Wide Gap Oxide Research Articles

Methane Chemisorption on Oxide-Supported Pt Single Atom.

Methane chemisorption has been recently demonstrated on the rutile IrO2 (110) surface. However, it remains unclear how the general requirements are for methane chemisorption or complexation with a single atom on an oxide surface. By exploring methane adsorption on Pt1 substitutionally doped on many rutile-type oxides using hybrid density functional theory, we show that the occupancy of the Pt dz2 orbital is the key to methane chemisorption. Pt single atom on the semiconducting or wide-gap oxides such as TiO2 and GeO2 strongly chemisorbs methane, because the empty Pt dz2 orbital is located in the gap and can effectively accept σ-electron donation from the methane C-H bond. In contrast, Pt single atom on metallic oxides such as IrO2 and RuO2 does not chemisorb methane, because the Pt dz2 orbital strongly mixes with the support-oxide electronic states and become more occupied, losing its ability to chemisorb methane. This study sheds further light on the impact of the interaction between a Pt single atom and the oxide support on methane adsorption.

Non-radiation creation of complex centers in wide-gap oxide crystals

It is shown that complex aggregate centers of oxygen divacancy type (F2 centers) and interstitial aluminum type near the anion vacancy (Ali) are formed in anion-deficient crystals of corundum (α-Al2O3-δ) and beryllium oxide (BeO1-δ) under thermo-optical treatment (TOT). These centers are similar to those created in stoichiometric α-Al2O3 and BeO crystals under neutron irradiation. It is important to note that thermal stability of the TOT-created complex centers is higher than that of similar neutron-induced centers. It is also established that the probability of their formation is related to the temperature of the TOO, the wavelength of the stimulating light, and the initial anionic deficiency manifested as F+ and F centers (anion vacancies with one and two electrons, respectively).

Carrier Generation in p-Type Wide-Gap Oxide: SnNb2O6 Foordite

Wide-gap oxides with their valence band maximum (VBM) composed of s orbitals are essential for realizing practical p-type transparent oxide semiconductors. We prepared a new p-type wide-gap oxide, SnNb2O6 foordite, with its VBM composed of Sn 5s orbitals. To discuss carrier generation, we prepared both p-type and n-type SnNb2O6 by controlling the annealing conditions. The carrier mobility and density were 3.8 × 10–1 cm2 V–1 s–1 and 3.7 × 1018 cm–3, respectively, for the p-type sample and 9.9 cm2 V–1 s–1 and 7.5 × 1015 cm–3, respectively, for the n-type sample. The crystal structure of SnNb2O6 foordite consists of two types of alternating layers, Sn and Nb2O6 octahedra, where three nonequivalent oxygen sites exist. Six oxygens in the chemical formula of SnNb2O6 are distributed at the three sites in pairs, where the oxygens in three nonequivalent sites were named O1–O3. Hole and electron carriers were considered to be generated by Sn4+-on-Nb5+ substitutional defects (SnNb′) and oxygen vacancies of O1 and O2...

Hydrogenation of the wide-gap oxide semiconductor as a room-temperature and 3D-compatible electron doping technique

A hydrogen atom, characterized by one unpaired electron and the smallest atomic radius, underlies the operations of various solid-state devices such as transistors, capacitors, solar cells, etc. Given its specific character as donor impurity in oxides, hydrogen may also facilitate efficient electron doping in a wide range of oxide devices. Here, we demonstrate room-temperature electrochemical hydrogenation of an archetypical oxide semiconductor (TiO2) thin film to achieve a 3D-compatible electron doping technique. The hydrogenated region can be precisely defined by photolithography without the influence of polycrystalline grain boundaries. Besides, secondary ion mass spectroscopy with deuterium isotope reveals considerable amount of hydrogen condenses around the TiO2 bottom interface indicating the critical influence of the interface on hydrogen stability. This hydrogen shows excellent stability in contrast to its high diffusivity in bulk TiO2, enabling robust electron doping for oxide thin film devices as well as suggesting stable interface hydrogen reservoir for electrochemical phenomena.

Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments

Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G0W0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS).

