Ga Bonds Research Articles

A Dimeric Gallium Hydrazide as an Active Lewis Pair – Complexation and Activation of Me2GaH and Various Heterocumulenes

Treatment of the dimeric gallium hydrazide [Me2Ga‐N(2‐Ad)‐NC5H10]2 (5) with Me2GaH resulted in the formation of an adduct 6 by Ga–N bond cleavage and coordination of the metal hydride via a Ga–N and a 3c–2e Ga–H–Ga bond. This reaction reflects the typical behavior of frustrated Lewis pairs. Reactions with heterocumulenes R–N=C=X (R = Ph, CMe3, Dipp, X = O; R = Ph, X = S) or X=C=X (X = O, S) resulted in the formation of the cyclic Ga–N insertion products Me2Ga–N(R)C(O)N(2‐Ad)‐NC5H10 (7a–c), Me2GaS2C‐N(2‐Ad)‐NC5H10 (8) or Me2GaX2C‐N(2‐Ad)‐NC5H10 [X = O (9); S (10)] in moderate to good yields. Three different structural motifs were observed in the solid state: Five‐membered GaNCN2 heterocycles with exocyclic C=O bonds for compounds 7a–c, four‐membered GaSCN or GaSCS heterocycles for compounds 8 and 9 (chelating coordination of the Ga atoms by SCN and CS2 ligands) and an eight‐membered (GaOCO)2 heterocycle for the dimeric CO2 insertion product 10. Treatment of 5 with PhCN or Ph2CO resulted in a completely different reaction and afforded a dimeric Ga imide 11a or an alcoholate 11b. These reactions may start by retro‐hydrogallation with the formation of H10C5N–N=C(C9H14) and Me2GaH and proceed by addition of the metal hydride to the polar multiple bonds of the nitrile or ketone.

Novel nitrogen/gallium precursor [Ga(bdma)H2] for MOVPE

Dilute nitrogen (N) containing III/V semiconductors are promising candidates for solar cell and laser applications. The N incorporation efficiency of 1,1-dimethylhydrazine (UMDHy) in metal organic vapor phase epitaxy (MOVPE), however, happens to be only in the one percentage range and below. This leads to an extremely high offer of UDMHy in the MOVPE reactor and, therefore, a drastic change in the growth conditions. Furthermore, the device efficiency of dilute nitride materials is currently hampered by carbon (C) incorporation, which is believed to be incorporated either jointly with the N from the dimethylamine radical of the UMDHy or from short hydrocarbon radicals originating from the decomposition of the other metal organics. Therefore, this work presents a novel N precursor N,N'-Bis(dimethylamino)acetamidinato-galliumdihydride [Ga(bdma)H2], which provides not only N but also gallium (Ga) during MOVPE. The direct N–Ga bond in this molecule might facilitate the N incorporation and hence increase the efficiency. For a systematic N incorporation study Ga(NAs)/GaAs heterostructures were grown by MOVPE. The N content was determined via high resolution X-ray diffraction and photoluminescence (PL) studies. Good structural quality and as grown room temperature PL were obtained. It will be also shown that the N incorporation efficiency in GaAs using [Ga(bdma)H2] is significantly higher than for growths using UDMHy under comparable conditions, making this class of molecules promising candidates for the growth of dilute nitride materials.

Possible new metastable Mo2Ga2C and its phase transition under pressure: a density functional prediction

The newly synthesized novel double-Ga layer nanolaminated carbide Mo2Ga2C is stimulating tremendous research interests. In the present exploration of Mo2Ga2C structure, another plausible metastable structure with close-packed Ga layers is predicted from density functional calculations. This new structure (denoted as m-structure) is slightly less stable by only 52 meV/atom than the experimentally determined structure (e-structure) with on-top stacked Ga layers. Importantly, this m-structure is dynamically stable from the calculated phonon dispersion. Moreover, the stability behavior of this m-structure under compression shows that possible phase transition from e-phase to m-phase could occur under a pressure above 24.3 GPa, which requires further experimental confirmation. Phase transition model is proposed, and the energy barrier for phase transition is further derived. The electronic structures show that Mo–C bonds and Ga–Ga bonds are weaker in metastable m-phase than in e-phase. During compression, more strengthening of Ga–Ga and Mo–Ga bonds and less weakening of Mo–Mo bonds in m-phase explain the stabilization of m-phase under pressure.

