T. V. Perevalov

Electronic structure of δ-Ta2O5 with oxygen vacancy: ab initio calculations and comparison with experiment

Electronic structure of oxygen vacancies in Ta2O5 have been studied theoretically by first-principles calculations and experimentally by x-ray photoelectron spectroscopy. Calculations of δ-Ta2O5 were performed using density functional theory within gradient-corrected approximation with the +U approach. Results indicate that the oxygen vacancy causes a defect level in the energy gap at 1.2 eV above the top of the valence band. To produce oxygen vacancies, amorphous films of Ta2O5 were bombarded with Ar+ ions. XPS results indicate that the Ar-ion bombardment leads to the generation of the oxygen vacancies in Ta2O5 that characterize the peak at 2 eV above the valence band. The calculated spectrum of crystalline δ-Ta2O5 demonstrates qualitative correspondence with the XPS spectrum of the amorphous Ta2O5 film after Ar-ion bombardment.

Electronic structure of α-Al2O3: Ab initio simulations and comparison with experiment

Al2O3 films 150 Å thick are deposited on silicon by the ALD technique, and their x-ray (XPS) and ultraviolet (UPS) photoelectron spectra of the valence band are investigated. The electronic band structure of corundum (α-Al2O3) is calculated by the ab initio density functional method and compared with experimental results. The α-Al2O3 valence band consists of two subbands separated with an ionic gap. The lower band is mainly formed by oxygen 2s states. The upper band is formed by oxygen 2p states with a contribution of aluminum 3s and 3p states. A strong anisotropy of the effective mass is observed for holes: mh⊥* ≈ 6.3m0 and mh‖* ≈ 0.36m0. The effective electron mass is independent of the direction me‖* ≈ me⊥* ≈ 0.4m0.

Electronic structure of TiO2 rutile with oxygen vacancies: Ab initio simulations and comparison with the experiment

The electronic structure of TiO2 rutile with oxygen vacancies, which is a promising insulator, has been analyzed. The ab initio density functional calculations, as well as the comparative analysis of the results obtained in the σ-GGA spin-polarized generalized approximation and those obtained by the σ-GGA + U method with allowance for Coulomb correlations of d electrons titanium atoms in the Hartree-Fock approximation for the Hubbard model, have been performed. It has been found that the effective electron mass in rutile is anisotropic and there are both light (me* = (0.6–0.8)m0, where m0 is the free-electron mass) and heavy (me* > 1m0) electrons, whereas holes in rutile are only heavy (me* ⩾ 2m0). It has been shown that the σ-GGA + U method gives a deep occupied level in the band gap and that an oxygen vacancy in rutile is an electron and hole trap.

Electronic structure of an oxygen vacancy in Al2O3 from the results of Ab Initio quantum-chemical calculations and photoluminescence experiments

The electronic structure of an oxygen vacancy in α-Al2O3 and γ-Al2O3 is calculated. The calculation predicts an absorption peak at an energy of 6.4 and 6.3 eV in α-Al2O3 and γ-Al2O3, respectively. The luminescence and luminescence excitation spectra of amorphous Al2O3 are measured using synchrotron radiation. The presence of a luminescence band at 2.9 eV and a peak at 6.2 eV in the luminescence excitation spectrum indicates the presence of oxygen vacancies in amorphous Al2O3.

Charge transport mechanism in thin films of amorphous and ferroelectric Hf0.5Zr0.5O2

The charge transport mechanism in thin amorphous and ferroelectric Hf0.5Zr0.5O2 films has been studied. It has been shown that the transport mechanism in studied materials does not depend on the crystal phase and is phonon-assisted tunneling between traps. The comparison of the experimental current–voltage characteristics of TiN/Hf0.5Zr0.5O2/Pt structures with the calculated ones provides the trap parameters: thermal energy of 1.25 eV and the optical energy of 2.5 eV. The trap concentration has been estimated as ~1019–1020 cm–3.

