Buffer layer ZnO-assistant fabrication of c-axis GaN films by using pulsed laser deposition on Si(111) substrate: annealing effects in ammonia ambience

ZnO buffer layers have been used to fabricate GaN thin films by using pulsed laser deposition on Si (111) substrates. c-axis GaN thin films were obtained by annealing in NH₃ atmosphere at 950°C for 15 min. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM) have been used for the characterizations of the crystalline quality, composition, and surface morphology of the films. The annealing in ammonia (NH₃) atmosphere markedly affects the preparation of GaN films and the least annealing time is 15 min under our experimental conditions. The mechanism of the effects of the ZnO buffer layers was studied. In the beginning, Zn–O bonds are destroyed in the interface of the films; a few O and Zn atoms depart from their positions, while N and Ga atoms fill in the empty positions and form a hexagonal structure of a special component. Many bonds (such as Ga–O bonds, Zn–N bonds) existed then. The number of Zn–O bonds decreases and the number of Ga–N bonds increases in the films with increasing of the annealing time. Many other bonds (such as Ga–O bonds, Zn–N bonds) also decreased and more Ga–N bonds formed with annealing time increasing. After having been annealed for 15 min under our experimental conditions, the quality of the hexagonal structure GaN films was markedly improved by the destroying of the Zn–O bonds during high-temperature annealing.     

Density–temperature properties of Ga–Sb alloy melt

In an experiment, the improved Archimedes method was used to test and analyze density–temperature properties of Ga, Sb and Ga–Sb melts. The experimental results indicated that the density of Ga₇Sb₉₃ melt shows a linear trend of decline when its temperature changes within the experimental range, whereas the densities of Ga_36.5Sb_63.5 and Ga₃₀Sb₇₀ melts show a non-continuous change near the melting point, which rises first and then drops abnormally. These results were analyzed from the perspective of a liquid–liquid structure transition according to the property changes of Ga–Sb melt.     

Modifications in magnetic anisotropy of M—type strontium hexaferrite crystals by swift heavy ion irradiation.

Using vibrating sample magnetometery (VSM) 50 MeV Li³⁺ ion irradiation effects on magnetic properties of single crystals of SrGa_xIn_yFe_12−(x+y)O₁₉ (where x=0, 5, 7, 9; y=0, 0.8, 1.3, 1.0), are reported. The substitution of Ga and In in strontium hexaferrite crystals decreases the value of magnetization sharply, which is attributed to shifting of collinear magnetic order to a non-collinear one. Reduction of magnetization is also explained to be as a result of the occupation of the crystallographic sites of Fe³⁺ by Ga³⁺ and In³⁺. The Li³⁺ ion irradiation decreases the value of magnetization, irrespective of whether the crystals are Ga–In substituted or unsubstituted crystals of SrFe₁₂O₁₉. The result is interpreted in terms of the occurrence of a paramagnetic doublet in crystals replacing magnetic sextuplet as a result of irradiation. Substitution of Ga–In in Strontium hexaferrite decreases the value of anisotropy constant. Irradiation with Li³⁺ ions increases the values of anisotropy field for both substituted as well as unsubstituted crystals. Substitution with Ga–In also decreases the Curie temperature (T_c) but the irradiation with Li³⁺ ions does not affect the curie temperature of either Ga–In substituted or pure SrFe₁₂O₁₉ crystals.     

Alkali displacements in intercalated 1T-TaSe2

Single crystals of 1T-TaSe₂ have been intercalated with different alkali metals by deposition in ultra-high vacuum onto in situ-cleaved (0001) surfaces. In a second step a different alkali metal, or Cl₂, has been deposited on top. The interactions among the deposited species and the substrate have been investigated using soft x-ray photoelectron spectroscopy (SXPS). Li and Na appear to compete in the intercalation process. Li replaces Na, by pushing it deeper into the crystal. Cs on Li:TaSe₂ does not intercalate, but stays on top, repelling intercalated Li⁺ deeper inside. For Li on Cs:TaSe₂ an exchange reaction takes place, and Cs is deintercalated. The same effect is induced by Na deposited on intercalated Cs. 1T→2H phase transition for TaSe₂ has been observed only for Li deposition. Cl₂ deposition on Na intercalated substrate induced deintercalation of Na. The experimental results are discussed in relation to thermodynamic, electronic and electrostatic effects. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Status report on theGALLEX experiment

Summary The GALLEX collaboration is performing an experiment for the detection of neutrinos coming principally from the p-p fusion reaction in the Sun, via the reaction ν_e+⁷¹Ga→⁷¹Ge+e⁻. The experiment is running in the Gran Sasso Underground Laboratory of INFN, using as a target 30.3 tons of gallium in the form of 8.13 molar aqueous GaCl₃ solution. A report is given of the status of GALLEX after the end of the operations devoted to the removal from the solution of the cosmogenically formed Ge isotopes, completed in the middle of 1991. The experiment is now collecting data on solar neutrinos and the data analysis is in progress. Preliminary results concerning the first year of measurement are presented. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