XPS study of impurities in Si‐doped AlN film

This paper reports an XPS study of impurities in a 100‐nm‐thick AlN film grown by metalorganic chemical vapor deposition (MOCVD) under low pressure on the n‐type 6H‐SiC substrate. The Si‐doped AlN film was characterized by the X‐ray photoelectron spectroscopy (XPS) in a high vacuum system, which reveals the content distribution and chemical states of impurities along depth. The XPS analysis of AlN film before and after argon‐ion etching indicates that there always exist Ga, O and C contaminations in AlN film. Especially, O contamination on the AlN film surface is mostly introduced during the growth of AlN layer by MOCVD. Meanwhile, most of O atoms bind with Al or Ga in Al―O and Ga―O chemical states. In particular, the Ga atoms in AlN film are always in two chemical states, i.e. Ga―Ga bond and Ga―O bond, which demonstrates that the aggregation of Ga is accompanying with AlN growth. Copyright © 2016 John Wiley & Sons, Ltd.

P-type Ge epitaxy on GaAs (100) substrate grown by MOCVD

In this work, Ga-doped Geranium (Ge) films have been grown on GaAs (100) substrates by metal-organic chemical vapor deposition (MOCVD). Undesired pillar structures have been observed on the epilayers prepared at relatively lower temperatures. Energy dispersive X-ray spectroscopy (EDX) indicated that the pillars are mainly consisted of Ga atoms, which is totally different from that of the Ge film. It was demonstrated that the pillar structures could be reduced by simply raising the growth temperature while keeping the other growth conditions unchanged. In this regard, the growth mechanism of the pillars was related to the Ge-Ga dimers formed during the growth of p-Ge films. By further studying the influence of a GaAs or Ge buffer layer on the growth of p-Ge layers, we found that the GaAs substrate with lower density of Ga or Ge dangling bonds was helpful in suppressing the formation of the undesired pillar structures.

Thermally assisted oxidation of GaSb(100) and the effect of initial oxide phases

The oxidation of GaSb(100) surface has been widely studied because it affects the functionality of various devices. However even initial stages of the oxygen incorporation are not completely understood. To clarify this issue, we have investigated the oxidized GaSb(100) surfaces, which have been recently probed by scanning tunneling microscopy and spectroscopy, with high resolution synchrotron radiation photoelectron spectroscopy, in order to interconnect these different measurements. The results give a clear support that the oxidation initiates through saturation of available Ga bonds with; i.e., replacing some of the SbGa bonds with OGa in the surface layers. Oxygen atoms have two different bonding environments in consistent with two dominating STM features. Also role of the plasmon features in the spectra have been elucidated.

Ab initiostudy of Ga-GaN system: Transition from adsorbed metal atoms to a metal–semiconductor junction

Ab initio studies of a GaN(0001)-Ga system with various thicknesses of a metallic Ga layer were undertaken. The studied systems extend from a GaN(0001) surface with a fractional coverage of gallium atoms to a Ga-GaN metal–semiconductor (m–s) contact. Electronic properties of the system are simulated using density functional theory calculations for different doping of the bulk semiconductor. It is shown that during transition from a bare GaN(0001) surface to a m–s heterostructure, the Fermi level stays pinned at a Ga-broken bond highly dispersive surface state to Ga–Ga states at the m–s interface. Adsorption of gallium leads to an energy gain of about 4 eV for a clean GaN(0001) surface and the energy decreases to 3.2 eV for a thickly Ga-covered surface. The transition to the m–s interface is observed. For a thick Ga overlayer such interface corresponds to a Schottky contact with a barrier equal to 0.9 and 0.6 eV for n- and p-type, respectively. Bond polarization-related dipole layer occurring due to an electron transfer to the metal leads to a potential energy jump of 1.5 eV, independent on the semiconductor doping. Additionally high electron density in the Ga–Ga bond region leads to an energy barrier about 1.2 eV high and 4 A wide. This feature may adversely affect the conductivity of the n-type m–s system.

