Showing papers on "Single crystal published in 1994"

Pulsed laser deposition of VO2 thin films

Abstract: High quality vanadium dioxide (VO2) thin films have been successfully deposited by pulsed laser deposition without postannealing on (0001) and (1010) sapphire substrates. X‐ray diffraction reveals that the films are highly oriented with (010) planes parallel to the surface of the substrate. VO2 thin films on (0001) and (1010) substrates show semiconductor to metal transistions with electrical resistance changes as large as 4×104, 105, respectively. Thin films on (1010) substrate have a transition at as low as 55 °C with a hysteresis less than 1 °C. These transition properties are comparable with single crystal VO2.

Blue‐green luminescence from porous silicon carbide

Abstract: Porous silicon carbide has been fabricated using single crystal 6H‐SiC that has a wider indirect band gap than silicon crystal. Intense blue‐green luminescence has been observed at room temperature. The peak wavelength is around 460 nm, below the band gap of crystalline SiC. The luminescence intensity is about 100 times stronger than that of crystalline 6H‐SiC. These results not only clarify the origin of luminescence in porous Si but also point to the possibility of the use of this new material for an intense blue‐green luminescent source.

Crystal structures and cation distributions in simple spinels from powder XRD structural refinements: MgCr2O4, ZnCr2O4, Fe3O4 and the temperature dependence of the cation distribution in ZnAl2O4

Abstract: The crystal structure and cation distributions in the spinels MgCr2O4, ZnCr2O4, Fe3O4 and a suite of ZnAl2O4 samples annealed at 900 to 1400° C and then rapidly quenched, have been determined by powder X-ray diffraction, using several different X-ray procedures and both conventional structure-factor refinement and whole-pattern (or Rietveld) refinement methods. The chromite spinels are expected from crystal chemical considerations to have an almost completely normal cation distribution (inversion parameter, x, equal to zero). In agreement with this expectation, three samples of MgCr2O4 annealed at 900, 1100 and 1300° C, and ZnCr2O4 were all found to have x=0 within two estimated standard deviations (esd), suggesting that the accuracy with which cation distributions in spinels may be determined by powder XRD is close to the estimated precision. Slightly better results are obtained assuming neutral-atom scattering curves rather than half-ionized or fully ionized, but the differences are small (within the esd). The results from the Rietveld refinements are similarly in good agreement with those using the conventional structure factor refinement approach (agreement within the combined esd's), although in detail the Rietveld procedure sometimes produces small systematic differences in refined parameters. The suite of ZnAl2O4 spinels show a smooth increase in x from 0.01 at 900° C to 0.05 at 1300° C, and this behaviour is well described by the simple thermodynamic model for disordering in spinels with αZn-Al=89 kJ/mol, assuming β=−20 kJ/mol. The oxygen positional parameters for Fe3O4 are similar to those from published single crystal studies, indicating that the powder method also yields accurate interatomic distances in spinels.

New monolayer phases of n-alkane thiols self-assembled on Au(111): Preparation, surface characterization, and imaging

Abstract: We report the observation by scanning tunneling microscopy (STM) and low energy atom diffraction, of new, striped, structures at the surface of monolayers of n‐alkane thiols [CH3 (CH2)n−1 SH with n=8,10,12] self‐assembled on the (111) face of single crystal gold. These structures can be prepared by slow (room temperature) or thermally accelerated treatment of the well known c(4√3×2√3)R 30° phase formed by self‐assembly in solution, or can be accessed directly by molecular beam deposition. With respect to the unit mesh of the gold substrate, the new striped structures can be described as p×√3 overlayers where 7.5≤p≤13. The discovery of these phases has implications for the understanding of the growth mechanisms and the pursuit of applications of this widely studied class of materials.

