M. Katsikini

Raman study of Mg, Si, O, and N implanted GaN

The effect of Mg, Si, N, and O ion implantation (with doses in the range 5×1013–1×1018 cm−2), in epitaxially grown GaN samples has been studied using Raman spectroscopy. It is found that implantation increases the static disorder and activates modes that were not allowed in the as-grown material. More specifically it causes the appearance of three additional Raman peaks at 300, 420, and 670 cm−1. It is found that the position of these peaks does not depend on the type of the implant and thus they do not correspond to local vibrational modes. They are attributed to disorder activated Raman scattering (300 cm−1) and/or to implantation induced N and Ga vacancies or interstitials (420 and 670 cm−1). Finally, ion implantation causes a marginal increase of the build-in hydrostatic stress.

A novel approach for arsenic adsorbents regeneration using MgO

An integrated procedure for the regeneration of iron oxy-hydroxide arsenic adsorbents by granulated MgO is proposed in this study. A continuous recirculation configuration, with a NaOH solution flowing sequentially through the saturated adsorbent (leaching step) and the MgO (adsorption step) column beds, was optimized by utilizing the high arsenic adsorption efficiency of MgO at strong alkaline environments. Experimental results indicated that the total amount of leached arsenic was captured by MgO whereas the regenerated iron oxy-hydroxide recovered around 80% of its removal capacity upon reuse. The improved adsorption capacity of MgO for As(V), which is maximized at pH 10, is explained by the intermediate hydration to Mg(OH)2 and the following As(V) oxy-anions adsorption on its surface through the formation of monodentate inner sphere complexes, as it is deduced from the AsK-edge X-ray absorption fine structure (EXAFS) analysis. In addition to the economical-benefits, corresponding tests proved that the solid wastes of this process, namely spent MgO/Mg(OH)2, can be environmentally safely disposed as stable additives in cement products, while the alkaline solution is completely detoxified and can be recycled to the regeneration task.

Experimental determination of the N-p-partial density of states in the conduction band of GaN: Determination of the polytype fractions in mixed phase samples

Hexagonal, cubic, and mixed phase GaN samples grown by molecular beam epitaxy are studied using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The spectra were recorded at the N-K-edge, at several angles of incidence. It is shown that the N-K-edge NEXAFS spectra, which are proportional to the p-partial density of states in the conduction band, are characteristic of the cubic or hexagonal structure of the examined crystal. The spectra of the cubic sample do not depend on the angle of incidence (θ), contrary to the spectra of the hexagonal sample in which the areas under the NEXAFS resonances depend linearly on cos2 θ. From the fitting of the lines Ai=A+B cos2 θ, where Ai are the areas under the resonances and A and B are constants, the directions of maximum electron charge density with respect to the normal to the surface are determined for the hexagonal sample. The energy positions of the absorption edge and the NEXAFS resonances in the spectra from the cubic sample are different from th...

Tetravalent manganese feroxyhyte: a novel nanoadsorbent equally selective for As(III) and As(V) removal from drinking water.

The development of a single-phase Fe/Mn oxy-hydroxide (δ-Fe0.76Mn0.24OOH), highly efficient at adsorbing both As(III) and As(V), is reported. Its synthesis involves the coprecipitation of FeSO4 and KMnO4 in a kilogram-scale continuous process, in acidic and strongly oxidizing environments. The produced material was identified as a manganese feroxyhyte in which tetravalent manganese is homogeneously distributed into the crystal unit, whereas a second-order hollow spherical morphology is favored. According to this structuration, the oxy-hydroxide maintains the high adsorption capacity for As(V) of a single Fe oxy-hydroxide combined with enhanced As(III) removal based on the oxidizing mediation of Mn(IV). Ion-exchange between arsenic species and sulfates as well as the strongly positive surface charge further facilitate arsenic adsorption. Batch adsorption tests performed in natural-like water indicate that Mn(IV)-feroxyhyte can remove 11.7 μg As(V)/mg and 6.7 μg As(III)/mg at equilibrium pH 7, before residual concentration overcomes the regulation limit of 10 μg As/L for drinking water. The improved efficiency of this material, its low cost, and the possibility for scaling-up its production to industry indicate the high practical impact and environmental importance of this novel adsorbent.

Identification of implantation-induced defects in GaN: A near-edge x-ray absorption fine structure study

We apply near-edge x-ray absorption fine structure spectroscopy, at the N K edge, in order to identify the signature of implantation-induced defects in the partial density of empty states in GaN implanted with O, Mg, and Si ions. The dose range was 1014–1018 cm−2. It is found that two of the implantation-induced defects introduce characteristic resonances (hereafter called RL1 and RL2) in the near-edge x-ray absorption fine structure spectra. RL1 appears 1.7 eV below the absorption edge, its formation is independent of the projectile and the implantation dose, and is attributed to nitrogen interstitials. RL2, which appears at about 1.0 eV above the absorption edge, is generated when the dose exceeds 1016 cm−2 and is attributed to nitrogen dangling bonds.

