Showing papers in "Materials Science and Engineering B-advanced Functional Solid-state Materials in 1998"
TL;DR: The potential of high temperature superconductors to generate large magnetic fields and to carry current with low power dissipation at 77 K is particularly attractive for a variety of permanent magnet applications as mentioned in this paper.
Abstract: The potential of high temperature superconductors to generate large magnetic fields and to carry current with low power dissipation at 77 K is particularly attractive for a variety of permanent magnet applications. As a result large grain bulk (RE)-Ba-Cu-O ((RE)BCO) materials have been developed by melt process techniques in an attempt to fabricate practical materials for use in high field devices. This review outlines the current state of the art in this field of processing, including seeding requirements for the controlled fabrication of these materials, the origin of striking growth features such as the formation of a facet plane around the seed, platelet boundaries and (RE)2BaCuO5 (RE-211) inclusions in the seeded melt grown microstructure. An observed variation in critical current density in large grain (RE)BCO samples is accounted for by Sm contamination of the material in the vicinity of the seed and with the development of a non-uniform growth morphology at ≈ 4 mm from the seed position. (RE)Ba2Cu3O7-gd (RE-123) dendrites are observed to form and broaden preferentially within the a/b plane of the lattice in this growth regime. Finally, trapped fields in excess of 3 T have been reported in irradiated U-doped YBCO and (RE)1+xBa2-xCu3Oy (RE = Sm, Nd) materials have been observed to carry transport current in fields of up to 10 T at 77 K. This underlines the potential of bulk (RE)BCO materials for practical permanent magnet type applications.
236 citations
TL;DR: In this article, two relaxation processes for frequencies from 20 Hz to 1 MHz are observed, attributed to space charge and dipolar relaxation, respectively, in the frequency range up to 100 kHz at 833 K and close to 0.5 kHz at 493 K.
Abstract: Dielectric measurements, carried out on Na0.5Bi0.5TiO3 ceramics, revealed behaviour-dependency of both temperature and frequency. Two relaxation processes for frequencies from 20 Hz to 1 MHz are observed. The first relaxation process could be attributed to space charge. The second is low-frequency dipolar relaxation in the frequency range up to 100 kHz at 833 K and close to 0.5 kHz at 493 K, and could be related to the existence of polar regions in Na0.5Bi0.5TiO3.
179 citations
TL;DR: In this paper, the growth of ZnO nanocrystal thin films on sapphire substrates was investigated by atomic force microscopy and transmission electron microscopy, and it was concluded that the grain boundaries between nanocrystals serve not only as potential barriers confining excitons but also as cavity mirrors.
Abstract: Hexagonally shaped ZnO nanocrystal thin films were fabricated on sapphire(0001) substrates by laser molecular beam epitaxy. Nanocrystal structure was investigated by atomic force microscopy and transmission electron microscopy. Epitaxial growth of ZnO nanocrystal thin films on sapphire substrates was found to occur in a spiral and grain growth mode. The grain growth mode was interpreted by taking higher order epitaxial relationship of oxygen sublattice units between ZnO and sapphire into account. Nanocrystal size could be tuned from 50 to 200 nm controlling film thickness, growth conditions and stoichiometry of the target. The films having small nanocrystal size of about 50 nm showed excitonic stimulated emission having peak energy of 3.2 eV at room temperature with a very low threshold (24 kW cm−2). Mode transition from excitonic stimulated emission to electron hole plasma appeared above another threshold (50 kW cm−2). Well defined Fabry–Perot cavity mode was observed in the emission spectra measured from side edge of the film. It was concluded that the grain boundaries between nanocrystals serve not only as potential barriers confining excitons but also as cavity mirrors.
171 citations
TL;DR: Growth kinetics of interfacial IMC layer and its effect on the shear strength on practical LCCC surface mount solder joints were studied for isothermal aging at 70, 120, 155 and 170°C.