Study of self-sensitization of wide-gap oxides to visible light by intrinsic defects: From Terenin to the present days

We present the results of the investigation of the role of intrinsic point defects in wide-bandgap oxides photo-sensitization. This is one of numerous directions of researches pioneered by academician A.N. Terenin in the Leningrad (St. Petersburg) State University and continued by his disciples. The step-by-step experimental investigations of photo-excitation of the electronic sub-system of wide-bandgap oxides and further relaxation channels are analyzed in situ in three phases: solid – adsorbate – gas. The spectroscopy of optical activation and the spectroscopy of diffuse reflection are used for identification of initial and final excited states of the system. Photoluminescence (PL) and thermo-stimulated luminescence (TSL) are used to study radiative channels. Thermo-programmed desorption (TPD), FTIR and ESR spectroscopies are used to study the precursors and adsorbed species in the excited system. The final steps – photo-adsorption/desorption and chemical reactions in gaseous phase are analyzed using the mass-spectrometry (MS). In situ UV (8.43eV) Photoelectron spectroscopy (UPS) is used to monitor the electro-physical parameters of the sample: Fermi level EF, band bending VS, density of occupied states (DOS) and dipole potential δ. Common and specific features of wide-bandgap oxides – insulators and semiconductors – are analyzed.

Defect levels and hyperfine constants of hydrogen in beryllium oxide from hybrid-functional calculations and muonium spectroscopy

The atomistic and electronic structures of isolated hydrogen states in BeO were studied by ab initio calculations and muonium spectroscopy (SR). Whereas standard density-functional theory with a semi-local GGA functional led to a detailed probing of all possible minimum-energy configurations of hydrogen further calculations with the hybrid HSE06 functional provided improved properties avoiding band-gap and self-interaction errors. Similarly to earlier findings for the other wide-gap alkaline-earth oxide, MgO, hydrogen in BeO is also predicted to be an amphoteric defect with the pinning level, E(), positioned in the mid-gap region. Both donor and acceptor levels were found too deep in the gap to allow for hydrogen to act as a source of free carriers. Whereas, hydrogen in its positively-charged state, , adopts exclusively hydroxide-bond OH configurations, and instead show a preference to occupy cage-like interstitial sites in the lattice. in particular displays a multitude of minimum-energy configurations: its lowest-energy ground state resembles closely a trapped-atom state with a nearly spherical spin-density profile. In contrast to the strongly ionic MgO, in BeO was further found to stabilise in additional higher-energy elongated-bond OH configurations whose existence should be traced to a partial covalent character of the Be–O bonding. Calculations of the proton-electron hyperfine coupling for all states showed that the ground-state interstitial configuration is dominated by an isotropic hyperfine interaction with a magnitude very close to the vacuum value, a finding corroborated by the SR-spectroscopy data.

RETRACTED: Thermodynamic Stability of ABX Heavy Elements of TaIrGe, TiIrSb, TaIrSn and ZrIrSb TCOs Using the Half-Heusler Technique

Electronic structure theory has recently been used to propose hypothetical compounds in presumed crystal structures, seeking new useful functional materials. The functional materials include transparent conductors needed in solar cell, light emitting diode and flat panel displays, which represent the usually contraindicated functionalities of optical transparency (generally associated with electrical insulators) coexisting with electrical conductivity (generally associated with optically opaque metals). Usually, such hypothetical materials are meta-stable, albeit with technologically useful long lifetimes. Yet, in other cases, suggested hypothetical compounds may be significantly higher in energy than their lowest-energy crystal structures or competing phases, making their synthesis and eventual device-stability questionable. Shifting focus from the previous searches of doping wide-gap metal oxides turns our focus to the newly predicted never before synthesized ABX compounds of TiIrSb, TaIrSn and ZrIrSb called “filled tetrahedral structure” (sometimes called Half-Heusler). We give interest to these compounds because; there stability range is located around X-richest and A/B-poor growth. The ‘inverse design’ principles are applied to ABX compounds. Then the effect of atomic number in their materials to tolerance of sufficient level of off-stoichiometry in creation of free holes that avoid the ionic sites and thus lead to high hole mobility at room temperature.

Fabricating Ohmic contact on Nb-doped SrTiO3 surface in nanoscale

Fabricating reliable nano-Ohmic contact on wide gap semiconductors is an important yet difficult step in oxide nanoelectronics. We fabricated Ohmic contact on the n-type wide gap oxide Nb-doped SrTiO3 in nanoscale by mechanically scratching the surface using an atomic force microscopy tip. Although contacted to high work function metal, the scratched area exhibits nearly linear IV behavior with low contact resistance, which maintains for hours in vacuum. In contrast, the unscratched area shows Fowler–Nordheim tunneling dominated Schottky rectifying behavior with high contact resistance. It was found that the Ohmic conductivity in the scratched area was drastically suppressed by oxygen gas indicating the oxygen vacancy origin of the Ohmic behavior. The surface oxygen vacancy induced barrier width reduction was proposed to explain the phenomena. The nanoscale approach is also applicable to macroscopic devices and has potential application in all-oxide devices.

