Showing papers on "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.

Single pentacene molecules detected by fluorescence excitation in a p-terphenyl crystal.

Abstract: Experiments clearly prove that narrow peaks in the fluorescence-excitation spectrum of a pentacene-doped p-terphenyl crystal stem from single molecules. This claim is supported by the distribution, width, and height of the peaks, as well as by the correlation of the emitted light and the sudden drops and surges of the emission of certain peaks. We attribute these to the hole burning of a single molecule. These results show the feasibility of the optical study of a single molecule and its local environment.

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.

A superplastic covalent crystal composite

Abstract: SUPERPLASTICITY is defined phenomenologically as the ability of a material to exhibit exceptionally large elongations during tensile deformation1. It is a property of some poly crystalline solids, and is well established for metals and alloys2. Superplasticity has also been observed in some ionic crystals, such as Y2O3-stabilized tetragonal ZrO2 polycrystals3,4, but has not been found previously for covalent crystals. Here we report superplastic elongation (by more than 150%) of a covalent crystal composite, Si3N4/SiC. The superplasticity is probably related to the presence of an inter-granular liquid phase. Combined with its hardness, this property suggests several useful applications for the novel material: for example, to form engine components—superplasticity will make it readily mouldable at high temperatures.

Optical-damage-resistant LiNbO(3):Zn crystal.

Abstract: Zinc doping is shown to reduce the photorefraction in LiNbO3:Zn. The damage-resistant crystal LiNbO3:Zn has demonstrated a conversion efficiency of approximately 50% for frequency doubling of 1.06-μm radiation. The dependence of optical characteristics on the ZnO concentration in the melt reveals a sharp change of the optical properties at the threshold concentration of 4–6 mol % Zn. The optical and nonlinear-optical data of LiNbO3:Zn are similar to those of LiNbO3:Mg, but the former shows better optical performance.

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.

Physics and Chemistry of Crystalline Lithium Niobate

Abstract: Introduction Physico-chemical properties of lithium metaniobate: Phase diagram for the Li^O2-0-Nb^O 2O^O5 system Crystal structure of lithium metaniobate Crystal-chemical features of metaniobates of alkali metals Phase formation in LN Crystals Methods of obtaining single crystals of lithium niobate Peculiarities of growth of LN single crystals Synthesis of the charge and preparation of the melt Choice of the optimum conditions for growth Growth conditions for LN crystals of a constant radius High-temperature annealing and formation of single-domains in crystal The Stepanov technique Electric phenomena arising in crystallization of LN Defect structure of single-crystal lithium niobate: Morphology and macro-defects Point defects in LN Formation of F centres in LN crystals Twinning in LN crystals The domain structure of lithium metaniobate crystals: Ferroelectric domains in lithium metaniobate Selective etching of LN crystals Growth domain structure Formation of a stationary domain structure Depolarisation mechanisms of lithium metaniobate crystals The influence of temperature gradients on domain formation in the process of crystal growth and annealing The effect of annealing on the near-surface domain structure The regular domain structure in LN crystals Electrical properties of lithium metaniobate: Electric conductivity Dielectric properties Thermal diffusion in lithium niobate crystals Relaxation phenomena in lithium niobate crystals Electric fields in lithium niobate crystals Electric effect and relaxation polarization of lithium niobate Thermionic emission of lithium niobate single crystals Effective ion charges and spontaneous electric moment of lithium niobate Optical and electro-optical properties of lithium metaniobate single crystals Optical properties of lithium metaniobate Electro-optical effect in dielectric crystals Phenomenological theory of the electro-optical effect Establishment of electro-optic coefficients Specific features of lithium niobate crystals applications in electro-optical devices Nonlinear optical properties of lithium niobate: Elements of nonlinear optics Methods used to establish nonlinear coefficients Relationship of birefringence and phase-matching temperature to lithium niobate crystal composition Criteria for nonlinear-optical quality of crystals Enhancement of SHG in lithium niobate crystals with periodic laminar ferroelectric domains Photoelectrical and photo-refractive properties: Model representations of the photo-refractive effect Occurrence of optical distortion in lithium niobate crystals exposed to cw laser radiation Occurence of optical distortion in lithium niobate exposed to pulsed laser radiation Laser-induced physical effects in lithium niobate Photo-induced distortion of the crystal structure in lithium niobate Optical inhomogeneity of crystals and methods of its investigation Nature of optical inhomogeneity Electrically induced optical inhomogeneity of crystals Doping and heat treatment effects on crystal optical inhomogeneity Methods used to observe optical inhomogeneities in lithium niobate crystals Conclusions References Index

