Gallium Sites Research Articles

Modeling of the chemically assisted ion beam etching process: Application to the GaAs etching by Cl2∕Ar+

The development of two-dimensional chemically assisted ion beam etching model of GaAs by Cl2∕Ar+ allowed the authors to analyze the role of some critical parameters of etch process on the morphology of trench and mesa structures. In a fact, the simulation results show that the etch rate variation with substrate temperature Ts reveals three regimes: For Ts<350K, the etch rate of GaAs is mainly controlled by ion assisted etching component. For 350<Ts<TM, where TM corresponds to the maximum of etch rate, a high variation of etch rate is observed. In this range of temperature, the etch rate is controlled by the chemical component and mainly by the exponential rise of desorption coefficient of GaClx,λ3, with Ts. For Ts>TM a fast decrease of the etch rate is observed because of the high decrease of the coverage rate of Cl, ξ, on the gallium sites with the substrate temperature. On the other hand, the increase of the Cl2 flow rate allows to improve the anisotropy and contributes to the elimination of both the microtrenching and the transfer of the facets from the mask into the substrate.

Solid‐state NMR characterization of 69Ga and 71Ga in crystalline solids

Gallium model systems containing four- and six-coordinate gallium sites have been investigated using solid-state NMR. Measurement of the isotropic chemical shift and electric field gradient (EFG) have been performed at 9.4 T on alpha-Ga2O3, beta-Ga2O3, LiGaO2, NaGaO2, KGaO2, Ga2(SO4)3, and LaGaO3 using a variety of techniques on both NMR active nuclei (69Ga and 71Ga) including static, high speed magic-angle spinning (MAS), satellite transition (ST) spectroscopy, and rotor-assisted population transfer (RAPT). The chemical shift is found to correlate well with the coordination number, with four-coordinate gallium having values of approximately 50 ppm and six-coordinate gallium having values near 225 ppm (referenced to 1 M gallium nitrate solution). The magnitude of the EFG is found to be correlated to the distortion of the gallium polyhedra, with the strained systems having EFGs of 3 x 10(21) Vm(-2) or more, while the less strained systems have values of 1.5 x 10(21) Vm(-2) or less. A plot of chemical shift versus EFG suggests that solid-state NMR of gallium oxyanions can be more discriminating than liquid state NMR chemical shifts alone.

Influence of crystal structure and formation energies of impurities (Mg, Zn and Ca) in zinc blende GaN

First-principles calculations on neutral metal impurities (Mg, Zn and Ca) in zinc blende GaN were studied. Formation energies were calculated for substitution on the gallium site, the nitrogen site and incorporation in the octahedral interstitial site and the tetrahedral interstitial sites. The calculated results show that the major defects studied have a high formation energy in excess of 5 eV, and the gallium substitutional site is favorable for incorporation. Mg Ga, has particularly low formation energy 1.19 eV and can be expected to incorporate readily into GaN. The local crystal structural changes around the impurity in the lattice were studied after metal atoms occupying the gallium substitutional site. It shows that the lattice constant becomes bigger and the tetrahedral angle between impurities and its nearest N atom becomes smaller mainly due to the extended M—N bond length and big size of impurities atoms, which results in a local lattice distortion. The Zn-N (2.04 Å) bond strength is the smallest among the three impurities which raises the formation energy. Ca Ga is unfavorable due to a large size mismatch in spite of a large bond strength (2.25 Å). The calculated results identify the two key factors determining impurities incorporation in zinc blende GaN: the atomic size of impurities comparing to that of host atoms and the bond strength between the impurities and its neighbors. The results are in well agreement with other calculated and experimental results.

