Showing papers on "Single crystal published in 1998"

On the optical band gap of zinc oxide

Abstract: Three different values (3.1, 3.2, and 3.3 eV) have been reported for the optical band gap of zinc oxide single crystals at room temperature. By comparing the optical properties of ZnO crystals using a variety of optical techniques it is concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at ∼3.15 eV that can dominate the optical absorption when the bulk of a single crystal is probed.

Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization

Abstract: ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a ver...

Quasi-ideal strontium titanate crystal surfaces through formation of stontium hydroxide

Abstract: In recent years, well-defined and nearly perfect single crystal surfaces of oxide perovskites have become increasingly important. A single terminated surface is a prerequisite for reproducible thin film growth and fundamental growth studies. In this work, atomic and lateral force microscopy have been used to display different terminations of SrTiO3. We observe hydroxylation of the topmost SrO layer after immersion of SrTiO3 in water, which is used to enhance the etch-selectivity of SrO relative to TiO2 in a buffered HF solution. We reproducibly obtain perfect and single terminated surfaces, irrespective of the initial state of polished surfaces and the pH value of the HF solution. This approach to the problem might be used for a variety of multi-component oxide single crystals. True two-dimensional reflection high-energy electron diffraction intensity oscillations are observed during homo epitaxial growth using pulsed laser deposition on these surfaces.

Method for manufacturing single crystal structures

Abstract: Producing single crystal structures, parts or workpieces of superalloys on substrates with one or more single crystal structures comprises: (a) melting the substrate surface with an energy beam; (b) supplying the molten region with material which is to be introduced into the single crystal structure; (c) completely melting the supplied material; and (d) regulating and/or controlling the energy supply such that the solidification velocity and temperature gradient lie in the dendritic crystalline region outside the globulitic region in the GV (gradient velocity) diagram. The supplied material may be the same as the substrate material or may be a different material having a similar crystal structure, preferably a Co, Fe or Ni base superalloy such as CMSX-4.

Lead-free high-strain single-crystal piezoelectrics in the alkaline–bismuth–titanate perovskite family

Abstract: Doped alkaline–bismuth–titanate perovskite single crystals have been grown in ferroelectric phases with high piezoelectric actuation. Rhombohedral-phase Na1/2Bi1/2TiO3–BaTiO3 crystals exhibit up to 0.25% free strain with low hysteresis along the cubic 〈001〉 direction (d33∼450 pC/N). Tetragonal phase crystals exhibit free strains as high as 0.85% with greater hysteresis characteristic of domain switching; low field d33 exceeds 500 pC/N. Strain energy densities exceed those of optimized polycrystalline lead perovskites, and actuation capability is retained at compressive stresses >100 MPa.

Magnetoresistance of magnetite

Abstract: The magnetoresistance behavior of Fe3O4 in polycrystalline thin film, powder compact, and single-crystal form are compared. Negative magnetoresistance with peaks at the coercive field, observed in thin films and powder compacts but not in the single crystal, is due to field-induced alignment of the magnetization of contiguous grains. The effect is associated with intergranular transport of spin-polarized electrons.

Fabrication of single-crystal lithium niobate films by crystal ion slicing

Abstract: We report on the implementation of crystal ion slicing in lithium niobate (LiNbO3). Deep-ion implantation is used to create a buried sacrificial layer in single-crystal c-cut poled wafers of LiNbO3, inducing a large etch selectivity between the sacrificial layer and the rest of the sample. 9-μm-thick films of excellent quality are separated from the bulk and bonded to silicon and gallium arsenide substrates. These single-crystal films have the same room-temperature dielectric and pyroelectric characteristics, and ferroelectric transition temperature as single-crystal bulk. A stronger high-temperature pyroelectric response is found in the films.

X‐Ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study of Passive Films Formed on (100) Fe‐18Cr‐13Ni Single‐Crystal Surfaces

Abstract: X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were combined to investigate the thickness, chemical composition, and structure of passive films formed in 0.5 M H 2 SO 4 on (100)Fe-18Cr-13Ni. The XPS measurements show that aging under polarization at +500 mV/SHE causes a dehydration reaction of the outer chromium hydroxide layer of the passive film. This reaction results in a thickening of the mixed Cr(III) and Fe(III) inner oxide layer and increases the Cr 2 O 3 enrichment. This reaction consumes, in addition to chromium hydroxide of the outer layer, chromium from the metallic phase underneath the passive film. Only traces of nickel (hydroxide) are detected in the passive film, whereas Ni enrichment is observed in the alloy underneath the passive film. High-resolution STM images reveal that aging under polarization causes a crystallization of the inner Cr 2 O 3 oxide layer in epitaxy with the substrate. The epitaxial relationship is (0001) α-Cr 2 O 3 ||(100) Fe-18Cr-13Ni with [2130]α-Cr 2 O 3 ||[011] Fe-18Cr-13Ni. The crystallization proceeds with a faster kinetics than on (110) Fe-22Cr in the same conditions. The crystallization rate is modified by the presence of Ni in the alloy, which is enriched in the metallic phase underneath the film and slows down the formation of Cr 2 O 3 in the inner part of the film. This favors a more complete process of crystallization. Aging under polarization is beneficial to the further stability of the passive film in air.

