Showing papers on "Single crystal published in 1993"

Single crystal metals encapsulated in carbon nanoparticles.

Abstract: Single-domain microcrystals of LaC2 encapsulated within nanoscale polyhedral carbon particles have been synthesized in a carbon arc. Typical particle sizes are on the order of 20 to 40 nanometers. The stoichiometry and phase of the La-containing crystals have been assigned from characteristic lattice spacings observed by high-resolution transmission electron microscopy and energy dispersive spectroscopy (EDS). EDS spectra show that La and C are the only elements present. Characteristic interatomic distances of 3.39 and 2.78 angstroms identify the compound inside the nanoparticle cavities as α-LaC2, the phase of LaC2 that is stable at room temperature. Bulk α-LaC2 is metallic and hydrolytic. Observation of crystals of pure encapsulated α-LaC2 that were exposed to air for several days before analysis indicates that the LaC2 is protected from degradation bythe carbon polyhedral shells of the nanoparticles. A high percentage of the carbon nanoparticles have encapsulated LaC2 single crystals. These carbon-coated metal crystals form a new class of materials that can be protected in their pure or carbide forms and may have interesting and useful properties.

Stress Measurement in Single-Crystal and Polycrystalline Ceramics Using Their Optical Fluorescence

Abstract: A general methodology is developed for determining the state of stress and the numerical value of the stresses from observed shifts and broadening of optical fluorescence lines. The method is based on the piezospectroscopic properties of single crystals. We present general relationships between the measured fluorescence shifts and the stress state for a number of illustrative cases, pertinent to both polycrystalline and single-crystal ceramics under stress. These include measuring the stresses applied to polycrystalline ceramics, the residual stress distribution due to crystallographically anisotropic thermal expansion, and the stresses applied to single crystals. Using the recently implemented technique of performing the fluorescence measurements in an optical microprobe, we also provide experimental tests of the relationships derived.

Direct observation of magnetic flux lattice melting and decomposition in the high-Tc superconductor Bi2.15Sr1.95CaCu2O8+x

Abstract: The flux line lattice inside a single crystal of Bi2.15Sr1.95CaCu2O8+x has been observed using small-angle neutron diffraction. The diffracted intensity goes rapidly to zero at a magnetic-field-dependent flux lattice melting temperature; this melting coincides with the appearance of finite resistance within the superconducting state. The flux lattice signal can also be made to disappear at low temperatures, by applying a sufficiently high field, probably because of the decomposition of flux lines into two-dimensional 'pancake' vortices.

Semiconductors for Solar Cells

Abstract: Physical principles of photovoltaic energy conversion technology of solar cell devices fundamental material parameters structural and electrical properties of lattice defects single crystal and polycrystalline silicon single crystal and epitaxial compound semiconductors thin-film compound semiconductors amorphous thin-film semiconductors.

Anomalous peak effect in single crystal Bi2Sr2CaCu2O8+y studied by Hall probe magnetometry

Abstract: We have studied an anomalous peak effect in a Bi 2 Sr 2 CaCu 2 O 8+ y single crystal using a specially designed Hall probe magnetometer. In order to investigate the origin of the anomalous peak effect which manifests itself in the magnetic hysteresis loop, the relaxation (10 -2 s t 4 s), and the local field imaging experiments were performed. Decoupling of 2D vortices in neighboring CuO layers, namely, a dimensional crossover in the pinning mechanism, is proposed as a possible mechanism of the peak effect.