The magnesium acceptor states in GaN: an investigation by optically‐detected magnetic resonance

AbstractWe observe a systematic dependence of the g ‐values of magnesium acceptors on the photoluminescence energy that is used for the optical detection of magnetic resonance signals. This effect leads to a model which assumes that, in the magnesium acceptors, the hole is located in an atomic orbital of p ‐character, presumed to be on a nitrogen atom, and which, in the wurtzite crystal field, is an orbital doublet. The remaining degeneracy is then removed by a reduction in local symmetry associated with the relative positions of the magnesium dopant and the nitrogen atom upon which the hole is localized. Further changes in the local crystal field are caused by the presence of nearby defects or by strain and are accompanied by changes in acceptor depth. The model leads to the correct g ‐values and the correct correlation between the g ‐values and acceptor depth and is consistent with a range of acceptor states of different depths being formed by simple substitution of a magnesium ion at a gallium site. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Methanol Adsorption on the β-Ga2O3 Surface with Oxygen Vacancies: Theoretical and Experimental Approach

Methanol adsorption on beta-Ga2O3 surface has been studied by Fourier transform infrared spectroscopy (FTIR) and by means of density functional theory (DFT) cluster model calculations. Adsorption sites of tetrahedral and octahedral gallium ions with different numbers of oxygen vacancies have been compared. The electronic properties of the adsorbed molecules have been monitored by computing adsorption energies, optimized geometry parameters, overlap populations, atomic charges, and vibrational frequencies. The gallia-methanol interaction has different behaviors according to the local surface chemical composition. The calculations show that methanol can react in three different ways with the gallia surface giving rise to a nondissociative adsorption, a dissociative adsorption, and an oxidative decomposition. The surface without oxygen vacancies is very reactive and produces the methanol molecule decomposition. The molecule is nondissociatively adsorbed by means of a hydrogen bond between the alcoholic hydrogen atom and a surface oxygen atom and a bond between the alcoholic oxygen atom and a surface gallium atom. Two neighbor oxygen vacancies on tetrahedral gallium sites produce the dissociation of the methanol molecule and the formation of a bridge bond between two surface gallium atoms and the methoxy group.

Crystal structure of hafnium palladium gallium, HfPdGa

GaHfPd, hexagonal, P62c (no. 190), a = 7.1572(2) A, c = 6.8945(4) A, V= 305.9 A, Ζ = 6, R&(F) = 0.019, η>RKf(F) = 0.031, 7 =293 K. Source of material A 1:1:1 mixture of the elemental components (hafnium pieces, 99.9 %, ChemPur; palladium wire, 99.9 %, ChemPur; gallium pieces, 99.999 %, ChemPur) was arc-melted in a protective atmosphere of purified argon. The homogenization was performed in an evacuated quartz tube at 800 °C for two weeks followed by subsequent quenching in cold water. Single crystals which were suitable for structure refinements were obtained by annealing of pre-reacted reguli at temperatures slightly below the melting point (controlled visually) using a high frequency furnace [1]. Experimental details Sample characterization was performed by X-ray powder diffraction (Huber Image Plate Guinier camera G670, CuK^i radiation, λ = 1.54056 A, 5° < 2Θ < 100°) and optical metallography as * Correspondence author (e-mail: schwaiz@cpfs.mpg.de) well as energy dispersive X-ray spectroscopy (EDXS). The compound was found to be a single phase. EDXS confirmed the equiatomic composition. Lattice parameters were obtained by a leastsquares refinement of 36 reflections determined from a powder diffraction pattern (LaBe as internal standard, a = 4.15692 A). The refined lattice parameters were used in the last runs of the structure refinement and for calculations of interatomic distances. Discussion The new ternary equiatomic compound HfPdGa is isostructural with HfRhSn [2] which represents a distorted and ordered variety of the well-known Fe2P structure type [3] and its ternary derivative ZrNiAl [4,5]. Α klassengleiche symmetry reduction of index 2 (fe) from space group P62m (ZrNiAl) to P62c (HfPdGa) is accompanied by a doubling of the lattice parameter c which is caused by a slight shift of the Hf atoms within the (001) plane and a movement of the Pdl along [001]. Alternatively, HfPdGa can be described as a variant of the Mg2Ga type [6] with an ordering of hafnium and gallium atoms on the magnesium positions (6A and 6g, respectively), while palladium atoms occupy the gallium sites (2/and 4b). The number of phases reported with a HfRhSntype crystal structure is relatively small: ZrlrSn, HfCoSn [2], ZrRhSn, ZrPtGa [5], ScPdSn, ScPtSn [η. Palladium and gallium atoms in the crystal structure of HfPdGa build a 3D framework (figure, top) which is exclusively linked by heteronuclear bonds. Hafnium atoms are embedded in voids of the network and form zig-zag chains within large pseudohexagonal [PdGa] tubes oriented parallel [001]. The Hf—Hf distance of 3.4567(4) A within the chain is significantly longer than the shortest Hf—Hf contacts of 3.126 A in hep hafnium metal [8]. Each crystallographically distinct atom in the [PdGa] network is differently coordinated (figure, bottom). Pdl atoms are located on the three-fold axis and are trigonally non-planar coordinated with d(¥dl—Ga) = 2.6581(6) A (figure, bottom left). The displacement of Pdl from the plane which is formed by Ga atoms amounts to ±0.21 A along [001] and results in a short (3.028(2) A) and a long (3.867(2) A) distance to adjacent Pdl atoms. Both values are significantly larger than the closest contacts of 2.751 A observed in bcc palladium [8], Along [001], an alternation of expansion (d(Hf—Hf) = 4.0032(6) A) and contraction (d(Hf—Hf) = 3.5860(7) A) in the Hf3 triangles is observed. The difference between the two types of triangles correlates with the movement of the Pd species towards one of the triangular planes of the trigonal prisms Hfe. The short and long Pdl—Hf interactions correspond to 2.7630(8) A and 2.8329(9) A, respectively. The coordination sphere of Pd2 is an undistorted trigonal prism formed by Ga atoms (figure, bottom middle). Three additional Hf atoms above the rectangular prism faces are situated in distances of 2.8541(3) A from the central atom. It is remarkable that the Pd2—Ga distances of2.5789(3) A for six-fold coordinated Pd2 is even slightly shorter than the Pdl—Ga distances of 2.6581(6) A for three-fold coordinated Pdl. Ga atoms are tetrahedrally coor-