Resonant magnetization tunneling in the trigonal pyramidal mnivmniii3 complex mn4o3cl(o2cch3)3(dbm)3

Abstract: The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the MnIVMnIII3 complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (DŜz2) with D = −0.53 cm-1 and a quartic zero-field splitting (B40O40)with B40 = −7.3 × 10-5 cm-1. Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426−2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each MnIVMnIII3 complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by...

Semiconductor thin film and semiconductor device

Abstract: After an amorphous semiconductor thin film is crystallized by utilizing a catalyst element, the catalyst element is removed by performing a heat treatment in an atmosphere containing a halogen element. A resulting crystalline semiconductor thin film exhibits {110} orientation. Since individual crystal grains have approximately equal orientation, the crystalline semiconductor thin film has substantially no grain boundaries and has such crystallinity as to be considered a single crystal or considered so substantially.

Growth of poly- and single-crystal ScN on MgO(001): Role of low-energy N2+ irradiation in determining texture, microstructure evolution, and mechanical properties

Abstract: ScN layers, 345 nm thick, were grown on MgO(001) substrates at 750 °C by ultrahigh-vacuum reactive magnetron sputter deposition in pure N2 discharges at 5 mTorr. The N2+ to Sc ratio incident at the substrate and growing film was maintained constant at 14, while the ion energy EN2+ was varied from 13 to 50 eV. All films were stoichiometric with N/Sc ratios of 1.00±0.02. However, microstructural and surface morphological evolution were found to depend strongly on EN2+. The nucleation and initial growth stages of ScN films deposited with EN2+=13 eV are dominated by the formation of 111- and 002-oriented islands, but preferred orientation rapidly evolves toward a purely 111 texture by a film thickness of ≃50 nm as 002 grains grow out of existence in a kinetically limited competitive growth mode. In distinct contrast, films deposited with EN2+=20 eV grow in a cube-on-cube epitaxial relationship with the substrate and exhibit no indication of 111-oriented grains, even in the earliest stages. Increasing EN2+ to ...

Comprehensive characterization of metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaN

Abstract: We report on the material, electrical, and optical properties of metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaN, with active layers of 1.5 and 4.0 μm thickness. We have modeled current transport in the 1.5 μm devices using thermionic field emission theory, and in the 4.0 μm devices using thermionic emission theory. We have obtained a good fit to the experimental data. Upon repeated field stressing of the 1.5 μm devices, there is a degradation in the current–voltage (I–V) characteristics that is trap related. We hypothesize that traps in the GaN are related to a combination of surface defects (possibly threading dislocations), and deep-level bulk states that are within a tunneling distance of the interface. A simple qualitative model is presented based on experimental results. For devices fabricated on wafers with very low background free electron concentrations, there is a characteristic “punch-through” voltage, which we attribute to the interaction of the depletion ...

Role and behaviour of μ phase during deformation of a nickel-based single crystal superalloy

Abstract: The effect of the presence of variable amounts of the topologically closed packed (TCP) μ phase on the mechanical properties of the MC2 nickel-based single crystal superalloy has been investigated. It is shown that moderate amounts of needle-like μ particles do not affect the tensile properties or the impact strength at 25°C. The precipitates behave similarly to brittle short fibres in a ductile matrix, thus internal cracking occurs under stress rather than interfacial decohesion. For a higher amount of μ phase, precipitates are of smaller size and of globular shape, exhibiting a weaker interface with the γ/γ′ matrix, thus they are more prone to decohesion. By separating effects due to the μ phase from those due to the rafted γ / γ ′ microstructure, it is shown that the low cycle fatigue (LCF) properties of MC2 alloy at 650 and 950°C are not affected by the presence of μ phase. Finally, some observations related to the behaviour of μ phase precipitates during high temperature creep are presented.

Mono-sized single-wall carbon nanotubes formed in channels of AlPO4-5 single crystal

Abstract: An alternative approach to the synthesis of mono-sized and parallel-aligned single-wall carbon nanotubes (SWCNs) is reported. The SWCNs are formed in 0.73 nm sized channels of microporous aluminophosphate crystallites by pyrolysis of tripropylamine molecules in the channels. They are characterized through transmission electron microscopy, polarized Raman scattering, and electrical transport measurements. Our results would open a door to further detailed studies on the intrinsic properties of carbon nanotubes now in progress.