The Electronic Structure and Bonding of Acetylene on PdGa(110)

We have studied the adsorption of acetylene on the PdGa(110) surface by Density Functional Theory calculation. Our results predict the hollow site is the most stable location for the adsorbate. In this site, both Pd and Ga atoms interact with acetylene. This molecule is bonded with the C–C bond almost parallel to the surface. A small tilt angle of 4.1° is computed. The C atoms present a rehybridization from sp → sp2. This rehybridization is also present in the bridge site but it is not present in the top configuration, where the C–C–H bond angle is about 160°. We computed the total density of state for the system and also the projection on Pd, Ga, C, and H atoms. These plots show a shift to lower energies on C and H projected states, which is an indication of stabilization after adsorption. At the same time, there is a reduction in the density for the Pd atom directly bonded to the C atom. The crystal orbital overlap population curves show an increase in the C–C overlap population (OP) after adsorption; while, the C2H2 rehybridizes to a near sp2 geometry. The acetylene withdraws charge from the surface indicating a donation–adsorption mechanism. The formation of Pd–C and Ga–C bonds and a decrease in OP for Pd–Pd and Pd–Ga bonds are detected. We also found, in the hollow site, a reduction in the C–C bond stretching vibrational frequency. This is an indication of the significant distortion in the adsorbed molecule. We have also found that the C–C bond breaking is unfavorable and that semihydrogenation is 2.27 eV more stable than C2H2 + H2 in the gas phase.

A DFT study of atomic geometry and electronic structures for oxidized Al0.25Ga0.75N (0 0 1) (2 × 2) reconstruction surfaces

In order to study formation energies, atomic geometry, electronic structures, and surface states of oxidized Al0.25Ga0.75N (001) (2×2) reconstruction surfaces, models of Al0.25Ga0.75N (001) (2×2) reconstruction surfaces with different coverages of oxygen atoms were built, and first-principles calculations were performed based on density functional theory (DFT). The results of these calculations show that formation energy of the oxidized Al0.25Ga0.75N (001) (2×2) reconstruction surfaces decreases as the increasing of O coverage under O-rich conditions. O atom prefers to locate on the top of GaGa and GaAl dimers. After relaxation, the length values of GaO and AlO bonds are 1.856Å and 1.853Å, respectively. With smaller electronegativity values, Ga and Al atoms donate electrons to O atom, and the dangling bonds of Ga and Al atoms on the surface were occupied. At Fermi level, the partial density of states (PDOS) of Ga and Al atoms on the topmost surface decrease significantly after Ga and Al atoms bond with O atoms, showing that surface states disappear. O atoms adsorption or replacement on Al0.25Ga0.75N (001) (2×2) reconstruction surface induces charge redistribution, which in turn forms Ga(Al)O dipoles oriented to O atoms. With Ga(Al)O dipoles, work functions of oxidized surfaces are larger than that of clean surface, and the work function increases as O coverage increases.

Supramolecular Chemistry Based on Gallium–Gallium Single Bonds – Formation of Large Heterocycles and Cages with up to Twelve Gallium Atoms

AbstractThe tetraalkyldigallium(II) compound R2Ga–GaR2 (1) [R = CH(SiMe3)2] reacted with amino and hydroxo functionalized carboxylic acids by retention of the Ga–Ga bond and release of CH2(SiMe3)2. New heterocyclic or cage‐like compounds were formed with three, two, or six Ga–Ga bonds in a single molecule. The latter dodecagallium compounds encapsulated THF or dioxane molecules in their molecular cavities (carcerands), the other compounds have up to six THF molecules coordinated by N–H···O hydrogen bonds.

