Ga 4s Research Articles

Electronic structure and optical properties of Al and Mg co-doped GaN

The electronic structure and optical properties of Al and Mg co-doped GaN are calculated from first principles using density function theory with the plane-wave ultrasoft pseudopotential method. The results show that the optimal form of p-type GaN is obtained with an appropriate Al:Mg co-doping ratio rather than with only Mg doping. Al doping weakens the interaction between Ga and N, resulting in the Ga 4s states moving to a high energy region and the system band gap widening. The optical properties of the co-doped system are calculated and compared with those of undoped GaN. The dielectric function of the co-doped system is anisotropic in the low energy region. The static refractive index and reflectivity increase, and absorption coefficient decreases. This provides the theoretical foundation for the design and application of Al—Mg co-doped GaN photoelectric materials.

Electronic properties on GaN(0001) surface – ab initio investigation

Density Functional Theory (DFT) calculations were used to determine physical properties of bulk GaN and GaN(0001), i.e. N-terminated surface. The bulk data are in agreement with the theoretical and experimental data reported by Magnuson et al. [M. Magnuson et al., Phys. Rev. B 81 (2010) 085125], demonstrating that bonding in GaN is of a mixed nature in which Ga 4s, Ga 4p, Ga 3d, and N 2s, N 2p orbitals are involved. Ga 4s and Ga 4p states are sp3 hybridized while the N states are involved independently in bonding. Ga sp3 states overlap with the N 2s states creating the upper subband extending down to 6 eV below the valence band maximum (VBM). The lower valence band (VB) subband is composed of Ga 3d states overlapping with N 2s states whose energy levels range from 13 to 17 eV below VBM. GaN bulk bonding enforces creation of the two surface states at clean GaN(0001) surface: the lower energy state, located 15 eV below VBM is composed of N 2s states. The upper energy surface state, located below VBM, is composed of N 2pz nonsaturated state, i.e. it is of the broken bond type. The Fermi energy is pinned by this dispersionless acceptor state. Ga adsorption leads to emergence of donor state, which pins the Fermi level at 1.1 eV above VBM which is compatible with the Fermi level pinned at the surface for p-type GaN and is inconsistent with n-type bulk. The total energy of slab has the energy lower by about 2 eV for the adsorbed than for the separate Ga atom. The inconsistent data obtained from thermodynamic stability and band diagram analyses suggest further investigations to explain inconsistencies between these two pictures.

A First-Principles Study of Structure-Property Correlation and the Origin of Ferrimagnetism in Gallium Ferrite

A first-principles study of structure property correlation and the origin of ferrimagnetism is presented based on LSDA+U method. In particular, the results for the ground state structure, electronic band structure, density of states, Born effective charges, spontaneous polarization and cationic disorder are discussed. The calculations were done using Vienna ab-initio simulation package (VASP) with projector augmented wave method. We find that the ground state structure is orthorhombic and insulating having A-type antiferromagnetic spin configuration. The cationic disorder is found to play an important role. Although the cationic site disorder is not spontaneous in the ground state, interchange of octahedrally coordinated Ga2 and Fe2 sites is most favored. We find that ferrimagnetism in gallium ferrite is primarily due to this exchange between Ga-Fe sites such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1 sites as well as between Fe spins at Ga2 and Fe2 sites. Further, the partial density of states shows noticeable hybridization of Fe 3d, Ga 4s, Ga 4p and O 2p states indicating some covalent character of Ga/Fe-O bonds. However, the charge density and electron localization functions show largely the ionic character of these bonds. Our calculation predicts spontaneous polarization of ~59 μC/cm2 along b-axis.