PEO-based composite lithium polymer electrolyte, PEO-BaTiO3-Li(C2F5SO2)2N

Polyethylene oxide (PEO) based polymer electrolytes with BaTiO₃ as filler and Li(C₂F₅SO₂)₂N as salt have been examined in lithium polymer batteries. The aluminum disolution potential in PEO-Li(C₂F₅SO₂)₂N was estimated to be 4.1 V vs. Li/Li⁺ at 80 °C, which was compared to that of 3.8 V vs. Li/Li⁺ in PEO-Li(CF₃SO₂)₂N. The electrical conductivity of the system was measured as a function of O/Li ratio. The highest conductivity was observed in O/Li=8. The conductivity was 1.65×10⁻³ S/cm at 80 °C and 1.5×10⁻⁵ S/cm at 25 °C. The interfacial resistance of Li/polymer electrolyte/Li annealed at 80 °C for 15 days was lower than 100 Ωcm². Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

Covalency Measurements via NMR in Lithium Metal Phosphates

³¹P nuclear magnetic resonance (NMR) shifts on the order of thousands of parts per million are observed for olivine LiMPO₄ (M = Mn, Fe, Co, Ni) samples, a promising class of Li ion rechargeable battery electrode materials. Variable-temperature ³¹P NMR measurements of shift are used to determine that the supertransferred hyperfine interaction is the dominant mechanism giving rise to these unusually large observed ³¹P shifts. Various models for predicting ³¹P and ⁷Li shifts in LiMPO₄ (M = Mn, Fe, Co, Ni) were investigated. Alloys of LiFe_1−x Mn _x PO₄, where x varies from 0 to 1, were also investigated by ⁷Li NMR. Covalency constants, calculated from variable-temperature NMR shifts and magnetic susceptibility data, are determined for the P–O–M bonds in LiMPO₄ (M = Mn, Fe, Co, Ni) and compared to the covalency constants of the Li–O–M bond. The sign and relative magnitude of the covalency constants are discussed in terms of positive and negative spin densities at the nuclei of interest. The covalency constants for the Li–O–M and P–O–M bonds were measured for Li_1.8Na_0.2FeMn₂(PO₄)₃ and compared to the covalency constants measured in the olivine LiMPO₄ (M = Mn, Fe, Co, Ni) samples. The Li_1.8Na_0.2FeMn₂(PO₄)₃ structure has a volume per transition metal atom and Li–O–M bond distances that are similar to those of the olivine LiMPO₄ (M = Mn, Fe, Co, Ni) samples. Authors' address: Jeffrey A. Reimer, Department of Chemical Engineering, University of California Berkeley, Berkeley, CA 94720, USA     

The effect of Ti on the wetting of CaF<Subscript>2</Subscript> substrate by In–Ti and Ga–Ti alloys. Ab-initio consideration

CaF₂ is a thermodynamically stable, non-reactive compound, displaying a relatively high contact angle with pure liquid metal melts. A remarkable decrease of this contact angle takes place when small amounts of Ti are added to liquid In (a decrease from 125 to 20°) or to liquid Ga (from 118 to 60°). Thermodynamic analysis indicates that this feature cannot be attributed to chemical reactions between the substrate and the melt. It was suggested that the reason for this behavior may be a preferential titanium adsorption from the liquid In–Ti or Ga–Ti solution at the substrate surface. In order to understand the nature of the In–Ti or Ga–Ti bonding in the vicinity of the CaF₂ surface, the adsorption energy of 0.5 monolayer of In and Ga was computed for three different surface conditions: (i) clean CaF₂(111), (ii) CaF₂(111) with In or Ga adatoms, and (iii) CaF₂(111) with Ti adatoms. The differences in adsorption energies for these configurations are related to the electron charge redistribution in the vicinity of the interface, and to the density of states of the electronic subsystems. It was found that the adsorption energy of In or Ga increases due to the lateral interactions with the adatoms. According to the analysis, a strong lateral interaction exists between Ti adatoms and Me, while relatively weak interaction exists between Me and Me adatoms. The difference of the lateral interactions was considered in order to explain the improvement of the wetting of CaF₂ substrate by Ti alloying of In and Ga.     

Composition dependence of the lattice constants in LiTaO3

Abstract

The composition range of lithium tantalate was found between 46.5 and 50.0 mole% Li. The congruent melt composition, from which homogeneous single crystals can be grown, was determined at about 48.5 mole% Li₂O. Due to Li₂O evaporation during the reaction of Li₂CO₃ and Ta₂O₅ the starting composition depends on the reaction conditions. The lattice constants change with the composition comparable to LiNbO₃.     

