annihilates most of the local elastic strain of their delimiting partial dislocations. What remains combines to yield partial dislocations that are always of screw character. As a result, TD nucleation becomes geometrically necessary, as well as energetically favorable, due to the coexistence of crystallographically equivalent prismatic facets surrounding the BSF domain. The presented model explains all observed BSF domain morphologies, and constitutes a physical mechanism that provides insight regarding dislocation nucleation in wurtzite-structured alloy epilayers.

The effects of ultrathin AlN prelayers, with nominal thicknesses between 0 and 1.5 nm, on the spontaneous growth of GaN nanowires (NWs) on Si (1 1 1) substrates were investigated. The morphological and structural characteristics of GaN NWs were analyzed by electron microscopy and X-ray diffraction techniques. The results quantify how the AlN prelayer thickness affects GaN NWs. The increase of AlN thickness gradually limits nitridation of the substrate surface and accelerates 3D GaN nucleation. The formation of amorphous SixNy by Si nitridation is completely avoided for 1.5 nm of AlN that fully covers the Si surface. The dependence of the height, diameter and density of GaN NWs on the AlN thickness was also determined. The 1.5 nm AlN provided the optimum condition for GaN NW nucleation and growth; the NWs exhibited a large homogeneous height with almost no parasitic GaN formation between them. High resolution transmission electron microscopy showed the full relaxation of misfit strain of AlN on Si (1 1 1) and of GaN NWs on AlN. In-situ reflection high energy electron diffraction during AlN nucleation revealed the immediate relaxation of the AlN prelayer before GaN NW nucleation. Formation of beta-Si3N4 before AlN nucleation was also observed.

The aim of this work is to elucidate how different growth mode and composition of barriers can influence the QW properties and their PL and to find optimal QW capping process, to suppress the In desorption from QWs and to maintain the QW PL efficiency. It concentrates on the technology procedure for growth of upper quantum well (QW) interfaces in InGaN/GaN QW structure when different temperature for QW and barrier epitaxy is used. We have found that optimal photoluminescence (PL) results were achieved, when the growth after QW formation was not interrupted, but immediately continued during the temperature ramp by the growth of (In) GaN capping layer with small introduction of In precursor into the reactor. Optimal barrier between QW with respect to PL results was found to be pure GaN. We have shown according to SIMS and HRTEM results that by this technological procedure the InGaN desorption was considerably suppressed and three times higher In concentration and two times thicker QWs were achieved for the same QW growth parameters without deterioration of PL intensity in comparison to sample with usually used thin GaN low temperature capping protection. Additionally, for samples covered by the QW capping layer during the temperature ramp the defect band is almost completely missing, thus we can conclude that this defect band is connected with quality of the upper QW interface.

Elastic model of continuum material is often used to simulate the relaxation of crystalline heterostructures. There are many reports on the successful application of the theory of elasticity to nano-sized crystalline heterostructures, even if the continuum condition for them is hardly fulfilled. On the other hand, progress in epitaxial growth allows for the preparation of stable ultra-thin layers with thickness of few monolayers. For such ultra-thin layers, results provided by continuum model and molecular statics/dynamics calculations become diverging. The key problem seems to be located at the modelling of the interface between layers, which is problematic in the continuum approach. By applying a step-wise substitutive compositional interfacial function, it is possible to obtain good agreement with molecular dynamics calculations, even for a single monolayer heterostructure. We propose another approach that uses composition as an extra parameter during finite element calculations, along with classical nodal displacements. Such an approach creates a chemo-elastic coupling that allows to interpolate the composition much like in the case of atomistic calculations.

Indentation techniques were utilised to induce deformation on polar (0001) c-plane and non-polar [Formula: see text] m-plane GaN single crystal. Cracking was more sensitively dependent on the orientation of the indenter tip, compared to hardness. The indentation-induced plastic deformation and fracture sequences were studied by cathodoluminescence imaging and optical microscopy, respectively. Polar GaN was harder than non-polar, while pop-in discontinuities occurred at lower loads in polar than non-polar GaN. Dislocation arrangements were more isotropic at the polar than the non-polar orientation. Polar GaN was more susceptible to cracking compared to non-polar. Indentation at the high load regime fostered radial and lateral crack formation at both indenter orientations in polar GaN. Post-indentation lateral crack propagation was observed in situ in polar GaN. This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.

