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.

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.

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.

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.

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.

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.

An efficient evolutionary structure prediction algorithm in combination with ab initio calculations is implemented in order to reveal energetically favorable superstructures of the III-Nitride ternary alloys. Several 2 x 2 x 2 32-atom supercells are used to explore the full range of concentrations, from x = 0 to 1.The formation enthalpies, bandgaps, clustering and/or ordering of the atoms are investigated and the results are discussed. The formation enthalpy plots show local minima at specific concentrations, namely for x = 0.25, 0.50 and 0.75, that correspond to ordered structures. The valance band maxima, conduction band minima, bandgaps and the composition-independent bowing parameters for 2nd order Vegard's equation are calculated. The bandgap deviations from 1st order Vegard's law show total maxima at specific concentrations. The formation enthalpies and the bandgaps cannot be accurately described by single composition-independent bowing parameters, but the bandgaps are sufficiently described by composition-dependent bowing parameters, that are established.In order to verify the rationality of the results against the size of the ab initio supercells, molecular dynamics calculations of 14 x 14 x 14 supercells of similar to 10(4) atoms using bond-order interatomic potentials are performed. The obtained local minima of the formation enthalpy for the specific alloy compositions concur with those predicted by ab initio calculations proving the results are not influenced by the super-cell size. (C) 2016 Elsevier B.V. All rights reserved.

Surface treatment of the foreign substrate is a critical factor influencing heteroepitaxial catalyst-free growth of nanowires, their crystal quality, their diameter and their areal density. To this end, catalyst-free growth of GaN nanowires on Al2O3(0001) by plasma-assisted molecular beam epitaxy was achieved using the following substrate surface treatments: (a) deposition of a SixNy layer on nitridated Al2O3 surface, and (b) deposition of Si on bare Al2O3 surface. The nanostructure of GaN nanowires and GaN/Al2O3 interfaces was explored by quantitative high-resolution transmission electron microscopy and related analytical methods. Spontaneous growth of GaN nanowires was realized on the amorphous SixNy layer, while a discontinuous crystalline zone in contact with Al2O3 was identified as partially strained AlN. Subsequently, GaN nanowires were directly grown on top of Al2O3 among stress-free Si islands. The orientation relation of these islands with the substrate was the [112](1¯1¯1)Si//[11¯00](0001)Al2O3, providing the minimum lattice misfit between the two structures. Increasing the Si deposition time a higher density of Si islands was realized, leading to non-coalesced nanowires of lower density and better structural quality. Hence, the presence of Si islands induced a mask-like effect on the nucleation of GaN nanowires that can be exploited for a controlled catalyst-free growth of nanowires.

A tannery sludge, produced from physico-chemical treatment of tannery wastewaters, was incinerated without any pre-treatment process under oxic and anoxic conditions, by controlling the abundance of oxygen. Incineration in oxic conditions was performed at the temperature range from 300 °C to 1200 °C for duration of 2 h, while in anoxic conditions at the temperature range from 400 °C to 600 °C and varying durations. Incineration under oxic conditions at 500 °C resulted in almost total oxidation of Cr(III) to Cr(VI), with CaCrO4 to be the crystalline phase containing Cr(VI). At higher temperatures a part of Cr(VI) was reduced, mainly due to the formation of MgCr2O4. At 1200 °C approximately 30% of Cr(VI) was reduced to Cr(III). Incineration under anoxic conditions substantially reduced the extent of oxidation of Cr(III) to Cr(VI). Increase of temperature and duration of incineration lead to increase of Cr(VI) content, while no chromium containing crystalline phase was detected.

AlN/GaN heterostructures have been studied using density-functional pseudopotential calculations yielding the formation energies of metal vacancies under the influence of local interfacial strains, the associated charge distribution and the energies of vacancy-induced electronic states. Interfaces are built normal to the polar <0 0 0 1> direction of the wurtzite structure by joining two single crystals of AlN and GaN that are a few atomic layers thick; thus, periodic boundary conditions generate two distinct heterophase interfaces. We show that the formation energy of vacancies is a function of their distance from the interfaces: the vacancy-interface interaction is found repulsive or attractive, depending on the type of the interface. When the interaction is attractive, the vacancy formation energy decreases with increasing the associated electric charge, and hence the equilibrium vacancy concentration at the interface is greater. This finding can reveal the well-known morphological differences existing between the two types of investigated interfaces. Moreover, we found that the electric charge is strongly localized around the Ga vacancy, while in the case of Al vacancies is almost uniformly distributed throughout the AlN/GaN heterostructure. Crucially, for the applications of heterostructures, metal vacancies introduce deep states in the calculated bandgap at energy levels from 0.5 to 1 eV above the valence band maximum (VBM). It is, therefore, predicted that vacancies could initiate ‘green luminescence’ i.e. light emission in the energy range of 2.5 eV stemming from electronic transitions between these extra levels, and the conduction band, or energy levels, due to shallow donors.

