Gallium Sites Research Articles

Optical and electrical transport properties of α-Ga2O3 thin films with electrical compensation of Sn impurities

Polycrystalline α-Ga2O3 thin films containing secondary phase SnO were grown on BaF2 substrates by magnetron sputtering. The impurity tin concentration, electron concentration, and room temperature mobility of the α-Ga2O3 films are 4.5 × 1020 cm−3, 1.5 × 1015 cm−3, and 26.9 cm2 V−1 s−1, respectively, determined by secondary ion mass spectrometry and Hall effect experiments. The mobility vs temperature dependence confirms that the electrons are mainly subject to polar optical phonon scattering and ionized impurity scattering in the temperature range of 160–400 K. Two ionization energies, 29 and 71 meV, were determined for different temperature ranges by logarithmic resistivity vs the reciprocal of temperature, where the former is the shallow donor SnGa formed by the incorporation of tin into gallium sites. The latter is the shallow acceptor VSn–H associated with secondary phase SnO, and it is the electrical compensation of this shallow acceptor that results in the very low carrier concentration of α-Ga2O3 films. The photoluminescence spectrum exhibits 280 and 320 nm UV radiation, where 280 nm is due to the radiation recombination of electrons trapped by the deep donor state (EC−1.1 eV) with holes trapped by the VSn–H complex. In addition, there are several narrow radiation peaks in the visible region, and the energy levels involved in the radiation transitions are determined one by one after excluding the effects of interference and diffraction.

Structure and properties of gallate and germanate langasite infrared optical glass materials

Glasses in the system BaO – La2O3 – Ga2O3 – GeO2 and Langasite crystalline phases with the same stoichiometry have been studied considering the possible use of these glasses for laser-induced space-selective growth of crystalline architectures in the glass compositions. For better understanding of mechanisms involved in such controlled crystallization process, crystalline and glass compounds with same stoichiometry and different La/Ba ratios have been synthesized to compare their local structure. Solid-state Nuclear Magnetic Resonance evidence 4- and 5-coordinated gallium units in the glass matrix, denying unambiguously the formation of 6-coordinated gallium sites observed in crystalline phases. Moreover vibrational spectroscopies suggest a link between a 3D network formed by 4- and 5-coordinated gallium, connected to each other and to 4- coordinated Q4 and Q3 germanium units in the glass and their congruent crystallization. The transition from 4- and 5-coordinated gallium to 6-coordinated gallium sites observed in Langasite crystalline phases remains not still elucidated.

Ferromagnetism in heavily Fe-doped GaAs: a DFT study

Computational calculations based on density functional theory were carried out to investigate ferromagnetism in gallium arsenide (GaAs) heavily doped with Fe atoms that can substitutionally occupy gallium (Ga) or arsenic (As) sites in the zinc-blende-like crystal structure. The calculations were performed within the spin polarized density functional theory (DFT) and generalized gradient approximation (GGA) with full potential linearized augmented plane wave (FP-LAPW) method. We investigate and discuss the ab initio calculation results focusing on properties intrinsically important for spintronic applications, such as spin polarization and magnetic anisotropy. The density of states (DOS) was calculated with substitution of Fe atoms in the As and Ga sites, giving the electronic properties, as well as the magnetic ground state. According to DFT calculations, heavily doped GaAs with 25 at. % of Fe becomes a ferromagnetic metal with spin polarization as high as 69% at Fermi level. This is corroborated by experimental results previously published in literature.

Coordination environment dominated catalytic selectivity of photocatalytic hydrogen and oxygen reduction over switchable gallium and nitrogen active sites

Coordination environment dominated catalytic selectivity of the HER and OER with switchable active centers.

Magnetic properties of defect induced β-Ga2O3: A first principles study

Electrical and magnetic properties of gallium vacancy (VGa) as well as 3d transition metal ion doped at gallium site (TMGa) in gallium oxide (β-Ga2O3) has been studied under the frame work of density functional theory. Ga vacancy in β-Ga2O3 shows p-type semiconducting behavior along with a ferromagnetic ordering. Spin-spin interaction study reveals that Fe, Ni and Cu substituted at the gallium site of β-Ga2O3 can induce significant amount of magnetic moment along with a stable ferromagnetic ordering. 2p orbital electrons of oxygen near the vacancy site give raise to the large magnetic moment in the gallium vacancy induced system. In the doped system, the 3d orbital electrons of the dopants are the main source of magnetism and a small contribution come from 2p electrons of oxygen. Bader charge analysis and the band-structure calculations have been carried out for all the systems. Gallium vacancy induced β-Ga2O3 could be a viable material for the spintronics applications.

