Inhomogeneities In Crystals Research Articles

Sound attenuation in X-cut quartz, a-cut sapphire, and (100)-cut diamond at 70 GHz

The recently developed method of linear frequency modulation of a continuous wave (FM-CW) in soundecho experiments was applied to measure the sound attenuation in single crystals at 70 GHz. The residual sound attenuation at low temperatures and the crystal inhomogeneity were investigated using the method of sound-beam topography in several crystals of X-cut quartz and of a-cut sapphire of different origin. The temperature dependent part of attenuation was also determined and compared with other investigations, especially at 9 GHz and 1 GHz. We also present sound-attenuation measurements in (100)-cut type IIa diamond which are the first ever made in diamond. Echoes were observed up to 230 K at 70 GHz.

Micropipes and polytypism as a source of lateral inhomogeneities in SiC substrates

Structural and electrical inhomogeneities in SiC bulk crystals grown by the modified Lely sublimation method [1] in 〈0001〉 direction are investigated. Inclusions of different polytypes such as 15R in 6H material are observed on a macroscopic as well as a microscopic scale. Their size and number can be associated with the growth rate of the crystal. Beside the familiar micropipes, ‘nanopipes’ were observed by atomic force microscopy. Micropipes and nanopipes form centers of growth spirals related to the growth mechanism of the crystals under study. These micro- and nanopipes are a major source of structural and electrical inhomogeneities of SiC which are discussed in this paper.

Slow relaxation of a polydomain ferroelectric in weak electric fields

Polarization relaxation of a polydomain ferroelectric (triglycine sulfate) in weak electric fields has been studied. A phenomenological description of the relaxation process is proposed, which is based on the concept of thermally activated motion of domain walls through energy barriers created by inhomogeneities, and of the existence distribution spectra of relaxation times τ and barrier energies Eg. Possible spectra were constructed from experimental data, and their characteristic parameters were determined for various electric fields E. It is shown that the width of the barrier energy spectrum ΔEg is practically independent of E and may probably be considered as a characteristic of crystal inhomogeneity.

Micro-probe Raman spectroscopy for detection of inhomogeneities in YVO4 single crystals

Several near quasicongruent undoped YVO4 single crystals grown by the Czochralski (CZ) technique as well as near stoichiometric YVO4 single crystals grown by the top-seeded solution growth technique, were investigated by micro-probe Raman spectroscopy using unpolarized argon–ion laser light. The line broadening effects of the Raman active mode at 379 cm−1 indicate that all CZ grown crystals possess wider peak widths than near stoichiometric YVO4. In the latter case a value of 9.23 cm−1 was measured for full width at half-maximum. Although the annealing process in O2 can improve the oxygen stoichiometry in the originally slightly oxygen deficient CZ grown YVO4 crystals, certain remaining peak broadening effects demonstrate a Y–V related stoichiometry problem by both lower (external) and higher frequency (internal) modes. Radial inhomogeneous distributions were observed in CZ specimens by the shift of external Raman modes at 157 and 162 cm−1 frequencies. In addition, nonassigned low intensity extra Raman peaks appeared over 1000 cm−1 frequencies in all CZ grown samples. The above detected changes in Y–V–O stoichiometry demonstrate a complex phase system around the homogeneity region of quasicongruent YVO4 specimens and enable a more realistic and unified interpretation of the segregated defects and stoichiometry changes in CZ grown YVO4 single crystals than previously available.

Nano-Tubes in GaN

ABSTRACTFormation of vertical hollow nano-tubes in GaN grown on different substrates using different growth methods is described. These defects are shown to be of several different types, some related to threading dislocations, but others originating at tubular inversion domains, or crystal inhomogeneities. Kinetic mechanism based on slow growth rate on polar {0111} surfaces is proposed to explain the origin of these defects.

