Ga K-edge Extended X-ray Absorption Research Articles

Experimental and theoretical evidence for hydrogen doping in polymer solution-processed indium gallium oxide

The field-effect electron mobility of aqueous solution-processed indium gallium oxide (IGO) thin-film transistors (TFTs) is significantly enhanced by polyvinyl alcohol (PVA) addition to the precursor solution, a >70-fold increase to 7.9 cm2/Vs. To understand the origin of this remarkable phenomenon, microstructure, electronic structure, and charge transport of IGO:PVA film are investigated by a battery of experimental and theoretical techniques, including In K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS); resonant soft X-ray scattering (R-SoXS); ultraviolet photoelectron spectroscopy (UPS); Fourier transform-infrared (FT-IR) spectroscopy; time-of-flight secondary-ion mass spectrometry (ToF-SIMS); composition-/processing-dependent TFT properties; high-resolution solid-state 1H, 71Ga, and 115In NMR spectroscopy; and discrete Fourier transform (DFT) analysis with ab initio molecular dynamics (MD) liquid-quench simulations. The 71Ga{1H} rotational-echo double-resonance (REDOR) NMR and other data indicate that PVA achieves optimal H doping with a Ga···H distance of ∼3.4 Å and conversion from six- to four-coordinate Ga, which together suppress deep trap defect localization. This reduces metal-oxide polyhedral distortion, thereby increasing the electron mobility. Hydroxyl polymer doping thus offers a pathway for efficient H doping in green solvent-processed metal oxide films and the promise of high-performance, ultra-stable metal oxide semiconductor electronics with simple binary compositions.

Short range order and topology of Ge Ga Te100-2 glasses

Chemical short range order and topology of Ge$_{x}$Ga$_{x}$Te$_{100-2x}$ glasses was investigated by neutron- and x-ray diffraction as well as Ge and Ga K-edge extended x-ray absorption fine structure (EXAFS) measurements. Large scale structural models were obtained by fitting experimental datasets simultaneously with the reverse Monte Carlo simulation technique. Models, relying only on experimental data and basic physical information without constraining the average coordination numbers, give 3.9 - 4.1 for the number of the atoms in the first coordination sphere of Ge atoms, while the average number of first neighbors of Ga atoms scatters around 3.8. The average coordination number of Te atoms is significantly higher than 2 for $x$ = 12.5 and 14.3. It is found that the vast majority of MTe$_4$ (M=Ge or Ga) tetrahedra have at least one corner sharing MTe$_4$ neighbor.

Local atomic structure of the GaN-side of the Al2O3/GaN interface revealed by X-ray absorption spectroscopy

The interface between a gate insulator (Al2O3) and a semiconductor (GaN) was investigated via surface-sensitive Ga K-edge extended X-ray absorption fine structure spectroscopy, achieved by detecting the GaLMM Auger electrons originated from the Ga K-shell absorption. This GaN-side interface study was conducted on Al2O3 thin films formed via atomic layer deposition. The determined atomic structures revealed GaN crystalline changes and the formation of Ga–O bonds due to nitrogen annealing.

Local atomic structure analysis of GaN surfaces via X-ray absorption spectroscopy by detecting Auger electrons with low energies.

GaN is a promising material for power semiconductor devices used in next-generation vehicles. Its electrical properties such as carrier mobility and threshold voltage are affected by the interface between the oxide and the semiconductor, and identifying the interface states is important to improve these properties. A surface-sensitive measurement of Ga K-edge extended X-ray absorption fine structure (EXAFS) by detecting Ga LMM Auger electrons that originate from Ga K-shell absorption is proposed for GaN. LMM Auger electrons with low energies were detected and the EXAFS oscillation was confirmed, providing information on the Ga atoms at the surface. Investigation of thermally oxidized GaN with an oxide film of defined thickness showed that the analysis depth was less than 10 nm, which is consistent with the inelastic mean free path of 2.3 nm estimated for LMM Auger electrons in GaN.

