Hexagonal Single Crystals Research Articles

Pressure‐ and temperature‐dependent Raman studies of KNbW2O9 hexagonal tungsten bronze

AbstractPressure‐ and temperature‐dependent Raman studies were performed on KNbW2O9 ferroelectric hexagonal tungsten bronze single crystal. The results suggest that some structural changes, possibly phase transitions, connected with tilting of the WO6 octahedra occur in this material at around 290–350 K and 1.0–1.4 GPa. No changes in the spectra could be observed at the 523 and 543–553 K phase transitions. The data indicate that these phase transitions lead to only weak changes in the vibrational properties of KNbW2O9, which were not observed owing to the large bandwidth of the Raman bands. The Raman studies suggest, therefore, that the structure of the unknown phase, existing below 553 K, differs slightly form the orthorhombic structure present above 553 K. The pressure‐dependent study revealed that the 52 cm−1 band, assigned to external vibrations of the WO6 octahedron, consists at ambient temperature and pressure of two overlapping bands. These bands become well separated at high pressure. Copyright © 2003 John Wiley & Sons, Ltd.

NaY3S3[SiS4]: a sodium-containing yttrium sulfide thiosilicate with channel structure

The reaction of sodium bromide (NaBr), yttrium (Y), sulfur (S) and silicon disulfide (SiS2) in the molar ratio 1:4:6:2 with an excess of NaBr as flux in evacuated silica ampoules at 850°C for 7 days leads to the formation of deep red, hexagonal pillar-shaped single crystals of the title compound. NaY3S3[SiS4] crystallizes hexagonally in the space group P63 (a=976.57(7), c=570.61(5) pm; Z=2). The sodium cations are situated in the trigonal antiprismatic holes (CN=6) of a channel along [001] that is built by non-silicon bonded sulfide anions, which on their part are surrounded by four Y3+ and two Na+ cations. The yttrium cations are surrounded by eight sulfur atoms in the shape of a square antiprism divided into four silicon- and four non-silicon-bonded sulfide anions. Finally, all discrete [SiS4]4− tetrahedra point in the same direction along the polar c-axis. Therefore, the whole structure of NaY3S3[SiS4] remains non-centrosymmetric.

Near-Band-Edge Photoluminescence from Very High Quality Hexagonal ZnO Bulk Crystals

The near-band-edge photoluminescence spectra of very high quality hexagonal ZnO single crystals in the temperature range between 9 and 305 K were measured. Based on the energetic positions and the evolutions of well-resolved photoluminescence lines with temperature and with excitation power density we interpret the observed photoluminescence lines as resulting from recombination of the free-exciton, bound-exciton, biexciton, inelastic exciton-exciton collision and electron-hole plasma.

Highly Ordered and Well-oriented Single-crystal CdTe Nanowire Arrays by Direct-current Electrodeposition

Highly ordered CdTe nanowire arrays were prepared by electrochemical deposition into the pores of anodic aluminum oxide templates from an ethylene glycol bath containing CdCl2, TeCl4, and KI. Electron microscopy results showed that the length, diameter, and direction of growth of the nanowires were quite uniform. X-ray diffraction, transmission electron microscopy, and high-resolution electron microscopy investigations showed that these nanowires had a crystalline structure of hexagonal CdTe single crystal with a uniform [001] growth direction.

Basal plane slip and formation of mixed-tilt boundaries in sublimation-grown hexagonal polytype silicon carbide single crystals

Optical microscopy, synchrotron white beam x-ray topography (SWBXT), and high resolution x-ray diffraction (HRXRD) were used to study the distribution of basal plane dislocations in bulk 4H silicon carbide crystals grown by the physical vapor transport method. An etch pit array was observed on the silicon face of KOH-etched off-cut wafers. The arrays were aligned parallel to each other and perpendicular to the off-cut direction. The etch pits were oval-shaped, which is characteristic of basal plane dislocations. Corresponding array images have been observed by SWBXT. Based on the characteristic distribution, the etch pit arrays are interpreted as the slip traces of high temperature deformation during the growth process. Thermoelastic stress is proposed as the plausible cause of the deformation. In addition, basal plane dislocation pileups were found in the proximity of polygonized threading edge dislocation arrays. SWBXT and HRXRD were used to study the misorientation related to such dislocation structure. The pileups are interpreted as the origin of the basal-plane-tilt domains in SiC crystals.

Cathodoluminescence study of h– and c–GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N2 in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN3 as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h-and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.