A Summary of the Physical Properties of Cirrus Clouds

Abstract: A review of existing literature is made to determine typical values for the physical properties cirrus clouds. The properties examined (with typical values and measured ranges) are cloud-center altitude (9 km, 4 to 20 km), cloud thickness (1.5 km, 0.1 to 8 km), crystal number density (30 L−1, 10−4 to 10−4 L−1), condensed water content (0.025 g m −3, 10−4 to 1.2 g m −3), and crystal size (250 μm, 1 to 8000 μm). A typical crystal size distribution is also reported.

Probing organic glasses at low temperature with variable time scale optical dephasing measurements

Abstract: Amorphous materials at low temperature have markedly different physical and thermal properties from crystals.' For example, the specific heats of crystals obey the Debye P law at low temperatures2 and the thermal conductivity also follows the same P temperature dependen~e .~ Many of these properties can be calculated because crystals have long-range spatial and orientational order.4 Twenty years ago, specific heat measurements on disordered materials revealed significant deviations from the Debye law at approximately 1 K.5-8 Measurements of thermal conductivity5y9 and dielectric responselOJ1 also deviated from crystalline behavior. These observations showed that the dynamics of ordered and disordered systems must be fundamentally different. Although ordered crystals are commonly found in nature, many other naturally occurring complex systems, such as proteins, are inherently disordered.12 Furthermore, many artificial materials, such as polymeric solids and amorphous semiconductors, are glasses. Therefore, understanding the microscopic behavior of the glassy state has been and continues to be13-15 an important problem in chemistry, physics, and materials science. Glasses are systems in which there is no translational or rotational order. More important, unlike a crystal, a glass is not in thermodynamic equilibrium. At low temperature, the equilibrium state of a substance like ethanol is crystalline. A glass is formed by rapid cooling, which traps the material in the glassy state. Thermodynamically the material should be a crystal, but kinetics prevent the system from finding the global potential minimum; Le., the kinetics at low temperature make the time scale for crystallization essentially infinitely long. Dynamics in simple crystals involve fast fluctuations about an equilibrium structure.16 In conlocal structures as well as time evolution of the nonequilibrium local structures them~e1ves.l~ Glass dynamics can occur through essentially all time scales from femtoseconds, to kiloseconds, and perhaps longer.lgZ0 Many concepts that are useful in describing dynamics in crystals cannot be extended to the amorphous state, e.g., translational symmetry, which gives rise to phonon bands and fast phonon fluctuations, providing a separation of time scales for a variety of processes.z1 In crystals at low temperatures, only the acoustic phonons are thermally excited. The phonon dispersion is well described by the Debye density of ~ t a t e s , ~ J ~ and this means that the distribution of fluctuation rates is known. A glass also has modes that are equivalent to a crystal's phonons. Even at low temperatures, however, there is a major contribution to the dynamical properties of glasses from the evolution of local structures, and it is these "extra" dynamics that make glasses fundamentally different from crystals. The anomalous heat capacities found in glasses show a term linear in temperature which is not present in crystals. This is true in such diverse substances as silicates and ceramics as well as poly(methy1 methacrylate) and Lexan polymer^.^-^>^^ In addition, heat capacities are time dependent; the heat capacity increases as time i n c r e a ~ e s . ~ ~ J ~ ~ ~ ~ ~ Thermal conductivity, which varies as P in crystals, varies as P? The phonon mean free path in a glass is substantially less than in the corresponding crystal, and a variation in the velocity of sound with temperature is seen in amorphous syst e m ~ . ~ ~ Many theoretical models have been developed to account for the various observations. The models invoke defect-induced scattering,26 localized electronic states,27 and defect and particle diffusionz8 to explain aspects of the observed behaviors. The two-level-system (TLS) model proposed independently by Anderson et al.29 and Phillips30 has been the most widely used, and it is frequently the point of departure in discussing the properties of low-temperature glasses. The TLS model postulates that, in an amorphous material, some atoms or molecules (or groups of atoms or molecules) can reside in not just one but two potential minima of the local structure potential surface (Figure 1). Each side of the double-well potential represents a distinct local structure of the glass. This is a simplified representation of what is almost certainly a complex multidimensional potential surface. At low temperatures, transitions from one side of the double well to the other represent changes in the local structure. Transitions occur by phonon-assisted tunneling. To model the complex distribution of local structures and transition rates, the TLS model states that there