Electronic structure, Born effective charges and spontaneous polarization in magnetoelectricgallium ferrite

We present a theoretical study of the structure–property correlation ingallium ferrite, based on first-principles calculations followed by a subsequentcomparison with experiments. The local spin density approximation (LSDA + U) of the density functional theory has been used to calculate the ground statestructure, electronic band structure, density of states and Born effective charges.The calculations reveal that the ground state structure is orthorhombicPc 21n having A-type antiferromagnetic spin configuration, with lattice parameters matching wellwith those obtained experimentally. Plots of the partial density of states of constituent ionsexhibit noticeable hybridization of Fe 3d, Ga 4s, Ga 4p and O 2p states. However,the calculated charge density and electron localization function show a largelyionic character of the Ga/Fe–O bonds which is also supported by a lack of anysignificant anomaly in the calculated Born effective charges with respect to thecorresponding nominal ionic charges. The calculations show a spontaneous polarization of ∼ 59 µC cm − 2 alongthe b-axis which is largely due to asymmetrically placed Ga1, Fe1, O1, O2 and O6 ions.

Na 5(Ga 4S)(GaS 4) 3·6H 2O: A three-dimensional thiogallate containing a novel octahedral building block

The new thiogallate Na 5(Ga 4S)(GaS 4) 3·6H 2O has been prepared solvothermally, using 3,5-dimethyl pyridine as a solvent, and characterised by powder and single crystal X-ray diffraction. This material, which exhibits a three-dimensional crystal structure, crystallises in the cubic space group F 4 ¯ 3 c ( a = 17.557(4) Å). The crystal structure contains octahedral building blocks [Ga 4S (GaS 4) 6] 20−, linked into a three-dimensional network with a perovskite-type topology, and sodium hydrate clusters, [Na 5(H 2O) 6] 5+, filling the cavities in the [Ga 4S(GaS 4) 6/2] 5− framework. UV–Vis diffuse reflectance measurements indicate that this material is a wide band gap semiconductor, with a band gap of ca. 4.4 eV.

Exchange interaction in GdGa from first-principles

The electronic structure and magnetic properties for GdGa have been studied from a first-principles density functional calculation. The energy band structure has been calculated in a local spin density approximation (LSDA), plus Hubbard U approach (LSDA+U). For Gd atoms, seven spin up 4f bands are fully occupied and situated at the bottom of Ga 4s states, while the spin down 4f hole levels are completely unoccupied and well above the Fermi level ( E f ). The p- and d-like states dominate at E f . The calculated magnetic moment is 7.37 μ B per formula unit (f.u.) and is not sensitive to the change of the unit cell volume. The effective exchange parameters, J 0, decrease from 2.6 to 0.9 mRy with increasing lattice volume from 35.7 to 55.3 Å 3/f.u., resulting in a pressure induced enhancement of the Curie temperature ( T C ). With the experimental lattice constants, the calculated mean field T C is 187 K, in good agreement with the experimental value ( T C exp. =183 K).

Electronic structures of AlB2-type superconducting YbGa1+xSi1−x alloys probed by NMR and Seebeck coefficient

We report the electronic properties of the AlB2-type compounds YbGa1+xSi1−x (x=0, 0.15, and 0.3) studied by means of the nuclear magnetic resonance (NMR) and the Seebeck coefficient measurements. These materials are of current interest due to the presence of superconductivity with Yb element. From the analysis of G69a NMR spin-lattice relaxation rates, we deduce the Ga 4s partial Fermi level density of states Ns(EF) for these compounds. The result indicates a gradual increase in Ns(EF) with increasing x in YbGa1+xSi1−x. In addition, the evolution of the Seebeck coefficient feature can be understood well within the band-filling scenario. From the Seebeck coefficient analysis, we find that the variation in the total Fermi level density of states N(EF) is not consistent with the trend of superconducting temperature Tc which shows a gradual decrease with Ga content. These observations support the hypothesis that the electronic Fermi level density of states is not the key factor in determining the superconducting transition temperature of YbGa1+xSi1−x.