Amorphization of SiC under ion and neutron irradiation

Abstract: This paper presents results on the microstructure and physical properties of SiC amorphized by both ion and neutron irradiation. Specifically, 0.56 MeV Si ions have been implanted in single crystal 6H–SiC from ambient through >200°C and the critical threshold for amorphization was measured as a function of the irradiation temperature. From a high resolution transmission electron microscopy (HRTEM) study of the crystalline to amorphous transition region in these materials, elongated pockets of amorphous material oriented parallel to the free surface are observed. Single crystal 6H–SiC and hot pressed and sintered 6H and 3C SiC were neutron irradiated at approximately 70°C to a dose of ∼2.56 dpa causing complete amorphization. Property changes resulting from the crystal to amorphous transition in SiC include a density decrease of 10.8%, a hardness decrease from 38.7 to 21.0 GPa, and a decrease in elastic modulus from 528 to 292 GPa. Recrystallization of the amorphized, single crystal 6H–SiC appears to occur in two stages. In the temperature range of ∼800–1000°C, crystallites nucleate and slowly grow. In the temperature range of 1125–1150°C spontaneous nucleation and rapid growth of crystallites occur. It is further noted that amorphized 6H (alpha) SiC recrystallizes to highly faulted fcc (beta) SiC.

Structure and properties of ion-beam-modified (6H) silicon carbide

Abstract: The ion-beam-induced crystalline-to-amorphous phase transition in single crystal ( 6 H) α -SiC has been studied as a function of irradiation temperature. The evolution of the amorphous state has been followed in situ by transmission electron microscopy in specimens irradiated with 0.8 MeV Ne + , 1.0 MeV Ar + , and 1.5 MeV Xe + ions over the temperature range from 20 to 475 K. The threshold displacement dose for complete amorphization in α -SiC at 20 K is 0.30 dpa (damage energy=15 eV atom −1 ). The dose for complete amorphization increases with temperature due to simultaneous recovery processes that can be adequately modeled in terms of a single-activated process. The critical temperature, above which amorphization does not occur, increases with particle mass and saturates at about 500 K. Single crystals of α -SiC with [0001] orientation have also been irradiated at 300 K with 360 keV Ar 2+ ions at an incident angle of 25° over fluences ranging from 1 to 8 Ar 2+ ions nm −2 . The damage accumulation in these samples has been characterized ex situ by Rutherford backscattering spectrometry–channeling (RBS/C) along the [0001] direction, Raman spectroscopy, cross-sectional transmission electron microscopy (XTEM), and mechanical microprobe measurements.

Polarity determination for GaN films grown on (0001) sapphire and high-pressure-grown GaN single crystals

Abstract: In order to resolve any doubt in lattice polarity calibrations, a given undoped GaN layer deposited on (0001) sapphire by metalorganic chemical vapor deposition and a given high-pressure-grown GaN single crystal have been studied by three different techniques: Hemispherically scanned x-ray photoelectron diffraction, convergent beam electron diffraction, and chemical etching. We conclude that Ga-polar surfaces are resistant to a 200 °C molten NaOH+KOH etching whereas N-polar surfaces are chemically active. All the observed flat GaN films grown on (0001) sapphire have Ga polarity. On the contrary, the native flat faces of undoped GaN bulk crystals have N polarity.

Red shift of plasmon resonance frequency due to the interacting Ag nanoparticles embedded in single crystal SiO2 by implantation

Abstract: A metal nanoparticle system has been prepared by 200 Kev Ag+ ion implantation into perfect single crystal SiO2 at room temperature to dose: 6.7×1016/cm2. The system presents quasidual-layer structure: the shallower implanted layer containing noninteracting small Ag nanoparticles and the deeper layer containing interacting large nanoparticles, in which great red shift, about 1 eV, comparing with the plasmon resonance frequency of the noninteracting nanoparticle, can be clearly observed. The red shift is attributed to the multipoles interaction among the high density nanoparticles at external electric field. Moreover, the magnitude of red shift increases with implanted dose.

Adsorption of H2O on a Single-Crystal α-Al2O3(0001) Surface

Abstract: The adsorption of H2O on a single-crystal α-Al2O3(0001) surface was examined using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. α-Al2O3(0001) mode...

Anisotropic complex permittivity measurements of mono-crystalline rutile between 10 and 300 K

Abstract: The dielectric properties of a single crystal rutile (TiO2) resonator have been measured using whispering gallery modes. Q factors and resonant frequencies were measured from 300 to 10 K. Q factors as high as 104, 105, and 107 were obtained at 300, 80, and 10 K, respectively. Using the whispering gallery mode technique we have determined accurately the loss tangent and dielectric constant of monocrystalline rutile and obtained much more sensitive measurements than previously reported. We show that rutile exhibits anisotropy in both the loss tangent and permittivity over the range from 10 to 300 K.