Electrical characterization of gate oxides by scanning probe microscopies

Ballistic electron emission microscopy (BEEM) and non-contact atomic force microscopy (NC-AFM) are used to characterize SiO₂ and Al₂O₃ layers grown on Si(1 0 0). The effective conduction band mass and its energy dispersion in SiO₂ and an offset between Al₂O₃ and Si conduction bands of 2.78 eV were obtained with BEEM. NC-AFM was used to image electrons, and in some instances holes, trapped in the oxide layers near the surface and in the bulk of the oxide. Modeling of the tip-surface interaction supports the interpretation of image features arising from a single electron occupying a trap. The polarity of the trapped charge was deduced from Kelvin (potential difference) images that were simultaneously recorded with the topographic images.     

Photoluminescence Spectra of GaS0.75Se0.25 Layered Single Crystals

Photoluminescence (PL) spectra of GaS_0.75Se_0.25 layered single crystals have been studied in the wavelength region of 500‐850 nm and in the temperature range of 10‐200 K. Two PL bands centered at 527 ( 2.353 eV, A‐band) and 658 nm (1.884 eV, B‐band) were observed at T = 10 K. Variations of both bands have been studied as a function of excitation laser intensity in the range from 8 × 10^‐3 to 10.7 W cm^‐2. These bands are attributed to recombination of charge carriers through donor‐acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located 0.043 and 0.064 eV below the bottom of conduction band to acceptor levels located 0.088 and 0.536 eV above the top of the valence band are suggested to be responsible for the observed A‐ and B‐bands in the PL spectra, respectively.     

Infrared photoluminescence from TlGaS2 layered single crystals

Photolimuniscence (PL) spectra of TlGaS₂ layered crystals were studied in the wavelength region 500‐1400 nm and in the temperature range 15‐115 K. We observed three broad bands centered at 568 nm (A‐band), 718 nm (B‐band) and 1102 nm (C‐band) in the PL spectrum. The observed bands have half‐widths of 0.221, 0.258 and 0.067 eV for A‐, B‐, and C‐bands, respectively. The increase of the emission band half‐width, the blue shift of the emission band peak energy and the quenching of the PL with increasing temperature are explained using the configuration coordinate model. We have also studied the variations of emission band intensity versus excitation laser intensity in the range from 0.4 to 19.5 W cm^‐2. The proposed energy‐level diagram allows us to interpret the recombination processes in TlGaS₂ crystals. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoconduction of glasses in the TeSeSb system

The photoconductivity of bulk glasses of the TeSeSb system is measured as a function of light intensity and photon energy. The relative sensitivity (ΔI/I_d) has linear and square-root dependences on light intensity in low and high illumination intensities, respectively, and is nearly proportional to the square-root of the resistivity at room temperature. The spectral response of photoconductivity, which is calculated by taking into account the effect of surface recombination of carriers, agrees qualitatively with the experimental results. The experimentally determined broad spectral response suggests the presence of band tails below the conduction band and above the valence band. The large residual dark conductivity in the decay response is associated with the presence of many deep trapping centers.     

Temperature dependence of photoluminescence from ordered GaInP2 epitaxial layers

The temperature behavior of the integrated intensity of photoluminescence (PL) emission from ordered GaInP₂ epitaxial layer was measured at temperatures of 10 ‐ 300 K. Within this temperature range the PL emission is dominated by band‐to‐band radiative recombination. The PL intensity temperature dependence has two regions: at low temperatures it quenches rapidly as the temperature increases, and above 100 K it reduces slowly. This temperature behavior is compared with that of disordered GaInP₂ layer. The specter of the PL emission of the disordered layer has two peaks, which are identified as due to donor‐accepter (D‐A) and band‐to‐band recombination. The PL intensity quenching of these spectral bands is very different: With increasing temperature, the D‐A peak intensity remains almost unchanged at low temperatures and then decreases at a higher rate. The intensity of the band‐to‐band recombination peak decays gradually, having a higher rate at low temperatures than at higher temperatures. Comparing these temperature dependencies of these PL peaks of ordered and disordered alloys and the temperature behavior of their full width at half maximum (FWHM), we conclude that the different morphology of these alloys causes their different temperature behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Slowing down of water dynamics in disaccharide aqueous solutions

