Dielectric/piezoelectric properties and temperature dependence of domain structure evolution in lead free single crystal

(K_0.5Na_0.5)NbO₃ (KNN) single crystals were grown using a high temperature flux method. The dielectric permittivity was measured as a function of temperature for [001]-oriented KNN single crystals. The ferroelectric phase transition temperatures, including the rhombohedral–orthorhombic T_R−O, orthorhombic–tetragonal T_O−T and tetragonal–cubic T_C were found to be located at −149  C, 205 C and 393 C, respectively. The domain structure evolution with an increasing temperature in [001]-oriented KNN single crystal was observed using polarized light microscopy (PLM), where three distinguished changes of the domain structures were found to occur at −150  C, 213 C and 400 C, corresponding to the three phase transition temperatures.     

A novel deposition method to grow ZnO nanorods: Spray pyrolysis

ZnO layers were deposited by chemical spray pyrolysis (CSP) using zinc chloride aqueous solutions onto indium tin oxide (ITO) glass substrates at growth temperatures in the region of 400–580 C. The layers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-temperature () photoluminescence (PL) measurements. The flat film of ZnO obtained at 400 C evolves to a structured layer by raising the temperature up to 500 C. Deposition around 550 C and above results in a layer comprising well-shaped hexagonal ZnO nanorods with diameter of 100–150 nm and length of up to 1 micron. XRD shows strong c-axis orientation of ZnO being in accordance with the SEM study. Deposition of nanorods was successful using ITO with grain size around 100 nm, whereas on fine-grained ITO (grain size < 50 nm) with smooth surface fat crystals with diameter up to 400 nm and length of about 300 nm were formed. Sharp near band edge (NBE) emission peaks centered at 3.360 and 3.356 eV dominated the PL spectra of ZnO at , originating from the exciton transition bound to neutral donors. PL and XRD results suggest that ZnO rods prepared by spray pyrolysis are of high optical and crystalline quality.     

Morphological control of ZnO nanostructures on silicon substrates

ZnO nanostructures are grown on Au-catalyzed Si substrates by vapour phase transport between 800 and 1150 C. Nanostructures grown at 800 C are mainly rod-like in structure with diameters of <200 nm. Increasing growth temperature yields combination growth modes with 2D structures (nanowalls/nanosheets) connecting 1D nanorods at intermediate temperatures and a 3D growth mode of foam-like appearance at the highest temperatures. The present work indicates that it may be possible to systematically control the morphology of ZnO nanostructures by varying the growth temperature.     

Enhancement of surface morphology and optical properties of nanocolumnar ZnO films

Nanocolumnar ZnO films were prepared by electrodeposition (ED) on a glass substrate covered with a conductive layer of thin oxide doped with fluorine (FTO). After deposition the samples were annealed in oxidizing or reducing atmosphere, at temperatures between 100 to 500 C, in order to follow the evolution of optical properties and morphology. The optical properties of these films were studied by means of photoluminescence spectroscopy (PL) and the morphology by scanning electron microscopy (SEM). Films annealed at 300 C exhibit a higher ultraviolet emission peak, originating from exciton transitions. A green band related to deep-level emission centered at 500 nm, shows a drastic increase at 500 C. These results are independent of the annealing atmosphere. An increase of coalescence is also observed after annealing at 500 C. These results are explained taking into account the contribution of different point defects.     

The effect on the annealing temperature of Li doped ZnO thin film for a film bulk acoustic resonator

We investigated the material and electrical properties of Li doped ZnO thin film (ZLO) with variation of the annealing temperature. In the 500 C sample, ZLO film showed well defined (002) c-axis orientation and a full width half-maximum property of 0.25. The electrical properties of ZLO thin films showed the excellent specific resistance of 1.5×10¹¹ Ω cm. Finally, the frequency characteristics of the ZLO thin film FBAR, according to the annealing temperature, showed improvement of the return loss from 24.48 to 30.02 dB at a resonant frequency of 1.17 GHz.     

Effects of annealing temperature on morphologies and optical properties of ZnO nanostructures

The effects of annealing temperature on the morphologies and optical properties of ZnO nanostructures synthesized by sol–gel method were investigated in detail. The SEM results showed that uniform ZnO nanorods formed at 900 C. The PL results showed an ultraviolet emission peak and a relatively broad visible light emission peak for all ZnO nanostructures sintered at different temperature. The increase of the crystal size and decrease of tensile stress resulted in the UV emission peak shifted from 386 to 389 nm when annealing temperature rose from 850 to 1000 C. The growth mechanism of the ZnO nanorods is discussed.     