Catalytic growth of high quality GaN micro-crystals

GaN micro-crystals were catalytically grown using Ni-mesh in direct reaction of gallium and ammonia. At 1100°C, the growth rate of the GaN crystals in the presence of Ni catalyst was almost ten times higher than in the absence of the catalyst. The use of the catalyst induced the increase of GaN crystal size. The structural characterization of the GaN crystal showed the growth of dislocation free hexagonal GaN single crystal. PL spectrum showed a strong band edge emission at the energy position of ∼3.35 eV with FWHM of ∼115 meV. The time-resolved photoluminescence measurements also revealed the growth of high quality GaN micro-crystal grains. The catalytic activity for the growth of the GaN crystals was discussed based on the in situ analyzed gas composition during the growth.

MBE growth of different hexagonal GaN crystal structures on vicinal (100) GaAs substrates

GaN thin films of different hexagonal crystal structures have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy (RFMBE) on vicinal (100) GaAs substrates. Polycrystalline hexagonal material occurred for high temperature (630°C) nitridation of the GaAs surface or low temperatures of the initial GaN buffer layer deposition. On the contrary, initial GaN growth at 540°C gave hexagonal single crystals with [0001] axis either inclined at approximately 43° from the growth axis or aligned parallel to it. The GaN orientation depended on the annealing or not, respectively, of the initial low temperature buffer layer.

An analytical approach to deformation of anisotropic ice-crystal aggregates

AbstractDeformation rates of single hexagonal crystals, deforming by glide on the basal plane, are described as a function of stress state and crystal orientation. These results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous. Analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric. This type of fabric is approximated by a uniform distribution of c-axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis. For simple shear stress acting on a single-maximum fabric there is a slight de-enhancement for cone angles of 60–90°. In uniaxial compression a maximum enhancement of ∼1.7 occurs at a cone angle of 57°. A pure shear stress state has similar features, with the additional complication that it causes a non-zero transverse strain rate, except for perfect vertical alignment of crystals and isotropic fabric. In combined states of stress the contribution of each stress component to the strain rate depends on fabric. A single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states.

Preparation and characterization of CdS nanowire arrays by dc electrodeposit in porous anodic aluminum oxide templates

Aligned CdS nanowires have been prepared by dc electrodeposition in porous anodic aluminum oxide template from dimethylsulfoxide solution containing cadmium chloride and elemental sulfur. These nanowires have uniform diameters of about 20 and 100 nm, which correspond to the pore sizes of the templates used, and lengths are up to tens of micrometers. HREM investigations demonstrate that these nanowires have a uniform crystalline structure of hexagonal CdS single crystal. It was proposed that the steady diffusing process of ions probably plays an important role in the growth of the nanowire by dc electrodeposition.

Depth-selective conversion-electron Mössbauer spectroscopy of the surface of Ba-M hexaferrite single crystals

The magnetic properties of a 50-to 2-nm-thick surface layer in hexagonal ferrite BaFe12O19 single crystals are investigated for the first time. Measurements are made on a conversion-electron Mossbauer spectrometer constructed on the basis of a unique ultrahigh-vacuum magnetostatic electron analyzer of the “orange” type. An analysis of the experimental Mossbauer spectra obtained from the surface layer of BaFe12O19 50-to 2-nm-thick single crystals reveals that (1) the spectral line widths are close to the natural widths of Mossbauer lines and neither changes nor a set of the values of effective magnetic fields, as well as a paramagnetic state of iron ions, could appear due to a defect such as the “surface,” and (2) the experimental spectra are best described only under the assumption that the surface of a hexaferrite BaFe12O19 single crystal contains a 2-nm-thick layer, in which the magnetic moments of iron ions are deflected through ∼20° from the crystallographic C axis along which the magnetic moments of ions located in the bulk of the crystal are oriented.

Synthesis of ZnO Particles by Ammonia-Catalyzed Hydrolysis of Zinc Dibutoxide in Nonionic Reversed Micelles