Magnetostriction and microstructure of melt-spun Fe77Ga23 ribbons prepared with different wheel velocities

A set of stacked ribbons with the composition of Fe77Ga23 were prepared with different wheel velocities of 7 m/s, 12.5 m/s and 25 m/s (named as S7, S12.5 and S25, respectively). High resolution X-ray diffraction patterns of these ribbons show that all the ribbons present the disordered A2 structure, whereas an additional modified-DO3 phase is detected in S12.5 and S25. S25 has stronger (100) texture than other two samples. Ga K-edge extended X-ray absorption fine structure results indicate that both bond distance and the number of Ga atoms in the second neighbor shell around Ga decrease with increasing wheel velocity. No Ga cluster is detected in the studied ribbons. A short-range ordering Ga-rich phase and large local strain have no obvious influence on magnetostriction of S7. It is believed that both the (100) texture and the additional modified-DO3 phase play a positive role in magnetostrictive properties of Fe77Ga23 ribbons.

Extended x-ray absorption fine structure in Ga1−xMnxN/SiC films with high Mn content

In this study, the local atomic structure of highly homogeneous Ga1−xMnxN alloy films (0.03 < x < 0.09) is analyzed by means of Mn and Ga K-edge extended x-ray absorption fine structure measurements. From the curve fitting, the structural parameters corresponding to the first two atomic shells surrounding both Ga and Mn atoms are reported. In the Ga1−xMnxN films, grown by molecular beam epitaxy, the Mn atoms are in tetrahedral configuration, independent of the Mn concentration; that is, they are in a substitutional site, MnGa, in the wurtzite structure. A small increase in the interatomic distances has been found with increasing Mn content. The Debye-Waller factor does not show a significant trend as Mn content increases, which suggests the presence of short-range disorder in the GaN lattice. Ab initio calculations of the structural parameter for two different Mn concentrations are consistent with the experimental results.

Preparation, structural characterisation and antibacterial properties of Ga-doped sol–gel phosphate-based glass

A sol–gel preparation of Ga-doped phosphate-based glass with potential application in antimicrobial devices has been developed. Samples of composition (CaO)0.30(Na2O)0.20−x(Ga2O3)x(P2O5)0.50 where x = 0 and 0.03 were prepared, and the structure and properties of the gallium-doped sample compared with those of the sample containing no gallium. Analysis of the 31P MAS NMR data demonstrated that addition of gallium to the sol–gel reaction increases the connectivity of the phosphate network at the expense of hydroxyl groups. This premise is supported by the results of the elemental analysis, which showed that the gallium-free sample contains significantly more hydrogen and by FTIR spectroscopy, which revealed a higher concentration of –OH groups in that sample. Ga K-edge extended X-ray absorption fine structure and X-ray absorption near-edge structure data revealed that the gallium ions are coordinated by six oxygen atoms. In agreement with the X-ray absorption data, the high-energy XRD results also suggest that the Ga3+ ions are octahedrally coordinated with respect to oxygen. Antimicrobial studies demonstrated that the sample containing Ga3+ ions had significant activity against Staphylococcus aureus compared to the control.

Local atomic structure of martensiticNi2+xMn1−xGa: An EXAFS study

The local atomic structure of Ni$_{2+x}$Mn$_{1-x}$Ga with 0 $\le$ $x$ $\le$ 0.16 alloys was explored using Mn and Ga K-edge Extended X-ray Absorption Fine Structure (EXAFS) measurement. Inorder to study the atomic re-arrangements that occur upon martensitic transformation, room temperature and low temperature EXAFS were recorded. The changes occurring in the L2$_1$ unit cell and the bond lengths obtained from the analysis enables us to determine the modulation amplitudes over which the constituent atoms move giving rise to shuffling of the atomic planes in the modulated structure. The EXAFS analysis also suggests the changes in hybridization of Ga-$p$ and Ni-$d$ orbitals associated with the local symmetry breaking upon undergoing martensitic transition.

Multiple-scattering effects in Ga K-edge extended x-ray absorption fine structure spectra ofGaP, GaAs and GaSb semiconductor compounds

The dependence of Ga K-edge multiple-scattering extended x-ray absorption fine structure(MS-EXAFS) effects on the nearest neighbours in GaP, GaAs and GaSb semiconductorcompounds with the zinc blende structure has been comprehensively investigated byconsidering the coordination environment within the first three shells around the Ga atoms.It is revealed that in the case of GaP with a light element as the first neighbourof the Ga absorber, the MS-EXAFS effects are negligibly weak with respect tothe single-scattering (SS) contribution. For GaAs and GaSb compounds withheavier elements as the first neighbour of the Ga absorber, the MS effects becomeincreasingly important and are dominated by a triangular double-scattering path DS2(). The EXAFS contribution of the DS2 path destructively interferes with that of thesecond shell single-scattering path (SS2), with the amplitude ratio of DS2 to SS2 risingfrom 7% for GaP to 25 and 70% for GaAs and GaSb, respectively. This indicates that thesecond shell peak magnitude for these compounds is increasingly damped by the MS effectsas the first nearest neighbour goes from P to Sb. Based on these results, we present ageneralized and simplified high-shell MS-EXAFS analysis method for compounds with theopen structure of zinc blende.

