V. Sh. Aliev

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.

Optical properties of the HfO 2 − x N x and TiO 2 − x N x films prepared by ion beam sputtering

Optical characteristics of the HfO2 − xNx and TiO2 − xNx films obtained by reactive ion beam sputtering have been investigated by spectral ellipsometry. The chemical composition of the films was determined using X-ray photoelectron spectroscopy. The nitrogen content in the oxynitride films (determined by the N2/O2 ratio in the gas mixture during synthesis) reached ≈9 at % for TiO2 − xNx and ≈ 6 at % for HfO2 − xNx. It is found that the dispersion relations n(λ) and k(λ) for the TiO2 − xNx films change from those characteristic of titanium dioxide to those typical of titanium nitride with an increase in the nitrogen content from 0 to ≈9 at %. The optical parameters of the HfO2 − xNx films depend weakly on the nitrogen content in the range 0–6 at %.

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.

Cathodo- and photoluminescence increase in amorphous hafnium oxide under annealing in oxygen

Cathodo- and photoluminescence of amorphous nonstoichiometric films of hafnium oxide are studied with the aim to verify the hypothesis that oxygen vacancies are responsible for the luminescence. To produce oxygen vacancies, hafnium oxide was enriched in surplus metal during synthesis. To reduce the oxygen concentration, the film was annealed in oxygen. A qualitative control of the oxygen concentration was carried out by the refractive index. In the initial, almost stoichiometric films we observed a 2.7-eV band in cathodoluminescence. Annealing in oxygen results in a considerable increase in its intensity, as well as in the appearance of new bands at 1.87, 2.14, 3.40, and 3.6 eV. The observed emission bands are supposed to be due to single oxygen vacancies and polyvacancies in hafnium oxide. The luminescence increase under annealing in an oxygen atmosphere may be a result of the emission quenching effect.

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.

Investigations of stochastic resonance in two-terminal device with vanadium dioxide film

The results of stochastic resonance investigation in a nonlinear system, consisting of a microstructure with a polycrystalline vanadium dioxide (VO2) film grown on sapphire and resistor in series are reported. Nonlinearity of the system was provided due to insulator-metal phase transition in VO2. In the stochastic resonance regime at 100 Hz signal frequency, the transition coefficient of signal-to-noise ratio reached 87 in contrast to 250 for microstructures with VO2 films grown on silica in our previous investigations. The measured characteristics of microstructures with VO2 films grown on silica and sapphire substrates were found to be qualitatively similar. For both substrates, a stochastic resonance was observed at threshold switching voltage from insulating to metallic state of VO2. For sapphire substrate the output signal-to-noise ratio rose at higher signal frequencies. The stochastic resonance phenomenon in VO2 films is explained in terms of the monostable damped oscillator model.

Optical properties of the HfO2 − xNx and TiO2 − xNx films prepared by ion beam sputtering

Optical characteristics of the HfO2 − xNx and TiO2 − xNx films obtained by reactive ion beam sputtering have been investigated by spectral ellipsometry. The chemical composition of the films was determined using X-ray photoelectron spectroscopy. The nitrogen content in the oxynitride films (determined by the N2/O2 ratio in the gas mixture during synthesis) reached ≈9 at % for TiO2 − xNx and ≈ 6 at % for HfO2 − xNx. It is found that the dispersion relations n(λ) and k(λ) for the TiO2 − xNx films change from those characteristic of titanium dioxide to those typical of titanium nitride with an increase in the nitrogen content from 0 to ≈9 at %. The optical parameters of the HfO2 − xNx films depend weakly on the nitrogen content in the range 0–6 at %.

The Nature of Defects Responsible for Transport in a Hafnia-Based Resistive Random Access Memory Element

Abstract A promising candidate for universal memory, which would involve combining the most favorable properties of both high-speed dynamic random access memory and nonvolatile flash memory, is resistive random access memory (ReRAM). ReRAM is based on switching back and forth from a high resistance state to a low resistance state. ReRAM cells are small, allowing for the creation of memory on the scale of terabits. One of the most promising materials for use as an active medium in resistive memory is hafnia (HfO 2 ). However, unresolved in physics is the nature of defects and traps that are responsible for charge transport in different states of resistive memory. In this study, we demonstrated experimentally and theoretically that oxygen vacancies are responsible for charge transport in resistive memory elements based on HfO 2 . We also demonstrated that transport in the low resistance state occurs through a mechanism described according to percolation theory. Based on the model of phonon-assisted tunneling between traps, and assuming that the electron traps are oxygen vacancies, a good quantitative agreement between the experimental and theoretical data of current–voltage characteristics was achieved. The thermal excitation energy of the traps in hafnia was determined based on the excitation spectrum and luminescence of the oxygen vacancies. The findings of this study demonstrate that oxygen vacancies play the key role in charge transport in hafnia-based resistive memory elements.

Optical properties of nonstoichiometric ZrO x according to spectroellipsometry data

Amorphous nonstoichiometric ZrOx films of different composition have been synthesized by the method of ion-beam sputtering deposition of metallic zirconium in the presence of oxygen at different partial oxygen pressures in the growth zone, and their optical properties have been studied in the spectral range of 1.12–4.96 eV. It is found that light-absorbing films with metallic conductivity are formed at the partial oxygen pressure below 1.04 × 10–3 Pa and transparent films with dielectric conductivity are formed at the pressure above 1.50 × 10–3 Pa. It is shown that the spectral dependences of optical constants of ZrOx films are described well by the corresponding dispersion models: the Cauchy polynomial model for films with dielectric conductivity and the Lorentz–Drude oscillator model for films with metallic conductivity.

Morphological transformations of vanadium oxide films during low-temperature reduction in hydrogen electron cyclotron resonance plasma

Morphological transformations of amorphous vanadium oxide films obtained by the sol-gel method and polycrystalline V2O5 films are studied during their low-temperature (295–623 K) reduction in a hydrogen electron cyclotron resonance plasma. The morphology of films is analyzed using atomic force microscopy and high-resolution electron microscopy. It is found that a homogeneous amorphous film during the reduction process transforms to an island film and then bulk amorphous islands of a regular shape appear. These islands resemble microcrystals, and their concentration depends on the temperature and the reduction time. The low-temperature reduction of polycrystalline V2O5 films leads to their amorphization; however, the microcrystals in the polycrystalline film do not change their shape in this process. A mechanism of the reduction process is proposed. This mechanism explains the regularities of morphological transformations in amorphous sol-gel films of vanadium oxides based on the suggestion of a competition between the ion-stimulated nucleation and growth of nuclei of the crystalline phase and the amorphization of the growing nuclei.