Method of fabricating a gallium nitride based semiconductor device with an aluminum and nitrogen containing intermediate layer

Abstract: Disclosed are a gallium nitride type semiconductor device that has a single crystal of (Ga 1-x Al x ) 1-y In y N, which suppresses the occurrence of crystal defects and thus has very high crystallization and considerably excellent flatness, and a method of fabricating the same. The gallium nitride type semiconductor device comprises a silicon substrate, an intermediate layer consisting of a compound containing at least aluminum and nitrogen and formed on the silicon substrate, and a crystal layer of (Ga 1-x Al x ) 1-y In y N (0≦x≧1, 0≦y≦1, excluding the case of x=1 and y=0). According to the method of fabricating a gallium nitride base semiconductor device, a silicon single crystal substrate is kept at a temperature of 400° to 1300° C. and is held in the atmosphere where a metaloganic compound containing at least aluminum and a nitrogen-containing compound are present to form a thin intermediate layer containing at least aluminum and nitrogen on a part or the entirety of the surface of the single crystal substrate, and then at least one layer or multiple layers of a single crystal of (Ga 1-x Al x ).sub. 1-y In y N are formed on the intermediate layer.

Substrate dependence of the surface structure and chain packing of docosyl mercaptan self‐assembled on the (111), (110), and (100) faces of single crystal gold

Abstract: Low‐energy helium diffraction has been used to study the surface structure, chain packing, and thermal vibrations of docosyl mercaptan [CH3(CH2)21SH] self‐assembled on single crystal Au(111), (110), and (100). The docosyl mercaptan molecules form monolayers with different periodicity on the different surfaces of gold. On Au(111) at low temperatures (≤100 K), the terminal methyl groups of the docosyl mercaptan molecules form domains of a hexagonal lattice with a unit mesh constant of 5.01±0.02 A. The sulfur head groups are arranged in a commensurate (√3×√3)R30° structure and are believed to adsorb on the triple hollow sites of the Au(111) lattice. The unit mesh parameters for CH3(CH2)21S/Au(110) are a=b=4.99±0.08 A and α=109.5°, suggesting that the chemisorbed sulfur atoms remove the ‘‘missing row’’ reconstruction of the Au(110) surface and form a commensurate c(2×2) lattice. The adsorption of docosyl mercaptan molecules on a Au(100) surface results in a complicated diffraction pattern. Analysis of the data reveals an oblique unit mesh with a=b=5.97±0.09 A and α=95°±5° with four kinds of equivalent domains present because of the fourfold symmetry of Au(100). The above results confirm that the sulfur–substrate interaction plays an important role in determining the periodicity and the packing density of the molecules within the monolayers. The estimated average domain size of the terminal methyl groups is 22.8, 38.6, and 23.4 A for CH3(CH2)21SH self‐assembled on Au(111), (110), and (100) faces, respectively. The chain packing and orientation within the unit cell are also discussed in this paper in conjunction with the latest results obtained via other techniques such as reflection IR spectroscopy and low‐energy electron diffraction.

Equation of state and phase diagram of solid 4He from single-crystal x-ray diffraction over a large P-T domain.

Abstract: Single-crystal x-ray diffraction has been performed on solid $^{4}\mathrm{He}$ from 1 to 58 GPa over the temperature range 46--400 K. The high-density properties of helium are pinned down: the hcp structure is stable apart from an fcc loop along melting in between 15 K and around 285 K; comparison of the 304 K hcp equation of state with a series of calculations demonstrates there are significant attractive many-body interactions; and the thermal pressure has been measured between 95 and 304 K and it differs from self-consistent phonon calculations.

Structure of the superconducting La 2 CuO 4 + δ phases (δ≊0.08,0.12) prepared by electrochemical oxidation

Abstract: Two powder samples of electrochemically oxidized La[sub 2]CuO[sub 4+[delta]] (nominally [delta][approx]0.08 and 0.12) and one single crystal ([delta][approx]0.1), with superconducting critical temperatures of 32, 42, and 40 K, respectively, were studied using neutron diffraction. All samples appear to be single phase, both at room temperature and at low temperature (10--18 K), as evidenced by sharp Bragg peaks, indicating that these samples have compositions beyond the phase-separated region of the phase diagram. A detailed analysis of the Bragg reflections demonstrated that the basic crystallographic structure of all samples has [ital Fmmm] symmetry, with the excess oxygen located between adjacent LaO layers. However, a number of low-intensity peaks in the powder data suggested the existence of a very large superstructure. The satellites could be clearly identified in the single-crystal data, allowing the propagation vectors of the modulation to be determined. Rietveld refinements of the average structure, based on the main Bragg peaks, are presented here for samples prepared with this technique.

