D. B. Migas

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.

Anisotropic photonic properties of III-V nanowires in the zinc-blende and wurtzite phase.

Some critical aspects of the anisotropic absorption and emission properties of quasi one-dimensional structures are reviewed in the context of III-V compound semiconductor nanowires. The unique optical and electronic properties of III-V nanowires stem from the combination of dielectric effects due to their large aspect ratio, and their specific crystallographic structure which can differ significantly from the bulk case. The growth conditions leading to single-crystal nanowires with either zinc blende or wurtzite phase are first presented. Dipole selection rules for interband transitions in common III-V compounds are then summarized for the two different phases, and corroborated by ab initio Density Functional Theory calculations of the oscillator strength. The optical anisotropy is discussed considering both the effect of refractive index mismatch between the nanowire and its surroundings and the polarization of the emitting dipoles set by the nanowire crystallographic structure and orientation. Finite Difference Time Domain simulations are finally employed to illustrate the influence of the emitting dipole orientation and the nanowire diameter on the distribution of radiation in the far-field. The importance of the correlation between structural and optoelectronic properties is highlighted in view of potential applications in future nanowire photonics.

Transport properties of semiconducting Rhenium silicide

Transport properties of semiconducting rhenium silicide ReSi1.75 were studied both experimentally and theoretically. For this high quality ReSi1.75 single crystals were grown by the zone melting technique with radiation heating and their resistivity and Hall coefficient were investigated. The room temperature carrier concentration in undoped material is appreciably high, about 1019 cm-3. The Hall mobility at room temperature is 30 cm2/V.s, that is considerably lower than previously reported values for single crystals. P-type conductivity persists across the entire interval investigated. Calculation of the charge carrier mobility involved the carrier effective masses, which were obtained from the ab initio electronic band structure and classical scattering mechanisms. A reasonable explanation of the hole mobility behavior is proposed and a strong anisotropy in hole effective masses and mobilities is predicted.

Electronic properties of osmium disilicide

Electronic property calculation of OsSi2 performed by the linear muffin-tin orbital method within the local density approximation scheme has shown the material to be an indirect gap semiconductor with a gap value of 0.95 eV. A direct transition with appreciable oscillator strength at 1.14 eV is predicted.

The role of morphology in stability of Si nanowires

By means of ab initio calculations we have investigated the morphology and stability of nonhydrogenated, free standing, single crystal silicon nanowires oriented along ⟨001⟩, ⟨011⟩, ⟨111⟩, and ⟨112⟩ axes and with diameters ranging from 2 to 5 nm. Different shapes and facet reconstructions have been carefully considered in order to provide a surface without any atoms with two dangling bonds and, eventually, to select the morphology more stable in energy. We have found the ⟨011⟩-oriented silicon nanowires to display the lowest total energy and we also discuss how our results support recent experimental observations.