Showing papers on "Single crystal published in 1990"

Anisotropic Etching of Crystalline Silicon in Alkaline Solutions I . Orientation Dependence and Behavior of Passivation Layers

Abstract: The anisotropic etching behavior of single‐crystal silicon and the behavior of and in an ethylenediaminebased solution as well as in aqueous , , and were studied. The crystal planes bounding the etch front and their etch rates were determined as a function of temperature, crystal orientation, and etchant composition. A correlation was found between the etch rates and their activation energies, with slowly etching crystal surfaces exhibiting higher activation energies and vice versa. For highly concentrated solutions, a decrease of the etch rate with the fourth power of the water concentration was observed. Based on these results, an electrochemical model is proposed, describing the anisotropic etching behavior of silicon in all alkaline solutions. In an oxidation step, four hydroxide ions react with one surface silicon atom, leading to the injection of four electrons into the conduction band. These electrons stay localized near the crystal surface due to the presence of a space charge layer. The reaction is accompanied by the breaking of the backbonds, which requires the thermal excitation of the respective surface state electrons into the conduction band. This step is considered to be rate limiting. In a reduction step, the injected electrons react with water molecules to form new hydroxide ions and hydrogen. It is assumed that these hydroxide ions generated at the silicon surface are consumed in the oxidation reaction rather than those from the bulk electrolyte, since the latter are kept away from the crystal by the repellent force of the negative surface charge. According to this model, monosilicic acid is formed as the primary dissolution product in all anisotropic silicon etchants. The anisotropic behavior is due to small differences of the energy levels of the backbond surface states as a function of the crystal orientation.

Mounting of crystals for macromolecular crystallography in a free-standing thin film

Abstract: A method for mounting single crystals in macromolecular crystallographic studies is described in which the crystal is suspended in a thin film. The film is formed from a mixture of the crystallization buffer and a hydrophilic viscous material, confined within a thin-wire loop by surface tension. Compared with conventional crystal mounting methods, this method greatly simplifies and speeds the mounting procedure, is well suited to shock freezing and to optical monitoring of the crystals, deforms fragile crystals less and gives a lower and more uniform background in the X-ray diffraction patterns.

Intercalation of sea urchin proteins in calcite: study of a crystalline composite material.

Abstract: Sea urchin skeletal elements are composed of single crystals of calcite. Unlike their synthetic counterparts, these crystals do not have well-developed cleavage and are consequently much more resistant to fracture. This phenomenon is due in part to the presence of acidic glycoproteins occluded within the crystals. By means of x-ray diffraction with synchrotron radiation, it is shown that the presence of the protein in synthetic calcite only slightly decreases the coherence length but significantly increases the angular spread of perfect domains of the crystals. In biogenic calcite, the coherence length is 1/3 to 1/4 as much as that in synthetic calcite and the angular spread is 20 to 50 times as wide. It is proposed that the presence of macromolecules concentrated at mosaic boundaries that are oblique to deavage planes is responsible for the change in fracture properties. These results may be important in the material sciences, because of the unusual nature of this material, namely, a composite based on the controlled intercalation of macromolecules inside single-crystal lattices.

Formation of Ultrathin Single-Crystal Silicide Films on Si: Surface and Interfacial Stabilization of Si-NiSi2Epitaxial Structures

Abstract: Continuous and planar single-crystal NiSi2films (<60 A thick) have been grown on Si(100) and (111) by ultrahigh-vacuum techniques. Nickel deposition (<20 A) on atomically clean Si at room temperature, followed by low-temperature heating at ~450°C, results in the growth of epitaxial NiSi2 films whose structure depends critically on the starting Ni thickness. The films are coherent, without misfit dislocations. The orientation of the (111) silicide can be controlled by the initial Ni thickness.

On the nature of the oxygen-related defect in aluminum nitride

Abstract: The oxygen-related defect in an aluminum nitride (AIN) single crystal and in polycrystalline ceramics is investigated utilizing photoluminescence spectroscopy, thermal conductivity measurements, x-ray diffraction lattice parameter measurements, and transmission electron microscopy. The results of these measurements indicate that at oxygen concentrations near 0.75 at.%, a transition in the oxygen accommodating defect occurs. On both sides of this transition, simple structural models for the oxygen defect are proposed and shown to be in good agreement with the thermal conductivity and lattice parameter measurements, and to be consistent with the formation of various extended defects (e.g., inversion domain boundaries) at higher oxygen concentrations.