The first polymeric thiogallate with lanthanide-containing counter cation, [Dy 2(en) 6(μ 2-OH) 2]Ga 4S 8

The title compound, [Dy 2(en) 6(μ 2-OH) 2]Ga 4S 8 (en = ethylenediamine), contains 1D infinite sinusoidal [ GaS 2 - ] n anions and hydroxy bridged dinuclear dysprosium complex cations. Two bridging hydroxy ions connect two inversion-related Dy 3+ ions to form a centrosymmetric dimer, in which a Dy 2O 2 rhomboidal core is formed. The title compound is the only example of the thiogallates with lanthanide-containing counter cation.

Crystal structure of the ternary semiconductor compound thallium gallium sulfide, TlGaS 2

Thallium gallium sulfide, TlGaS 2, a semiconductor compound, was prepared by solid-state reaction technique. Its crystal structure was determined by single-crystal X-ray diffraction. This material crystallizes in the monoclinic system with space group C2/c (No. 15), Z = 16 and unit cell parameters a = 10.2990 ( 8 ) Å , b = 10.2840 ( 8 ) Å , c = 15.1750 ( 18 ) Å , β = 99.603 ( 4 ) ° . The structural refinement converged to R ( F ) = 0.0999 , R ( F 2 ) = 0.0764 and S = 1.067 . The structure consists of a three-dimensional arrangement of distorted TlS 8 and GaS 4 polyhedrons. Four GaS 4 tetrahedra form adamantine-like units of the type Ga 4S 10, which in turn connect through the corners forming layers that run along the [1 0 0] direction.

Electronic structure of HgGa 2S 4

The electronic structure and chemical bonding in HgGa 2S 4 crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa 2S 4 is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3–7 eV and the components at BE=7–11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS 2 and SrGa 2S 4 compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa 2S 4 crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.

Electronic structure of the Ga 1 − xCr xN studied by high-energy photoemission spectroscopy

Valence band spectra of Ga 1 − x Cr x N have been investigated by high-energy photoemission spectroscopy at the photon energy of 5.95 keV. Cr doping does introduce a novel electronic structure in the bandgap and causes some change in valence band structure. Based on the first-principle calculation, Cr-associated electronic levels in the bandgap are assigned to nonbonding and antibonding d states while the change of the valence band suggests that the Ga 4s originated states are significantly modified through hybridization with the Cr 3d orbital. The present result evidences that the Ga valence electrons are considerably modified through the interaction with the second nearest-neighbour Cr atoms.

Ga-doping effects on electronic and structural properties of wurtzite ZnO

The electronic properties of Ga-doped ZnO with wurtzite structures are studied using the frozen-core projector augmented wave method based upon the density functional theory with the Perdew-Wang generalized gradient approximation. Among a series of doped cases with atomic ratio of 1.85%, 2.78%, 6.25%, and 12.5%, the system with a 12.5% level of doping yields the lowest cohesive energy in the hexagonal structure. A sharp resonance attributed to the Ga 4s orbital shows up just below the low-lying Zn 3d valence bands. The width of this peak becomes broader as the amount of the Ga dopant is increased, reflecting an increased interaction with the 2p orbitals of the surrounding O atoms. The calculated result reproduces the pronounced Burstein–Moss shift and the shrinkage of the fundamental band gap. Furthermore, an increase of the Ga content leads to the amplification of the volume of the unit cell. The present work seems to indicate that, when doping is introduced, the effective mass of the electron in conduction bands becomes slightly smaller than that in pure ZnO.

Element resolved spin configuration in ferromagnetic manganese-doped gallium arsenide.

We report induced Ga and As moments in ferromagnetic Ga(1-x)MnxAs detected using x-ray magnetic circular dichroism at the Mn, Ga, and As L(3,2) edges. Across a broad composition range, we find As and Ga dichroism signals which indicate an As 4s moment coupled antiparallel to the Mn 3d moment, and a smaller parallel Ga 4s moment. The Ga moment follows that of Mn in both doping and temperature dependence. These results are consistent with recent predictions of induced GaAs host moments and support the model of carrier-mediated ferromagnetic ordering involving As-derived valence band states.