Univalent Gallium Complexes of Simple and ansa-Arene Ligands: Effects on the Polymerization of Isobutylene.

Using [Ga(C6 H5 F)2 ]+ [Al(ORF )4 ]- (1) (RF =C(CF3 )3 ) as starting material, we isolated bis- and tris-η6 -coordinated gallium(I) arene complex salts of p-xylene (1,4-Me2 C6 H4 ), hexamethylbenzene (C6 Me6 ), diphenylethane (PhC2 H4 Ph), and m-terphenyl (1,3-Ph2 C6 H4 ): [Ga(1,4-Me2 C6 H4 )2.5 ]+ (2+ ), [Ga(C6 Me6 )2 ]+ (3+ ), [Ga(PhC2 H4 Ph)]+ (4+ ) and [(C6 H5 F)Ga(μ-1,3-Ph2 C6 H4 )2 Ga(C6 H5 F)]2+ (52+ ). 4+ is the first structurally characterized ansa-like bent sandwich chelate of univalent gallium and 52+ the first binuclear gallium(I) complex without a GaGa bond. Beyond confirming the structural findings by multinuclear NMR spectroscopic investigations and density functional calculations (RI-BP86/SV(P) level), [Ga(PhC2 H4 Ph)]+ [Al(ORF )4 ]- (4) and [(C6 H5 F)Ga(μ-1,3-Ph2 C6 H4 )2 Ga(C6 H5 F)]2+ {[Al(ORF )4 ] - }2 (5), featuring ansa-arene ligands, were tested as catalysts for the synthesis of highly reactive polyisobutylene (HR-PIB). In comparison to the recently published 1 and the [Ga(1,3,5-Me3 C6 H3 )2 ]+ [Al(ORF )4 ]- salt (6) (1,3,5-Me3 C6 H3 =mesitylene), 4 and 5 gave slightly reduced reactivities. This allowed for favorably increased polymerization temperatures of up to +15 °C, while yielding HR-PIB with high contents of terminal olefinic double bonds (α-contents=84-93 %), low molecular weights (Mn =1000-3000 g mol-1 ) and good monomer conversions (up to 83 % in two hours). While the chelate complexes delivered more favorable results than 1 and 6, the reaction kinetics resembled and thus concurred with the recently proposed coordinative polymerization mechanism.

Study of lattice deformation and atomic bond length for AlxGa1−xN epi-layers with synchrotron radiation X-ray absorption spectroscopy

The lattice deformation in wurtzite AlxGa1−xN epi-layers (0.19 ≤ x ≤ 0.58) grown on sapphire substrates by metal organic chemical vapor deposition has been studied with synchrotron radiation X-ray absorption spectroscopy and X-ray diffraction. The result reveals that Al substitution for Ga in AlGaN epi-layers induces a significant contraction in crystal lattice and c/a ratio (c and a are the lattice constants for AlGaN epi-layer, respectively), while the substitution only results in a slight expansion in internal parameter u, which is used to describe the lattice deformation of wurtzite structure. This fact suggests that u is not sensitive to Al substitution for Ga. A detailed analysis of extended X-ray absorption fine structure of Ga K-edge spectra show that the Ga–N bond length has only a weak composition dependence, while the Ga–Ga (Al) bond lengths are strongly dependent on the Al composition.