Abstract: Growth kinetics of interfacial Cu‐Sn intermetallic (IMC) layer and its effect on the shear strength on practical LCCC surface mount solder joints were studied for isothermal aging at 70, 120, 155 and 170°C. Only normal Cu6Sn5 (h-phase) intermetallic was found in the interfacial IMC layer of as-soldered solder joint, whereas the duplex structure of both h-phase and o-phase Cu3Sn existed in all annealed joints. The growth kinetics of the overall interfacial IMC layer can simply be described by classical kinetic theory for solid-state diffusional growth with an activation energy of 1.09 eV and interdiffusion coefficient of 1.61 10 4 m 2 s 1 . The relatively higher activation energy, as compared with that found for bi-metallic couple of eutectic Sn‐Pb solder on copper, is attributed to the dissolution of Ni from the component metallization into the bulk Sn‐Pb solder. In addition, the shear fractures in all the solder joints investigated are shown to be ductile in nature and confined in the bulk solder rather than through the interfacial IMC layer. A linear reduction in shear joint strength was observed with an increase in intermetallic layer thickness up to 5.6 mm. Such a reduction in joint strength is due to a continuous removal of Sn from the bulk solder for the growth of interfacial IMC layer and flattening of the solder:IMC layer during isothermal aging of the solder joint. © 1998 Elsevier Science S.A. All rights reserved.
160 citations
TL;DR: In this paper, the XPS Pt4f spectra have shown the presence of Pt0, PtII and PtIV at the metal-oxide interface, whereas the Pt04f doublet was not observed.
Abstract: Two kinds of tantalum oxide films have been studied by XPS: dry and electrogenerated anodic oxides. XPS spectra of Ta4f and O1s have been used to determine the chemical composition of the different films. Ta2O5 is the main constituent of thick films (15 nm≤dox≤60 nm), although the concomitant presence of sub-oxides (mainly TaO) is observed. In thin films (dox<15 nm), the amount of TaII is larger and depends on the preparation procedure. Estimations of the thickness of the oxide layers are given. Ta/Ta2O5/Pt structures were prepared by depositing Pt by photoinduction. The XPS Pt4f spectra have shown the presence of Pt0, PtII and PtIV at the metal–oxide interface. On the contrary, the spectra of electrodeposited Pt present only the Pt04f doublet. Accordingly, these two kinds of structures have different electrochemical behaviours in the presence of a redox couple in solution. Ta/Ta2O5 structures exhibit a diode effect, whereas Ta/Ta2O5/Pt behave rather like Pt electrodes.
130 citations
TL;DR: In this paper, a systematic study has been made on the influence of the sputtering parameters (total pressure, oxygen partial pressure and r.f. power) on the film composition and on their structural and optical properties.
Abstract: Thin films of zirconium oxide were prepared by r.f. magnetron sputtering from a ZrO2 target. A systematic study has been made on the influence of the sputtering parameters (total pressure, oxygen partial pressure and r.f. power) on the film composition and on their structural and optical properties. The zirconia films crystallize either in the cubic or in the monoclinic phases depending on the sputtering gas. The crystallinity and the compactness of the films were found to increase with the kinetic energy of the sputtered particles. The stresses are compressive and become very important in thick films deposited at a high power density. Films are generally substoichiometric and their O/Zr atomic ratio was found to increase with the oxygen partial pressure. On the contrary, films deposited at high sputtering pressures (more than 5 Pa) contain an oxygen excess. This overstoichiometry results, as it was revealed by F. T. I. R. analyses, from the incorporation of water and hydroxyl groups into the ZrO2 structure. The optical constants (refractive index and extinction coefficient) vary also in a wide range with the deposition conditions. These variations were correlated mainly to structural properties.
128 citations
TL;DR: In this paper, the authors have grown epitaxial Mg x Zn 1− x O alloy films and characterized their structures by X-ray diffraction, showing that the structure of the superlattices was greatly improved by the use of a ZnO buffer layer on sapphire substrate prior to the deposition of super-lattice.
Abstract: We have grown epitaxial Mg x Zn 1− x O alloy films and ZnO/Mg x Zn 1− x O ( x =0.20) superlattices on sapphire(0001) substrates by laser molecular beam epitaxy and characterized their structures by X-ray diffraction. Single phase Mg x Zn 1− x O could be obtained up to x =0.33, whereas MgO impurity phase with (111) orientation segregated at x >0.33. The bandgap of Mg x Zn 1− x O was successfully controlled as verified by the photoluminescence peaks shifting 3.36 eV ( x =0) to 3.87 eV ( x =0.33). It was found that the structure of the superlattices was greatly improved by the use of a ZnO buffer layer on sapphire substrate prior to the deposition of superlattice. Small angle X-ray diffraction peaks corresponding to the period of the superlattices ranging from 8 to 18 nm could be clearly observed.