Surface core-level shifts of some 4d-metal single-crystal surfaces: Experiments and ab-initio calculations

Abstract: High resolution measurements are reported of the surface core-level shift of the 3d level for the Rh(111), Rh(110), Pd(111), Pd(110), and Ag(111) single-crystal surfaces. These measurements and earlier ones for the Mo(110), Rh(100), and Pd(100) surfaces are analyzed by ab initio calculations of the surface core-level shift. The calculations are found to reproduce well the trends of the experimental shifts with the 4d metal and with the crystal plane. The comparison between these experimental and theoretical results demonstrates the importance of proper inclusion of final-state effects for accurate calculations of surface core-level shifts. A core hole in a surface atom is found to be better screened than one in a bulk atom for the 4d metals to the left of Pd in the Periodic Table. The use of the Z+1 approximation to describe the core hole is investigated both by explicit use of this approximation and by performing calculations for 1s and 3d core holes, respectively. The Z+1 approximation is found to be well obeyed in the case of Ag whereas for the rest of the 4d transition metals it is less precise, introducing errors of typically 0.1 eV.

Temperature Dependence of Band Gap Change in InN and AlN

Abstract: The optical band gap of InN and AlN single crystal films was measured through absorption spectra in the temperature range of 42 to 300 K The samples were grown on (0001)α-Al2O3 substrates by metalorganic vapor phase epitaxy The Varshni equation and the Bose-Einstein expression have been compared with the experimental results The nitride III-V compound semiconductors have smaller temperature dependence of band gap change In III-V compound semiconductors, the variation of the band gap change increases in the order of N, P and As

Optical properties of lattice defects in α-Al2O3

Abstract: Experimental optical properties of lattice vacancy defects in single crystal α-Al 2 O 3 are reviewed. Specific ultraviolet absorption and luminescence bands are associated with F-type centers. Following extended exposures to epithermal neutron sources, photobleaching and anisotropic properties allowed the association of several of the visible and near infrared bands with F 2 -type centers. Photoconductivity, photostability and charge state considerations provided tentative placement of defect energy levels with respect to the conduction band edge.

XPS and STM Investigation of the Passive Film Formed on Cr(110) Single‐Crystal Surfaces

Abstract: A combined ex situ x‐ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) investigation of the passive films formed on Cr(110) single‐crystal surfaces in 0.5M is reported. The composition, thickness, and structure of the passive films were studied as a function of polarization time and potential in the passive region. XPS measurements evidence the formation of a hydroxide layer of trivalent chromium in the outer part of the film, and of a layer of oxide of trivalent chromium in the inner part. Some amount of oxyhydroxide may be present in the inner part of the film. Aging of the passive film under polarization is critical for the development of the oxide inner part. The surface topography, assigned to the hydroxide layer, is characterized by disordered protrusions of 1 to 4 nm lateral dimensions and 4 to 8 A vertical dimensions. Ordered domains of limited extension (≤3 nm), assigned to chromium oxide, are detected, showing the nanocrystalline character of the inner part of the passive film. These nanocrystals are separated by areas in which no order could be found on the surface and where no crystalline defects could be detected. The relationship between the measured structure of the passive film and the resistance to breakdown is discussed.

Hetero-epitaxially grown compound semiconductor substrate

Abstract: A method of growing a gallium arsenide single crystal layer on a silicon substrate comprises steps of growing a buffer layer of aluminium arsenide on the silicon substrate by atomic layer epitaxy, and growing the gallium arsenide single crystal layer on the buffer layer epitaxially.

GaN single crystal

Abstract: A GaN single crystal having a full width at half-maximum of the double-crystal X-ray rocking curve of 5-250 sec and a thickness of not less than 80 μm, a method for producing the GaN single crystal having superior quality and sufficient thickness permitting its use as a substrate and a semiconductor light emitting element having high luminance and high reliability, comprising, as a substrate, the GaN single crystal having superior quality and/or sufficient thickness permitting its use as a substrate.

