V. E. Borisenko

Electronic and related properties of crystalline semiconducting iron disilicide

Band structure calculations for β‐FeSi2 have been performed by the linear muffin‐tin orbital method within the local density approximation scheme including exchange and correlation effects. A detailed analysis of the conduction and valence band structure around high‐symmetry points has shown the existence of a quasidirect band gap structure in the material. It is experimentally confirmed that between the threshold energy of optical interband transition of 0.73 eV and the first direct gap transition with appreciable oscillator strength at about 0.87 eV there is a region in which direct transition of low oscillator strength and indirect transitions overlap. That explains the tricky behavior of β‐FeSi2 in experimental investigations demonstrating it to be either a direct or indirect gap semiconductor.

Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO3

In this part we present results of our ab initio calculations indicating that dispersion of the bands near the gap region for different phases of WO3 (namely, e-WO3, δ-WO3, γ-WO3, β-WO3, orth-WO3, α-WO3, and hex-WO3) is rather close. The rapid increase in the absorption coefficient starts at the lower energy range for α-WO3 and hex-WO3 than for the other phases in accordance with the calculated band gaps. An oxygen vacancy has turned out to decrease the gap by 0.50 eV and to shift the absorption coefficient to the lower energy range in the room temperature γ-WO3 phase. We have also traced changes caused by molybdenum and sulfur doping of γ-WO3. Only sulfur doped γ-WO3 has been revealed to display the formation of the impurity band along with a sizable reduction in the gap and the shift in the absorption coefficient to the lower energy range.

Intrinsic nanofilamentation in resistive switching

Resistive switching materials are promising candidates for nonvolatile data storage and reconfiguration of electronic applications. Intensive studies have been carried out on sandwiched metal-insulator-metal structures to achieve high density on-chip circuitry and non-volatile memory storage. Here, we provide insight into the mechanisms that govern highly reproducible controlled resistive switching via a nanofilament by using an asymmetric metal-insulator-semiconductor structure. In-situ transmission electron microscopy is used to study in real-time the physical structure and analyze the chemical composition of the nanofilament dynamically during resistive switching. Electrical stressing using an external voltage was applied by a tungsten tip to the nanosized devices having hafnium oxide (HfO2) as the insulator layer. The formation and rupture of the nanofilaments result in up to three orders of magnitude change in the current flowing through the dielectric during the switching event. Oxygen vacancies and metal atoms from the anode constitute the chemistry of the nanofilament.Resistive switching materials are promising candidates for nonvolatile data storage and reconfiguration of electronic applications. Intensive studies have been carried out on sandwiched metal-insulator-metal structures to achieve high density on-chip circuitry and non-volatile memory storage. Here, we provide insight into the mechanisms that govern highly reproducible controlled resistive switching via a nanofilament by using an asymmetric metal-insulator-semiconductor structure. In-situ transmission electron microscopy is used to study in real-time the physical structure and analyze the chemical composition of the nanofilament dynamically during resistive switching. Electrical stressing using an external voltage was applied by a tungsten tip to the nanosized devices having hafnium oxide (HfO2) as the insulator layer. The formation and rupture of the nanofilaments result in up to three orders of magnitude change in the current flowing through the dielectric during the switching event. Oxygen vacancies and...

Tungsten oxides. II. The metallic nature of Magnéli phases

In the first part [D. B. Migas et al., J. Appl. Phys. 108, 093713 (2010)] electronic and optical properties of different phases of WO3 have been considered. In this part we present results of our ab initio calculations which clearly show that all Magneli phases of tungsten oxides WOx (namely, W32O84, W3O8, W18O49, W17O47, W5O14, W20O58, and W25O73) are characterized by metal-like properties. Their band structures display an energy gap in the valence band just below the Fermi level. We discuss how addition (removal) of oxygen atoms to (from) the unit cell of W18O49 affects the position of the Fermi level with respect to the energy gap and the charge carrier concentration. A possible mechanism has been suggested in order to switch from metallic to semiconducting properties for W18O49 and to explain experimental observations.