Memory Glass: An Amorphous Material Formed from AlPO4

Abstract: A glass exhibiting structural memory has been produced through the compression of a single crystal of AlPO4 berlinite to 18 gigapascals at 300 kelvin. The unique and extraordinary characteristic of this glass is that upon decompression below 5 gigapascals it transforms back into a single crystal with the same orientation as the starting crystal. This glass has a "memory" of the previous crystallographic orientation of the crystal from which it forms.

The instability of polycrystalline thin films: Experiment and theory

Abstract: Dense polycrystalline thin films of ZrO2 (3 and 8 mol % Y2O3) were produced by the pyrolysis of zirconium acetate precursor films, which were deposited on single crystal Al2O3 substrates by spin-coating aqueous solutions of zirconium acetate and yttrium nitrate. Dense films were heat treated to encourage grain growth. With grain growth, these films broke into islands of ZrO2 grains. Identical areas were examined after each heat treatment to determine the mechanism that causes the polycrystalline film to uncover the substrate. Two mechanisms were detailed: (a) for a composition which inhibited grain growth and produced a polycrystalline film with very small grains, the smallest grains would disappear to uncover the substrate, and (b) for a composition which did not inhibit grain boundary motion, larger grains grew by enveloping a smaller grain and then developed more spherical surface morphologies, uncovering the substrate at three grain junctions. In both cases, the breakup phenomenon occurred when the average grain size was larger than the film thickness. Thermodynamic calculations show that this breakup lowers the free energy of the system when the grain-size-to-film-thickness ratio exceeds a critical value. These calculations also predict the conditions needed for polycrystalline thin film stability.

Growth and properties of oxygen- and ion-doped Bi 2 Sr 2 CaCu 2 O 8+δ single crystals

Abstract: A directional solidification method for growing large single crystals in the ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ system is reported. Ion doping, with replacement of La for Sr and Y for Ca, as well as oxygen doping in these crystals has been explored. Doped and undoped crystals have been characterized using microprobe analysis, x-ray diffraction, thermogravimetric analysis, and magnetic and Hall measurements. Ion doping results in little change of the superconducting transition for substitution levels below 20--25%, while beyond this level the Meissner signal broadens and the low-temperature Meissner signal decreases. Microprobe analysis and x-ray diffraction performed on these more highly substituted single crystals provide evidence for inhomogeneity and phase segregation into regions of distinct composition. Annealing unsubstituted crystals in increasing partial pressures of oxygen reversibly depresses the superconducting transition temperature from 90 (as made) to 77 K (oxygen pressure annealed), while the carrier concentrations, as determined from Hall effect measurements, increase from n=3.1(3)\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (0.34 holes per Cu site) to 4.6(3)\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (0.50 holes per Cu site).No degradation of the Meissner transition or other indications of inhomogeneity or phase segregation with doping are noted, suggesting that oxygen-doped ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ is a suitable system for pursuing doping studies. The decrease in ${\mathit{T}}_{\mathit{c}}$ with concentration for 0.34\ensuremath{\le}n\ensuremath{\le}0.50 indicates that a high-carrier-concentration regime exists in which ${\mathit{T}}_{\mathit{c}}$ decreases with n and suggests that this decrease does not arise from material inhomogeneity or other materials problems. An examination of the variation of ${\mathit{T}}_{\mathit{c}}$ with the density of states and lattice constants for all of the doped and undoped superconducting samples considered here indicates that changes in ${\mathit{T}}_{\mathit{c}}$ with doping are primarily affected by changes in the density of states (or carrier concentration) rather than by structural variation induced by the doping.

Liquid crystal display having essentially single crystal transistors pixels and driving circuits

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

Growth and Luminescence Properties of Mg‐Doped GaN Prepared by MOVPE

Abstract: Growth and luminescence properties of Mg‐doped prepared by metal‐organic vapor phase epitaxy, in which or is used as the Mg source gas, are reported for the first time. It was found that the Mg concentration in is proportional to the flow rate of the Mg source gas; the doping efficiency of Mg into is independent of the substrate temperature from 850° to 1040°C; and Mg in acts as an acceptor and forms blue luminescence centers. By using, an efficient near‐UV and blue LED can be fabricated.

The formation of ordered ω-related phases in alloys of composition Ti4Al3Nb

Abstract: During cooling of an alloy of composition Ti4Al3Nb from a B2 phase field above 1100°C, a metastable trigonal (P3m1) ω-related phase, designated ω″, forms along with small amounts of D019 and L10 phases. The ω″ phase exhibits partial collapse of 111 planes and reordering relative to its B2 parent. An apparently equilibrium low temperature phase with the B82 structure was found after 26 days of annealing at 700°C. Both ω″ and B82 structures were verified by means of transmission electron microscopy and by single crystal X-ray diffraction. The latter permitted detailed analysis of the collapse parameters and site occupancies. The observed transformation path, B2(Pm3m)→ω″(P3m1)→B82 (P63/mmc), occurs in two steps. The first involveds a subgroup transition during cooling that is primarily displacive with reordering consistent with the trigonal symmetry imposed by the ω-collapse. The second involves a supergroup transition during prolonged annealing that is primarily replacive and constitutes a chemical disordering. The direct equilibrium transformation, B2→B82, without the formation of an intermediate trigonal phase, can only occur by a reconstructive transformation.