APPLICATION OF NEAR-EDGE X-RAY ABSORPTION FINE STRUCTURE FOR THE IDENTIFICATION OF HEXAGONAL AND CUBIC POLYTYPES IN EPITAXIAL GAN

Cubic and hexagonal GaN thin films are studied using the angular dependence of the near‐edge x‐ray absorption fine structure (NEXAFS) spectra recorded at the N–K edge. It is shown that the energy positions of the NEXAFS resonances are characteristic of the cubic and hexagonal structures and, thus, the NEXAFS spectra can be used as a fingerprint of the symmetry of the examined crystal. Deviations from the zinc‐blende or wurtzite phases are clearly detectable since they lead to a systematic energy shift in the NEXAFS resonances. Finally, a method is proposed for the quantitative estimation of the percentages of the cubic and wurtzite phases present in a mixed crystal.

Structural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy

Heteroepitaxial non-polar III-Nitride layers may exhibit extensive anisotropy in the surface morphology and the epilayer microstructure along distinct in-plane directions. The structural anisotropy, evidenced by the “M”-shape dependence of the (112¯0) x-ray rocking curve widths on the beam azimuth angle, was studied by combining transmission electron microscopy observations, Raman spectroscopy, high resolution x-ray diffraction, and atomic force microscopy in a-plane GaN epilayers grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). The structural anisotropic behavior was attributed quantitatively to the high dislocation densities, particularly the Frank-Shockley partial dislocations that delimit the I1 intrinsic basal stacking faults, and to the concomitant plastic strain relaxation. On the other hand, isotropic samples exhibited lower dislocation densities and a biaxial residual stress state. For PAMBE growth, the anisotropy was correlated to N-rich (or Ga-poor) condit...

Optimization of tetravalent manganese feroxyhyte's negative charge density: A high-performing mercury adsorbent from drinking water

This study demonstrates an optimization procedure for the development of an Hg-specified adsorbent able to comply with the regulation limit for drinking water of 1μg/L. On this purpose, the synthesis of Mn(IV)-feroxyhyte was modified to achieve high negative charge density by combining alkaline and extreme oxidizing conditions. In particular, precipitation of FeSO4 at pH9 and excess of KMnO4 follows a very fast nucleation step providing a product with very small nanocrystal size (1-2nm), high specific surface area (300m2/g) and maximum negative charge density (1.8mmol H+/g). The adsorbent was validated for Hg removal in batch experiments and column tests using natural-like water indicating an adsorption capacity as high as 2.5μg/mg at equilibrium concentration 1μg/L under reliable conditions of application. Importantly, the adsorption is an exothermic spontaneous process, resulting in the formation of inner sphere complexes by sharing both A-type and B-type oxygen atoms with the metal surface octahedral as revealed by the X-ray absorption fine structure results.

NITROGEN K-EDGE NEXAFS MEASUREMENTS ON GROUP-III BINARY AND TERNARY NITRIDES

Abstract It is demonstrated that the NEXAFS spectra are a “fingerprint” of the microstructure and the composition of the ternary alloys In0.16Ga0.84N and AlyGa1−yN (y=0.25, 0.5) and the parent binary compounds GaN, AlN and InN. From the angular dependence of the N K-edge NEXAFS spectra, the hexagonal symmetry of the under study compounds is deduced and the (px, py) or pz character of the final state is identified. The energy position of the absorption edge (Eabs) of the binary compounds GaN, AlN and InN is found to red-shift linearly with the atomic number of the cation. The Eabs of the AlyGa1−yN alloys takes values in between those corresponding to the parent compounds AlN and GaN. Contrary to that, the Eabs of In0.16Ga0.84N is red-shifted relative to that of GaN and InN, probably due to ordering and/or phase separation phenomena.

Determination of the local microstructure of epitaxial AlN by x-ray absorption

Hexagonal AlN, epitaxially grown on GaAs, is studied using x-ray absorption measurements at the N-, O-, and Al-K edges. The measured distances of the nitrogen nearest neighbors (nn) are found shorter than expected by 0.04–0.13 A for the first to third nn shells, respectively. Nitrogen atoms are fourfold coordinated with Al as first nearest neighbors (nn), while Al atoms are fourfold coordinated with N and O atoms. The Al–N distance is 1.85 A, i.e., it is smaller by 0.04 A than expected. A reduction in the nn distances is also observed in the second and third nn shells where the N–N and the N–Al distances are found 0.07 and 0.13 A shorter than expected, respectively. The reduction in the nn distances is attributed to the presence of oxygen contamination. The Al–O distance is only 0.18 A smaller than the Al–N distance, i.e., O is a substitutional impurity in the N sublattice. The characteristic angular dependence of the N-K edge NEXAFS spectra verifies the hexagonal structure of the AlN film. Moreover, from...