Spatially Resolved Spectral Inhomogeneities in Small Molecular Crystals Studied by Near-Field Scanning Optical Microscopy

Near-field scanning optical microscopy (NSOM) has been employed to spatially resolve mesoscopic inhomogeneous spectral features in small crystals of the dye 1,1‘-diethyl-2,2‘-cyanine iodide (PIC). The small crystals show strong absorption perpendicular to their long direction of growth and no absorption in either of the two other orthogonal directions. This polarization is seen uniformly throughout the crystals. Topographic images reveal the crystals are composed of platelike single-crystalline domains separated by defect planes. The individual plates share a common orientation that gives the crystal its single polarization. Despite the uniformity of the polarization throughout the crystal, dramatic spatial variation is seen in the fluorescence spectrum. NSOM images and spectra reveal two distinct emission sources in the crystals. Strong self-absorption is seen in the PIC emission, and red-shifted fluorescence is also observed from lower energy emissive traps. These traps are seen to form in distinct regi...

Structure and properties of domain walls in a (111)-oriented plate of crystals with combined anisotropy

We theoretically study the physical properties of magnetic domain wall inhomogeneities in crystals possessing both natural cubic and induced uniaxial anisotropies (combined anisotropy). The structure and stability of Bloch type domain walls (DWs) has been investigated for plates with (111)-oriented surfaces in the anisotropy constant range in which bubbles exist. We have shown the possibility of hysteresis phenomena during reorientation the DWs relative to crystallographic axes. Dynamic characteristics of steady and oscillatory motion have been found to be functions of sample material parameters and their peculiarities are examined in the vicinity of spin-reorientation phase transition as well as in the vicinity of the DW reorientation line phase transition relative to crystallographic axes. The dynamic DW parameter values are shown to be affected by temperature and external stresses.

Quantum diffusion of muonium in an inhomogeneous crystal

Abstract The tunnelling kinetics of muonium (Mu) atoms in solid nitrogen (s-14N2) have been studied using the technique of Mu spin relaxation in weak longitudinal magnetic fields. At low temperatures the results are inconsistent with diffusion models using a single correlation time τc-. Spatial inhomogeneity of solid nitrogen crystals arising at low temperatures results in a two-component behaviour of the muon polarization function.

Advances in high-Q piezoelectric resonator materials and devices

In order to compare piezoelectric materials and devices, an intrinsic parameter, the motional time constant tau(1)/sup (m/)=(omega/sub m/Q/sub m/)(-1) for a particular mode m is employed. The use of tau(1)/sup (m/) follows from the accommodation of acoustic loss in the elastic compliance/stiffness and the establishment of material coefficients that are elements of viscosity matrices. Alternative and fully equivalent definitions of tau(1) are given based on the RC time constant derived from the equivalent circuit representation of a crystal resonator, acoustic attenuation, logarithmic decrement, and viscosity or damping. For quartz devices, the variation of tau(1): for any simple thickness mode, for the Y'X shear mode for rotated Y-cuts, and with diameter-thickness ratio for AT-cuts is discussed. Other factors such as mounting loss and loss caused by crystal inhomogeneities (dislocations, defect positions in the resonator, and impurity migration under vibrational stress) are briefly considered with quartz devices as the model. Some new piezoelectric materials/material constants/devices are reviewed and their motional time constants are compared. A physical parameter, composed of acoustic velocity, piezoelectric coupling, and tau(1) is identified which aids in understanding the maximum frequency limitations of plate resonators.

Depth profilometry of near-surface inhomogeneities via laser-photothermal probing of the thermal diffusivity of condensed phases

Several applications are presented of the Hamilton-Jacobi formulation of thermal-wave physics to the problem of laser photothermal depth profilometry of the thermophysical transport parameter (the thermal diffusivity) of inhomogeneous condensed phases (solids and liquids) with arbitrary, continuously varying thermal diffusivity profiles. A working general method for solving the inverse problem and obtaining arbitrary diffusivity depth profiles from the laser beam-intensity modulation frequency dependence of the photothermal signal (amplitude and phase) is described. Specific examples of profile reconstructions are presented, including magnetic field-induced thermophysical inhomogeneities in liquid crystals, laser processing inhomogeneities in steels and Zr-Nb alloys, and finally, evaluation of machining damage in metals. Whenever possible, profiles obtained photothermally are compared with those resulting from destructive methods, such as microhardness testing.