High-energy photoemission spectroscopy of ferromagnetic Ga 1− xMn xN

Here we report investigation of valence band electronic states of ferromagnetic Ga 0.96Mn 0.04N by bulk-sensitive X-ray photoemission, which is realized at high flux X-ray undulator beamline BL29XU of SPring-8, at photon energy of 5.95 keV. We have observed that Mn doping introduces a new structure in the band gap region near the top of the valence band, and also a broader structure in deeper valence band region. Basing upon the first principle calculation, these structures are assigned as Ga 4s originated states, which are raised by hybridization between 3d orbitals of Mn with GaN host orbitals. The present result evidences the second nearest Ga bonds are affected by that Mn–N bond formation, suggesting the long-range interaction of Mn in this host material.

Adsorption of [( tBu)GaS] 4 on the GaAs(0 0 1)-(4×2) surface

We have studied the adsorption of [( t Bu)GaS] 4 in ultrahigh vacuum on the single-domain Ga-rich GaAs(0 0 1)-(4×2) surface using scanning tunneling microscopy (STM), Auger electron spectroscopy, and low-energy electron diffraction (LEED). This study has focused on deposition at 650 K where adsorption is carbon-free. For submonolayer coverages, STM images show the presence of protrusions on the surface. The height distribution of these protrusions (which is insensitive to bias voltage) is peaked between 0.7 and 1.4 Å. Since the expected height of an intact Ga 4S 4 cubic core of the [( t Bu)GaS] 4 precursor residing on the surface is expected to be between 3 and 4 Å, our data suggest that the Ga 4S 4 cubic core dissociates upon adsorption on the (4×2) surface. Adsorption of a GaS overlayer by exposure of [( t Bu)GaS] 4 at 650 K, results in a reasonably ordered surface exhibiting a (2×1) reconstruction, as judged by LEED and STM. Unlike adsorption on the As-rich GaAs(0 0 1)-(2×4) surface, post-deposition annealing to temperatures in excess of 650 K is not necessary for the fabrication of an ordered GaS overlayer. Possible structural models for the (2×1) surface are discussed along with the practical implications of these results.

Adsorption of [( tBu)GaS] 4 on GaAs(001)-(2×4)

We have conducted an ultrahigh vacuum investigation of [( tBu)GaS] 4 adsorption on GaAs(001)-(2×4). Adsorbing [( tBu)GaS] 4 at a surface temperature of 650 K results in a carbon-free overlayer as judged by Auger electron spectroscopy. For submonolayer coverages, scanning tunneling microscopy (STM) images show the presence of protrusions on the surface. The height distribution of these protrusions, which is insensitive to bias voltage, is peaked between 0.7 and 1.4 Å, i.e., less than would be expected for the adsorption of intact Ga 4S 4 cubane cores, suggesting that the Ga 4S 4 cubane core of the [( tBu)GaS] 4 molecular precursor has dissociated upon adsorption at 650 K. Further exposure to [( tBu)GaS] 4 results in a nearly complete overlayer which contains small domains of a (2×1) GaS-induced reconstruction as judged by STM. Residing on the overlayer with (2×1) domains are features with heights between 3 and 4 Å suggesting that the Ga 4S 4 cubane core of the [( tBu)GaS] 4 molecular precursor remains intact as adsorption at 650 K proceeds to the second layer of GaS. Upon annealing the overlayer to 790 K, STM images show adsorbates on a well-ordered (2×1) reconstructed surface. The height distribution of these adsorbates is peaked between 0.7 and 1.4 Å, suggesting that annealing to temperatures above 700 K results in the dissociation of the second layer Ga 4S 4 cubane cores that were adsorbed intact at 650 K. By annealing to 830 K, it is possible to desorb the second layer of GaS and obtain a (2×1) surface. Our STM images of this (2×1) reconstruction are consistent with a previously proposed model consisting of GaS heterodimers. Further adsorption of [( tBu)GaS] 4 on the (2×1) surface at a temperature of 650 K results in clustering of the Ga 4S 4 cubane cores into anisotropic GaS islands. The distribution of island aspect ratios ( d [1̄10]/ d [110]) is peaked between 2 and 3.