In the present study, the effects of various types of strain and indium concentration on the total energy and optoelectronic properties of GaN nanowires (NWs) with embedded InxGa1−xN nanodisks (NDs) are examined. In particular, the bi-axial, hydrostatic, and uniaxial strain states of the embedded InxGa1−xN NDs are investigated for multiple In concentrations. Density functional theory is employed to calculate the band structure of the NWs. The theoretical analysis finds that the supercell-size-dependent characteristics calculated for our 972-atom NW models are very close to the infinite supercell-size limit. It is established that the embedded InxGa1−xN NDs do not induce deep states in the band gap of the NWs. A bowing parameter of 1.82 eV is derived from our analysis in the quadratic Vegard's formula for the band gaps at the various In concentrations of the investigated InxGa1−xN NDs in GaN NW structures. It is concluded that up to ∼10% of In, the hydrostatic strain state is competitive with the bi-axial due to the radial absorption of the strain on the surfaces. Above this value, the dominant strain state is the bi-axial one. Thus, hydrostatic and bi-axial strain components coexist in the embedded NDs, and they are of different physical origin. The bi-axial strain comes from growth on lattice mismatched substrates, while the hydrostatic strain originates from the lateral relaxation of the surfaces.

The devitrification routes of vitrified materials containing chromium-loaded ash were studied in this work. Chromium-loaded ash originates from the incineration of tannery sludge. Vitrification was applied using SiO2, Na2O and CaO. Three different batch compositions were studied with the relative proportions of SiO2 and Na2O kept constant, and varying proportions of chromium-loaded ash and CaO. All vitrified products were thermally treated in order to produce glass–ceramic materials, i.e. to induce devitrification. Thermal treatment temperatures were selected by application of differential thermal analysis. All products were characterized with X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. The resulting glass–ceramic products possessed different microstructures, i.e. composition, morphology and spatial distribution of separated ceramic phases, depending on the chromium-loaded ash content and thermal treatment temperature. The results show that a combination of differential thermal analysis with morphological, structural and elemental characterization methods renders microstructural tailoring and control of physical properties feasible.

We investigate the structural properties of a series of high alloy content InGaN epilayers grown by plasma-assisted molecular beam epitaxy, employing the deposition temperature as variable under invariant element fluxes. Using transmission electron microscopy methods, distinct strain relaxation modes were observed, depending on the indium content attained through temperature adjustment. At lower indium contents, strain relaxation by V-pit formation dominated, with concurrent formation of an indium-rich interfacial zone. With increasing indium content, this mechanism was gradually substituted by the introduction of a self-formed strained interfacial InGaN layer of lower indium content, as well as multiple intrinsic basal stacking faults and threading dislocations in the rest of the film. We show that this interfacial layer is not chemically abrupt and that major plastic strain relaxation through defect introduction commences upon reaching a critical indium concentration as a result of compositional pulling. Upon further increase of the indium content, this relaxation mode was again gradually succeeded by the increase in the density of misfit dislocations at the InGaN/GaN interface, leading eventually to the suppression of the strained InGaN layer and basal stacking faults.

Epitaxial growth of InAs on semi-insulating GaAs was a subject of various attempts to reduce the influence of the ~7% lattice mismatch on the InAs layer properties. The most cost effective and promising method appears to be the growth of low temperature buffer (LTB) InAs layer at ~400°C followed by a thick InAs layer at ~600°C. There is a scarcity of available information about the structural properties of the LTB layers, their background conductivity type and level of doping. We have found that a predominant part of the threading dislocations generated at the interface annihilate within the first 400nm.The average misfit dislocation spacing is 6.15nm, proving that the LTB InAs/GaAs interface is nearly completely relaxed. XRD measurements have revealed a well pronounced deformation decrease in the LTB layers for thicknesses above 300nm. The LTB InAs layer is n-type with carrier concentration of the order of 5×1016cm−3 and can be additionally doped with Te and hence can serve as a bottom contact layer. The morphology of unintentionally doped InAs layer grown at 600°C upon the LTB shows sub-nanometer flatness and carrier concentration of the order of 5×1015cm−3.

An important step towards optimization of InN/GaN based devices is identification of the structural characteristics of the corresponding interfaces since the favourable bonding configurations determine materials polarity and consequently the direction of spontaneous polarization. We have addressed this issue through calculations on InN/GaN interfaces comprising misfit dislocations (Kioseoglou et al., J. Mater. Sci. 43, 3982 (2008) [5]). In our present study, additional calculations are performed on subcritical thickness InN/GaN QWs, which exhibit lower dislocation densities as well as reduced InN decomposition and are currently implemented in the fabrication of near-UV light emitting diodes. Ab initio calculations are performed under modified pseudopotentials, accurately reproducing the InN and GaN band gap values, on supercells comprising multilayers of one monolayer (ML) thick InN elastically strained in 5 nm thick GaN barriers having a wurtzite or zinc blende stacking at the interface. The former is found to be energetically favorable. Subsequent calculations on supercells comprising 1 ML thick InN in 8 and 11 nm thick GaN barriers as well as 3 ML InN in 11 nm thick GaN, depict a variation in III-N bond lengths. This variation becomes more significant as the barrier thickness decreases or the QW thickness increases. Hence the strain and consequently piezoelectric polarization are modified. Our results scrutinize recent experimental observations and could prove beneficial for tailoring the optoelectronic properties of InN/GaN QWs. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Based on bicrystal symmetry and transmission electron microscopy observations, we elaborate on the coexistence of the two orientation variants of semipolar s-plane ($1{\bar {1}}01$) InN epilayers grown on r-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE). It is shown that variant coexistence is favored by a high order of coincident symmetry ensuring significant lattice continuity. The (0002)?||?($01{\bar {1}}0$) low-energy grain boundary was identified to principally delimit the two InN variants. Aside of the variant coexistence, the InN/sapphire interface was observed to comprise protrusions attributed to the InN buffer layer growth. Rapid thermal annealing was employed in order to improve the epilayer quality and it was found to induce defect reduction attributed to dislocation glide. However, the InN/sapphire interface was adversely affected by this process.