Inhomogeneity study of alanine-doped TGS crystal using micro-probe raman spectroscopy

Abstract Micro-probe Raman spectroscopy is used for studying positional inhomogeneity in alanine-doped TGS crystals. 20 data points are measured along the c- and a-axes. Several known Raman peaks are observed unchanged, but most other peaks are found to be changed remarkably. The major difference in spectra are found to be related to SO4 vibrations. The vibrational frequencies are found to be lowered near the seed position. The most unstable spectra are detected in the vicinity of the seed region.

Localized modes in inhomogeneous one-dimensional anharmonic lattices.

We present numerical calculations for the determination of localized modes in one-dimensional finite chains of atoms with free ends containing harmonic and quartic anharmonic interactions. By adding step by step the quartic term we can follow the formation of even and odd localized modes arising from the highest harmonic frequency mode. We have studied the role of crystal inhomogeneity by introducing a modification of the fourth-order force constant between neighboring atoms at the center of the chain, where the localized mode has its maximum displacement. For large weakening of this force constant the localized mode develops a double-peaked structure, as has been found in the continuum limit. In the case of asymmetrical local inhomogeneity the localized mode remains stable and moves toward the atom with the inhomogeneity. We also show the existence of anharmonic surface modes localized at the end of the chain.

Growth from solution-melt in KCl and properties of high-quality low-T c phase (2 2 0 1) single crystals

High Sr-deficient Bi 2+ x Sr 2−( x+ y ) Cu y O z (2 2 0 1)- phase) single crystals from solution-melt in KCl keeping constant both growth temperature and temperature gradient were obtained. Superconducting crystals were produced in the temperature range 830–845°C up to Bi/Sr ratio of 1.7 provided the Cu concentration was increased. The T c values measured by d.c. resistive and a.c. susceptibility methods are equal to 3−13 K. The best single crystals with T c0 = 3–5 K, ΔT c = 1 K have size up to 2 × 2 × 0.01 mm 3 and consist of blocks with a relative misorientation in the ab-plane no larger than three tenths of a degree. Measurements of supercell structure parameters have revealed that the in-plane and out-of-plane components of the modulation vector vary with opposite manner. The superlattice monoclinic angle of the superconducting crystals ranges from 123.5° to 129° whereas that of nonsuperconducting ones from 110° to 121°. It was found that the parameters of modulated structure can be successfully used to assess the crystal inhomogeneity.

Modulated cathodoluminescence for extended defect characterization

The basic principles of cathodoluminescence with electron beam energy and intensity modulation and its application for the characterization of depth inhomogeneities in semiconductor crystals have been discussed. The results of numerical simulation, illustrating the possibility of exponential profile reconstruction, are presented.

A TEM and EDS Study on the effect of back melting on directionally solidified HgZnTe

Bulk growth of II-VI semiconductors such as HgZnTe (MZT) is difficult because of the gravity-induced convection that occurs during solidification. This convection can lead to crystal defects (primarily dislocations and subgrain boundaries) and composition inhomogeneities which may limit device performance. Crystals grown in the low-gravity environment of space should have fewer of these defects. However, the limited duration of space shuttle experiments and the need for uniform composition make it imperative that the initial composition transient that occurs in directionally solidified alloys be eliminated. One proposed solution to this problem is back-melting. In the first stage of this process, a section of the specimen is directionally solidified and the remainder is quenched. The quenched portion is then remelted and directionally solidified. In the flight experiment, the remelting and second directional solidification step will be carried out in space.

Microstructure of single crystals of austenitic stainless steel

Auger spectroscopy and x-ray structure analysis have been used to study the chemical and phase inhomogeneity in single crystals of the austenitic nitrided stainless steel Cr22Ni17Mo3 with various morphologies of the dendritic structure. In single crystals grown in the 〈100〉 direction with a classical dendritic structure the interaxial regions are highly enriched with S, N, and C and weakly enriched with Mo, V, and Mn. At the same time these regions have a high density of particles of the type Me23C6, Me7C3, and vanadium carbonitrides, oriented in the growth direction. The axial regions have a subcellular structure, with oxygen-rich subcell boundaries. In single crystals with growth direction close to 〈110〉, in which only secondary dendrite axes are formed, the alloying elements, impurities, and second-phase particles are distributed relatively uniformly throughout the ingot.