Theory for the instability of the diamond structure of Si, Ge, and C induced by a dense electron-hole plasma.

Abstract: The effect of a dense electron-hole plasma on the stability of the diamond lattice of the crystalline group-IV elemental semiconductors C, Si, and Ge is examined with use of a tight-binding model. Such a plasma may result, for example, from a short, intense laser pulse. We find that the transverse-acoustic phonons of Si become soft if about 9% of the electrons are excited from the valence band into the conduction band. At higher densities of the electron-hole excitations the cubic symmetry of the diamond lattice is destroyed within less than 100 fs after the creation of the electron-hole plasma. This is much shorter than the time needed for the crystal to melt. The instability of the lattice then leads directly to a very rapid melting of the crystal structure. Our results are in agreement with recent experiments using pulsed lasers to induce disorder in crystalline Si surfaces. We obtain for C and Ge essentially the same theoretical results as for Si.

Transport through a finite one-dimensional crystal.

Abstract: We have studied the magnetotransport properties of an artificial one-dimensional crystal. The crystal consists of a sequence of fifteen quantum dots, defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure by means of a split-gate technique. At a fixed magnetic field of 2 T, two types of oscillations with different amplitude and period are observed in the conductance as a function of gate voltage. A simple model demonstrates that the oscillations arise from the formation of a miniband structure in the periodic crystal, including energy gaps and minibands which contain fifteen discrete states.

Thermodynamic parallels between solid-state amorphization and melting

Abstract: A thermodynamics-based description, in the form of an extended phase diagram, of melting and solid-state amorphization is proposed which brings out the parallels between these two phenomena and suggests that their underlying causes are apparently the same. Through molecular dynamics simulations we demonstrate that every crystal, in principle, can undergo two different types of melting transitions with characteristic features that are also observed in radiation- and hydrogenation-induced amorphization experiments on ordered alloys. The first type, defined in terms of free energies, is shown to involve the heterogeneous nucleation of the liquid or amorphous phase at extended lattice defects (such as grain boundaries, free surfaces, voids, or dislocations) and subsequent thermally-activated propagation of solid-liquid/amorphous interfaces through the crystal. The second type, arising from a mechanical instability limit described by Born, is homogeneous and does not require thermally-activated atom mobility. It is suggested that the role of chemical and structural disordering, a prerequisite for irradiation- but not hydrogenation-induced solid-state amorphization, is merely to drive the crystal lattice to a critical combination of volume and temperature at which the amorphous phase can form either heterogeneously or homogeneously.

Crystal to amorphous transformation of NiTi induced by cold rolling

Abstract: A NiTi intermetallic compound was cold rolled at room temperature by 30% and 60% thickness reductions, and microstructures were studied by means of transmission electron microscopy (TEM). In the cold-rolled samples we observed both a phase of nanometer-sized crystals and an amorphous phase. A substantially high dislocation density, 1013 to 1014/cm2, was evident in the transition region between crystalline and amorphous phases. A simple estimate of the elastic energy arising from this dislocation density is of the same order as the crystallization energy, suggesting that dislocation accumulation is a major driving force for amorphization in cold-rolled NiTi.

Comparison between calculated and experimental values of the lowest excited electronic state of small CdSe crystallites

Abstract: The lowest excited electronic state of small CdSe crystallites is calculated within a tight-binding approximation. Two types of crystal shape and a spatially varying dielectric constant are considered. The results are found to compare favorably to recent experiments.

Direct measurement of crystal surface stress.