Structural disorder and distribution of impurity atoms in β-Ga2O3 under boron ion implantation

Gallium oxide (Ga2O3) has recently attracted much attention due to the prospect of its application in power electronics and in other areas. An important task is the development of controlled and reproducible methods for doping this material with various impurities, among which boron is of considerable interest. The regularities of Ga2O3 doping with this isovalent (for Ga) impurity are poorly studied. Even for ion implantation as the most common semiconductor doping method these studies are still at the initial stage. In this work, the structural properties of β-Ga2O3 layers upon boron ion irradiation and subsequent annealing have been studied using X-ray Diffraction, Reflection High-Energy Electron Diffraction, Secondary Ion Mass Spectrometry, as well as theoretical calculations from first principles. Among the most interesting results are the establishment of a strong redistribution of implanted boron during post-implantation annealing and the conclusion about low degree of substitution of gallium sites by boron atoms.

Electronic structure of intrinsic defects in c -gallium nitride: Density functional theory study without the jellium approximation

We report the first nonjellium, systematic, density functional theory (DFT) study of intrinsic and extrinsic defects and defect levels in zinc-blende (cubic) gallium nitride. We use the local moment counter charge (LMCC) method, the standard Perdew-Becke-Ernzerhoff (PBE) exchange-correlation potential, and two pseudopotentials, where the Ga $3d$ orbitals are either in the core $({d}^{0})$ or explicitly in the valence set $({d}^{10})$. We studied 64, 216, 512, and 1000 atom supercells, and demonstrated convergence to the infinite limit, crucial for delineating deep from shallow states near band edges, and for demonstrating the elimination of finite cell-size errors. Contrary to common claims, we find that exact exchange is not required to obtain defect levels across the experimental band gap. As was true in silicon, silicon carbide, and gallium arsenide, the extremal LMCC defect levels of the aggregate of defects yield an effective LMCC defect band gap that is within 10% of the experimental gap (3.3 eV) for both pseudopotentials. We demonstrate that the gallium vacancy is more complicated than previously reported. There is dramatic metastability--a nearest-neighbor nitrogen atom shifts into the gallium site, forming an antisite, nitrogen vacancy pair, which is more stable than the simple vacancy for positive charge states. Our assessment of the ${d}^{0}$ and ${d}^{10}$ pseudopotentials yields minimal differences in defect structures and defect levels. The better agreement of the ${d}^{0}$ lattice constant with experiment suggests that the more computationally economical ${d}^{0}$ pseudopotentials are sufficient to achieve the fidelity possible within the physical accuracy of DFT, and thereby enable calculations in larger supercells necessary to demonstrate convergence with respect to finite size supercell errors.

Effect of chloride adlayer on the oxide-derived gallium microspheres catalysts for electroreduction of CO2 to CO

Developing highly efficient catalysts containing earth-abundant elements for carbon dioxide (CO2) reduction is essential for practical application. Ga is a nonprecious metal and offer extraordinary properties in catalysis due to its simultaneous metallic and liquid natures. In present work, we synthesis carbon-supported gallium microspheres (Ga/C-CP) catalyst by sonication in an ethanolic solution and investigate the performance of Ga microspheres in CO2 reduction. Potentiostatic electrolysis test shows current density increased gradually in the beginning of electrolysis and then became stable at 14.5 mA/cm2 on Ga/C-CP electrode. XPS results reveal that this is because a portion of surface Ga2O3 was reduced to Ga0 during the CO2 reduction. The oxide-derived active gallium sites efficiently enhance the reaction rate of CO2 conversion on the electrode. In addition to the effect of the oxide-derived active gallium sites, we found that the adsorbed chloride adlayer significantly favor the protonation of CO2 and suppress the proton reduction. During the electrolysis process, the CO Faradic efficiency measured on the oxide-derived gallium in Bu4NCl/AN achieved 84%, which is obviously higher the compared bulk Ga electrode in 0.1 M KHCO3 solution (23%).