Ab initio simulation of the electronic structure of Ta2O5 crystal modifications

Ab initio simulation of the electronic structure crystalline β and δ phases of tantalum(V) oxide (Ta2O5), representing a promising dielectric material for microelectronics, has been carried out. Both ideal crystals and those with neutral oxygen vacancies in various coordination positions have been studied. The simulation has been performed using the density functional theory with hybrid functionals involving the Hartree-Fock exchange energy. This approach gives a correct description of the bandgap width: 4.1 eV for β-Ta2O5 and 3.1 eV for δ-Ta2O5. The energy levels related to oxygen vacancies in various positions have been determined for the spectra of electron states in β- and δ-Ta2O5 polymorphs. It is established that the presence of oxygen vacancies in Ta2O5 crystal modifications leads to the formation of characteristic absorption peaks in their electron energy loss spectra.

Ab initio simulation of the electronic structure of δ-Ta2O5 with oxygen vacancy and comparison with experiment

The electronic structure of a Ta2O5 insulator with oxygen vacancies is studied theoretically and experimentally. The ab initio calculations of δ-Ta2O5 are performed in terms of density functional theory using the generalized gradient (GGA) and GGA + U approximations. The electronic structure of Ta2O5 is experimentally studied by X-ray photoelectron spectroscopy (XPS). To study oxygen vacancies, an amorphous Ta2O5 film is irradiated by argon ions. The calculated XPS spectra of the valence band of δ-Ta2O5 agree satisfactorily with the corresponding experimental spectra of the amorphous films. The oxygen vacancy in δ-Ta2O5 is found to be a trap for holes and electrons. The minimum and maximum effective masses of electrons and holes in δ-Ta2O5 are calculated.

Origin of the blue luminescence band in zirconium oxide

The photoluminescence excitation and steady-state photoluminescence spectra of nonstoichiometric zirconium oxide films with a high concentration of oxygen vacancies have been investigated. A band with energy of about 2.7 eV in the blue spectral region dominates in photoluminescence spectra of prepared films. The photoluminescence intensity of this band increases as the depletion of zirconium oxide films with oxygen increases. The excitation maximum of the blue photoluminescence band corresponds to energy of 5.2 eV. It has been established by quantum-chemical modeling that the optical absorption peak of the oxygen vacancy in crystalline zirconium oxide is located at energy of 5.1 eV. The analysis of the results has demonstrated that the blue photoluminescence band at 2.7 eV with the excitation peak near 5.2 eV is caused by oxygen vacancies in zirconium oxide.

Atomic and electronic structures of lutetium oxide Lu2O3

The chemical composition, electronic structure, structure, and physical properties a lutetium oxide Lu2O3 film are studied by X-ray photoelectron spectroscopy, ellipsometry, and X-ray absorption spectroscopy. The short-range order in Lu2O3 is found to correspond to its cubic modification. The binding energies of the 1s and 2p levels of oxygen and the 4d5/2 and 4f7/2 levels of lutetium are 529.2, 5.0 and 7.4, 195.9 eV, respectively. The energy gap determined from the electron energy loss spectrum of the film is 5.9 eV. The electron energy loss spectra have two peaks at 17.4 and 22.0 eV, which can be attributed to the excitation of bulk plasma oscillations. The dispersion of the refractive index is measured by spectral ellipsometry. The refractive index is shown to increase from 1.82 at 1.5 eV to 2.18 at 5.0 eV, and the high-frequency permittivity of Lu2O3 is 3.31.

Charge Transport and the Nature of Traps in Oxygen Deficient Tantalum Oxide

Optical and transport properties of nonstoichiometric tantalum oxide thin films grown by ion beam deposition were investigated in order to understand the dominant charge transport mechanisms and reveal the nature of traps. The TaOx films composition was analyzed by X-ray photoelectron spectroscopy and by quantum-chemistry simulation. From the optical absorption and photoluminescence measurements and density functional theory simulations, it was concluded that the 2.75 eV blue luminescence excited in a TaOx by 4.45 eV photons, originates from oxygen vacancies. These vacancies are also responsible for TaOx conductivity. The thermal trap energy of 0.85 eV determined from the transport experiments coincides with the half of the Stokes shift of the blue luminescence band. It is argued that the dominant charge transport mechanism in TaOx films is phonon-assisted tunneling between the traps.