The dynamics of water in aqueous solutions of three homologous disaccharides, namely trehalose, maltose and sucrose, has been analyzed by means of molecular dynamics simulations in the 0-66 wt.% concentration range. The low-frequency vibrational densities of states (VDOS) of water were compared with the susceptibilities χ″ of 0-40 wt.% solutions of trehalose in D₂O obtained from complementary Raman scattering experiments. Both reveal that sugars significantly stiffen the local environments experienced by water. Accordingly, its translational diffusion coefficient decreases when the sugar concentration increases, as a result of an increase of water-water hydrogen bond lifetimes and of the corresponding activation energies. This induced slowing down of water dynamics, ascribed to the numerous hydrogen bonds that sugars form with water, is strongly amplified at concentrations above 40 wt.% by the percolation of the hydrogen bond network of sugars, and may partially explain their well-known stabilizing effect on proteins in aqueous solutions.     

Broadband terahertz time-domain spectroscopy and low-frequency Raman scattering of glassy polymers: Boson peak of PMMA

The low-energy vibrational properties of polymethyl methacrylate (PMMA) were studied by terahertz time-domain spectroscopy and Raman scattering. The real and imaginary parts of a complex dielectric constant were accurately determined in the frequency range from 0.2 to 5.0 THz. The imaginary part of the dielectric constant shows a line-shape similar to the imaginary part of the Raman susceptibility measured by Raman scattering. These two spectra show broad peaks at about 2.3 THz. We also separately determined the Raman and far-infrared light-vibration coupling constants of the PMMA using the vibrational density of states determined by cold neutron inelastic scattering in the literature.     

Temperature Dependence of Raman Spectrum in Solid Carbon Tetrabromide

Laser Raman spectra of carbon tetrabromide have been observed for the lattice and intramolecular vibrational regions over a temperature range of 80–370 K. Many bands were observed in the lattice region and the spectrum at the lower temperature in the monoclinic phase is similar to that of γ-Si(CH₃)₄. The temperature dependence of each band has been studied. The v₃ fundamental indicates a distinct variation corresponding to the phase transition.     

Atomic dynamics in different regions of the potential energy surface

The atomic dynamics in two (bulk) metallic glasses, Ni₄₀Pd₄₀P₂₀ and Zr₅₅Cu₃₀Al₁₀Ni₅, were investigated by neutron inelastic scattering in different regions of the potential energy landscape, which are reached by slow cooling the bulk glasses and by hyper-quenching the same alloys. The results prove that the atomic dynamics depends also on the fictive temperature, i.e. the region of the potential energy surface, in which the glass is frozen in. Obviously the shapes of the basins or inherent structures are not the same everywhere on the potential energy surface, and the glass with a higher fictive temperature has more low energy modes than has the same glass with a lower fictive temperature. As results from computer simulation have suggested already, on moving to regions of lower mean potential energy (aging), part of theses low energy modes are transferred to the energy region of the calculated Debye cut-off energy. The difference between the vibrational entropies, calculated from the generalized vibrational density-of-states, which have been determined for both fictive temperatures, shows that the contribution from the vibrational entropy to the total entropy change, when moving through the potential energy landscape, is small for the two metallic glasses investigated. Structural relaxation of the hyper-quenched glass removes part of the additional low energy modes, but quantitatively possibly only at the low and perhaps also at the high-energy limit of the density-of-states. The wavelength dependence of the dynamics suggests that the additional low energy modes in the glass with the higher fictive temperature are not dominated by extended but more likely by localized modes.     

Fictive temperature of GeO2 glass: Its determination by IR method and its effects on density and refractive index

A simple IR method of determining the fictive temperature of GeO₂ glass, both surface and bulk, was developed using the same technique developed earlier for silica glass. Specifically, IR absorption and reflection peak wavenumbers of GeO₂ structural bands were found to be correlated with the fictive temperature of the glass. Using this method structural relaxation kinetics can be investigated. Density and refractive index of GeO₂ glass were also measured as a function of fictive temperature.     

Radiation induced color centers in 50PbO–50P2O5 glass

The effect of γ-irradiation in the dose range of 5 kGy up to 25 kGy on the optical absorption spectra of 50PbO–50P₂O₅ glasses is reported. The spectral absorption of these glasses before and after γ-irradiation was measured in the spectral range of 300–900 nm at room temperature. The radiation induced absorption in this spectral range shown to consist of two bands centered approximately at 545 nm and 730 nm. The fundamental absorption edge shifts generally to lower energies with increasing γ-irradiation dose up to 25 kGy in this glass sample. The intensity of the induced absorption bands increases linearly with increasing γ-irradiation dose. The higher energy band (HEB) may be due to a hole in a singly bonded non-bridging oxygen distant from a modifier cation, while the lower energy band (LEB) is also due to a hole in similar oxygen which is interacting with a neighboring cation.     