Abstract: Mesure des contraintes internes d'une surface propre Si(111)7×7 par comparaison avec une surface de reference sur laquelle est adsorbee une couche monomoleculaire de Ga sous ultravide

The synthesis of high-quality diamond in combustion flames

Abstract: High‐quality diamond with good crystallinity has been successfully synthesized on a substrate using an oxy‐acetylene combustion flame in the atmosphere. The crystal grains under some conditions have good optical transparency. The deposition rate of transparent diamond depended strongly on substrate temperatures and the O2/C2H2 ratio and averaged ∼30 μm/h. The substrate temperature for the growth of optically transparent crystals was 500–750 °C, which is relatively low compared with other chemical vapor deposition methods. The optical transparency is attributed to the low defect densities in the crystals, as determined by transmission electron microscope, which results from the low substrate temperatures and moderate growth rates. Raman spectroscopy and x‐ray diffraction data on the synthesized crystals were comparable with that of natural diamond. The synthesis conditions and corresponding diamond quality as well as emission spectrum analysis of the combustion flame during diamond synthesis are described.

Pattern formation in growth of snow crystals occurring in the surface kinetic process and the diffusion process

Abstract: We propose a model of pattern formation in the growth of snow crystals that takes into account the actual elemental processes relevant to the growth of crystals, i.e., a surface kinetic process for incorporating molecules into a crystal lattice and a diffusion process. This model gives a clear correspondence between the patterns produced and the actual growth conditions such as supersaturation and the diffusion coefficient. Circular patterns due to kinetic roughening, hexagonal patterns, and dendritic patterns are obtained starting from a circular crystal under various growth conditions. We analyze these patterns and discuss the mechanisms of appearance of round patterns, the development of hexagonal patterns, and the formation of dendritic patterns of snow crystals. Finally, it is shown that the dimensionless crystal size with reference to the mean free path of a water molecule plays an important role in the pattern formation of growing snow crystals.

The structure of vitreous and liquid GeSe2: a neutron diffraction study

Abstract: High-quality neutron diffraction data are presented for GeSe 2 glass and liquid. In GeSe 2 glass at 10 K there is direct, clearly resolved evidence for edge-sharing of tetrahedra. C Ge (Ge) for dge-sharing tetrahedra measured in the glass (0.14), approaches that of the crystal (0.17). The Ge(Se 4 ) tetrahedron is the unit of short-range order in the glass, as it is in the crystal. The angle between neighboring corner-sharing tetrahedra in the glass is similar to that in the crystal. The tetrahedral building block persists in the liquid state at 1084 K, but it is broken up. The anomalous temperature dependence of the first sharp diffraction peak (FSDP) observed in the glass persists in the liquid. Experimentally, the height of the FSDP in the 1084 K liquid is 10% less than in the 10 K glass. The positions and the half-widths are identical. GeSe 2 retains considerable intermediate-range order in the liquid state. A comparison with molecular dynamics computer simulations is also presented.

CdS microcrystal-doped silica glass prepared by the sol-gel process

Abstract: Glass of composition of 44CdO·956SiO2 was prepared by the sol-gel process from Si(OC2H5)4 and Cd(CH3COO)2·2H2O CdS crystals were precipitated by exposing the glass of H2S gas, which were identified with hexagonal wurtzite CdS crystals From the X-ray diffraction analyses and transmission electron micrographs, these crystals were 2 to 6 nm in diameter In the optical absorption spectra, the absorption edge exhibited a blue shift compared to that of the bulk CdS and its energy shift was reciprocally proportional to the square of the crystal size Thus the quantum size effect could be found for the glass containing CdS prepared by the sol-gel process

Preparation of Small-Particle-Size, Semiconductor CdS-Doped Silica Glasses by the Sol–Gel Process

Abstract: The sol–gel process has been applied successfully to the preparation of small-particle-size CdS-doped silica glasses with a significant quantum size effect. Gels prepared through the hydrolysis of a complex solution of Si(OC2H5)4 and Cd(CH3COO)2·2H2O were heated at 500°C, then reacted with H2S gas to form fine, hexagonal, CdS-microcrystal-doped glasses. The optical absorption edge is blue shifted by ∼0.4 eV compared with the bulk absorption value of CdS crystal. This result is interpreted in terms of a quantum-confinement effect of small crystal size.