Coupling between adjacent crystal planes in heterogeneous catalysis by propagating reaction–diffusion waves

Abstract: UNDERSTANDING of the mechanisms and kinetics of heterogeneous catalytic reactions has come largely from the study of gas–solid interactions on well defined single-crystal surfaces1,2. But real catalysts usually consist of nanometre-sized particles on which several different crystal planes are exposed. In general, it has been assumed that their properties can be regarded as a superposition of the contributions from each individual structural element. Here we show that this assumption may be invalid, even qualitatively, in certain cases. We have studied the oxidation of hydrogen on platinum surfaces at low pressure and room temperature. On a macroscopic Pt(lOO) single crystal the reaction reaches a steady state with a uniform distribution of adsorbates. But on the platinum tip of a field ion microscope, on which several different crystal planes are exposed, the reaction has a very different character. The tip contains a region of the (100) plane just 40 nm in diameter, on which the reaction rate displays sustained temporal oscillations. This effect is associated with continuously changing distributions of the adsorbed species in the form of propagating waves, which are generated by coupling of reactions occurring on adjacent crystal planes. This kind of interaction between different crystal planes may exert a profound influence on the kinetics of heterogeneous catalysis.

Single molecule lines and spectral hole burning of terrylene in different matrices

Abstract: We observe fluorescence excitation lines of single terrylene molecules in three new polymer matrices (polyvinylbutyral, polymethylmethacrylate, and polystyrene) and in two crystals, n‐hexadecane (polycrystalline Shpol’skii matrix) and anthracene single crystal. We also burn persistent spectral holes in bulk samples of these solutions for comparison to single molecule lines. In all matrices where hole burning is efficient enough, we find good agreement between the average width determined from the distribution of single molecules’ linewidths and the homogeneous width deduced from spectral holes, which demonstrates the consistency and complementarity of the two techniques.

Structural, optical, and electrical properties of epitaxial chalcopyrite CuIn3Se5 films

Abstract: Single crystal CuIn3Se5 epitaxial films have been synthesized on GaAs(001) by a hybrid sputtering and evaporation technique The microstructure, microchemistry, and selected electrical and optical properties of the films have been investigated by scanning electron microscopy, energy dispersive x‐ray spectroscopy, transmission electron microscopy, cathodoluminescence, optical absorption and reflection, and four‐point probe resistivity measurements The results showed that the CuIn3Se5 crystals have an ordered point defect structure, a band gap of ≥118 eV, an optical absorption coefficient of about 15 000 cm−1 at a photon energy of 135 eV, and a film resistivity of ≳105 Ω cm The results suggest the presence of band tails giving rise to subgap radiative recombination and absorption Antiphase domain boundaries, stacking faults, and nanotwins were observed in the epitaxial layers and were reduced in number by rapid thermal annealing

Investigation of the structure of barium titanate thin films by Raman spectroscopy

Abstract: Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal‐organic chemical vapor deposition (MOCVD) and laser‐assisted deposition The spectra were compared with the spectra of a ceramic specimen and a single crystal Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films Other Raman peaks were ascribed to impurity (non‐BaTiO3) phases in the films or to the substrates (fused quartz, MgO) Some of the Raman peaks showed a strong polarization dependence The MOCVD films were also characterized by x‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and transmission electron microscopy The film‐to‐film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity‐phase features, was qualitatively consistent with the x‐ray diffraction and electron microscopy results Spatially resolved Raman measurements showed that the structure of the laser‐deposited film varies significantly over the deposited area The temperature dependencies of the Raman spectra of two MOCVD films were examined in the 25–175 °C range Raman peaks due to the tetragonal phase of BaTiO3 were observed at temperatures well above the Curie temperature of bulk single‐crystal BaTiO3 (132 °C) This observation suggests that the tetragonal ferroelectric phase is stabilized by an anisotropic film‐substrate interaction that gives rise to a two‐dimensional stress in the plane of the film

Method of making a single crystals Ga*N article

Abstract: A method of making a single crystal Ga*N article, including the steps of: providing a substrate of crystalline material having a surface which is epitaxially compatible with Ga*N; depositing a layer of single crystal Ga*N over the surface of the substrate; and etchably removing the substrate from the layer of single crystal Ga*N, to yield the layer of single crystal Ga*N as said single crystal Ga*N article. The invention in an article aspect relates to bulk single crystal Ga*N articles, such as are suitable for use as a substrate for the fabrication of microelectronic structures thereon, and to microelectronic devices comprising bulk single crystal Ga*N substrates, and their precursor structures.