Optical and FTIR studies of CuO-doped lead borate glasses and effect of gamma irradiation

CuO doped lead borate glasses of the composition PbO 70%–B₂O₃ 30% with varying CuO contents were prepared. UV–visible and infrared spectroscopic studies were measured before and after successive gamma irradiation with two different doses namely 2 and 8 Mrad. The experimental results indicate that the undoped sample reveals strong UV–near visible absorption while copper doped samples show additional broad visible band due to (Cu^2 +) ions.FT infrared absorption spectrum reveals vibrational bands due to triangular and tetrahedral borate groups together with the sharing of Pb–O vibrations.CuO-doped glasses have been found to show a shielding behavior towards the effects of progressive gamma irradiation causing the maintenance of the spectral curves. The changes in the UV–visible and infrared spectral data are discussed in relation to the states of copper ions and structural evolution caused by the change in glass composition including the CuO.     

Raman spectra and structure of sodium aluminogermanate glasses

Polarized Raman spectra of x NaAlO₂·(100 ? x) GeO₂ glasses (x = 0, 5, 10, 15, 20, 25, 33, 42, and 50) are presented. Analyses of the Raman data indicate that the aluminogermanate glasses have three-dimensional network structures consisting of interconnected AlO₄ and GeO₄ tetrahedra; Na⁺ ions are present in cavities and charge balance the Al³⁺ ions. Systematic changes are observed in the frequencies, intensities and polarization characteristics of spectral bands with variations in the NaAlO₂ content of these glasses. The antisymmetric stretching mode [ν_as (TOT), where T = Al, Ge] in the high-frequency region of the spectra (800–1000 cm^?1) appears as a doublet consisting of well-defined bands in the spectra of glasses along the entire join. Both components of the high-frequency doublet shift to a lower frequency with increasing NaAlO₂ content, indicating that the ν_as (GeO₄) and ν_as(AlO₄) stretching modes are coupled. The variations in the TO force constants and TOT bond angles with change in composition most likely cause the bands to shift. The frequencies of the Raman bands of sodium aluminogermanate glasses are compared with those of the corresponding bands in isostructural sodium gallogermanate glasses. On the basis of this comparison, the origin and delocalization of the vibrational modes producing characteristic Raman bands in the spectra of these glasses are discussed. The changes observed in the Raman spectra of aluminogermanate glasses with variation in NaAlO₂ content are analogous to those observed in the spectra of glasses along the NaAlO₂SiO₂ join.     

Optical spectra of borate glasses containing Ti and Co in relation to their structure

The optical absorption spectra of borate glasses of the base composition (mol%) 35 (Na₂O+BaO)·65 B₂O₃, to which Ti and Co ions were introduced, have been measured in the range 190–900 nm. The addition of increasing amounts of TiO₂ up to 16 g/100 g glass, introduces an intense charge transfer band in the UV region that shifts towards longer wavelengths without imparting any coloration to the glass samples.Titanium ions are present as the Ti⁴ state and its addition produces glass of more coherent structure. The cobalt ions are found to be present in two symmetries; as CoO₆ units with absorption bands (denoted here as bands a and b) around 525–530 nm and 580–595 nm (respectively), and as CoO₄ units with absorption bands (denoted here as band-c) around 620–635 nm. Replacing Na ions by Ba ions or increasing the TiO₂ content favours the conversion of CoO₄ units into the CoO₆ ones.     

Initiation and propagation of shear bands in Zr-based bulk metallic glass under quasi-static and dynamic shear loadings

The effect of strain rate on the initiation and propagation of shear bands in the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass under shear loading was investigated. The quasi-static (at a strain rate of 1.5 × 10⁻³ s⁻¹) and the dynamic shear tests (at a strain rate of 1.4 × 10³ s⁻¹) were conducted at room temperature using a GATAN Microtest-2000 instrument and a split Hopkinson pressure bar (SHPB) with a specially designed ‘Plate-shear’ specimen, respectively. The complete process of shear band initiation, propagation, and shear band unstable propagation-induced fracture was revealed. The experimental results demonstrated that the macroscopic shear strength is relatively insensitive to the strain rate, whereas shear band initiation and fracture are significantly dependent on strain rate. A dimensionless Deborah number was introduced to characterize the effect of the strain rate on the formation of shear bands. Additionally, the observed numerous liquid droplets and melted belts on the fracture surface at high-strain rates demonstrate that the adiabatic heating exerts a significant effect on fracture behavior of the material.     

