Plasma-assisted hot filament chemical vapor deposition of AlN thin films on ZnO buffer layer: toward highly c-axis-oriented,uniform, insulative films

c-Axis-oriented aluminum nitride (AlN) thin film with improved quality was deposited on Si(111) substrate using ZnO buffer layer by plasma-assisted hot filament chemical vapor deposition. The optical and electrical properties and surface morphology as well as elemental composition of the AlN films deposited with and without ZnO buffer layer were investigated using a host of measurement techniques: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and current–voltage (I–V) characteristic measurement. The XRD and XPS results reveal that the AlN/ZnO/Si films are free of metallic Al particles. Also, cross-sectional FESEM observations suggest formation of a well-aligned, uniform, continuous, and highly (002) oriented structure for a bi-layered AlN film when Si(111) is covered with ZnO buffer. Moreover, a decrease in full width at half maximum of the E₂ (high)-mode peak in Raman spectrum indicates a better crystallinity for the AlN films formed on ZnO/Si substrate. Finally, I–V curves obtained indicate that the electrical behavior of the AlN thin films switches from conductive to insulative when film is grown on a ZnO-buffered Si substrate.     

Raman scattering in sodium nitrite crystals near the phase transition

Optical Raman spectra of a ferroelectric sodium nitrite crystal have been detected in a wide spectrum range at various temperatures, including the region of the ferroelectric phase transition. A manifestation of a transverse soft polar mode of the A ₁(z) type responsible for the ferroelectric phase transition has been discovered in the spectrum at room temperature. This mode has been found to become overdamped even far from the ferroelectric phase transition temperature. This mode also appears as a central peak under heating. It has been found that the pseudoscalar mode of the A ₂ type has the highest intensity in the Raman spectrum of sodium nitrite. The frequency corresponding to the maximum intensity of this mode in the Raman spectrum varies from 130 cm^–1 at 123 K to 106 cm^–1 at T = 513 K. A fair agreement of the experimental data for the A ₁(z) mode with the Lyddane–Sachs–Teller relation has been established. The polariton curves for the A ₁(z) polar mode and the dispersion curves for axinons has been plotted.     

Post-collision interaction in inner-shell ionisation of ArL 3 by electron impact

The shifts and shapes of ArL ₃ Auger lines due to the post-collision interaction (PCI) in the inner-shell ionisation of theL ₃-shell of argon by electron impact have been measured for the range of excess energiesE ₁=10 to 1,500 eV. The experimental shifts {ie67-01} could be fitted by a relation {ie67-02} withc=(5.3±0.4) andn= (?0.45±0.04) whereГ(ArL ₃)=130 meV is the ArL ₃-level width. In a classical model the PCI shifts have been calculated for two limiting cases:a) whereE ₁ is large enough (E ₁>50 eV) for the motion of the two slow electrons to be treated as uncorrelated, andb) forE ₁→0. The calculated shifts forE ₁=250, 750 and 1,750 eV are in good agreement with the experimental results. Also the calculated PCI distorted shapes of Auger lines for differentE ₁ agree reasonably with the experimental Auger line shapes.     

Resonant Raman Scattering of synchrotron X-rays by xenon: Test of angular and polarization dependence of RRS

Using monochromatized synchrotron radiation with variable photon energyE _i=33.94keV...34.54keV, Resonant Raman Scattering (RRS) by free Xenon atoms (B _1s=34.566keV) was investigated. The measured double-differential RRS cross sections are in excellent agreement with those calculated in the non-relativistic dipole approximation, modified by some relativistic corrections, and including the interference corrections predicted by Tulkki and Åberg. Within the experimental error of 5 % the RRS cross section is found to be independent of the scattering angle and of the polarization of the incident photons.     

Resonant Raman Scattering of synchrotron X-rays by neodymium: Observation of fine structure in K-L-RRS and of K-N-RRS

Using monochromatized synchrotron radiation with variable photon energyE _i =43.01...43.35 keV, Resonant Raman Scattering (RRS) by Neodymium atoms (K-shell binding energyB _1s=43.57 keV) was investigated. For the first time the fine structure splitting in a K-L-RRS spectrum due to the energy difference of the L₂ and L₃ subshells was observed. In addition, the first observation of K-N-RRS in the spectrum of scattered photons is reported. The measured double-differential RRS cross sections are, on an absolute scale, in very good agreement with those calculated in the non-relativistic dipole approximation, modified by relativistic corrections.     