Extensive high resolution transmission and scanning transmission electron microscopy observations were performed in In(Ga)N/GaN multi-quantum well short period superlattices comprising two-dimensional quantum wells (QWs) of nominal thicknesses 1, 2, and 4 monolayers (MLs) in order to obtain a correlation between their average composition, geometry, and strain. The high angle annular dark field Z-contrast observations were quantified for such layers, regarding the indium content of the QWs, and were correlated to their strain state using peak finding and geometrical phase analysis. Image simulations taking into thorough account the experimental imaging conditions were employed in order to associate the observed Z-contrast to the indium content. Energetically relaxed supercells calculated with a Tersoff empirical interatomic potential were used as the input for such simulations. We found a deviation from the tetragonal distortion prescribed by continuum elasticity for thin films, i.e., the strain in the relaxed cells was lower than expected for the case of 1 ML QWs. In all samples, the QW thickness and strain were confined in up to 2 ML with possible indium enrichment of the immediately abutting MLs. The average composition of the QWs was quantified in the form of alloy content.

Computational analysis via molecular dynamics and density functional theory simulations elucidated the structural and electronic properties of a-type basal edge dislocations lying in the ⟨1–100⟩ direction in wurtzite GaN. As a particular and predominant type of misfit dislocations, experimentally identified in coherently grown semipolar heterostructures, understanding of their properties at the atomistic level is crucial for exploring the growth conditions of the material and improving its performance in its various applications. A total of six core configurations are revealed for the first time and investigated systematically. The energetic hierarchy of these core configurations and their electronic structures are determined. The two shuffle core configurations 3-1 and 4-2 are found to be energetically favorable with respect to the glide 2-2 core in both polarities. It is demonstrated that all the core configurations of the a-type basal dislocations introduce multiple gap states which leads to a narrowing of the bandgap in comparison to that of the pristine material, with the N-polar configurations having a more pronounced and detrimental impact. The presence of Ga-related dangling bonds in the vicinity of the core is widely observed, leading to a shift of the Fermi level to the related s orbitals.

The effect of different spacer materials (MgO, W, and Pt) on the magnetic coupling in FePt/spacer/FePt trilayers has been carefully investigated. MgO results in magnetically coupled FePt layers with perpendicular magnetic anisotropy (PMA); W gives rise to a magnetically coupled system consisting of layers with PMA and in-plane magnetic anisotropy whereas Pt results in magnetically decoupled FePt layers with PMA. The trilayer microstructure is essential for explaining the obtained results. The growth mode of the top FePt layer is strongly affected by the underlying non-magnetic spacer, with occurrence of different morphologies; in particular, L1(0) FePt islands grow on MgO, a continuous FePt layer with fcc crystal structure is obtained on W, whereas a continuous layer with L10 structure is observed when the top layer is deposited on Pt.

The aim of this work is to investigate the microstructure and microhardness of glassy and glass-ceramic products synthesized through vitrfication and devitrification of chromium containing tannery ash with SiO2, Na2O and CaO glass forming oxides. Four different batch mixtures were studied with varying proportions of chromium ash and glass formers. Electron Microscopy study was performed that included Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) to study the morphology and composition of the products in the microscale. Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM) observations were conducted to investigate structures up to the atomic scale and to correlate the structural properties with the stabilization process. Static microindentation was applied, using Knoop geometry, to determine the microhardness of the products while Vickers geometry was used to study crack propagation on their surfaces in order to test their potential use in various applications. Out of one, the initial vitrified products were amorphous with Eskolaite (Cr2O3) flakes dispersed inside the silicate matrix while the product with the lowest relative proportion of chromium ash was a homogenous glass. Furthermore, thermal treatment was conducted to the vitrified products and resulted to the separation of Devitrite, Combeite and Wollastonite crystalline phases from the silicate matrix depending on the initial batch composition of the glasses and the relative thermal treatment. The microhardness of the products was higher after thermal processing and it has been shown that the morphology of the separated crystal phases affected also the crack propagation.