Bulk GaN single crystals: growth mechanism by using Li<Subscript>3</Subscript>N and Ga

Colorless transparent hexagonal GaN platelet crystals with a size of 1–4 mm have been grown under more moderate conditions (800 °C, 12 atm) using Li₃N and Ga. Growth mechanism of this method was revealed by a series of experiment results. The morphologies of GaN single crystals were observed with an optical microscope and a scanning electron microscope. The crystal was characterized by X-ray powder diffraction and Raman scattering spectrum. PACS 81.05.Ea; 81.10.Dn     

The influence of lithium excess in the target on the properties and compositions of Li<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>2</Subscript>O<Subscript>4−<Emphasis Type="Italic">δ</Emphasis></Subscript> thin films prepared by PLD

Li–Mn–O thin films were deposited by pulsed laser deposition (PLD) onto stainless steel substrates using targets containing different concentrations of added Li₂O. The influence of the target composition on the stoichiometry of the resulting thin films, the surface morphology and the electrochemical properties was studied. The application of the target with added 7.5 mol% Li₂O results in an almost ideal lithium content, while all films were still oxygen deficient. The thin films were applied as electrodes in Li//Li_1+x Mn₂O_4−δ cells (i.e. model cells for a rechargeable Li-ion battery) and characterized by cyclic voltammetry and galvanostatic charge/discharge experiments. The electrochemical measurements of the thin films confirmed that the thin films can serve as good model systems and that they show a sufficient cyclability.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

Dynamical properties and electronic structure of (LaLi)TiO3 conductors

A computer simulation by a molecular dynamics method is performed to study the properties of structure and Li ion diffusion in La_4/3???x Li (LaLi)TiO₃ ;LLTO;Li ion conductors;Superionic conductors;Perovskite;Off-site;Electronic structureA computer simulation by a molecular dynamics method is performed to study the properties of structure and Li ion diffusion in La_{4/3 − x} Li Ti₂O₆ =(LaLi)TiO₃ =LLTO, which is the perovskite-type Li ion conductor. In the low Li concentration, Li ions conduct a two-dimensional motion, while Li ions diffuse a three-dimensional motion in the high Li ion concentration. The partial distribution function for Li–Ti and the diffusion paths of Li ions suggest that Li ions stay for a long time at off-site positions, which are 2.7? away from a body-centered Ti ion. The Li ion concentration dependence to the conductivity σ is in approximate agreement with experiments. The energy band dispersion and the density of states are calculated using the linear-muffin-tin-orbital method. The energy contour map shows the stable position of Li ions is off center of the vacant La sites. Both calculations suggest that the stable position of Li ions is off center of the vacant La sites. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

A study on the influence of ytterbium and impurities on lattice parameters and the phase transition temperature of Czochralski-grown LiNbO3

LiNbO₃ crystals were grown from a congruent melt by the Czochralski method. Special care was taken with respect to purity and an analysis of impurity levels. Present crystals are as pure as 3N-4N. In particular, doping by Yb³⁺ was investigated. An upper limit for the inclusion of Yb³⁺ in LiNbO₃ may be set at 2.5 mol% for a congruent melt. Present results demonstrate the complex interaction between intrinsic and extrinsic defects in LiNbO₃. The variation of lattice parameters supports evidence for the Li⁺ vacancy model and an occupation of Li sites for Yb³⁺. Data on purities, laser properties and analytical methods provide a short review on a field, where analytical results gained by different techniques often may disagree.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Electrochemical studies on polymer electrolytes based on poly(methyl methacrylate)-grafted natural rubber for lithium polymer battery

MG30 is natural rubber grafted with 30% poly(methyl methacrylate). Gel polymer electrolytes containing MG30–LiCF₃SO₃–X (X = propylene carbonate, ethylene carbonate) are prepared by solution casting technique. The polymer–salt complexes were investigated using Fourier-transformed infrared. The ionic conductivity of the electrolytes are determined by the ac impedance studies over the temperature range of 303–383 K and is observed to obey the Vogel–Tamman–Fulcher (VTF) rule. The Li⁺ transference number obtained using the Bruce and Vincent method is <0.3. The Li/Li⁺ interface stability is established and the electrolytes were found to be able to withstand a voltage of more than 4.2 V.     