Analysis of the structure and magnetic properties of an interface in multilayered (Fe/Si) N nanostructures with the surface-sensitive XMCD method

The structural and magnetic properties of (Fe/Si) _N nanostructures obtained by successive deposition on the SiO₂/Si(100) surface at a temperature of the substrate of 300 K have been studied. The thicknesses of all Fe and Si layers have been determined by transmission electron microscopy measurements. The magnetic properties have been studied by the X-ray magnetic circular dichroism (XMCD) method near the Fe L _{3, 2} absorption edges. The orbital (m _l ) and spin (m _S ) contributions to the total magnetic moment of iron have been separated. The thicknesses of magnetic and nonmagnetic iron silicide on the Si/Fe and Fe/Si interfaces have been determined with the surface sensitivity of the XMCD method and the model of the interface between the nonmagnetic and weakened magnetic phases.     

Specific features of optical phonons in raman spectra of an array of vertical ZnO microrods on silicon

The polarized Raman spectra of an array of vertical zinc oxide microrods prepared by the carbothermal synthesis on a Si(001) substrate have been investigated. The length and diameter of the rods are 9 ± 0.5 μm and 210 ± 50 nm, respectively. X-ray diffraction studies have confirmed the mutual crystallographic orientation [001]_ZnO ‖ [001]_Si in the direction normal to the substrate plane. In the Raman spectra of the rods, there are modes forbidden by selection rules for the single crystal: the A ₁(TO) E ₂ ^high and modes in the scattering geometry $x\left( {yz} \right)\bar x$ and the E ₁(LO) mode in the geometry $x\left( {zz} \right)\bar x$ . It has been found that the spectra contain quasi-modes Q(TO) and Q(LO), the spectral position of which shifts with a variation in the angle of incidence of the exciting radiation. The angle range of propagation of the exciting radiation in the array of rods is estimated as ～25° for a fixed angle of incidence by comparing the spectral shift of the quasi-modes with experimental data for the single crystal.     

Dielectric Spectroscopy of Strongly Correlated Electronic States of Vanadium Dioxide

The thermal evolution of the conductivity of a VO₂ film and database-obtained band gap E_g of film nanocrystallites is traced in the temperature range of –196°C < T < 100°C (77 K < T < 273 K); the level position of donor impurity centers is determined to be E_d = 0.04 eV. It is shown that energy E_g decreases from 0.8 to ～0 eV with an increase in temperature in the range of 273 K < T < 300 K, which is caused by the narrowing of the energy gap due to correlation effects and considered as the temperature-extended Mott “insulator–metal” electron phase transition with the monoclinic lattice symmetry retained. The subsequent jump in the symmetry from monoclinic to tetragonal with a further increase in temperature is considered as the Peierls structural phase transition, the temperature of which is in the vicinity of 340 K and determined by the size effects, nonstoichiometry of VO₂ film nanocrystallites, and degree of their adhesion to the substrate.     

Surface-enhanced Raman scattering (SERS)-based volatile organic compounds (VOCs) detection using plasmonic bimetallic nanogap substrate

In this paper, we present surface-enhanced Raman scattering (SERS)-based volatile organic compounds (VOCs) detection with bimetallic nanogap structure substrate. Deep UV photolithography at the wavelength of 250 nm is used to pattern circular shape nanostructures. The nanogap between adjacent circular patterns is 30 ± 5 nm. Silver (30 nm) and gold (15 nm) plasmonic active layers are deposited on the nanostructures subsequently. SERS measurements on different concentrations of acetone vapor ranged from 0.7, 1.5, 3.5, 10.3, 24.5 % and control have been performed with the substrate. The measurement results are found reproducible, and the detection limit is found to be 9.5 pg (acetone molecule). The detection sensitivity is 28.7 % higher than that of the recent reported leaning silicon nanopillar substrate. With further system miniaturization, the sensing technique can work as a portable SERS-based VOCs detection platform for point-of-care breath analysis, homeland security, chemical sensing and environmental monitoring.     