Treatment of the Digallium Compound R 2 Ga–GaR 2 [R = CH(SiMe 3 ) 2 ] with Functionalized Dicarboxylic Acids: Macrocycles, Hydrogen Bonding, and Sulfur–Sulfur Closed‐Shell Interactions

Ga–Ga bonds act as effective templates for the generation of macrocyclic compounds with dicarboxylato spacer ligands in the bridging positions. Hydrogen bonds or sulfur–sulfur closed-shell interactions were observed with functionalized amino or dithio spacer ligands.

Si doping at GaN inversion domain boundaries: an interfacial polar field for electron and hole separation

Using first-principles calculations, we investigated the phenomenon of Si doping at the GaN inversion domain boundaries (IDB) perpendicular to the wurtzite [0001] axis. The results reveal that the half monolayer Si doped GaN IDB is more stable than the abrupt monolayer Si doped IDB. This finding is vital to understanding the unique growth mechanism of Si-induced IDBs in N-polar GaN nanowires that are embedded in a Ga-polar layer [Nano Lett., 2012, 12, 6119]. The lower-energy boundary exhibits the characteristics of intrinsic semiconductor and fulfils the electron counting rule. Charge neutrality is achieved by transferring electrons from the Si–N to the Ga–Ga bonds. Moreover, a potential step is induced by the asymmetric substitution of Si for Ga atoms at the interface, which facilitates the spatial separation of excited carriers at this neutral boundary. Our results suggest an alternative strategy for designing novel and highly efficient photovoltaic devices.

Structure and Charge Compensation of Heteropolar SiC/GaN Interfaces

We present studies of the morphology and charge distribution at the 4H-SiC/wz-GaN heteropolar junctions. Our investigations are based on the rst principles calculations in the framework of the density functional theory where the interfaces between the SiC substrate and GaN layers are represented by means of a slab. These studies reveal possible charge compensation patterns at the interfaces that lead to charge redistribution from monopole to dipole character and increase the stability of the junctions. It turns out that the interfaces with C Ga and Si Ga bonds across the junction and reconstructions involving substitution of group IV elements into Ga layer are the most favorable energetically.

Na8Au9.8(4)Ga7.2 and Na17Au5.87(2)Ga46.63: The diversity of pseudo 5-fold symmetries in the Na–Au–Ga system

The Na-rich part (~30% Na) of the Na–Au–Ga system between NaAu2, NaGa4, and Na22Ga39 has been found to contain the ternary phases Na8Au9.8(4)Ga7.2 (I) and Na17Au5.87(2)Ga46.63 (II), according to the results of single crystal X-ray diffraction measurements. I is orthorhombic, Cmcm, a=5.3040(1), b=24.519(5), c=14.573(3) Å, and contains a network of clusters with local 5-fold symmetry along the a-axis. Such clusters are frequent building units in decagonal quasicrystals and their approximants. II is rhombohedral, R3¯m, a=16.325(2), c=35.242(7) Å, and contains building blocks that are structurally identical to the Bergman-type clusters as well as fused icosahedral units known with active metals, triels and late transition elements. II also contains a polycationic network with elements of the clathrate V type structure. Tight-binding electronic structure calculations using linear muffin–tin-orbital (LMTO) methods on idealized models of I and II indicate that both compounds are metallic with evident pseudogaps at the corresponding Fermi levels. The overall Hamilton bond populations are generally dominated by Au–Ga and Au–Au bonds in I and by Ga–Ga bonds in II; moreover, the Na–Au and Na–Ga contributions in I are unexpectedly large, ~20% of the total. A similar involvement of sodium in covalent bonding has also been found in the electron-richer i-Na13Au12Ga15 quasicrystal approximant.

Structural evolution of Ga-Ge-Te glasses by combined EXAFS and XPS analysis

The structural evolution of GaxGeyTe100-x-y glasses in the vicinity of GeTe4-GaTe3 pseudo-binary tie-line is determined with high-resolution X-ray photoelectron (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The analysis of XPS data is complicated by similar electronegativity values for the constituent chemical elements, but then the interpretation is facilitated by information from complementary EXAFS analysis of the structure around each element independently. The results show 4∕4∕2 coordination for Ga∕Ge∕Te atoms and absence of Ga(Ge)-Ge(Ga) bonds or extended Te clusters in significant concentrations within the whole range of studied composition. The observed structural features correlate well with the measured basic physical properties of Ga-containing germanium telluride glasses.