128 citations
TL;DR: In this article, the authors observed ultraviolet laser emission from ZnO nanocrystal thin films at room temperature at 3.2 eV, increasing with a power of eight as increasing pumping intensity.
Abstract: We have observed ultraviolet laser emission from ZnO nanocrystal thin films at room temperature. ZnO films were epitaxially grown on sapphire substrates by laser molecular beam epitaxy. Closely packed and hexagonally shaped nanocrystals were formed in a spiral growth mode presumably due to a large lattice mismatch (∼18%) between ZnO and sapphire. Photoluminescence of these films clearly showed a sharp emission due to free excitons at room temperature. Above a threshold intensity as small as 24 kW cm −2 of pumping laser pulses (355 nm, 15 ps), we observed stimulated emission at 3.2 eV, increasing with a power of eight as increasing pumping intensity. This emission is shown to be due to an exciton–exciton collision process. Excitons are confined in these nanocrystals to show the giant oscillator strength effect and resulting in an excitonic stimulated emission even at room temperature.
116 citations
TL;DR: In this paper, the integration of the (Ba,Sr)TiO 3 capacitors for the next-generation dynamic random access memories (DRAMs) is reviewed from the point of view of the mass-production compatible process.
Abstract: Integration processes of the (Ba,Sr)TiO 3 (BST) capacitors for the next-generation dynamic random access memories (DRAMs) are reviewed. Various integration schemes utilizing different processes, different electrode materials and structures are reviewed from the point of view of the mass-production compatible process. Integration issues, mostly related to the electrode and the barrier material and their fabrication techniques, are described. The current status and the problems of the two most viable techniques for the BST deposition, sputtering and metal-organic chemical vapor deposition (MOCVD), are comparatively described. Plate electrode technologies with the back-end processing issues are also briefly described, and some valuable experimental results are presented to show the feasibility of the BST capacitor.
112 citations
TL;DR: In this article, the metastable VO 2 (B) transforms irreversibly into thermochromic VO 2(R) powder between 400 and 500°C, and this transformation with constant mass and high increases of density is studied by XRD, IRTF spectroscopy, thermal analyses and electronic microscopies.
Abstract: The metastable VO 2 (B) transforms irreversibly into thermochromic VO 2 (R) between 400 and 500°C. Differential scanning calorimetry (DSC) of this transition shows a broad transformation starting with a first endothermic step at 420°C followed by a second exothermic step at 455°C. During this process, the initial platelet like VO 2 (B) morphology leads to a submicronic VO 2 (R) powder. This low temperature thermal treatment avoids the grain growth commonly observed in the case of classical routes at higher temperatures. This transformation with constant mass and high increases of density has been studied by XRD, IRTF spectroscopy, thermal analyses and electronic microscopies. The structure and properties of the VO 2 (R) issued from this irreversible transition are very similar to those of the VO 2 (R) obtained by the powder metallurgy process, except the optical properties. Indeed, this thermochromic VO 2 (R) powder shows an unexpected transition contrast in the IR spectral range during the insulator–metal transition at 68°C. Experimental observations by scanning electron microscopy (SEM) correlated with calculations could explain this high efficiency by the unusual morphology of the submicronic particles.
112 citations
TL;DR: Tin oxide and fluorine doped tin oxide (FTO) thin films of different thicknesses and varying impurity concentrations have been prepared by spray technique as discussed by the authors, and the XRD results are in good agreement with ASTM data.
Abstract: Tin oxide (TO) and fluorine doped tin oxide (FTO) thin films of different thicknesses and varying impurity concentrations have been prepared by spray technique. The XRD results are in good agreement with ASTM data. From the XPS (X-ray photoelectron spectroscopy) studies, the presence of tin and oxygen in tin oxide has been confirmed and also the ratio of Sn/O has been found out. Chemical analysis of FTO films has been carried out by SAM (Scanning Auger Microprobe) studies and from the peak to peak height (PPH) calculations, Sn, O and F concentrations have been determined. The compositional analysis of FTO films at different depths (depth propfiles) has been studied. The results have been explained.