Ultrafine ZnO particles were synthesized by ammonia-catalyzed hydrolysis of zinc di-n-butoxide (ZDB) in polyoxyethylene (6) nonylphenyl ether (NP-6)/cyclohexane solutions in both the reversed and swollen micelle regions of the solubilization diagram of aqueous NH3. In all micelle regions, rodlike ZnO particles were produced which were determined to be hexagonal single crystals having a wurtzite structure. The effects of changing Rw (the molar ratio of water to surfactant = [H2O]/[NP-6]), Ra (the molar ratio of ammonia to surfactant = [NH3]/[NP-6]), and ZDB on the length (〈PL〉) and width (〈PW〉) of the ZnO particles were examined. The values of both 〈PL〉 and 〈PW〉 increased slowly from about Rw = 4. This increase reflected a change in the system, from reversed micelles with water bound to the polar groups of the surfactants to swollen micelles with semibound water interacting with hydrated polar groups. At a constant Rw (Rw = 6.0), as Ra was increased, both 〈PL〉 and 〈PW〉 slowly decreased. The 〈PL〉 and 〈PW〉 profiles of the ZnO particles at various Rw or Ra values were similar to the micellar droplet size profile at various Rw or Ra values, respectively. These facts indicate that the particle size of ZnO is controlled by both the solubilized states of water and the size of the micellar droplets. However, the 〈PL〉 and 〈PW〉 of the synthesized particles were independent of the ZDB. The 〈PL〉 and 〈PW〉 values also correlated to the average crystallite sizes of the (002) and (110) faces of the particles as determined by X-ray diffraction measurements. The long side of the rodlike particle was also found to be parallel to the c-axis of the hexagonal system.

Investigation of surface instability in an elastic anisotropic cone by the use of surface waves of weak discontinuity

The problem of surface instability of a right circular cone with an arbitrary opening made of a hexagonal single crystal (the cone axis coincides with the crystal's axis of isotropy) is investigated. The surface of the cone is free from normal and tangential stresses, but in the layer near the surface initial constant tensile or compressive stresses act in the hoop direction and in the direction of the cone's generators. Surface instability is analyzed by the use of weak nonstationary disturbances which propagate along the surface of the cone in the form of the two types of surface waves: the nonstationary Rayleigh waves polarized in the sagittal plane, and the nonstationary wave of the “whispering gallery” type polarized perpendicular to the sagittal plane. The weak nonstationary surface waves are interpreted as the lines of discontinuity (diverging circles) on which partial derivatives of the stress and strain tensor components with respect to coordinates and time have a discontinuity, but the components of these tensors are continuous. Each of the lines of discontinuity propagates with a constant normal velocity along the cone's surface in the direction of its generators and is obtained as a result of the exit onto the cone surface either of two conic complex wave surfaces of weak discontinuity intersecting along this line (Rayleigh wave) or of one real conic wave surface of weak discontinuity (wave of the “whispering gallery” type). The analysis is carried out within the framework of the theory of discontinuities based on the kinematic, geometric and dynamic conditions of compatibility; using them the velocities of the surface wave propagation and their intensities have been found. It has been shown that the surface wave velocities are dependent only on the initial stress acting in the direction of the propagation of a surface disturbance whereas the damping coefficients for the intensities of the surface waves are dependent not only on this stress but also on the initial stress acting in the hoop direction as well. The relationships for two critical magnitudes of the force compressive in the hoop direction have been obtained, and it has been shown that under the hoop compressive forces in excess of one of these magnitudes the intensity of the Rayleigh wave or the surface wave of the “whispering gallery” type begins to increase without bounds during its propagation, i.e., the surface of the cone loses stability with respect to either of two types of weak nonstationary disturbances.

Synthesis of GeO 2 nanorods by carbon nanotubes template

GeO 2 nanorods have been synthesized by means of a carbon nanotubes-confined reaction, carbon nanotubes are used as templates and pure metallic germanium is a reactant instead of the common-used volatile metal or non-metal oxides. The products are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectrometer (EDS) as well as X-ray diffraction (XRD). It is found that the GeO 2 nanorods are perfect single hexagonal crystals covered by an amorphous layer of Ge and O, and have diameters of 50–200 nm and lengths of several μm.

Preparation of II-VI group semiconductor nanowire arrays by dc electrochemical deposition in porous aluminum oxide templates

Abstract II-VI group compound semiconductors such as CdS, CdSe, and CdTe nanowire arrays have been prepared by direct current electrodeposition in porous anodic aluminum oxide template from nonaqueous electrolyte. SEM and TEM results show that these nanowires have a highly anisotropic structure of aligned nanowires with diameters of 15-200 nm, which are consistent with the diameters of the templates used. Electron diffraction and HREM investigations demonstrate that the crystalline structures of these nanowires are uniform hexagonal single crystal. This approach can be used to fabricate single crystal nanowire arrays of a wide range of semiconductors and other materials.