Temperature and Dislocation Density Effects on the Thermal Conductivity of Bulk Gallium Nitride

The performance of III-Nitride high power, high frequency transistors and laser diodes is intimately connected with the ability to dissipate heat from the junction to the substrate. The thermal conductivity was characterized by the three omega method for undoped and doped gallium nitride bulk substrates grown by HVPE from room temperature to 450 K. The thickness of the samples varied from thin film epilayers on sapphire to 2 millimeter thick free standing samples. Dislocation density of the substrates was measured by imaging cathodoluminescence, SIMS was used to measure impurity levels of oxygen, hydrogen, silicon, and iron, while carrier concentrations and resistivity were determined from electrical measurements and EPR. A semi-insulating, 2 mm thick iron doped sample had the highest thermal conductivity of 230W/K-m at room temperature. Undoped samples had comparable, but lower thermal conductivities throughout the temperature range from 300-450 K. By comparing these results with previously reported experimental results including those on MOCVD grown GaN free of grain boundaries, we establish an empirical relationship in a compact formula that relates the thermal conductivity of GaN and the dislocation density with three different regimes of low, intermediate, and high dislocation densities. In the high dislocation regime, the thermal conductivity improves significantly with reduction of dislocation densities. As material quality continues to improve it remains to be seen if in the low dislocation density regime, thermal conductivities will approach 300 W/K-m or plateau out near 250 W/K-m. As point defects start to limit the thermal conductivity when dislocation density becomes very low, gallium vacancies are expected to play an increasing role. Iron is postulated to substitute on the gallium site. The indication from this study is that iron doping at concentration of 1018 cm−3 is not limiting the thermal conductivity in the 300-450 K range.