TL;DR: In this article, the NiO thin film formation by atomic layer epitaxy (ALE) was studied including aspects of surface chemistry and structure of the prepared films, and the reactivity of the Ni-precursors towards various oxygen sources was studied at the deposition temperature of 250°C using soda-lime glass substrates.
Abstract: Nickel oxide thin film formation by atomic layer epitaxy (ALE) was studied including aspects of surface chemistry and structure of the prepared films. The applied source materials were Ni(acac)2, Ni(apo)2, Ni(dmg)2 and O2, H2O, N2O, O3, CH3COOH for Ni and O, respectively. The thermal behaviour of the Ni-containing precursors was studied by thermogravimetry and by experiments in the ALE reactor. The reactivity of the Ni-precursors towards various oxygen sources was studied at the deposition temperature of 250°C using soda-lime glass substrates. The crystalline rocksalt-type NiO was obtained only when O3 was used as an oxygen source. The film was in this case (100) oriented NiO independent on the applied Ni-precursor. The NiO ALE process was optimized for the reaction between Ni(acac)2 and O3. The surface-saturated growth rate was limited by the steric hindrance of the ligand and was 0.62 A cycle−1. A slightly increased growth rate (0.72 A cycle−1) was observed, when H2O was supplied together with O3 as an oxygen source. Structure of the deposited films was studied by atomic force microscopy and X-ray diffraction techniques and some structural variations related to high ozone doses were observed.
TL;DR: In this article, the authors showed that the transfer of oxygen through strontium cobaltite-based ceramics was limited both by the rate of oxygen exchange between oxides and gas phase and by ionic conductivity of the materials.
Abstract: Oxygen permeability of Sr 0.65− x La 0.35 CoO 3− δ ( x =0–0.10), Sr 0.65 La 0.35 Co 0.90 O 3− δ and Sr 0.7 Ln 0.3 CoO 3− δ (Ln=La, Nd, Sm, Gd) ceramics was studied under temperatures of 950–1250 K. The experiments held proved that the transfer of oxygen through the strontium cobaltite-based ceramics was limited both by the rate of oxygen exchange between oxides and gas phase and by ionic conductivity of the materials. It has been established that the permeability of materials studied increases rapidly with increase in oxygen partial pressure. A creation of vacancies in both cation sublattices of ABO 3 perovskite results in decreasing oxygen permeation flux. Therewith, cation deficiency in the sublattice B of the cobaltites have a lower influence on oxygen permeability in comparison with ones in the sublattice A. Oxygen permeation flux through Sr(Ln)CoO 3− δ ceramics decreases with decreasing mean radius of the cations in the sublattice A. Electrical conductivity of the cobaltites reduces when oxygen partial pressure diminishes or concentration of the cation vacancies increases.
TL;DR: In this article, the addition of uranium to large single grains of Y123 improves J c prior to irradiation, by over 80%. Observations on the U chemistry and microstructure studies are presented.
Abstract: The addition of uranium to large single grains of Y123 improves J c prior to irradiation, by over 80%. Observations on the U chemistry and microstructure studies are presented. Irradiation with thermal neutrons, to fission the U235 component, results in further increases in J c . Total increases of J c by factors of, e.g. > 30 at 77 K, 0.25 Tesla, and > 20 at 50 K, 10.0 Tesla, result. Values of J c of, e.g. 290000 A cm −2 at 77 K, 0.25 Tesla, and 1 million A cm −2 at 50 K, 0.25 Tesla, result. T c is reduced by 0.5 K and creep rate is increased by 1/5. The chemical and irradiation effects are multiplicative and neither is yet optimized.
TL;DR: In this article, thin films of vanadium pentoxide were prepared by the electron-beam evaporation technique onto Corning 7059 glass and silicon substrates maintained at Ts=553 K by varying the oxygen partial pressure in the range of 20 mPa.