A double-temperature-gradient technique for the growth of single-crystal fullerites from the vapor phase

Abstract: Fullerite single crystals were prepared by a sublimation-condensation method in a closed evacuated glass tube situated in a double-temperature-gradient furnace. Crystals of various size and up to 9 mg weight with well expressed smooth and shiny faces were obtained. X-ray analysis, interfacial angle measurements and observed morphological habits of selected crystals of C60 confirm the theoretically predicted and experimentally well established fcc structure at room temperature with two types of morphological faces, namely {111} and {100}. A strong tendency to twinning was observed. In the case of C70 crystals, a pure fcc structure was observed. Information on growth kinetics and on instability versus exposure to air and light were obtained from surface studies. Characteristic changes in a thin surface layer were observed when crystals were exposed to air and light. A new phase of C60 stabilized by oxygen was characterized.

Electron mobility in single- and polycrystalline Ga2O3

Abstract: Ga2O3 is a purely n‐conducting metal oxide at high temperatures. Increasing interest is being shown in this material as the basic material for gas sensors since its electrical conductivity is gas dependent. To investigate its electrical conductivity mechanism in the temperature range between 800 and 1000 °C, conductivity and Hall measurements were performed on single crystals and on polycrystalline ceramics. In the conductivity measurements, identical results were obtained with dc and low‐frequency ac, thus confirming the notion of purely electron current transport. A thermally activated specific conductivity is observed in both cases, that of the single crystals lying around three and a half orders of magnitude above that of the ceramic. The carrier densities are determined from the Hall measurements, a thermally activated behavior always being observed. The carrier density of the ceramic is only 2×1013 cm−3 at 1000 °C, that of the single crystals lying somewhat more than three orders of magnitude above ...

High resolution neutron powder diffraction: a case study of the structure of C60

Abstract: High resolution time-of-flight neutron powder diffraction has been used to determine the detailed structure of C$\_{60}$ as a function of temperature. Rapid data collection coupled with high resolution has enabled subtle aspects of the 86 K orientational glass transition and precursor effects of the 260 K order-disorder transition to be observed. This surveying capability complements traditional single crystal methods. The power of the Rietveld method of profile refinement is demonstrated in the elucidation of the detailed crystal structure of the orientationally-ordered low temperature phase and in the evaluation of the departure from isotropic scattering of the C$\_{60}$ molecule in the disordered high temperature phase. The counter-intuitive success in obtaining high-order cubic-harmonic coefficients, albeit to poorer precision than single crystal X-ray measurements, confirms the efficacy of the Rietveld profile refinement method. The collapse of three dimensions of diffraction information on to the one dimension of a high resolution powder diffraction pattern can still lead to an impressive amount of structural information that substantiates the assertion made by W. H. Bragg `the second method [powder diffraction], first used independently by Debye and Hull, can be used when the crystal is in powder, and can, therefore be employed when no single crystal can be obtained of sufficient size. All the spectra of the different planes are thrown together on the same diagram or photograph, and must be disentangled. This is not as difficult as it may seem...'.

Low differential stress and controlled chemical environment in multianvil high-pressure experiments