Oxidation of Single‐Crystal Silicon Carbide Part I . Experimental Studies

Abstract: The oxidation of single crystal in dry oxygen (10−3–1 atm and 1200°–1500°C) followed parabolic kinetics. Two different apparent activation energies were calculated for oxidation of the (0001) C faces of , approximately 120 kJ/mol below 1350°C and 260 kJ/mol above 1350°C. Two regimes were not apparent for oxidation of the (0001) Si faces, and apparent activation energies lay between 223 and 298 kJ/mol. Double oxidation experiments using and indicated that the process is dominated by the transport of molecular oxygen at lower temperatures (<1300°C) with a substantial contribution from diffusion of ionic oxygen at higher temperatures. Epitaxially grown Si 13C films on alpha‐Si 12C substrates via CVD were used to study carbon transport behavior during oxidation of . Depth profiles for carbonaceous species using SIMS showed that carbon can transport quickly through the oxide layer, which eliminates the possibility that transport of carbonaceous species is rate controlling in the oxidation of . Oxidation mechanisms of are discussed on the basis of these results.

Enhanced hardness in lattice‐matched single‐crystal TiN/V0.6Nb0.4N superlattices

Abstract: Single‐crystal TiN/V0.6Nb0.4N superlattice films were deposited on MgO(100) substrates using ultrahigh vacuum reactive sputtering from two magnetron sources with a computer‐controlled shutter. X‐ray diffractometer measurements showed first‐ and second‐order superlattice peaks while electron diffraction showed up to seventh‐order superlattice reflections. Direct observations using cross‐sectional transmission electron microscopy showed that the superlattices had flat, abrupt interfaces, but contained threading dislocations. Microhardness values for the superlattices ranged from to 3100 to 4100 kg/mm2, depending on the superlattice period (λ), compared with 1500 for an alloy with equal average composition. The hardness values for these lattice‐matched superlattices agreed with those previously reported for TiN/VN superlattices with a lattice mismatch of 2.4%, although the hardness peak was less pronounced. The present results indicate that coherency strains do not play the major role in enhancing the hardne...

Optical properties of reduced lithium niobate single crystals

Abstract: The optical transmission of LiNbO3 single crystals has been measured in the wavelength range 200–900 nm, for different degrees of reduction, to study the effect of reduction on the optical characteristics of LiNbO3 near the fundamental absorption edge. The optical transitions in LiNbO3 were found to be indirect and the band gap decreased with increasing degree of reduction. The band observed at 2.48 eV in the absorption spectrum in heavily reduced samples has been attributed to the formation of polarons, and the theoretical model of Reik and Heese [J. Chem. Solids 28, 581 (1967)] for small polarons is used to correlate the optical and electrical properties.

Raman study of CuO single crystals.

Abstract: Raman scattering in single-crystalline CuO samples has been studied. From the polarization dependence, the symmetries of the three Raman-active optical phonons have been identified. The ${\mathit{A}}_{\mathit{g}}$ Raman mode with frequency of 290 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was found to be strongly polarized along one of the crystal axes. This suggests that there are cancellations between different components of its Raman tensor.

Detailed in situ scanning tunneling microscopy of single crystal planes of gold(111) in aqueous solutions

Abstract: An in situ electrochemical scanning tunneling microscope (ESTM) was applied to a gold(111) surface with atomically flat terraces in aqueous perchloric acid solutions The pits, a single layer deep, were formed during the electrochemical reduction of oxide layers with higher oxidation states The surface diffusion of gold atoms in a pure HClO{sub 4} solution was quite slow and comparable to that observed in ultrahigh vacuum (UHV) reported in the previous literature An electrochemical dissolution of gold was investigated in a HClO{sub 4} solution in the presence of chloride ion Strongly adsorbed chloride ions on gold(111) remarkably enhanced the surface diffusion of gold