Incorporation of Si and Sn donors in β-Ga2O3 through surface reconstructions

Tin and silicon incorporate on gallium sites in Ga2O3 and act as shallow donors. The monoclinic structure of Ga2O3 has two inequivalent Ga sites; density functional theory calculations for bulk show that Sn prefers the octahedral site, while Si prefers the tetrahedral site. Experiments have indicated that Si and Sn can also incorporate on the thermodynamically less preferred site. We use density functional theory to study the adsorption of Si and Sn and also the co-adsorption of these impurities with Ga and O adatoms on the Ga2O3(010) surface. We identify a number of surface reconstructions in which Si adatoms prefer octahedral sites and Sn adatoms prefer tetrahedral sites. By applying the electron counting rule, we also study the mechanisms of the preferred adsorption sites for Si and Sn. We conclude that Si and Sn can also occupy the thermodynamically unfavored site due to surface reconstructions during the growth, which potentially leads to Si and Sn occupying both octahedral and tetrahedral sites in Ga2O3.

Mechanism leading to semi-insulating property of carbon-doped GaN: Analysis of donor acceptor ratio and method for its determination

Carbon impurities in GaN form both acceptors and donors. Donor-to-acceptor ratios (DARs) determine the semi-insulating behavior of carbon-doped GaN (GaN:C) layers and are still debated. Two models are discussed; both can theoretically achieve semi-insulating behavior: the dominant acceptor model (DAM, DAR<1) and the auto-compensation model (ACM, DAR=1). We perform a capacitance–voltage analysis on metal/GaN:C/nGaN (n-doped GaN) structures, exhibiting Fermi-level pinning in GaN:C, 0.7 eV above the valence band maximum. This observation coupled with further interpretation clearly supports the DAM and contradicts the ACM. Furthermore, we reveal a finite depletion width of a transition region in GaN:C next to nGaN, where carbon acceptors drop below the Fermi level becoming fully ionized. Calculation of the potential drop in this region exhibits DAR values of 0.5–0.67 for GaN:C with total carbon concentrations of 1018 cm−3 and 1019 cm−3. Based on those results, we re-evaluate formerly published density functional theory (DFT)-calculated formation energies of point defects in GaN. Unexpectedly, growth in thermodynamic equilibrium with the bulk carbon phase contradicts our experimental analysis. Therefore, we propose the consideration of extreme carbon-rich growth conditions. As bulk carbon and carbon cluster formation are not reported to date, we consider a metastable GaN:C solid solution with the competing carbon bulk phase being kinetically hindered. DFT and experimental results agree, confirming the role of carbon at nitrogen sites as dominant acceptors. Under N-rich conditions, carbon at gallium sites is the dominant donor, whereas additional nitrogen vacancies are generated under Ga-rich conditions.

Nuclear and Electron Spin Resonance Studies on Skyrmion‐Hosting Lacunar Spinels

Magnetic resonance techniques at nuclei and electrons are applied to characterize the electronic structure and collective magnetic excitations throughout the magnetic phase diagrams of the lacunar spinels GaV4S8 and GaV4Se8 showing cycloidal, Néel‐type skyrmion lattice and ferromagnetically polarized phases. 71Ga nuclear magnetic resonance (NMR) provides a local probe of the rhombohedral distortion and the resulting uniaxial magnetic anisotropy via the detection of electric field gradients (EFGs) and hyperfine coupling at the gallium sites of these lacunar spinels. Broadband electron spin resonance (ESR) allows identifying clockwise, counterclockwise, and breathing modes of the skyrmion‐lattice phase supported by theoretical simulations.

Identification of the spintronic NiGaVN center in c-GaN and its qubit applications

The presence of NiGa center in cubic gallium nitride (c-GaN), a candidate material for qubit applications, has been proven through first-principles calculation and theoretical analysis. The NiGa center is composed of a nitride-vacancy (VN) and an adjacent substitutional nickel atom (NiGa) at the gallium site, possessing the same C 3v structure with the negatively charged nitrogen-vacancy center (NV– center) as the diamond. The NiGa center is a spin-triplet, existing in n-type c-GaN. Combined with the group theory, the molecular orbitals (MOs), and the electron distribution of the center were analyzed. Furthermore, regarding the optical properties of the NiGa center and maintaining spin-conserving, the electron occupation number in the MOs a 1(2) and exy could be changed, which opens up the prospects for spin manipulation in the NiGa center. As a result, the zero-phonon line is found to be relatively low (at about 0.575 eV), which means the optical excitation can be achieved at lower energy. The Stokes and anti-Stokes shifts are shown to be 240 and 153 meV, respectively. Therefore, the findings of this study provide an important basis for quantum computing and networks. In particular, combining the results of the magnetic coupling and hyperfine interaction to achieve the best characteristics of the spintronic center makes it promising for qubit application.