Electrical properties and conduction mechanisms in La2/3Ca1/3MnO3 thin films prepared by pulsed laser deposition on different substrates

Perovskite manganite La_2/3Ca_1/3MnO₃ thin films were directly grown on MgO(100), Si(100) and glass substrates by pulsed laser deposition. From the XRD patterns, the films are found to be polycrystalline, single-phase orthorhombic. The metal–insulator transition temperature is 209 K for LCMO/MgO, 266 K for LCMO/Si and 231 K for film deposited on the glass substrate. The conduction mechanism in these films is investigated in different temperature regimes. Low-temperature resistivity data below the phase transition temperature (T _P) have been fitted with the relation \( \rho = \rho_{0} + \rho_{2} T^{2} + \rho_{4.5} T^{4.5} \) , indicating that the electron–electron scattering affects the conduction of these materials. The high-temperature resistivity data (T > T _P) were explained using variable-range hopping (VRH) and small-polaron hopping (SPH) models. Debye temperature values are 548 K for LCMO/Cg, 568 K for LCMO/Si and 508 K for LCMO/MgO thin films. In all thin films, the best fitting in the range of VRH is found for 3D dimension. The density of states near the Fermi level N (E _F) for LCMO/MgO is lower due to the prominent role of the grain boundary in LCMO/MgO and increase in bending of Mn–O–Mn bond angle, which decreases the double exchange coupling of Mn³⁺–O²–Mn⁴⁺ and in turn makes the LCMO/MgO sample less conducting as compared to the other films.     

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E ₂(high) and A ₁(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 10³) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.     

High-pressure behavior of the bond-bending mode of AIN

Lattice vibrations of the wurtzite-type AIN have been studied by Raman spectroscopy under high-pressure up to the phase transition to the rock salt structure at 20 GPa. Five fundamental bands E ₂ ² , A₁(TO), E₁(TO), A₁(LO), and E₁(LO) have a strong, positive pressure shift, whereas the shift of the low-frequency E ₂ ¹ band is weakly positive. We have found that the bond-bending mode has a positive mode Grüneisen parameter γ_i = 0.04, which is qualitatively consistent with the recently reported value γ_i = 0.12 [21]. Thus, we confirm that AIN remains stable with respect to the bond-bending mode, while in most tetrahedral semiconductors, bond-bending modes soften on compression. Experimental results are compared with the first-principle calculations.     

Conductivities and ion association constants of ferric chloride and chromic chloride in DMF and DMSO as a function of temperature

The equivalent conductivities of anhydrous ferric chloride (FeCl₃) and anhydrous chromic chloride (CrCl₃) were measured in nonaqueous aprotic solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) at temperatures between 278.15 and 318.15 K. In both DMF and DMSO, conductivity values for FeCl₃ were found to be higher than those for CrCl₃. In addition, the conductivity values for both FeCl₃ and CrCl₃ in DMF were higher than those in DMSO at all temperatures. The conductivity data were analyzed by the Robinson–Stokes equations. The limiting equivalent ionic conductivities for ferric ion (Fe³⁺) and chromic ion (Cr³⁺) and the ion association constants (K _A ) for FeCl₃ and CrCl₃ were determined in DMF and DMSO. The K _A values calculated for both FeCl₃ and CrCl₃ in DMF were higher than those in DMSO. This can be ascribed to an increase of the ion association constants with a decrease of the relative permittivity of solvents used in this study. The K _A values increased with the increase in temperature in the studied solvents. Thermodynamic functions (Gibbs’ free energy, entropy, and enthalpy of ion association) were estimated from the temperature dependence of the ion association constant. The positive values of entropy and enthalpy found for FeCl₃ and CrCl₃ at all temperatures indicate that the association process in DMF and DMSO is endothermic.     

Electronic structure of low-pressure and high-pressure phases of silicon disulfide

Self-consistent density functional theory calculations of band spectra, densities of states as well as the spatial distribution of valence electron charge density were carried out for the low-pressure α-phase and the high-pressure β-phase of SiS₂. Group-theoretical analysis performed for both phases enabled the symmetry of wave functions in a number of high-symmetry points of the Brillouin zone as well as the structure of valence band representations to be found. Based on the calculations of the band structure, the orthorhombic α-phase of SiS₂ was determined to be an indirect-gap semiconductor with the band gap E _gi = 2.44 eV (T ₁ → X ₈ transition), while the β-phase was shown to be direct gap with E _gd = 2.95 eV (Г ₃ → Г ₂ transition). The calculated energy distribution of the total density of states in the valence band of α-SiS₂ qualitatively and quantitatively correlates with the main experimental features of the X-ray photoelectron spectrum.     

SERS scaling rules

We demonstrate the qualitative analysis of surface-enhanced Raman scattering (SERS) intensity and optical extinction by experimentally and numerically. This analytical methods are well matched not only the simple square lattice array of nanostructures, but also the rectangular lattices. We also demonstrate SERS selectivity of modes controlling the optical extinction of excitation and scattering wavelength. Both square lattice and rectangular lattice have similar tendency, but the rectangular lattice structures have much higher selectivity of SERS modes.     