Energetics and migration of point defects inGa2O3

The results of a theoretical study on the point defects of monoclinic $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ are reported here. The point defects considered here are vacancies, interstitials together with dopant ions such as Be, Mg, In, Cr, Si, Ge, Sn, and Zr. Since the low symmetry of the monoclinic lattice does not provide an unambiguous location of interstitial sites and migration paths, we propose a unique way for their identification in terms of the electron density topology. Special attention has also been given to the preference among the lattice and interstitial sites for the impurity defects, and its explanation in terms of structural, electrostatic, and electron density arguments. The calculated results find the most prominent features in the lattice to be the existence of (i) empty channels along the $\mathbf{b}$ direction, and (ii) atomic layers perpendicular to them. Their interplay governs the stability and mobility of the point defects in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. The anionic Frenkel pair consisting of the oxygen vacancy and oxygen interstitial is predicted to dominate the defect structure in the lattice. The dopants considered here are likely to be stabilized at the octahedral gallium sites, except for ${\mathrm{Be}}^{+2}$, which prefers a tetrahedral gallium site in the lattice. Some of the possible migration paths have been determined, and the pseudoactivation energies for the intrinsic, oxygen-rich, and oxygen-deficient conditions are computed as a function of temperature. It is suggested that tuning the concentration of oxygen can lead to a change in the anisotropy of the ionic conductivity in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$.

Influence of Structural Defects on the Mechanical Stress in the Impurity Diffusion Zone of GaAs Single Crystals

Data are presented on the mechanical stress σ developing in the diffusion one of gallium arsenide single crystals at different dislocation densities and relationships between the concentrations of arsenic and gallium vacancies. The results are analyzed in terms of impurity diffusion through gallium, arsenic, and interstitial sites. The influence of the defect system on σ is shown to depend on the mechanism of impurity diffusion. A correction coefficient which eliminates the discrepancy between experimental stress data and theoretical predictions is determined for different diffusion mechanisms.

Nature of acceptor states in magnesium-doped gallium nitride

The most effective method of producing $p$-type gallium nitride is currently through incorporation of magnesium. However, such doping leads not only to the desired shallow acceptors but also to the creation of much deeper energy levels. Magnesium-related acceptors have been observed in many optically detected magnetic resonance experiments and their magnetic properties, as characterized by the $g$ values of the holes which they trap, have been found to vary significantly according to the growth conditions and doping levels. The purpose of the present paper is to present a model that accounts for these observations. The model assumes that, in the deep acceptors, the hole is located in an atomic orbital of $p$ character, presumed to be on a nitrogen atom. The orbital degeneracy is partly removed by the wurtzite crystal field and finally by a reduction in local symmetry associated with the relative positions of the magnesium dopant and the nitrogen atom upon which the hole is localized. Further changes in the local crystal field are caused by the presence of nearby defects or by strain. These changes in the crystal field are accompanied by changes in acceptor depth. The approach leads to the correct $g$ values and the correct correlation between the $g$ values and acceptor depth for reasonable choices of the parameters. In the limit that the low symmetry fields become small, the model evolves to one that is consistent with the correct forms of the ground and near-ground Kramers doublets that are observed by other workers in studies of shallow acceptors in material that is not doped with magnesium. Finally, the model is shown to be entirely consistent with a range of acceptor states of different depths being formed by simple substitution of a magnesium ion at a gallium site, rather than by the creation of more complicated defects. The conclusion also highlights the need for the GaN to be of high crystalline quality if effective $p$-type doping is to be achieved.

Amperometric hydrocarbon sensor using La(Sr)Ga(Fe)O 3 solid electrolyte for monitoring in exhaust gas

Emission of hydrocarbon at low temperature, especially in the cold start condition of the automobile engine, has attracted attention these days. For the first time an electrochemical sensor based on LaGaO 3 with iron doping was demonstrated to have very high sensitivity to propene (C 3H 6) at low temperature (423–573 K). In the amperometric mode, the output current signal was found to increase linearly with the C 3H 6 concentration (500–2000 ppm) in logarithm scale. Effect of iron doping at the gallium site was found to influence the sensing properties greatly and the electrolyte composition La 0.7Sr 0.3Ga 0.7Fe 0.3O 3 showed the highest sensitivity of 976 μA/decade at 473 K. The sensors were fast, reproducible and highly selective to hydrocarbons.

Ab Initio Studies of the Energy Characteristics and Magnetic Properties of Point Defects in GaAs

The formation energies of intrinsic point defects and solution energies of transition metal impurities in gallium arsenide are determined on the basis of ab initio calculations using the method of a locally self-consistent Green’s function, which is a generalization of the coherent potential approximation. Based on the calculated energies, the conclusion is made that the AsGa antisite defect is the most common intrinsic defect in GaAs. Calculations showed that transition metal impurities, except for Ni, preferentially occupy gallium sites substitutionally. The magnetic moments of impurity atoms are calculated as a function of the chemical environment. It is shown that, in compensated GaAs, Mn atoms tend to form clusters.