Carbon complexes in highly C-doped GaN

We investigate the properties of heavily C-doped GaN grown by hydride vapor phase epitaxy using both optical experiments and hybrid density functional theory calculations. Previous work has established that carbon acceptors $({\mathrm{C}}_{\mathrm{N}})$ give rise to a yellow luminescence band near 2.2 eV along with a blue luminescence band near 2.9 eV. Photoluminescence measurements show the yellow band shifting as a function of carbon concentration, suggesting a change in the behavior of carbon species as carbon content increases. With hybrid density functional theory we calculate the electrical and optical behavior of carbon centers containing multiple carbon impurities, which may arise in heavily doped material. We compare the behavior of these complexes to the isolated centers, and find that the dicarbon donor-acceptor $({\mathrm{C}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{C}}_{\mathrm{N}})$ complex is a candidate to explain the shift in the yellow luminescence peak. Tricarbon complexes have high formation energies and modest binding energies, and also give rise to optical transitions that are inconsistent with the observed spectra. We also identify the split dicarbon interstitial on the gallium site as a low-energy species with a large binding energy that may act to compensate carbon acceptors. Local vibrational modes are calculated for carbon impurity centers, and we compare these results to recent experiments. Dicarbon and tricarbon complexes involving ${\mathrm{C}}_{\mathrm{Ga}}$ and ${\mathrm{C}}_{\mathrm{N}}$ exhibit modes that are only slightly higher than the isolated species, while carbon interstitials and related complexes give rise to vibrational modes much higher than ${\mathrm{C}}_{\mathrm{Ga}}$ and ${\mathrm{C}}_{\mathrm{N}}$.

Atomistic origin of compositional pulling effect in wurtzite (B, Al, In)xGa1−xN: A first-principles study

Some fluctuations in composition are commonly observed in epitaxial-grown III-V multinary alloys. These fluctuations are attributed to compositional pulling effects, and an insight into their atomistic origin is necessary to improve current epitaxial growth techniques. In addition, the crystallinity of III-V multinary alloys varies widely depending on the constituent atoms. Using first-principles calculations, we then investigated different geometric configurations of gallium nitride (GaN)-based ternary alloy, X0.125Ga0.875N where X is the minority atom which is boron (B), aluminum (Al), or indium (In). The minority atoms are presented as two atoms in the simulation cell, and the energetics of five geometric configurations are analyzed to estimate the most stable configuration. For the B0.125Ga0.875N alloy, the most stable configuration is the one where the minority atoms occupy gallium (Ga) sites in a collinear orientation along the c-axis. On the contrary, the configurations along the in-plane direction result in a higher energy state. In0.125Ga0.875N and Al0.125Ga0.875N also show the same trend with a small relative energy difference. These preferential sites of minority atoms are consistent with composition pulling effects in wurtzite nitride phases. Moreover, the degree of crystallinity for wurtzite nitride alloys can be well described by the order of calculated relative energy.

Boron doping in gallium oxide from first principles

We study the feasibility of boron doping in gallium oxide (Ga2O3) for neutron detection. Ga2O3 is a wide band gap, radiation-hard material with potential for neutron detection, if it can be doped with a neutron active element. We investigate the boron-10 isotope as possible neutron active dopant. Intrinsic and boron induced defects in Ga2O3 are studied with semi-local and hybrid density-functional-theory calculations. We find that it is possible to introduce boron into gallium sites at moderate concentrations. High concentrations of boron, however, compete with the boron-oxide formation.

Propane Dehydrogenation on Single-Site [PtZn4] Intermetallic Catalysts

Summary Propane dehydrogenation (PDH) is a commercial propylene production technology that has received much attention, but high reaction temperatures result in a decrease of propylene selectivity and catalyst stability. This paper describes a single-site [PtZn4] catalyst by assembling atomically ordered intermetallic alloy (IMA) as a selective and ultrastable PDH catalyst. The catalyst enables more than 95% of propylene selectivity from 520°C to 620°C. No obvious deactivation is observed within the 160-h test, superior to PtSn/Al2O3 and state-of-the-art Pt-based catalysts. Based on in situ X-ray absorption fine structure, X-ray photoelectron spectroscopy measurements, and density functional theory calculations, we reveal that the surface [PtZn4] ensembles in PtZn IMAs serve as the key active site structures, wherein the geometry-isolated and electron-rich Pt1 site in [PtZn4] ensembles readily promotes the first and second C–H cleavage of propane but inhibits further dehydrogenation of surface-bounded propylene. This significantly improves the selectivity and stability by prohibiting coke side reactions.