Abstract: Studies of mass transport and kinetics in minerals at high pressure often require a sample environment in which the stress is near-hydrostatic and the chemical environment is carefully controlled. We report here details of a multianvil sample assembly in which these requirements are fulfilled and which has been used to study the effect of pressure on the kinetics of dislocation recovery in olivine up to 10 GPa. Annealing experiments have been performed on single crystals of San Carlos olivine at 8.5 GPa and 1400° C in a 1200 tonne split-sphere multianvil apparatus. The sample assembly consists of an 18 mm MgO octahedron with a LaCrO3 heater of variable wall thickness to give a small temperature variation (∼20° C) along the 3 mm length of the sample capsule. To minimize the differential stress on the sample, the olivine single crystal is surrounded by NaCl and both pressurization and depressurization are performed slowly at a temperature of 600° C (to minimize the strength of the NaCl). The silica activity \(({\text{a}}_{{\text{SiO}}_{\text{2}} } )\) is buffered by orthopyroxene powder in contact with the olivine and the oxygen fugacity is buffered by Ni + NiO within the sample capsule. The H2O-content of the sample assembly is minimized by drying all components at 230° C under vacuum. Olivine single crystals recovered after annealing at 1400° C and 8.5 GPa show no evidence of deformation, either ductile or brittle. Dislocation densities of 109–1010 m-2 are similar to those observed prior to high-pressure annealing and indicate differential stresses of <10 MPa. Infrared spectroscopy indicates that the hydrogen content of a sample annealed at 10 GPa, 1500° C for 21 h is ∼13 H/106Si, which, although low, is higher than that of the crystals prior to high-pressure annealing. Finally, the effectiveness of the fO2 buffer has been verified by estimating the fO2 at the surface of the sample from the solubility of Fe in Pt metal in equilibrium with the olivine and orthopyroxene.

Raman scattering from PbTiO3 thin films prepared on silicon substrates by radio frequency sputtering and thermal treatment

Abstract: Raman scattering has been carried out on PbTiO3 thin films prepared on platinum‐coated (100) silicon by radio‐frequency (rf)‐magnetron sputtering without substrate heating and a post‐deposition thermal treatment. The Raman spectra obtained from the thin film are characteristic of powder Raman spectra: In comparison with the single crystal spectra, the intensity of the background is relatively high at low frequencies and the Raman lines are broad. The lattice phonon modes corresponding to the observed lines are identified by comparison with the data on single crystals and powder. The Raman frequencies for the thin film remarkably shift to low frequencies compared with single‐crystal data. It is shown that the phenomenon of the frequency shifts is similar with the hydrostatic pressure effect on single crystals of PbTiO3. The result indicates that the thin films are composed of grains that are stressed depending on the grain size by neighboring grains of different orientations when they are split up into ferroelectric domains at the paraelectric‐to‐ferroelectric transition. This stress effect is significant even for a grain size of ∼0.5 μm. It is found that the lowest frequency E transverse optical (TO) mode in the thin film shows softening with increasing temperature as was reported in previous studies on single crystals.

Microscopy and microindentation mechanics of single crystal Fe-3 wt. % Si: Part I. Atomic force microscopy of a small indentation

Abstract: Atomic force microscope measurements of elastic-plastic indentation into an Fe−3 wt. % Si single crystal showed that the volume displaced to the surface is nearly equal to the volume of the cavity. The surface displacement profiles and plastic zone size caused by a 69 nm penetration of a Vickers diamond tip are reasonably represented by an elastic-plastic continuum model. Invoking conservation of volume, estimates of the number of dislocations emanating from the free surface are reasonably consistent with the number of dislocations that have formed in the plastic zone to represent an average calculated plastic strain of 0.044.

Method for the manufacture of large single crystals

Abstract: A method is disclosed for producing large single crystals. In one embodiment, a single crystal of electronic grade diamond is produced having a thickness of approximately 100-1000 microns and an area of substantially greater than 1 cm2. and having a high crystalline perfection which can be used in electronic, optical, mechanical and other applications. A single crystalline diamond layer is first deposited onto a master seed crystal and the resulting diamond layers can be separated from the seed crystal by physical, mechanical and chemical means. The original master seed can be restored by epitaxial growth for repetitive use as seed crystal in subsequent operations. Large master single crystal diamond seed can be generated by a combination of oriented smaller seed crystals by lateral epitaxial fusion. Since there is no limit to how many times seed combination step can be repeated, large diamond freestanding wafers comparable in size to silicon wafers can be manufactured.