Specific effects of nanometer scale size on magnetic ordering in La1−xCaxMnO3 (x = 0.1, 0.3 and 0.6) manganites

To check the impact of nano-size originated effects on the magnetic ordering in doped manganites, X-band electron magnetic resonance measurements were performed on nanometer sized and bulk samples of La_1?xCa_xMnO₃ (x = 0.1, 0.3 and 0.6). The model fittings of EPR signal parameters and complementary magnetic measurements indicate that bulk La_0.9Ca_0.1MnO₃ shows inhomogeneous confinement of charge carriers, leading to its mixed magnetic ordering. The carriers mobility within impurity-like band and ferromagnetic (FM)-like ground state are observed on nanocrystals of the same composition. The bulk La_0.7Ca_0.3MnO₃ demonstrates homogeneous FM order and band-like carrier mobility. The surface magnetic disorder in its nano-counterpart leads to appearance of two magnetic phases – a core phase (bulk-like in properties) and a surface phase with notably reduced temperature of magnetic ordering; neither double-exchange interaction nor carriers mobility exist between these phases. The size reduction induced effects in La_0.4Ca_0.6MnO₃ are: an increase of surface FM-like component and a decrease of the charge-ordering temperature. These findings allow us to conclude that the nano-scale effect on magnetic ordering in La_1?xCa_xMnO₃ system weakens progressively upon stabilization of the low temperature magnetic ground state with Ca-doping.     

A model for photostructural changes in the amorphous AsS system

The photodarkening effect was studied in thin film, bulk glasses and powdered glasses of composition As₂₉S₇₁, As₄₀S₆₀ and As_42.5S_57.5. The amount of photodarkening produced in a sample depended strongly on the samples state and composition as well as temperature, incident light intensity and wavelength. Using Raman spectroscopy, vibrational bands at 231 and 491 cm^?1 (assigned to AsAs and SS bond vibrations) were found to be enhanced in photodarkened samples. We present a model in which photodarkening is due to the formation of As_n clusters (n ? 2) which are loosely coupled to the amorphous network.     

Excitation pattern of the blue emission in Ge-doped silica

The luminescence of Ge-doped glassy SiO₂ has been investigated at room temperature using a deuterium lamp as the excitation source in the ultraviolet and vacuum ultraviolet energy range. Two emission bands are observed around 3 and 4 eV as the excitation increases from 4.5 to 8 eV. The peak shift and bandwidth broadening of the emission around 3 eV indicate the overlapping of several bands. We carried out a Gaussian best-fit approach of the detected spectra and extracted the excitation spectra of three distinct emission: a 2.9 eV band with two excitation channels at 4.7 and 5.3 eV and no contribution above 6 eV and the well-known composite β-band (3.1 eV). The latter is resolved into two Gaussian components whose excitation spectra present three main excitations around 5.1, 6.5 and 7.3 eV.     

Energy bands of the BCC metals rubidium and cesium

Energy bands of BCC rubidium and cesium are calculated for the first time using the full non-local but energy-independent Shaw's optimized model potential. The energy dependence of the potential is then included as first order perturbation. The effects of changing the model potential and of reversing the sign of the depletion charge d are reported. The Fermi energy E_F is calculated in all cases. Our results are compared with those obtained by other authors. The band gap at N is very small for both metals, except when all model potential parameters are increased considerably (50% to 100%).     

Thermally stimulated depolarization currents in OH−-doped NaCl and KCl crystals

The main features of the high-temperature ITC band are analyzed on the basis of experimental results obtained for NaCl and KCl crystals containing the hydroxyl ions. The unusual behaviour of this band has been compared with that described previously for alkali halides, AH, (LiF, NaCl, KCl) doped with some divalent cations, Me⁺⁺, (Be, Eu, Ni, Pb). It has been shown that — similar to the case of the Me⁺⁺-doping-most of the results obtained can be explained in frames of the Maxwell-Wagner-Sillars model of interfacial polarization induced by the impurities forming the charge clouds to the dislocation lines. An ITC band can be detected due to the fact that these regions exhibit electric conductivity and dielectric constants different from those characteristic of the bulk material. The role of the charge of dislocations in the induced polarization phenomenon is shortly discussed.