Growth of GaN layers onto misoriented (0001) sapphire by metalorganic chemical vapor deposition

The growth process of GaN layers grown by metalorganic chemical vapor deposition on sapphire is characterized by transmission electron microscopy and atomic force microscopy. The nitridation of the sapphire substrate and GaN buffer layers as well as film structure and the nature of defects are studied. Nitridation causes the formation of a 4 nm thick AlN layer on sapphire. GaN buffer layers grown at 510 °C are found to be hexagonal single crystals in their as-grown state with a mosaic structure. Annealing of the buffer layers leads to substantial smoothening of their surfaces due to the coalescence of the grains. GaN layers themselves are single crystalline, hexagonal, and epitaxial to the substrate. Layers grown on exactly oriented (0001) type substrate as well as on miscut substrate are compared. Smooth surfaces have been achieved on exactly oriented and on miscut substrates as well, but the range of the deposition parameters is wider when miscut substrates are used.

Electronically induced dislocation glide motion in hexagonal GaN single crystals

Dislocations in crystalline powder of hexagonal GaN intentionally deformed by crushing were examined by transmission electron microscopy (TEM) in order to see if the recombination enhanced dislocation glide (REDG) effect is present in this solid. It was found that dislocations with a Burgers vector of a-type (a/3〈112̄0〉) on the (0001) basal plane and a {11̄0n} pyramidal plane exhibit glide motion at room temperature under the influence of the electron beam used for TEM observations. From a quantitative comparison with the thermal mobility of basal dislocations empirically predicted for h-GaN, the authors concluded that the dislocation glides arose from the REDG effect induced by electronic excitation due to the TEM electron beam irradiation. Some implications of the results were discussed.

Photoreflectance study of hexagonal CdSe

Photoreflectance (PR) spectra have been measured to determine the fundamental-absorption-edge (E0) structure of hexagonal CdSe single crystal for both E c and E|| c polarizations at temperatures T between 77 and 300 K. The measured PR spectra are found to be successfully explained by an excitonic line shape for temperatures T below ~200 K and by a three-dimensional one-electron line shape for T>200 K. No clear evidence of the exciton interference effect has been observed within our experimental temperature range T = 77-300 K. The temperature dependence of the E0-exciton parameters (exciton energy, amplitude and broadening parameter) has been determined and analysed using the Varshni equation and an empirical expression of Bose-Einstein type.

Dislocations, kink bands, and room-temperature plasticity of Ti3SiC2

Transmission electron microscopy (TEM) of aligned, macrograined samples of Ti3SiC2, deformed at room temperature, shows that the deformed microstructure is characterized by a high density of perfect basal-plane dislocations with a Burgers vector of 1/3〈112 0〉. The dislocations are overwhelmingly arranged either in arrays, wherein the dislocations exist on identical slip planes, or in dislocations walls, wherein the same dislocations form a low-angle grain boundary normal to the basal planes. The arrays propagate across entire grains and are responsible for deformation by shear. The walls form as a result of the formation of kink bands. A dislocation-based model, that builds on earlier ideas proposed for kink-band formation in hexagonal metallic single crystals, is presented, which explains most of the microstructural features. The basic elements of the model are shear deformation by dislocation arrays, cavitation, creation of dislocation walls and kink boundaries, buckling, and delamination. The delaminations are not random, but successively bisect the delaminating sections. The delaminations and associated damage are contained by the kink boundaries. This containment of damage is believed to play a major role in endowing Ti3SiC2 and, by extension, related ternary carbides and nitrides with their damage-tolerant properties.

Molten metal flux growth and characterizations of a new compound PrRh4.8B2 single crystals

The hexagonal plate-like single crystals of a new compound PrRh4.8B2 were obtained by a molten metal flux growth method using Cu as a flux. PrRh4.8B2 is orthorhombic with the space group Fmmm; a=0.9697(4) nm, b=0.5577(2) nm and c=2.564(3)nm. The structure of the compound is of a modified CeCo3B2-type which is explained by a stacking model of Pr–B and rhodium layers. The temperature dependence of inverse static magnetic susceptibility, χ−1 obeys the Curie–Weiss law at higher temperature above 50 K. The μeff is 4.2 μB and Θp is −13 K. The value of the observed effective magnetic moment is larger than the value of 3.6 μB for the free trivalent ion Pr3+. The susceptibility deviates from the Curie–Weiss law, below 50 K, suggesting some magnetic ordering. The value of MVH for c-plane with the orthorhombic symmetry of PrRh4.8B2 is 6.7±0.6 GPa. The TG curve represents that oxidation of the PrRh4.8B2 proceeds above 600°C. Weight gain of the specimen heated up to 1200°C in air is 13%. After TG–DTA measurement, final product is PrBO3 and Rh.