IN SITU INVESTIGATION OF NUCLEATION, GROWTH, AND DISSOLUTION OF SILICATE CRYSTALS AT HIGH TEMPERATURES

Imperfections and inhomogeneities in single crystals of minerals and the textures of rocks and ores appear primarily as a result of nucleation and growth of crystals, and secondarily through post-growth histories such as dissolution, exsolution, and phase transition. How and under what conditions mineral crystals nucleated, grew, dissolved, or transformed, how rock textures appeared, and what sort of post-growth histories they experienced have been deduced by analyzing these characteristics. Inves­ tigations of earth and planetary materials along this line are as important as equilibrium thermodynamic analyses of mineral parageneses, since they provide information on kinetic processes. Various sophisticated methods have been used to detect and analyze these characteristics, including optical and electron microscopies, electron probe micro-analysis and other surface analysis methods, cathode luminescence topography, X-ray topography, laser-beam tomography, etc. A deductive analysis of this type, however, should be supplemented by a theoretical and experimental understanding

Characterization of inhomogeneity in high-quality quartz crystals

Infrared and near-infrared spectrometry, helium–neon laser absorption measurements and γ-irradiation were used together as non-destructive techniques to identify mechanical and growth defects, hydrogen content and aluminium defect centres in high-quality natural and synthetic quartz crystals. These measurements were compared with the results of chemical analyses obtained using inductively coupled plasma atomic emission spectrometry in order to correlate all these results with the intrinsic quality of the material used in the manufacture of high-performance quartz resonators.

Inhomogeneities in n-Type and Semi-Insulating InP Crystals Studied by Photoluminescence Topography and Raman Scattering

The homogeneity of n-type Sn and S doped and semi-insulating Fe doped InP crystals is studied by luminescence topography, low temperature luminescence, and Raman scattering. As in GaAs nonrelational striations and faceting are the main sources of inhomogeneous impurity distribution in doped crystals and dislocations in semi-insulating ones. The fluctuations in free carrier concentration influencing the luminescence intensity are independently proved by Raman scattering spectra. Die Homogenitat von n-leitenden, Sn- und S-dotierten In P-Kristallen sowie von semi-isolierendcn, Fe-dotierten InP-Kristallen wird mittcls Lumineszenztopografie. Tieftemperaturlumineszenz und Ramanstreuung untersucht. Wie im GaAs bilden Striations und Fazettenbildung die Hauptquellen fur inhomogene Strorstellenvertcilung in dotiertcn Kristallen und Vcrsetzungen in semi-isolierenden. Die Fluktuationen der Konzentration freier Trager, die die Lumineszenzintcnsitat beeinflussen, werden unabhangig mittels Ramanstreuspektren uberpruft.

Acoustic, opto-acoustic, and thermal properties investigated around the phase transitions of NaCN and Na(CN)xCl1-x mixed crystals.

The elastic and opto-acoustic properties of sodium cyanide crystals NaCN and sodium cyanide--sodium chloride mixed crystals Na(CN${)}_{\mathit{x}}$${\mathrm{Cl}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ (1\ensuremath{\ge}x\ensuremath{\ge}0.63) were investigated below room temperature using high-resolution Brillouin spectroscopy. Employing special scattering techniques, we were able to measure the elastic behavior in both the high- and low-temperature phases. Changing from systems with high ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ concentrations, which undergo structural phase transitions of first order, to systems with low ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ concentrations, which show glass transitions, the discontinuities of the extremely soft shear stiffness of these cubic crystals gradually vanish. Using a space- and angular-resolving Brillouin technique, we report data on the inhomogeneity of the Czochralski-grown mixed crystals. The consequences of these inhomogeneities for the phase-transition behavior have been studied by thermal analyses. We try to describe our shear elastic stiffness data in the framework of current theoretical approaches.