Structural assessment of the electrochemical performance of LixCoO2 membrane electrodes by X-ray diffraction and absorption refinements

Li_xCoO₂ samples with different nominal load x were prepared submitting HT-LiCoO₂ membrane electrodes to electrochemical polarisation, followed by aging in open circuit conditions. Care was taken to obtain samples for both positive and negative overcharge beyond the canonical regime (0.6≤x≤1) of the Li_xCoO₂ electrode. For 11 samples in the range 0.37≤x≤1.43, Rietveld refinements of X-ray diffraction data were performed, with Li at 3a, Co at 3b and O at 6c in the space group R-3m as starting model. In the range 0.37≤x≤1, results show a continuous variation of theac unit cell and structural parameters such as the lithium site occupancy and the Li-O and Co-O interatomic distances. For samples in the range 1≤x≤1.43, Li₂CO₃ was detected as an impurity which might be due to decomposition of the electrolyte, and, if the negative overcharge was high enough, an abnormally high electron density at the 3a site indicates some Li→Co substitution. Results from Co-K edge X-ray absorption spectroscopy for the samples with x=0.37, 0.89, 1 and 1.43 show a clear shift of the absorption maximum with the oxidation state of Co and support the conclusions from Rietveld refinement. GNXAS refinement of EXAFS spectra for the samples with x=0.37 and 1 suggest that cation disordering may occur also for deep lithium deintercalation (positive overcharge), a fact which was not detectable from diffraction data.     

Anisotropy of atomic bonds formed by p-type dopants in bulk GaN crystals

The anisotropy of atomic bonds formed by acceptor dopants with nitrogen in bulk wurtzite GaN crystals was studied by means of linearly polarized synchrotron radiation used in measurements of X-ray-absorption spectra for the K-edgeof Mg and Zn dopants. These spectra correspond to i) a single acceptor N bond along the c-axis and ii) three bonds realized with N atoms occupying the ab-plane perpendicular to the c-axis. The Zn dopant formed resonant spectra similar to that characteristic for Ga cations. In the case of the Mg dopant, similarity to Ga cations was observed for triple bonds in the ab-plane, only. Practically no resonant structure for spectra detected along the c-axis was observed. The absorption spectra were compared with ab initio calculations using the full-potential linear muffin-tin-orbital method. These calculations were also used for determination of the bond length for Mg–N and Zn–N in wurtzite GaN crystals and show that introducing dopants causes an increase of the lengths of the bonds formed by both dopants. Extended X-ray-absorption fine-structure measurements performed for bulk GaN:Zn confirmed the prediction of the theory in the case of the Zn–N bond. Finally, it is suggested that the anisotropy in the length of the Mg–N bonds, related to their larger strength in the case of bonds in the ab-plane, can explain preferential formation of a superlattice consisting of Mg-rich layers arranged in ab-planes of several bulk GaN:Mg crystals observed by transmission electron microscopy. Within the sensitivity of the method used, no parasitic metallic clusters or oxide compounds formed by the considered acceptors in GaN crystals were found. Received: 1 March 2001 / Accepted: 19 September 2001 / Published online: 20 December 2001 / Published online: 20 December 2001     

Luminescence and recombination processes in bulky Li6Gd x Y1 − x (BO3)3:Eu crystals

The luminescence and recombination processes in crystals of lithium borates Li₆Gd _x Y_{1 − x} (BO₃)₃:Eu have been investigated. The steady-state X-ray luminescence (XRL) spectra, the temperature dependences of the XRL intensity, and the thermally stimulated luminescence (TSL) spectra have been measured for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃:Eu, and Li₆Gd(BO₃)₃:Eu compounds in the temperature range of 90–500 K. It is established that the band at 312 nm, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ ions and the group of lines at 580–700 nm, which are due to the ⁵ D ₀ → ⁷ F _J (J = 0 … 4) transitions in Eu³⁺ ions dominate in the XRL spectra. The XRL intensity in these bands increases several times with a change in temperature from 100 to 400 K for undoped crystals. The likely mechanisms of the observed temperature dependence of the XRL intensity and their relationship with the features of electronic-excitation energy transfer in these crystals are discussed. The spectra of all crystals under study exhibit a dominant composite TSL peak with a maximum at 110–160 K and a series of weaker peaks, the composition and structure of which depend on the crystal type. The nature of the shallow traps, which are responsible for the TSL at temperatures below room, and their relationship with the defects of lithium cation sublattice are discussed.     

Mössbauer study of ultrafine amorphous Fe−B alloys

The chemical composition of ultrafine amorphous Fe−B powders prepared by a chemical reduction depends on the mixed molar ratio of KBH₄ to Fe ions. We propose the following reaction processes for the formation of ultrafine Fe−B powders: (1) 4Fe²⁺+2BH₄₋+6OH⁻→2Fe₂B+6H₂O+H₂ and (2) 4Fe²⁺+2BH₄₋+7OH⁻→2Fe₃B+Fe+BO₂+5H₂O+5/2H₂.