Abstract: Thin films of vanadium pentoxide were prepared by the electron-beam evaporation technique onto Corning 7059 glass and silicon substrates maintained at Ts=553 K by varying the oxygen partial pressure in the range 01–20 mPa These films have been characterized by studying their chemical state, structure, optical and electrical properties V2O5 films of thickness 06 μm prepared at an oxygen partial pressure of 20 mPa exhibit an orthorhombic layered structure with an optical band gap of 23 eV The room temperature electrical conductivity of the films is 2×10−5 S cm−1 with an activation energy of 042 eV in the temperature range 303–523 K
TL;DR: In this paper, the authors obtained a value of Z equal to 0.21×10 −4 K −1 for α =40 μV-K −1 and ρ =50 μΩ·m for P-type material and Z of about 0.17×10−3 K − 1 for α=90 μV−K − 1 and π =30 μξm for N −type material (at 300 K).
Abstract: (Bi 2 Te 3 ) 0.9 (Bi 2 Se 3 ) 0.1 for N-type material powder and (Bi 2 Te 3 ) 0.25 (Sb 2 Te 3 ) 0.75 for P-type material powder were evaporated by a flash evaporation technique. We obtained a value of Z equal to 0.21×10 −4 K −1 for α =40 μV K −1 and ρ =50 μΩ·m for P-type material and Z of about 0.17×10 −3 K −1 for α =90 μV K −1 and ρ =30 μΩ·m for N-type material (at 300 K), for 1 μm layer thickness deposited over polyimide substrate, before annealing. We show that after annealing at 250°C under He atmosphere, the figure of merit of the layers increases to Z =3.2×10 −3 K −1 for α =240 μV K −1 and ρ =12 μΩ·m for P-type material and Z =1.6×10 −3 K −1 for α =200 μV K −1 and ρ =15 μ Ω·m for N-type material (at 300 K). With these fabrication parameters, we realised three different structures of micromodule Peltier (MMP) junction and we obtained a maximum value for temperature drop between hot and cold sides of ≈3.4 K. This result is very promising in order to develop MMP.
TL;DR: In this article, the Au/InP Schottky diode at different temperatures (in the range 300-425 K) is characterized with an InSb thin film with several monolayers and the doping concentration and the diffusion voltage are calculated using the C(V) characteristics.
Abstract: This work attempts to characterize the Au/InP Schottky diode at different temperatures (in the range 300–425 K). The InP surface is restructured with an InSb thin film with several monolayers. I(V) analysis versus different temperatures gives the saturation current variation Is (2×10−5–7×10−5 A), the mean ideality factor (1.7–1.24), the barrier height (0.47–0.45 V), and finally the serial resistance Rs variations (85–19 Ω). The doping concentration Nd and the diffusion voltage Vd are calculated using the C(V) characteristics. The concentration Nd is 3×1015 cm−3 at room temperature and increases with thermal activation to 7×1015 cm−3 at 425 K. Nevertheless, the diffusion voltage Vd is reversibly proportional to the doping concentration Nd and decreases from 33.7×10−2 to 29×10−2 V. The mean interfacial state density Nss decreases with increasing temperature, from 4.33×1012 to 1012 cm−2.eV−1. This improvement is the result of molecular restructuring and reordering at the Au/InP interface. For temperatures less than 375 K, the C(V) characteristic is controlled by an important interfacial state density and/or the presence of deep donor levels in the semiconductor bulk. At temperatures greater than 375 K, the C−2(V) curve is linear and the deep donor levels disappear. The traps effect is also reduced.
TL;DR: In this paper, thin-films (thickness ∼1 μm) of SnO2(Pd, Pt) have been synthesized on oxidized silicon in the temperature range of 460-560°C using pyrolysis of an aerosol.
Abstract: For gas sensor applications, thin-films (thickness ∼1 μm) of SnO2(Pd, Pt) have been synthesised on oxidized silicon 〈100〉 in the temperature range of 460–560°C using pyrolysis of an aerosol. By doping with Pd or Pt metallic particles (1–12 wt.%) selectivity and sensitivity were improved. Electrical measurements under 300 ppm of CO were performed on thin films in steady-state and dynamic modes. Sensitivity, rate and response time were studied. The optimum synthesis conditions were determined in order to obtain the best microstructure of the films and the best concentration of doping additives for the detection of the carbon monoxide in the range of room temperatures. The mechanism of the sensor response to CO and the role of the additives (Pd, Pt) on the basis of chemical and electronic mechanisms were discussed.