Bulk and Surface Electronic Structure of GaN Measured Using Angle-Resolved Photoemission, Soft X-ray Emission and Soft X-ray Absorption

ABSTRACTThe electronic structure of thin film wurtzite GaN has been studied using a combination of angle resolved photoemission, soft x-ray absorption and soft x-ray emission spectroscopies. We have measured the bulk valence and conduction band partial density of states by recording Ga L- and N K- x-ray emission and absorption spectra. We compare the x-ray spectra to a recent ab initio calculation and find good overall agreement. The x-ray emission spectra reveal that the top of the valence band is dominated by N 2p states, while the x-ray absorption spectra show the bottom of the conduction band as a mixture of Ga 4s and N 2p states, again in good agreement with theory. However, due to strong dipole selection rules we can also identify weak hybridization between Ga 4s- and N 2p-states in the valence band. Furthermore, a component to the N K-emission appears at approximately 19.5 eV below the valence band maximum and can be identified as due to hybridization between N 2p and Ga 3d states. We report preliminary results of a study of the full dispersion of the bulk valence band states along high symmetry directions of the bulk Brillouin zone as measured using angle resolved photoemission. Finally, we tentatively identify a non-dispersive state at the top of the valence band in parts of the Brillouin zone as a surface state.

Electronic structure and electron transport properties of amorphous CaAlGa and CaMgGa alloys

Both electronic structure and electron transport properties of the amorphous CaAlGa and CaMgGa alloys have been studied by measurements of the low temperature specific heat, the resistivity and the X-ray photoemission spectroscopy (XPS) spectra. The electrical resistivity ϱ 300 K at 300 K and the electronic specific heat coefficient γ exp are found to increase with increasing Ga concentration in both amorphous Ca 60Al 40− x Ga x and Ca 60Mg 40− x Ga x (0 ⩽ x ⩽ 30) alloy systems. The XPS valence band spectra clearly showed that the introduction of Ga atoms results in the split band structure in both systems. We explain that the Al(Mg) 3s states and Ga 4s states are probably isolated from the p states, which form a narrow band across the Fermi level. This unique electronic structure gives rise to a resistivity at 300 K higher than 600 μω cm, while prossessing a large electronic specific heat coefficient exceeding 5 mJ mol −1 K −2. This results in an electron diffusion coefficient as small as 0.19 m 2 s −1, the smallest value so far found in amorphous alloys.

The development of the 3p and 4p valence band of small aluminum and gallium clusters

Photoelectron spectra of Al−n and Ga−n clusters in the size range n=1–15 are presented. Using 5.0 and 3.68 eV UV light (KrF excimer and nitrogen laser, respectively), electrons from molecular orbitals corresponding to the 3p and 4p orbitals of the atoms are detached. The spectra reveal a rich fine structure not observed in earlier experiments. The data are compared with the results of quantum chemical calculations. A change in the pattern of the spectra near n=6 can be interpreted as a transition from planar to compact 3D structures. The spectrum of Al−13 agrees with the icosahedral structure predicted for this particle. The data do not agree with jellium model predictions. Differences between Al and Ga data can be correlated with the larger binding energy of the Ga 4s band.

Novel adamantane-like thio- and selenoanions from aqueous solution: Ga 4S 108−, In 4S 108−, In 4Se 108−

Novel adamantane-like thio- and selenoanions from aqueous solution: Ga 4S 108−, In 4S 108−, In 4Se 108−