Improvement of cubic silicon carbide crystals grown from solution

Cubic-silicon carbide crystals have been grown from carbon-rich silicon solutions using the travelling-zone method. To improve the growth process, we investigated the effect of controlling more tightly some of the growth parameters. Using such improved growth conditions, our best sample is a 12 mm diameter and 3 mm long 3C–SiC crystal. It is grown on a (0001) 2 off, 6H–SiC seed and has 111-orientation. The low amount of silicon inclusions results in a reduced internal stress, which is demonstrated by the consideration of μ-Raman spectra collected at room temperature on a large number of samples.     

Al Schottky contact on p-GaSe

A new Schottky diode, Al/p-GaSe, was presented in this study. It shows an effective barrier height of 0.96 eV with an ideality factor of 1.24 over five decades and a reverse leakage current density of 4.12×10⁻⁷ A/cm² at −2 V after rapid thermal annealing at 400 C for 30 s. The generation–recombination effect of the Schottky diode was decreased as the annealing temperature was increased. The formation of Al_1.33Se₂ was observed by X-ray diffraction analysis after the diode was annealed at 400 C for 30 s. Owing to the grains’ growth, the surface morphology of the 400 C-annealed diode was rougher than that of the unannealed diode, which was observed both by the AFM and the SEM analysis.     

The role of a ZnO buffer layer in the growth of ZnO thin film on Al2O3 substrate

A ZnO buffer layer and ZnO thin film have been deposited by the pulsed laser deposition technique at the temperatures of 200 C and 400 C, respectively. Structural, electrical and optical properties of ZnO thin films grown on sapphire (Al₂O₃) substrate with 1, 5, and 9 nm thick ZnO buffer layers were investigated. A minute shift of the (101) peak was observed which indicates that the lattice parameter was changed by varying the thickness of the buffer layer. High resolution transmission electron microscopy (TEM) was used to investigate the thickness of the ZnO buffer layer and the interface involving a thin ZnO buffer between the film and substrate. Selected area electron diffraction (SAED) patterns show high quality hexagonal ZnO thin film with 30 in-plane rotation with respect to the sapphire substrate. The use of the buffer can reduce the lattice mismatch between the ZnO thin film and sapphire substrate; therefore, the lattice constant of ZnO thin film grown on sapphire substrate became similar to that of bulk ZnO with increasing thickness of the buffer layer.     

Photoluminescence of ZnO nanostructures grown by the aqueous chemical growth technique

Zinc oxide nanostructured films were grown by the aqueous chemical growth technique using equimolar aqueous solutions of zinc nitrate and hexamethylenetetramine as precursors. Silicon(100) and glass substrates were placed in Pyrex glass bottles with polypropylene autoclavable screw caps containing the precursors described above, and heated at 95 C for several hours. X-ray diffraction 2θ/θ scans showed that the only crystallographic phase present was the hexagonal wurtzite structure. Scanning electron microscopy showed the formation of flowerlike ZnO nanostructures, consisting of hexagonal nanorods with a diameter of a few hundred nanometers. The photoluminescence spectra of the ZnO nanostructures were recorded at 18–295 K using a cw He–Cd laser (325 nm) and a pulsed laser (266 nm). The ZnO nanostructures exhibit an ultraviolet emission band centered at 3.192 eV in the vicinity of the band edge, which is attributed to the well-known excitonic transition in ZnO.     

UV-Raman scattering study of lattice recovery by thermal annealing of Eu -implanted GaN layers

Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2×10¹⁴ to 4×10¹⁵ cm⁻² and subsequently annealed at 1000 C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 C. The Raman scattering spectra show resonant A₁(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery.     