Elastic constants of MoSi2 and WSi2 single crystals

Abstract: Single crystals of MoSi2 and WSi2 with a body-centred-tetragonal C 1 1b structure were fabricated using a floating-zone method. The elastic wave velocity was measured for samples with various orientations using a simple pulse echo method at room temperature, and six elastic stiffness constantsc ij were calculated. The stiffness constants were a little higher for WSi2 than for MoSi2.c 11 andc 33 of these compounds were approximately equal toc 11 of tungsten and molybdenum, respectively, althoughc ij (i ≠j) was a little higher for these compounds than for molybdenum and tungsten. Young's modulus 1/s 11 was the highest in the direction, and the lowest in the direction. The shear modulus 1/s 66 was high on the {0 0 1} plane and independent of shear direction. It was generally low on the close-packed {1 1 0} plane and largely dependent on shear direction. The elastic constants for the polycrystalline materials were estimated fromc ij ands ij . Poisson's ratiov was 0.15 for MoSi2 and for WSi2, and these values were much lower than for ordinary metals and alloys. The Debye temperature θD was estimated using the elastic-wave velocity of the polycrystalline materials via the elastic constants such as Young's modulus and shear modulus: it was 759 K for MoSi2 and 625 K for WSi2.

Anchoring structure of smectic liquid-crystal layers on MoS 2 observed by scanning tunnelling microscopy

Abstract: THE molecular alignment of liquid crystals on solid substrates is important both to fundamental issues in physics and biology and in practical applications such as optoelectronic devices. The structure of adsorbed phases is determined to a large extent by the balance of molecule–molecule and substrate–molecule forces at the solid-adsorbate interface. Scanning tunnelling microscopy (STM) now allows the possibility of direct observation of the interfacial structure with molecular resolution, and has been used to study positional and orientational order in monolayers of organic molecules on graphite1–4. Here we report STM imaging of smectic liquid crystals (4'-n-octyl-4-cyanobiphenyl (8CB)) condensed on a molybdenum disulphide single crystal. The anchoring structure is different from that on graphite4, and is unusual in that the molecules are aligned so that intermolecular interactions are energetically unfavourable. The influence of interactions with the substrate is clearly important in this case, and this is further supported by an apparent degree of epitaxy in the adsorbed layer.

The Effect of Anion Vacancies on the Tribological Properties of Rutile (TiO2-x), Part II: Experimental Evidence

Abstract: Friction and wear tests were completed with the (001) and (110) planes of single crystal rutile (TiO 2-x ) specimens sliding against selected ceramic counterfaces, in well-defined crystallographic directions. The purpose of the experiments was to investigate the environmental influences on anion vacancy formation, as related to a recent hypothesis connecting oxygen substoichiometry with predictable variations in the tribological properties of rutile. The data were obtained with two, entirely different test machines operating at various loads, speeds, temperatures, sliding directions and durations, as well as test specimen atmospheres. The results independently confirmed the predicted, anion vacancy-controlled formation of certain low and high lattice (strain) energy crystallographic shear systems (i.e., Magneli phases) and that their generation is overwhelmingly environment-dependent. The stoichiometry-controlled lattice energy of these rutile phases influences the surface and bulk shear strength (τS) of ...

Epitaxy of Y-Ba-Cu-O thin films grown on single-crystal MgO

Abstract: The epitaxy of a thin‐film Y‐Ba‐Cu‐O (YBCO) superconductor deposited on a single‐crystal [001] MgO substrate was examined by transmission electron microscopy. The large lattice mismatch (8–10%) in the basal plane of YBCO and MgO is accommodated mainly by the formation of a polycrystalline, mosaic structure. The grain boundaries correspond to unique crystallographic interfaces, determined by the crystal symmetry of the substrate and the thin film.

Atomically layered heteroepitaxial growth of single‐crystal films of superconducting Bi2Sr2Ca2Cu3Ox

Abstract: Atomic layering of high‐temperature superconducting compounds has been employed to grow films of Bi2Sr2Ca2Cu3Ox, in situ, on SrTiO3 substrates. Atomic monolayers of the constituent atoms were sequentially deposited by shuttering the fluxes from thermal effusion cells, and oxidation of the film was accomplished using a beam of ozone. The films were superconducting as‐grown, with complete resistive transitions as high as 86 K. Moreover, reflection high‐energy electron diffraction patterns observed during growth, as well as post‐growth analysis by x‐ray diffraction and high‐resolution scanning electron micrography, indicate the films to be single crystal and heteroepitaxial, with in‐plane a‐b twinning.

Phase equilibria and crystal chemistry in portions of the system SrO-CaO-Bi2O3-CuO. II: The system SrO-Bi2O3-CuO

Abstract: Symmetry data and unit cell dimensions bases on single crystal and powder x-ray diffraction measurements are reported for all the binary SrO-Bi 2 O 3 phases, including a new phase identified as Sr 6 Bi 2 O 9 . The ternary system contains at least four ternary phases which can be formed in air at ∼900˚C