Incorporation of Ga into the structure of Ge–Se glasses

The structure of Ga-containing Ge–Se glasses is studied by high-resolution X-ray photoelectron and extended X-ray absorption fine-structure spectroscopies. The results show that in GaxGeySe100–x–y glasses both Ga and Ge atoms are 4-fold coordinated, whereas Se atoms are 2-fold coordinated. For Se-deficit samples with Se content less than 67 at.%, Ge–Ge(Ga) bonds appear. First, Ge–Ge bonds are formed indicating preference for the formation of (Se)2 > Ga < (Se)2 units over the regular GeSe4/2 tetrahedra. Metal-Ga bonds appear only in the most Se-deficit sample. Addition of Ga facilitates more uniform Se distribution in comparison to binary Ge–Se or ternary As(Sb)–Ge–Se systems. The observed structural features are correlated with some basic thermo-mechanical properties of the investigated glasses.

Catalytic Mechanism of Pd Adsorption on S-Terminated GaAs(001)-(2 × 6) Surface

Structural and electronic properties of Pd adsorption on clean and S-terminated GaAs(001)-(2 × 6) surfaces are studied using first-principle simulations. Our calculations show that the Pd atom prefers to occupy the HH3 site. The Pd atom is lower than the S atom with 0.15 Å. The density of states analysis confirms that S–Ga bond plays an important role in Heck reaction. We also find that the Pd catalysis activity for Pd adsorption on clean GaAs(001)-(2 × 6) surface is weak while it is enhanced when the Pd atom is adsorbed on the S-terminated GaAs(001)-(2 × 6) surface, which is in good agreement with the experiments.

Electronic and atomic structure of Ba8Ga16Ge30−xSix type I clathrates: Ge and Ga XAFS study

X-ray absorption fine structure spectroscopy at the Ga and Ge K-edges was used to study changes in the Ga and Ge electronic structure and local coordination geometry as a function of composition in Ba8Ga16Ge30−xSix type I clathrates (x = 0, 7.5, 9.1, 10.7, 13.4 and 30). Based on XANES data, the partial density of unoccupied states with p character is modified for both Ga and Ge upon Si substitution. Among the specimens which contain both Ge and Si, we found that the specimen with the highest measured power factor (i.e., x = 7.5) has the lowest density of unoccupied states for both Ga and Ge. Our experimental results are qualitatively consistent with computational results based on density functional theory, indicating that a series of pertinent electronic states are modified by Si p states. This suggests that an increase in the electron density near the Fermi level for an optimal Si substitution leads to an increase in the Seebeck coefficient and consequently in the power factor, according to the Cutler–Mott relation. Based on quantitative analysis of EXAFS spectra, we found that Ga has more Si neighbors than Ge, indicating that Si resides preferentially next to Ga. Both the Ge–Ga/Ge and Ga–Ge/Ga coordination distances remain relatively unchanged (∼2.51 Å) regardless of the degree of Si substitution. Furthermore, the Ge–Si and Ga–Si coordination distances remain relatively unchanged at ∼2.41 and ∼2.45 Å, respectively, regardless of the degree of Si substitution. For the Ba8Ga16Si30 specimen, on average, Ga is coordinated with 0.9 Ga and 3.1 Si at roughly the same distance of ∼2.50 Å. The number of Ga–Ga bonds is consistent with the fact that Ga is distributed on the framework sites in a way which reduces the number of Ga–Ga bonds relative to that based on a random distribution. An understanding of the underlying physics of the structure–property relationship provides for potential additional routes for tuning the electronic properties of clathrates for thermoelectric applications.