Epitaxial growth of 4H–SiC{0001} and reduction of deep levels

Homoepitaxial growth of 4H–SiC{0001} by hot-wall chemical vapor deposition (CVD) and characterization of deep levels in both n- and p-type epilayers have been investigated. On 4 off-axis 4H–SiC(0001), formation of macrosteps can be reduced by decreasing the C/Si ratio during CVD, though the growth condition leads to the increase in nitrogen incorporation. The 4H–SiC() face is promising, owing to its very smooth surface morphology even on 4 off-axis substrates and to its superior quality of the oxide/SiC interface. Deep level transient spectroscopy measurements in the wide temperature range from 100 K to 820 K on both n- and p-type 4H–SiC epilayers have revealed almost all the deep levels located in the whole energy range of the bandgap. Thermal annealing at 1350–1700 C of epilayers has resulted in reduction of deep level concentrations by one order of magnitude.     

Study of structural, optical and electrical properties of ZnO and SnO2 thin films

The investigation of structure, optical and electrical properties of tin and zinc oxide films on glass substrates by using magnetron sputtering are carried out. X-ray data show the formation of textured tin oxides film during deposition and its transformation to SnO₂ polycrystalline film at low temperature (200 C) if the concentration of oxygen in the chamber is high (O₂ — 100%, Ar — 0%). Optimal conditions of SnO₂ polycrystalline film deposition (pressure of Ar–O₂ mixture in chamber — 2.7 Pa, concentration of O₂ — 10%) are determined. Low resistivity of as-deposited ZnO film and increasing ZnO crystallite sizes and phase volume at temperatures higher than the melting point of Zn (419.5 C) are explained by formation of conductive Zn and ZnO particle chains and their destruction, respectively.     

Low temperature homoepitaxy of GaN by LP-MOVPE using Dimethylhydrazine and nitrogen

Thin films of GaN with the V/III≈10 ratio were grown by low-pressure metal organic vapour phase epitaxy (LP-MOVPE) using N₂ and Dimethylhydrazine (DMHy) as a carrier gas and nitrogen precursor, respectively. For the growth temperatures in the range from 550 to 690 C the GaN layers exhibited good surface morphology. In the temperature range from approximately 550 to 610 C, the growth rate increases with increasing temperature, characteristic of the process limited by surface kinetics with the activation energy of approximately 36 kcal/mol. For the temperatures between 620 and 690 C, the growth rate was nearly independent of temperature, which is indicative of a mass transport limited growth. The activation energy was about 4.6 kcal/mol. Micro Raman spectroscopy revealed a significant relaxation of the selection rules for the scattering by the optical phonons in the films grown at lower temperatures. Variation of the intensity ratio for and E₁ phonon modes has been attributed to the changes in the structural quality of the films grown at different temperatures.     

Modified Kolmogorov wave turbulence in QCD matched onto “Bottom-up” thermalization

We investigate modification of Kolmogorov wave turbulence in QCD calculating gluon spectra as functions of time in the presence of a low energy source which feeds in energy density in the infrared region at a time-dependent rate. Then considering the picture of saturation constraints as has been constructed in the “bottom-up” thermalization approach we revisit that picture for RHIC center-mass energy, W=130 GeV, and also extend it to LHC center-mass energy, W=5500 GeV, thus for two cases having an opportunity to calculate the equilibration time, τ_eq|therm, of the gluon system produced in a central heavy ion collision at mid-rapidity region. Thereby, at RHIC and LHC energies we can match the equilibration time, obtained from the late stage gluon spectrum of the modified Kolmogorov wave turbulence, onto that of the “bottom-up” thermalization and other evolutional approaches as well. In addition, from the revised “bottom-up” approach we find the gluon liberation coefficient to be on the average, ε0.81–1.06 at RHIC and ε0.50–0.56 at LHC. We also present other phenomenological estimates of τ_therm which, at QCD realistic couplings, yield 0.45–0.65 fmτ_therm0.97–2.72 fm at RHIC and 0.31–0.40 fmτ_therm0.86–2.04 fm at LHC. We show that the second upper-bounds of τ_therm in both cases are due to the late stage gluon spectrum of the original Kolmogorov wave turbulence in QCD, previously deduced with a low energy source which feeds in energy density at a constant rate. On the other hand, the lower-bounds and first upper-bounds of τ_therm are due to the late stage gluon spectrum of the modified QCD wave turbulence, deduced here at the specific time-dependent rate. In the latter case, at certain conditions, taking also into account both very small and realistic couplings we give estimates: 0.65 fmτ_therm1.29 fm at RHIC and 0.52 fmτ_therm1.16 fm at LHC, as well as at realistic couplings we find 0.53<τ_therm<0.7 fm at RHIC and 0.41<τ_therm<0.65 fm at LHC.     