Creep deformation of single crystal superalloys—modelling the crystallographic anisotropy

Abstract: A generalised model of creep deformation in cubic single crystals is developed that considers the combined effects of viscous glide on two (or more) slip systems and accounts for tertiary creep by the accumulation of mobile dislocations with plastic strain. The model is applied to analyse a database of creep curves for the nickel-base single crystal superalloy SRR99 with the assumption that creep deformation occurs by glide on both the {111}〈 1 01〉 and {001}〈110〉 systems. A procedure for the calculation of creep curves and associated crystal rotations for arbitrary crystal orientations is described. The model predicts changes in the anisotropy of creep behaviour with stress and temperature that is in general agreement with the limited available experimental data. It also includes the contributions of all possible slip vectors to predict crystal rotations that are consistent with observations.

Quantum‐confinement effects in CdTe‐glass composite thin films produced using rf magnetron sputtering

Abstract: Quantum‐confinement‐induced shifts in the fundamental absorption edge of isolated CdTe crystallites are reported in CdTe‐glass composite thin films produced using a sequential rf magnetron sputtering process employing two separate sputtering sources. Films ranging in thickness from 0.5 to 4.5 μm and containing as much as 30 vol % CdTe have been produced, illustrating the versatility of this technique over a more conventional melting approach. Post‐deposition heat treatments were used to produce average crystallite sizes in the range 46–158 A. An improved fit to theory at larger crystal sizes is found if a cylindrical crystal morphology is assumed. The effective mass of the confined specie, which governs the shift of the absorption edge with crystal size, is found to be 0.20m0 (spherical morphology) and 0.12m0 (cylindrical morphology), both of which are greater than the exciton‐reduced mass in bulk CdTe. The data suggests, therefore, that a non‐negligible Coulomb interaction may still exist in crystals eve...

Computer experiments on the internal dynamics of crystalline polyethylene: Mechanistic details of conformational disorder

Abstract: The atomistic details of the internal dynamics of a polyethylene‐like crystal are studied using molecular dynamics. Crystals with up to 6100 chain atoms have been studied for up to 30 ps. A microscopic description of the atomic motion has been examined and a link to available experimental data on the macroscopic and microscopic motion is provided. The results show that the onset of a significant population of rotational isomers is strongly altered by the intermolecular forces. Typical rates for the formation of isomers are 1010 to 1012 s−1 at 350 K (depending on the size of the simulated crystal, which changes the overall nature of the intermolecular forces) and increase exponentially with temperature. The large number of created defects causes a continuous decrease in the end‐to‐end distance. Specific defects, however, have extremely limited lifetime (i.e., those suggested by molecular mechanics calculations). These results suggest that at the temperatures where annealing or deformation of metastable crystals is possible, only randomly generated defects cause the macroscopically observed changes. The defects should move under the free enthalpy gradient set up within the crystal toward a more stable location. The activation energy required for motion which ultimately results in mass transport or lamellar thickening can be shown to be temperature and chain‐length dependent. The highly uncorrelated behavior of the creation and annealing of defects reveals the underlying chaotic nature of the ‘‘transition’’ from an ordered crystal to a conformationally disordered crystal (CONDIS crystal). In the simulated case, the transition to the conformationally disordered state occurs gradually, involving little or no cooperative motion. This continuous transition to the condis state was suggested earlier on the basis of experimental evidence and is expected to occur in many other polymers in addition to and at lower temperature than possible additional first‐order transitions to the condis state. Thermodynamic and kinetic parameters of the simulations have been determined and compared to the available experimental data with good agreement.

Synthesis and Crystal Structure of a Novel Organic Ion-Complex Crystal for Second-Order Nonlinear Optics

Abstract: The novel organic ion-complex crystal composed of protonated merocyanine and p-toluenesulfonate anion, i.e., 1-methyl-4-(2-(4-hydroxyphenyl)vinyl)pyridinium 4-toluenesulfonate (MC-PTS), was synthesized for second-order nonlinear optics. Crystal structure analysis revealed that MC-PTS crystallized in the space group of P1, i.e., the most desired space group for waveguide applications where molecular dipoles are perfectly aligned in one direction. It was also pointed out that the tetrahedral sulfonate anion plays the role of a chiral handle to give noncentrosymmetric space groups.