TL;DR: In this article, the reactions induced in a vacuum furnace (5×10−7 Torr) between an electron-beam-evaporated Ni film a few hundred nm thick and a (001)-oriented (i.e. Si-face-oriented) single crystalline 3C-SiC substrate are investigated by 3.2 MeV 4He2+ backscattering spectrometry, X-ray diffraction, secondary ion mass spectrometer, and scanning electron microscopy.
Abstract: The reactions induced in a vacuum furnace (5×10−7 Torr) between an electron-beam-evaporated Ni film a few hundred nm thick and a (001)-oriented (i.e. Si-face-oriented) single crystalline 3C–SiC substrate are investigated by 3.2 MeV 4He2+ backscattering spectrometry, X-ray diffraction, secondary ion mass spectrometry, and scanning electron microscopy. Samples are characterized before and after annealing at temperatures of 400–700oC for 30 min. At 450oC, carbon diffuses throughout the Ni film and forms a carbon-rich layer at the Ni surface of a thickness of a few nm which remains unchanged during subsequent annealing. Some nickel silicides were detected at this initial stage but could not be clearly identified. At 450oC (after 120 min) the Ni31Si12 phase starts to form. This is the only detected phase at 500oC. The Ni2Si phase, the silicide that is thermodynamically stable with SiC and carbon, forms first at the surface and grows toward the SiC substrate. At 600oC, this reaction has consumed about half of the Ni31Si12 phase and at 700oC, Ni2Si is the only silicide in the reacted film. In all the reacted samples the carbon distribution is alike and consists of three distinct layers: a first zone with a constant carbon concentration that extends from near the SiC/silicide interface through most of the films thickness. The second zone is ∼70 nm thick and is deficient of carbon. The third zone is the thin graphite layer at the surface. There is oxygen in the film too, the distribution of which is related increasingly clearly to the carbon profile as the annealing temperature rises.
TL;DR: In this paper, the authors investigated the characteristics of diamond-like carbon (DLC), DLC doped with Cu, and DLCdoped with Ti deposited by a sequential pulsed laser ablation of two targets.
Abstract: We have investigated the characteristics of diamond-like carbon (DLC), DLC doped with Cu, and DLC doped with Ti deposited by a sequential pulsed laser ablation of two targets. The composition of these films was determined by Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and transmission electron microscopy studies showed typical features of DLC with a high fraction of sp 3 bonded carbon in the doped films as well as in the undoped films. Wear resistance measurements made on the samples by means of the ‘crater grinding method’ showed that DLC2.75% Ti has the highest wear resistance, while that of pure DLC has the lowest amongst the samples. Careful analysis of the Raman data indicates a significant shift to shorter wavelength with the addition of metal, which means that the compressive stress in the DLC films has been reduced. We envisaged that the reduction in the compressive stress promotes the wear resistance of the coatings. The XPS studies showed evidence for the formation of Ti‐C bonding in the Ti doped sample. Thus metal-doped DLC coatings are expected to improve the tribological properties and enhance the performance of components coated with metal-doped DLC. © 1998 Elsevier Science S.A. All rights reserved.
TL;DR: In this article, the authors consider the implications of grain boundaries on the electronic band structures of high-Tc cuprates and show that the normal state properties observed may be caused by depletion layers formed at the interfaces.
Abstract: The normal state transport properties of grain boundaries in high-Tc superconductors, of central importance for a large number of electronic applications, still lack a thorough explanation In this contribution we consider the implications of grain boundaries on the electronic band structures of high-Tc cuprates It is shown that the normal state properties observed may be caused by depletion layers formed at the interfaces
TL;DR: In this article, phase composition and microstructural properties of Feoxide powders were investigated using Fourier transform infrared spectroscopy, Mossbauer and X-ray diffraction and transmission electron microscopy.