Atomic layer deposition and characterization of Ga-doped ZnO thin films

Low-resistivity n-type ZnO thin films were grown by atomic layer deposition (ALD) using diethylzinc (DEZ) and H₂O as Zn and O precursors. ZnO thin films were grown on c-plane sapphire (c- Al₂O₃) substrates at 300 C. For undoped ZnO thin films, it was found that the intensity of ZnO () reflection peak increased and the electron concentration increased from 6.8×10¹⁸ to 1.1×10²⁰ cm⁻³ with the increase of DEZ flow rate, which indicates the increase of O vacancies () and/or Zn interstitials (Zn_i). Ga-doping was performed under Zn-rich growth conditions using triethylgallium (TEG) as Ga precursor. The resistivity of 8.0×10⁻⁴ Ω cm was achieved at the TEG flow rate of 0.24 μmol/min.     

Fabrication and properties of ZnO:Cu and ZnO:Ag thin films

Thin films of ZnS and ZnO:Cu were grown by an original metal–organic chemical vapour deposition (MOCVD) method under atmospheric pressure onto glass substrates. Pulse photo-assisted rapid thermal annealing of ZnO:Cu films in ambient air and at the temperature of 700–800 C was used instead of the common long-duration annealing in a vacuum furnace. ZnO:Ag thin films were prepared by oxidation and Ag doping of ZnS films. At first a closed space sublimation technique was used for Ag doping of ZnO films. The oxidation and Ag doping were carried out by a new non-vacuum method at a temperature >500 C. Crystal quality and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL). It was found that the doped films have a higher degree of crystallinity than undoped films. The spectra of as-deposited ZnO:Cu films contained the bands typical for copper, i.e. the green band and the yellow band. After pulse annealing at high temperature the 410 and 435 nm photoluminescent peaks were observed. This allows changing of the emission colour from blue to white. Flat-top ZnO:Ag films were obtained with the surface roughness of 7 nm. These samples show a strong ultraviolet (UV) emission at room temperature. The 385 nm photoluminescent peak obtained is assigned to the exciton–exciton emission.     

The extraction of total cross section from

We report on the first measurement of the differential cross section of -meson photoproduction for the d(γ,pK⁺K⁻)n exclusive reaction channel. The experiment was performed using a tagged-photon beam and the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. A combined analysis using data from the d(γ,pK⁺K⁻)n channel and those from a previous publication on coherent production on the deuteron has been carried out to extract the −N total cross section, σ_N. The extracted −N total cross section favors a value above 20 mb. This value is larger than the value extracted using vector-meson dominance models for photoproduction on the proton.     

Asymmetric di-jet production in polarized hadronic collisions

Using the collinear QCD factorization approach, we study the single-transverse-spin dependent cross section Δσ(S) for the hadronic production of two jets of momenta P₁=P+q/2 and P₂=−P+q/2. We consider the kinematic region where the transverse components of the momentum vectors satisfy PqΛ_QCD. For the case of initial-state gluon radiation, we show that at the leading power in q/P and at the lowest non-trivial perturbative order, the dependence of Δσ(S) on q decouples from that on P, so that the cross section can be factorized into a hard part that is a function only of the single scale P, and into perturbatively generated transverse-momentum dependent (TMD) parton distributions with transverse momenta .     

Enhanced sonochemical degradation of bisphenol-A by bicarbonate ions

Sonochemical elimination of organic pollutants can take place through two degradation pathways. Molecules with relatively large Henry’s law constants will be incinerated inside the cavitation bubble, while nonvolatile molecules with low Henry’s law constants will be oxidised by the OH ejected from the bubble of cavitation. Taking bisphenol-A as a model pollutant, this study points out an alternate degradation route, mediated by bicarbonate ions, which is significant for the elimination of micro-pollutants at concentrations present in natural waters. In this process, OH radicals react with bicarbonate ions to produce the carbonate radical, which, unlike the OH radical, can migrate towards the bulk of the solution and therefore induce the degradation of the micro-pollutants present in the bulk solution. As a consequence, initial degradation rate is increased by a factor 3.2 at low concentration of bisphenol-A (0.022 μmol l⁻¹) in presence of bicarbonate in water.