Characterization of Co-evaporated Cu-Ag-In-Se Thin Films

In this study, annealing effect on the structural, electrical, and optical characteristics of the quaternary Cu-Ag-In-Se (CAIS) thin films was investigated. These samples were deposited by co-evaporation of the Cu, Ag, In₂Se₃, and Se sources at the substrate temperature of 300 °C. The structural properties of the thin films were analyzed by means of X-ray diffraction, and the results indicated that all of the films were in the polycrystalline structure with the preferred orientation along (112) direction. From the optical measurements, the band gap values were found to vary between 1.38 and 1.45 eV with annealing processes. The temperature-dependent electrical conductivity of the samples was measured in the temperature range of 90–400 K. The films gained degenerate behavior with increasing annealing temperature. The carrier conduction mechanism was determined at high- and low-temperature regions by comparing thermionic emission and hopping parameters. Photoconductivity of the as-grown film showed that there was an increase in conductivity with increasing illumination intensity. From this measurement, the variation of photocurrent as a function of illumination intensity was determined.     

Molecular mobility of nanocellulose hydrogels

The molecular mobility of nanocellulose hydrogels isolated from microcrystalline cellulose is evaluated using the spin probe method, from the correlation time τ (s) and rotational frequency ν = 1/τ(s^–1) of stable nitroxyl radicals introduced into the medium under study. In an aqueous gel medium, the EPR spectrum of the probe features an anisotropic triplet of frozen particles over a temperature range of 77 to 265 K. In an aqueous–ethanolic gel solution, the temperature of onset of rotation of the radical is 85 K lower. The rotational correlation time is determined from the parameters of the EPR spectrum recorded in the temperature range of 180–290 K. The resulting Arrhenius temperature dependence logν = f(1/T) is used to evaluate the activation energy of rotation E of the radical and the preexponential factor ν₀(s^–1), the frequency of rotational vibrations of the particle around the equilibrium position. For the aqueous medium, E = 11.2 kcal/mol; in the presence of ethanol, E = 5.2 kcal/mol; the preexponential factors for the aqueous and aqueous–ethanolic media are ν₀ = 7 × 10¹⁸ and 6 × 10¹⁴ s^–1, respectively. The parameters E and ν₀ measured in the pure solvents and in the samples containing nanocellulose differ little, which is indicative of a high hydrophobicity of the probe molecule (and hydrogel particles) and of their weak interaction with the environment. The high value (~10¹⁸ s^–1) of the preexponential factor is explained in terms of the compensation effect of water.     

Polarisation studies on the Raman spectra of cubic crystals

Polarisation characters of the Raman lines of calcium fluoride (fluorspar) and potassium aluminium sulphate (alum) were investigated under the following conditions. Unpolarised light was incident normally on a face of the crystal making an angle 22.5° with a cubic face and the light scattered transversely along a cubic axis was analysed by a double image prism kept with its principal axes inclined at 45° to the vertical. Under these conditions the depolarisation factors of the Raman lines belonging to the totally symmetric (A), the doubly degenerate (E) and the triply degenerate (F) modes should be respectively =1, >1 and <1. The characteristic Raman line of CaF₂ at 322 cm^?1 exhibited a depolarisation value less than 1, showing thereby that the corresponding mode is a triply degenerate one (F). The Raman lines observed in the spectrum of K-alum were also classified and the results were compared with those given by previous investigators using standard crystal orientations.     

Elastic constants and Debye temperature of wz-AlN and wz-GaN semiconductors under high pressure from first-principles

First-principles calculations were performed to study the elastic stiffness constants (C _ij ) and Debye temperature (?? _D) of wurzite (wz) AlN and GaN binary semiconductors at high pressure. The lattice constants were calculated from the optimized structure of these materials. The band gaps were calculated at Γ point using local density approximation (LDA) approach. The unit cell volume, lattice parameters, c/a, internal parameter (u), elastic constant (C _ij ), Debye temperature (?? _D), Hubbard parameter (U) and band gap (E _g) were studied under different pressures. The bulk modulus (B ₀), reduced bulk modulus ( \(B_{0}^{\prime })\) and Poisson ratio (υ) were also calculated. The calculated values of these parameters are in fair agreement with the available experimental and reported values.