Effects of carbon related defects on opto-electronic properties of β-Ga2O3: The first principle calculation

Calculations using the revised Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE06) reveal the opto-electronic effects of carbon in β-Ga2O3. Stable charged states with carbon dopant are proved by phonon calculations which can result in the deep levels in the band gap. Stronger distortion can happen around oxygen sites substituted by carbon than those at gallium sites leading to interactions changing. Temperature and oxygen pressure can have huge effect on the formation of substitutions and the electronic conductivity. As partial oxygen pressure increase, the substitution concentration increases, and decrease with temperature increasing. Electronic conductivity of substitution of carbon at oxygen sites can get enhanced hugely, and it increases with temperature increasing. Besides electronic properties, carbon substitution can also have great impact on optical properties. Red-infrared PL emissions can be induced with substitutions at gallium sites, and yellow and blue emissions can be achieved by substitutions with oxygen in different charged states. The work can effectively predict the impact of carbon substitution in β-Ga2O3 and be useful for improvement of device designs and optimizations.

Ti- and Fe-related charge transition levels in β−Ga2O3

Deep-level transient spectroscopy measurements on β-Ga2O3 crystals reveal the presence of three defect signatures labeled E2a, E2b, and E3 with activation energies at around 0.66 eV, 0.73 eV, and 0.95 eV below the conduction band edge. Using secondary ion mass spectrometry, a correlation between the defect concentration associated with E3 and the Ti concentration present in the samples was found. Particularly, it is found that E3 is the dominant Ti-related defect in β-Ga2O3 and is associated with a single Ti atom. This finding is further corroborated by hybrid functional calculations that predict Ti substituting on an octahedral Ga site, denoted as TiGaII, to be a good candidate for E3. Moreover, the deep level transient spectroscopy results show that the level previously labeled E2 and attributed to Fe substituting on a gallium site (FeGa) consists of two overlapping signatures labeled E2a and E2b. We tentatively assign E2a and E2b to Fe substituting for Ga on a tetrahedral or an octahedral site, respectively.

The Effect of Cr3+ and Mg2+ Impurities on Thermoluminescence and Deep Traps in β-Ga2O3 Crystals

Thermally stimulated luminescence (TSL) of β-Ga2O3 single crystals doped with Cr3+ and Mg2+ impurities was investigated. Based on the correlation between the Cr3+ concentration and light sum accumulated in the thermoluminescence (TL) glow peak at 285 K, it was concluded that doping of β-Ga2O3 with Cr3+ ions leads to the formation of electron traps manifested in this peak. The activation energy of peak at 285 K is equal to Ec-0.55 eV and close to E1. Thus the Cr3+e− centers can be a candidate for E1. The high-temperature TL glow peak at 385 K (Ec-0.94 eV) is related to oxygen vacancies which are created in gallium oxide doped by Mg2+ ions to compensate for the negative charge formed by the substitution of gallium sites by magnesium ions.The co-doping of β-Ga2O3 crystals with Cr3+ and Mg2+ impurities leads to the appearance of a new TL glow peak at 320 K with an energy close to E*2 (Ec-0.7). It is suggested that this peak is formed by defect complex, in particular, oxygen vacancies with Cr3+ or Fe3+ ions.

XAFS study on the boron substituted LiGaO2 semiconductor material

An XAFS study was performed on the electronic and crystal structure properties of LiGa1-xBxO2 (LGO) material in where boron atoms were substituted at the gallium sites. Studies were carried out by the X-ray powder diffraction (XRD) patterns and supported by the Extended-XAFS data. The analysis on the substituted materials revealed interesting mechanisms at the boron sites that preserved the crystal symmetry in the entire bulk. It was determined that, boron atoms do not lie in the Ga sites due to inequivalent ionic radii and formed a crystal LiB3O5, which has the same crystal geometry and space group of the parent LGO. With the presence of the boron atoms on the vicinity of the gallium atoms, tiny shifts on the main edge spectra resulted from the change in the oxidation of the gallium atoms.