Features of carrier tunneling between the silicon valence band and metal in devices based on the Al/high-K oxide/SiO2/Si structure

The features of electron tunneling from or into the silicon valence band in a metal–insulator–semiconductor system with the HfO2(ZrO2)/SiO2 double-layer insulator are theoretically analyzed for different modes. It is demonstrated that the valence-band current plays a less important role in structures with HfO2(ZrO2)/SiO2 than in structures containing only silicon dioxide. In the case of a very wide-gap high-K oxide ZrO2, nonmonotonic behavior related to tunneling through the upper barrier is predicted for the valence-band–metal current component. The use of an insulator stack can offer certain advantages for some devices, including diodes, bipolar tunnel-emitter transistors, and resonant-tunneling diodes, along with the traditional use of high-K insulators in a field-effect transistor.

Influence of complex impurity centres on radiation damage in wide-gap metal oxides

Different mechanisms of radiation damage of wide-gap metal oxides as well as a dual influence of impurity ions on the efficiency of radiation damage have been considered on the example of binary ionic MgO and complex ionic–covalent Lu3Al5O12 single crystals. Particular emphasis has been placed on irradiation with ∼2GeV heavy ions (197Au, 209Bi, 238U, fluence of 1012ions/cm2) providing extremely high density of electronic excitations within ion tracks. Besides knock-out mechanism for Frenkel pair formation, the additional mechanism through the collapse of mobile discrete breathers at certain lattice places (e.g., complex impurity centres) leads to the creation of complex defects that involve a large number of host atoms. The experimental manifestations of the radiation creation of intrinsic and impurity antisite defects (Lu|Al or Ce|Al – a heavy ion in a wrong cation site) have been detected in LuAG and LuAG:Ce3+ single crystals. Light doping of LuAG causes a small enhancement of radiation resistance, while pair impurity centres (for instance, Ce|Lu–Ce|Al or Cr3+–Cr3+ in MgO) are formed with a rise of impurity concentration. These complex impurity centres as well as radiation-induced intrinsic antisite defects (Lu|Al strongly interacting with Lu in a regular site) tentatively serve as the places for breathers collapse, thus decreasing the material resistance against dense irradiation.

Design and discovery of a novel half-Heusler transparent hole conductor made of all-metallic heavy elements.

Transparent conductors combine two generally contradictory physical properties, but there are numerous applications where both functionalities are crucial. Previous searches focused on doping wide-gap metal oxides. Focusing instead on the family of 18 valence electron ternary ABX compounds that consist of elements A, B and X in 1:1:1 stoichiometry, we search theoretically for electronic structures that simultaneously lead to optical transparency while accommodating intrinsic defect structures that produce uncompensated free holes. This leads to the prediction of a stable, never before synthesized TaIrGe compound made of all-metal heavy atom compound. Laboratory synthesis then found it to be stable in the predicted crystal structure and p-type transparent conductor with a strong optical absorption peak at 3.36 eV and remarkably high hole mobility of 2,730 cm(2) V(-1) s(-1) at room temperature. This methodology opens the way to future searches of transparent conductors in unexpected chemical groups.

Luminescence and optical spectroscopy of charge transfer processes in solid solutions NiCMg1−CO and NixZn1−xO

In this work photoluminescence spectra for NicMg1−cO and NixZn1−xO solid solutions with the rock-salt crystal structure were obtained under synchrotron radiation excitation. Periodical peaks in the photoluminescence excitation spectrum of NicMg1−cO (c=0.008) have been discovered for a wide-gap oxide doped with 3d impurities for the first time. They can be considered as LO phonon repetitions of the narrow zero phonon line resulted from the optical transitions into the p–d charge transfer exciton [d9h] state. A close coincidence in energy of different peculiarities in the optical absorption and photoluminescence excitation spectra for the NicMg1−cO and NixZn1−xO solid solutions is due to the practically equal interatomic distances Ni–O in the investigated materials. The bulk of new experimental results is the trustworthy evidence that only the p–d charge transfer transitions manifest themselves in the spectral region of 3.5–6.5eV.

Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked againstGWband structure calculations and experiments

We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO2,ZrO2, and WO3. We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO2 into Ti2O3, involving a change in oxide stoichiometry. We show that the dielectric-dependent hybrid functional is generally good at reproducing both ground-state (lattice constants, phase stability sequences, and reaction energies) and excited-state (photoemission gaps) properties within a single, fully ab initio framework.

Aqueous transition-metal cations as impurities in a wide gap oxide: the Cu(2+)/Cu(+) and Ag(2+)/Ag(+) redox couples revisited.