The effect of strain on the elastic constants of GaN and InN, elaborating the stain state of pseudo-morphically grown heterostructures comprising these materials, is investigated. Towards this direction, density functional theory (DFT) calculations using local density approximation (LDA) are performed for studying the electronic and elastic properties of GaN and InN. This systematic study constitutes the first assessment of the performances of the Christiansen modified pseudopotentials in reproducing the elastic constants of nitride materials. Calculation of the elastic constants is not limited up to the previously reported range of strain variable delta = 0.02 [1] but their dependence on the strain state is investigated in an extended range of strain, up to delta = 0.1, which is the strain in pseudomorphic GaN/InN multiple quantum wells (QWs). The elastic constants C-11, C-12, and C-13 are more influenced by the type of applied strain as it is deduced from the interrelated energy density curves. The plateau depicted in the U-2 Energy Density curve over delta = +0.07, in both GaN and InN, could be attributed to the nonlinear behaviour that should be expected in the highly strained GaN/InN heterostructures, and which should affect their electronic properties. (c) 2017 Elsevier B.V. All rights reserved.

We carry out ab initio electronic structure calculations of (SixSn1−x)3N4 using density functional theory with projector augmented-wave potentials under the generalized gradient approximation. We find that the energetically favorable structure of Sn3N4 is the face-centered cubic spinel structure, followed by the hexagonal structure which has energy band gaps of 1.85eV and 1.44eV respectively. The (SixSn1−x)3N4 ternary compound can exhibit both cubic and hexagonal crystal structures over the full range of x. However, the cubic structure is found to be energetically favorable for x<0.3 above which the hexagonal structure of (SixSn1−x)3N4 dominates. The energy band gap can be tuned continuously from 1.44eV up to 5.8eV in the case of the hexagonal crystal structure of (SixSn1−x)3N4 and from 1.85eV to 4.82eV in the case of cubic (SixSn1−x)3N4. Nevertheless the energy gap of (SixSn1−x)3N4 is direct only for x<0.3 when it is cubic and for x<0.5 when hexagonal.

Interatomic potential based molecular dynamics and ab initio calculations are employed to investigate the structural, thermal, and electronic properties of polar GaN/AlN core/shell nanowires. Nanowire models for the molecular dynamics simulations contain hundreds of thousands of atoms with different shell-to-nanowire ratios. The energetic and structural properties are evaluated through a detailed examination of the strain, the stress, and the displacement fields. It is found that the relaxation of the AlN shell is initiated at the edges, with the shell becoming increasingly stress free when the shell-to-nanowire ratio is increased. The basal lattice parametera of the AlN shell is found to have a smaller value than the value predicted by the elasticity theory. The stresses on the GaN core are strongly influenced by the shell. The core retains the alattice parameter of bulk GaN only up to a shell-to-nanowire ratio equal to 0.10 and is significantly compressed beyond this point. Concerning the thermal properties, the molecular dynamics simulations conclude that there is a linear relationship between the thermal conductivity and the shell-to-core area ratio of the GaN/AlN core/shell nanowires. The bandgaps of the nanowires are calculated through ab initio calculations of 103 atoms and the influence of the structural characteristics on the electronic properties is investigated. A well-defined relationship that predicts the bandgap of the GaN/AlN nanowires, follows the 2nd order Vegard's law and taking into account the shell-to-nanowire ratio, is established. Finally, the valence band maximum is found to be dominated by the surface N-2p levels, while the conduction band minimum is dominated by the core and interface Ga-3s, and the surface Al-2s levels.

The vitrification process was applied for the stabilization and solidification of a rich in chromium ash that was the by-product of incineration of tannery sludge. Six different batch compositions were produced, based on silica as the glass former and sodium and calcium oxides as flux agents. As-vitrified products (monoliths) were either composed of silicate matrices with separated from the melt Eskolaite (Cr2O3) crystallites or were homogeneous glasses (in one case). All as-vitrified products were thermally treated in order to transform them to partially crystallized, i.e. devitrified products. Devitrification is an important part of the work since studying the transformation of the initial as-vitrified products into glass-ceramics with better properties could result to stabilized products with potential added value. The devitrified products were diversified by the effective crystallization mode and separated crystal phase composition. These variations originated from differences in: (a) batch composition of the initial as-vitrified products and (b) thermal treatment conditions. In devitrified products crystallization led to the separation of Devitrite (Na2Ca3Si6O16), Combeite (Na4Ca4Si6O18) and Wollastonite (CaSiO3) crystalline phases, while Eskolaite crystallites were not affected by thermal treatment. Leaching test results revealed that chromium was successfully stabilized inside the as-vitrified monoliths. Devitrification impairs chromium stabilization, only in the case where the initial as-vitrified product was a homogeneous glass. In all other cases, devitrification did not affect successful chromium stabilization.