Theoretical and experimental study of interband optical transitions in semiconducting iron disilicide

The interband optical spectra of the semiconducting β phase of iron disilicide (β-FeSi2) were investigated in the energy range from 0.5 to 5.0 eV. The dielectric function and other optical functions were deduced from ellipsometric experiments and calculated within the local-density approximation by using the semirelativistic linear muffin-tin orbital method. Reasonable agreement between the calculated and measured data has been obtained.

Role of oxygen vacancies in HfO2-based gate stack breakdown

We study the influence of multiple oxygen vacancy traps in the percolated dielectric on the postbreakdown random telegraph noise (RTN) digital fluctuations in HfO2-based metal-oxide-semiconductor transistors. Our electrical characterization results indicate that these digital fluctuations are triggered only beyond a certain gate stress voltage. First-principles calculations suggest the oxygen vacancies to be responsible for the formation of a subband in the forbidden band gap region, which affects the triggering voltage (VTRIG) for the RTN fluctuations and leads to a shrinkage of the HfO2 band gap.

Structural, electronic and optical properties of Ru2Si3, Ru2Ge3, Os2Si3 and Os2Ge3

We have performed a comparative study of structural, electronic and optical properties of Ru 2 Si 3 , Ru 2 Ge 3 , Os 2 Si 3 and Os 2 Ge 3 by means of ultrasoft pseudopotential and full-potential linearized augmented plane wave methods. The estimated difference in the cohesion energy between the low-temperature orthorhombic phase and the high temperature tetragonal one for all these compounds indicates that the former phase is lower in energy with respect to the latter one. All materials in the orthorhombic structure are found to be direct band-gap semiconductors, still some of them in the tetragonal structure display an indirect nature (Os 2 Si 3 ) or a competitive direct-indirect character (Ru 2 Ge 3 ) of the gap. Optical properties are discussed by analyzing the imaginary part of the dielectric function and the dipole matrix elements corresponding to different interband transitions indicating for osmium silicide and germanide the presence of low-energy transitions with an appreciable value of the oscillator strength.

Fast exothermic processes in porous silicon

It is found that rapid oxidation of porous Si (por-Si) layers in air may occur in the form of combustion or explosion. Combustion occurs in por-Si layers thinner than 60 μm and impregnated with potassium nitrate, while explosion is observed in thicker porous layers. It is suggested that explosion develops by a thermal mechanism resulting from an exponential increase in the reaction rate with temperature.

Combustion and explosion of nanostructured silicon in microsystem devices

Combustion and explosion in layers of nanostructured porous silicon has been studied in relation to the layer thickness and duration of sample storage after electrochemical anodization. The amount of hydrogen adsorbed on the surface of porous silicon after anodic treatment is evaluated. The amount of hydrogen accu-mulated in porous silicon is 4 wt %. Prototype microsystem devices employing the energy of the processes studied have been developed and fabricated. The combustion of porous silicon can be used as an energy source for silicon microactuators and the microexplosion can be employed both in self-destructing silicon chips and for dividing silicon wafers into separate crystals.

Room-temperature formation of erbium-related luminescent centers in anodic alumina

Luminescent Er-doped Al2O3 films have been fabricated at room temperature by a technique including magnetron deposition of Er-doped Al film on a silicon substrate and its subsequent electrochemical anodization. The films demonstrate strong Er-related photoluminescence at about 1.53 μm as recorded in the temperature range of 4.2–300 K. The effect is not influenced by annealing of the samples up to 200 °C. Upon annealing at 300–500 °C the luminescence intensity decreases, while above 600 °C it starts to recover. Annealing at 1000 °C restores the photoluminescence spectra to the initial level. The annealing peculiarities observed have been explained by dominant hydrogen outdiffusion at 300–500 °C, rearrangement of point defects at 600–800 °C, and recrystallization processes above 850 °C in the alumina film. Activation energies of these processes have been estimated to be 0.76 eV (for parabolic rate), 0.58 eV (for linear rate), and 0.46 eV (for linear rate), respectively.