Ferroelectric properties of pyridinium tetrafluoroborate.

Abstract: The differential thermal analysis of a single crystal of pyridinium tetrafluoroborate was performed revealing two solid-solid phase transitions at 205 and 240 K. The real part of the dielectric permittivity was measured between 190 and 290 K. The large anomaly at the higher phase transition is interpreted as the Curie point of a new ferroelectric crystal.

Electrically isolated released microstructures

Abstract: A single mask, low temperature reactive ion etching process for fabricating high aspect ratio, released single crystal microelectromechanical structures independently of crystal orientation.

First observation of' amorphization in single-crystal MgA12O4 spinel

Abstract: We have observed the amorphization of single-crystal MgA12O4 spinel induced by 400 keV Xe2+ -ion irradiation at 100K to a peak damage level of 25 displacements per atom (dpa). Rutherford backscattering spectrometry combined with ion channelling showed that the amount of radiation damage accumulated with increasing dose and reached a fully random level at a dose of 1 × 1016Xecm−2. Cross-sectional transmission electron microscopy examination of the high-dose sample revealed the presence of a continuous amorphous layer of about 130nm, followed by a radiation-damaged but crystalline transition layer on the undamaged spinel substrate. The irradiation-damaged crystalline layer is highly defected, and electron diffraction from this region exhibits a decrease in or complete loss of intensity at the spinel (220) reciprocal-lattice positions. These observations suggest that the principal driving force in the amorphization of MgA12O4 spinel is through the accumulation of chemical disorder (antisite defects)...

Single crystal cleavage of brittle materials

Abstract: Cleavage of brittle single crystals is reviewed and the historical criteria for the phenomenon are critically examined. Previously proposed criteria, including those based on crystal structure (crystal growth planes, the planes bounding the unit cell, and planar atomic packing) and crystal properties (ionic charge of possible cleavage planes, bond density, elastic modulus, and surface free energy), are found to be applicable only to particular crystals or to isostructural groups, but each lacks universal application. It is concluded that the fracture toughness (K Ic) of the crystallographic planes is the most appropriate criterion. Measurements reveal that the ‘cleavage toughnesses’ of brittle single crystals are usually about 1 MPa m1/2 or less. Measurements of the fracture toughnesses of brittle polycrystalline aggregates are then compared to the single crystal cleavage values in those instances where reliable results are available for the same crystal structures. Polycrystalline toughnesses are consistently higher, in part because of the lack of continuity of cleavage cracks through the polycrystalline aggregates. However, the increment of toughness increase is only 1–2 MPa m1/2. The role of grain texture or preferred crystal orientation is also addressed. It is concluded that polycrystalline aggregate toughnesses are often highly anisotropic and that the values for intensely oriented microstructures may approach those for single crystal cleavage.

Method of fabricating single crystal silicon arrayed devices for display panels

Abstract: A display panel is formed using essentially single crystal thin-film material that is transferred to substrates for display fabrication. Pixel arrays form light valves or switches that can be fabricated with control electronics in the thin-film material prior to transfer. The resulting circuit panel is than incorporated into a display panel with a light emitting or liquid crystal material to provide the desired display.

Second harmonic generation in transparent barium borate glass-ceramics

Abstract: s-BaB2O4 (BBO) has been considered to be an excellent non-linear optical material in single crystal form; however, BaO·B2O3 can also form a glass. BaO·B2O3 glass has a density of about 3.9 g/cm3, a glass transition around 525°C, an onset of crystallization around 560°C, a maximum crystallization at 585°C and a refractive index of 1.59. Optically transparent BaO·B2O3 glass-ceramics have been successfully prepared by controlled crystallization. Formation of submicrometer, spherical crystallites of s-BaB2O4 within the glass was verified by X-ray diffraction and scanning electron microscopy. These glass-ceramics exhibit a UV cutoff at 220 nm, a refractive index of 1.61 and second harmonic generation efficiency 0.01–0.12 times that of urea.