Abstract: The precipitation of Fe–oxides was performed by adding NaOH or urotropin to FeCl3 solutions and aging the precipitation systems at 90°C. Phase composition and microstructural properties of Fe–oxide powders were investigated using Fourier transform infrared spectroscopy, Mossbauer spectroscopy, X-ray diffraction and transmission electron microscopy. β-FeOOH was obtained in all precipitation systems with final pH values up to 1.80. However, when the precipitation was for a long time at pH≤1.80 the appearance of α-Fe2O3 phase was also observed. In the presence of urotropin the size of submicrometer β-FeOOH rods increased with time of aging thus forming a fibril structure of near micrometer size. The crystallite size of β-FeOOH varied between 7 and 11 nm and this indicated that β-FeOOH fibrils possessed a substructure. After 30 days of aging, the presence of a α-Fe2O3 phase was also detected by XRD. α-Fe2O3 was obtained as a single phase by precipitation with NaOH at pH 4.20–4.40 and aging of the precipitation systems up to 3 days. When the pH of precipitation was increased to 6.00–6.50 the formation of α-FeOOH particles, in addition to α-Fe2O3, was observed. TEM showed that the size and morphology of particles strongly depended on the precipitation conditions. The mechanism of the formation of Fe–oxide phases is discussed.
TL;DR: In this paper, a front-side bulk micromachining based on 0.2 μm GaAs HEMT MMIC technology is presented, where several chemical solutions have been used to perform the etching procedure characterization in respect to the obtained vertical profiles.
Abstract: Front-side bulk micromachining based on 0.2 μm GaAs HEMT MMIC technology is presented. Several chemical solutions have been used to perform the etching procedure characterization in respect to the obtained vertical profiles. It has been verified that citric acid solution is the most appropriate selective etchant to build suspended GaAs/AlGaAs mesa-shaped structures, while both H 3 PO 4 and NH 4 OH based anisotropic systems seem to be the most suitable for the free-standing triangular prism-shaped structures. Moreover, all these three solutions could be applied to suspend only metal and intermetallic materials. Etch rates as well as cross-section profiles were obtained. Furthermore, the compatibility of the etching procedure with the integrated electronics and the pad metallization has been successfully tested. The features and applications linked to the obtained structures are also discussed.
TL;DR: In this paper, the microstructural properties of boehmite formed in an alkaline medium under hydrothermal conditions were investigated using X-ray diffraction (XRD), Fourier transform IR (FT-IR) spectroscopy, transmission electron microscopy (TEM) and microelectrophoresis.
Abstract: The microstructural properties of boehmite formed in an alkaline medium under hydrothermal conditions were investigated using X-ray diffraction (XRD), Fourier transform IR (FT-IR) spectroscopy, transmission electron microscopy (TEM) and microelectrophoresis. The bayerite formed at the beginning of the precipitation process converted to boehmite by the dissolution/reprecipitation mechanism. Boehmite crystallites were in the nanosize range and of anisotropic shape, as determined by XRD. A tendency toward an increase in the crystallite size with prolonged duration of autoclaving was evident both at 160 and 120°C. The boehmite particles produced by autoclaving at 160°C for 10 h looked like thin microfoils, i.e. different from the boehmite morphology usually obtained (rods, fibrils and very small plates). The factors that influence the pHiep of boehmite particles are discussed. The FT-IR spectra of bayerite and boehmite are interpreted.
TL;DR: In this paper, Europium and terbium complexes with 1,10-phenanthroline were introduced into silica gel by the sol-gel method and the luminescence behavior of the complexes in silica gels was studied compared with the corresponding solid state complexes by means of emission, excitation spectra and lifetimes.
Abstract: Europium and terbium complexes with 1,10-phenanthroline were introduced into silica gel by the sol–gel method. The luminescence behavior of the complexes in silica gels was studied compared with the corresponding solid state complexes by means of emission, excitation spectra and lifetimes.
TL;DR: In this article, a modified chemical bath method was used to precipitate quantum dots of ZnSe in the presence of polyvinyl pyrrolidine, which achieved an average particle diameter of only 15 versus 34 A for the uncapped material.
Abstract: Quantum dots of ZnSe have been precipitated in the presence of polyvinyl pyrrolidine using a modified chemical bath method. Addition of the polymeric capping agent serves to limit particle growth and stabilize nanocrystals. Analysis of X-ray diffraction (XRD) measurements indicates that capped particles attain an average particle diameter of only 15 versus 34 A for the uncapped material. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) results show nanocrystals to be single crystals encapsulated in polymeric material, while SAED patterns confirm that the cubic phase of ZnSe was obtained with this synthesis route. Observation of a blue shift in the optical absorption and luminescence data is consistent with the reported reduction in nanocrystal size below the ZnSe bulk exciton Bohr diameter of ∼90 A.