Synthesis of NiGa Layered Double Hydroxides. A Combined EXAFS, SAXS, and TEM Study. 3. Synthesis at Constant pH

The present paper focuses on the direct synthesis of NiGa layered double hydroxides (LDHs) by concomitant addition of Ni and Ga chloride in solution with NaOH solution, at a constant pH value of 6.5. Various Ni/Ga ratios (Ni/Ga = 2, Ni/Ga = 4, or Ni/Ga = 6) were investigated, and samples were collected after successive additions. Each sample was then analyzed by small-angle X-ray scattering (SAXS) to derive information about the shape of the objects formed upon synthesis. In parallel Ni K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS) analysis were carried out on relevant samples to obtain information on the local structure of the species in suspension. SAXS curves reveal significant changes. At the beginning of the synthesis, for low added volumes (V(s)), the suspension appears to contain both monodimensional and bidimensional objects. For higher V(s), the monodimensional objects disappear and only platelike particles are detected in the suspension. EXAFS results show that all the added gallium atoms are involved in a solid Ni-Ga LDH phase throughout the whole synthesis. In contrast, some nickel cations do not precipitate in the Ni-Ga LDH. At the beginning of the synthesis, for low added volumes they are also involved in the formation of fibrous polycations. For higher added volumes, the fibers are not observed anymore and monomeric nickel species are then present in the suspension.

Synthesis of NiGa Layered Double Hydroxides. A Combined EXAFS, SAXS, and TEM Study. 2. Hydrolysis of a Ni2+/Ga3+Solution

Takovites are nickel-based layered double hydroxides (LDH) with a general formula that can be written as Ni(1-x)Al(x)(OH)2, A(z-)(x/z), yH(2)O, where A is a compensating interlayer anion. As in some other LDH samples, the positive charge of the layer can be adjusted upon synthesis and various anions can be exchanged in the interlayer region. It is then important to understand the synthesis pathway of these materials. We then undertook a study on the hydrolytic behavior of pure Ni salts and mixtures of Ni and Ga salts. This paper focuses on the hydrolysis of Ni(2+) and Ga(3+) ions, together in solution, carried out by base addition. The samples will be defined by their hydrolysis ratio R = [OH(-)]/([Ni(2+)] + [Ga(3+)]). Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to obtain information on the colloidal species size and shape on a large scale. Each hydrolyzed sample was also studied by Ni K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS) to obtain information on the local structure of the species in suspension. SAXS curves reveal the presence of bidimensional objects whatever the R values. The platelets formed for R values >/=1.1 are slightly thicker and smaller in size, which may be linked to their different natures. Complementary information is provided by TEM analysis: the first colloids formed have a structure very close to that of alpha-GaOOH, as shown by electronic diffraction. Those structures are progressively replaced by Ni-Ga LDH platelets with increasing hydrolysis ratio, which are the only species in suspension for R = 2.0, as shown by XRD. EXAFS results confirm the complete hydrolysis of gallium before the formation of Ni-Ga LDH phases.

Ga K-edge EXAFS analysis on the coordination of gallium in PbO–Ga 2O 3 glasses

Ga K-edge extended X-ray absorption fine structure spectra of ( X)PbO–(1− X)Ga 2O 3 glasses, where X is 0.7, 0.75 and 0.80 in mole fraction were recorded at room temperature to understand the coordination scheme of gallium in glasses. Analyses of the spectra indicate that most of the gallium forms GaO 4 tetrahedra at the Ga–O bond distance between 0.1856 and 0.1861 nm with a negligible amount (less than 5% of total Ga-polyhedra) of GaO 6 octahedra. The longer Ga–O bond distance in glasses compared to that in β-Ga 2O 3 crystal composed of similar GaO 4 tetrahedra indicated the presence of oxygens bonded to three independent cations.