Analysis of the GaN-rich side of GaNP using x-ray diffraction

We investigated the growth of the GaN-rich side of GaNP films using metal-organic chemical-vapor deposition. The results of Auger electron spectroscopy suggest that phosphorus atoms incorporated into the film would be substituted on gallium sites rather than on nitrogen sites. Using x-ray diffraction, we have revealed that such GaNP films belong to a hexagonal system and that the relationship of the orientation between the GaNP film and its underlying GaN layer is [0001]GaNP‖[0001]GaN and [101¯0]GaNP‖[101¯0]GaN. Moreover, it was found that the lattice constants of GaNP were smaller than those of bulk GaN and there was no phase separation. We have also confirmed that the changes in these lattice constants were almost proportional to the phosphorus concentration included in the entire GaNP film thickness and that they decreased with an increase in the phosphorus concentration.

Early stages of cesium adsorption on the As-richc(2×8)reconstruction of GaAs(001): Adsorption sites and Cs-induced chemical bonds

We investigate the adsorption of Cs on the As-rich $c(2\ifmmode\times\else\texttimes\fi{}8)/(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction of GaAs(001) at low coverages using a combination of theoretical and experimental techniques. Density-functional-theory local-density-approximation total-energy calculations and x-ray diffraction experiments find only minimal Cs-induced surface relaxation and identify three preferential adsorption sites within the partially disordered overlayer. These sites are, in order of decreasing occupation probability, the arsenic dimer bridge D site, the gallium dangling bond ${T}_{2}^{\ensuremath{'}}$ site, and the arsenic ${T}_{3}$ trench site. Detailed analysis of the wave functions and electronic charge densities allows us to clarify the bonding mechanisms at the three sites. At the gallium site, the bonding is strongly ionic and involves significant charge transfer to a new Cs-induced state reminiscent of the ${p}_{z}$ orbital of the gallium atom in the ${\mathrm{sp}}^{{}_{2}}$ configuration. In sharp contrast, at the arsenic sites, the charge transfer is minimal and the bonding rather occurs through mixing with a relatively delocalized state of the clean surface. The ionization energy decreases are estimated and compared for the three sites.

Ternary Gallides REMgGa (RE = Y, La, Pr, Nd, Sm–Tm, Lu) – Synthesis and Crystal Chemistry

The title compounds have been synthesized by reacting the elements in sealed niobium or tantalum tubes in a high-frequency furnace. They crystallize with the hexagonal ZrNiAl type structure, space group P62m. All gallides have been characterized through their X-ray powder diffractogram. The cell volume decreases from the lanthanum to the lutetium compound as expected from the lanthanoid contraction. The structures of LaMgGa, PrMgGa, NdMgGa, SmMgGa and TmMgGa have been refined from single crystal diffractometer data. The structures contain two crystallographically independent gallium sites which both have a trigonal prismatic coordination: Ga1 by six RE and Ga2 by six Mg atoms. These trigonal prisms are capped on the rectangular sites by three Mg (RE) atoms, leading to coordination number 9 for each gallium atom. Together, the gallium and magnesium atoms form a three-dimensional [MgGa] network in which the rare earth atoms fill distorted hexagonal channels. Within the network the magnesium atoms have short Mg-Mg contacts, i. e. 312 pm in SmMgGa. The Mg-Ga distances in that gallide range from 284 to 287 pm. Bonding in the network is thus governed by strong Mg-Ga and Mg-Mg bonding. EuMgGa crystallizes with the orthorhombic TiNiSi type: Pnma, a = 783.1(2), b = 472.8(1), c = 829.8(2) pm.