TL;DR: In this paper, an epitaxial buffer layer of MgO on Si will be needed for optical waveguide formation and for enabling the growth of BaTiO3, which is motivated by the potential integration of electrooptical functions onto silicon.
Abstract: The epitaxy of thin films of BaTiO3 is motivated by the potential integration of electrooptical functions onto silicon. An epitaxial buffer layer of MgO on Si will be needed for optical waveguide formation and for enabling the growth of BaTiO3. In a first step, epitaxial waveguide structures of c-axis oriented BaTiO3 thin films on MgO (001) have been grown by pulsed laser deposition (PLD). The structural properties of the samples have been characterized by Rutherford backscattering spectrometry/ion channeling (RBS/C) and X-Ray diffraction (XRD). We found excellent crystalline quality even up to a thickness of a few microns. Waveguide losses of 2.9 dB cm−1 have been demonstrated.
TL;DR: In this article, optical and electrical properties of al-doped ZnO films, deposited by R.F. magnetron sputtering, have been investigated as a function of preparation conditions in an attempt to develop transparent films with low electrical resistivity.
Abstract: Optical and electrical properties of Al-doped ZnO films, deposited by R.F. magnetron sputtering, have been investigated as a function of preparation conditions in an attempt to develop transparent films with low electrical resistivity. The electrical resistivity as well as the Hall mobility of sputtered films depend on the R.F. power density and thickness of the films when it is less than about 300 nm. The free carrier concentration is almost independent on the film thickness and the R.F. power. The optical transmission of the films in the visible range does not depend on the thickness as well as on the R.F. power and is about 90% from the substrate transmission there. In the near-infrared, where the absorption is due to free carriers, the transmission depends strongly on the film thickness and on the preparation conditions.
TL;DR: In this paper, a polycrystalline ZnO film was deposited on quartz and silicon substrates by a spray-CVD method and the deposited film was annealed by thermal and by laser heating.
Abstract: A polycrystalline ZnO film was deposited on quartz and silicon substrates by a spray-CVD method. The deposited film was annealed by thermal and by laser heating. Both CW CO 2 laser and pulsed excimer (XeCl) lasers were used for the purpose. The as-grown film and the annealed film were analysed for crystallinity and compositions by XRD, for surface morphology by SEM, for band gap by optical absorption and for electrical parameters by Van der Pauw and Hall effect measurements. While the excimer laser annealing improved the crystallinity with higher laser power density, CW CO 2 laser incorporated the crystallinity only at low power. High power CO 2 laser annealing damaged the ZnO surface with the creation of more defects. This was verified by SEM morphology studies. There was no change in band gap which ensures no formation of any other phase in ZnO due to annealing effects. The increase in resistivity both by thermal and pulsed laser annealing was confirmed by Van der Panw resistivity measurements. The majority carrier concentration and mobility were determined by Hall effect measurements.
TL;DR: In this paper, the band gap energy of some semiconducting sulphides (ZnS, CdS and Bi 2 S 3 ) and selenides were determined by optical absorption measurements while the activation energy was determined from resistance measurements.
Abstract: Thin films of some semiconducting sulphides (ZnS, CdS and Bi 2 S 3 ) and selenides (ZnSe, CdSe and Bi 2 Se 3 ) have been prepared using the chemical deposition method. The band gap energy of the films was determined by optical absorption measurements while the activation energy was determined from resistance measurements. The band gap energies obtained from the absorption measurements ranged from 1.42 eV for Bi 2 Se 3 to 3.72 eV for ZnS while the activation energies obtained from the resistance measurements ranged from 0.31 eV for Bi 2 Se 3 to 0.59 eV for ZnSe. The high optical band gap obtained for the films could be attributed to very small size in chemically deposited films which lead to electrical isolation of individual grains, or quantum well structure. The small values of the activation energies are due to the presence of impurity levels in the energy band gap of the films.