The interactions of the d electrons of transition-metal aqua ions with the solvent are usually divided in short-range electronic interactions with ligand water molecules and long-range electrostatic interactions with molecules beyond the first coordination shell. This is the rationale behind the cluster continuum and QM/MM methods developed for the computation of the redox potentials. In the density functional theory based molecular dynamics (DFTMD) method, the electronic states of the complex are also allowed to mix with the extended band states of the solvent. Returning to the Cu(+) and Ag(+) oxidation reaction, which has been the subject of DFTMD simulation before, we show that coupling to the valence band states of water is greatly enhanced by the band gap error in the density functional approximation commonly used in DFTMD (the generalized gradient approximation). This effect is analyzed by viewing the solvent as a wide gap oxide and the redox active ions as electronic defects. The errors can be reduced significantly by application of hybrid functionals containing a fraction of Hartree-Fock exchange. These calculations make use of recent progress in DFTMD technology, enabling us to include sp core polarization and Hartree-Fock exchange in condensed-phase model systems.

Antiferromagnetic superexchange interaction between Fe3+ ions in Fe3+ doped wide-gap oxide of β-Ga2−Fe O3 (x=0.1, 0.2, 0.3, 0.4)

In the Fe 3+ doped wide-gap oxide of β-Ga 2− x Fe x O 3 ( x =0.1, 0.2, 0.3, 0.4), Rietveld refinements of the X-ray diffraction data show that the Fe 3+ ions are preferentially occupied on the octahedral sites. From the measurements of magnetization as a function of magnetic field and inverse molar susceptibility as a function of temperature, it is found that the superexchange interaction between Fe 3+ ions via an intermediate oxygen ion is antiferromagnetic. The optical bandgap ( E g ) is found to decrease monotonically in energy upon increasing the Fe 3+ concentration. Below the E g edge, optical absorption subspectra are observed to occur at ~1.8 and ~2.7 eV, which are attributed to the d – d transitions of the crystal-field splitting of 3 d 5 multiplets of the octahedral Fe 3+ ion. • Preferential replacement of Ga 3+ by Fe 3+ ions occurs on the octahedral sites of β-Ga 2 O 3 . • The superexchange interaction between Fe 3+ ions in β-Ga 2 O 3 is antiferromagnetic. • The optical bandgap decreases monotonically upon increasing Fe 3+ content.

Fabrication and characterization of wüstite-based epitaxial thin films: p-type wide-gap oxide semiconductors composed of abundant elements

We report on the growth of FeO (wüstite)-based thin films using pulsed laser deposition. Epitaxial films of iron oxide with different crystal structures (corundum, spinel, and rock-salt-type) were separately fabricated by controlling the Si content in the Fe2O3-SiO2 mixture target. The rock-salt-type Fe0.92Si0.08O was found to form a solid solution with MgO, whose bandgap could be widely tuned (2.7–6.0 eV) by altering the MgO fraction. Moreover, Fe0.92Si0.08O-MgO films show p-type semiconducting behavior with high electrical conductivity (0.1–7.8 S/cm) at 300 K. These results indicate that Fe0.92Si0.08O-MgO films are good candidates for transparent p-type semiconductors.

Adsorption of thioether molecules on an alumina thin film

Low-temperature scanning tunneling microscopy has been employed to study the adsorption of (bis(3-phenylthio)-phenyl)sulfane (BPPS) molecules on an aluminum-oxide film grown on NiAl(110). Large variations in the molecular coverage on incompletely oxidized samples indicate substantial differences in the binding strength of BPPS to metallic (NiAl) versus dielectric (alumina) surfaces. From atomically resolved images, we obtain possible BPPS adsorption geometries on the oxide, in which the sulfur centers and not the phenyl rings of the molecule govern the interaction. A local hexagonal ordering of BPPS, as deduced from pair correlation functions, suggests a preferential binding of the BPPS sulfur atoms to Al ions with distorted pyramidal coordination in the oxide surface. Our work provides insight into rarely explored binding schemes of organic molecules on wide-gap oxide materials.

Dopant compensation in HfO2 and other high K oxides

The theory of doping limits in semiconductors and insulators is applied to the case of wide gap oxides, crystalline, or amorphous, and used to explain that impurities do not in general give rise to gap states or a doping response. Instead, the system tends to form defect complexes or undergo symmetry-lowering reconstructions to expel gap states out of the band gap. The model is applied to impurities, such as trivalent metals, carbon, N, P, and B, in HfO2, the main gate dielectric used in field effect transistors.

Grain Boundary Controlled Electron Mobility in Polycrystalline Titanium Dioxide

Grain Boundary Controlled Electron Mobility in Polycrystalline Titanium Dioxide