Calculated properties of point defects in Be-doped GaN

The properties of several point defects in hexagonal gallium nitride that can compensate beryllium shallow acceptors $({\mathrm{Be}}_{\mathrm{Ga}})$ are calculated using the AIMPRO method based on local density functional theory. ${\mathrm{Be}}_{\mathrm{Ga}}$ itself is predicted to have local vibrational modes (LVM's) very similar to magnesium acceptors. The highest frequency is about $663{\mathrm{cm}}^{\ensuremath{-}1}.$ Consistent with other recent studies, we find that interstitial beryllium double donors and single-donor beryllium split interstitial pairs at gallium sites are very likely causes of compensation. The calculations predict that the split interstitial pairs possess three main LVM's at about 1041, 789, and $738{\mathrm{cm}}^{\ensuremath{-}1}.$ Of these, the highest is very close to the experimental observation in Be-doped GaN. Although an oxygen donor at the nearest-neighboring site to a beryllium acceptor $({\mathrm{Be}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{O}}_{\mathrm{N}})$ is also a prime suspect among defects that are possibly responsible for compensation, its highest frequency is calculated to be about $699{\mathrm{cm}}^{\ensuremath{-}1}$ and hence is not related in any way to the observed center. Another mode for this defect is estimated to be about $523{\mathrm{cm}}^{\ensuremath{-}1}$ and is localized on the ${\mathrm{O}}_{\mathrm{N}}$ atom. These two vibrations of ${\mathrm{Be}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{O}}_{\mathrm{N}}$ are thus equivalent to those for the isolated substitutional centers perturbed by the presence of their impurity partners.

Carbon-Based Defects in GaN: Doping Behaviour

We present first-principles calculations for the substitutional carbon impurity on the nitrogen and gallium sites, and substitutional and split-interstitial dicarbon pairs (2C) in cubic GaN. The pseudopotential approach and a plane-wave expansion are applied in the framework of density functional theory (DFT) and local density approximation (LDA). The stability of carbon-based defects versus the Fermi level and preparation conditions is analysed. Calculations taking into account several impurities and native defects in their charge states are performed in order to determine the defect which is formed at the highest concentration. The results provide information about consequences of carbon doping in cubic GaN. They are used to elucidate the compensation mechanism seemingly observed with the increment of the high carbon concentrations in recent experiments.

Study of zinc impurity atoms in GaP, GaAs, and GaSb 67Ga(67Zn) and 67Cu(67Zn) by emission Mössbauer spectroscopy

The Mossbauer spectra of 67Ga(67Zn) and 67Cu(67Zn) impurity atoms in the bulk of GaP, GaAs, and GaSb samples correspond to isolated zinc centers at Ga sites. The observed shift of the spectral centers of gravity to higher positive velocities at the transition from p-to n-type samples corresponds to the recharging of a shallow zinc impurity center. Mossbauer spectra of 67Cu(67Zn) impurities at the surface of samples represent a superposition of spectra corresponding to isolated zinc centers at gallium sites with those corresponding to zinc associates with an arsenic vacancy.

The absorption spectra of gallium nitride crystals doped with erbium ions

We have measured the absorption spectra of Er3+ ions introduced by diffusion into bulk GaN crystals grown by vapor phase epitaxy in a chloride system. The wavelength interval of 518–527 nm corresponding to the region of the 4I15/2→2H11/2 transition was studied in detail at 293, 77, and 2 K. At 2 K, the spectrum of this transition displays six lines, which correspond to the theoretically possible number of sublevels of the 2H11/2 state of Er3+ ion occurring in a noncubic crystal field. The positions of levels in the 2H11/2 multiplet correspond to 2.360, 2.361, 2.365, 2.369, 2.379, and 2.386 eV. Both the number and narrow width of lines observed at 2 K indicate that Er3+ ions predominantly occupy the same regular position in GaN crystals. Most probably, erbium ions occupy gallium sites in the crystal lattice.

The nature of arsenic incorporation in GaN

A systematic study of the nature of arsenic incorporation in GaN grown by molecular-beam epitaxy is presented. The samples were grown with concentrations of arsenic ranging from 3.4×1017 to 4.2×1018 cm−3. Secondary ion mass spectroscopy data show that increasing the As concentration has the effect of increasing the amount of As in the nitrogen site as compared to As in the gallium site. This trend is used to explain the reduction in carrier mobility with increasing As concentration.