The effect of growth temperature on the coaxial InxGa1 − xN/GaN nanowires grown by metalorganic chemical vapor deposition

We report on the growth of coaxial In_xGa_1 − xN/GaN nanowires (NWs) on Si(111) substrates by using pulsed flow metalorganic chemical vapor deposition. The coaxial In_xGa_1 − xN/GaN NWs were grown by a two step process in which the core (GaN) structure was grown at a higher temperature followed by the shell (In_xGa_1 − xN) structure at a lower temperature. Dense and well-oriented coaxial In_xGa_1 − xN/GaN NWs were grown with an average diameter and length of about 300 ± 50 nm and 1.5-2.0 μm, respectively. The coaxial In_xGa_1 − xN/GaN NW was confirmed by cathodoluminescence mapping and high-resolution transmission electron microscopy. It is proposed that the critical dissociation of precursors at an elevated growth temperature can lead to a clear formation of an outer-shell in coaxial In_xGa_1 − xN/GaN NWs.     

Interface structure and strain development during compression tests of Al2O3/Nb/Al2O3 sandwiches

The interface structure of an Al₂O₃/Nb/Al₂O₃ sandwich produced by solid-state diffusion bonding was investigated in detail by various transmission electron microscopy (TEM) methods. The joint possessed at one interface a , , and on the other interface a and orientation relationship. At both interfaces, misfit dislocations formed to compensate the lattice mismatch as found by high-resolution transmission electron microscopy (HRTEM). Electron energy-loss near edge structure (ELNES) studies revealed that the interface is terminating with an Al layer resulting in Al–Nb bonds. Identical sandwiches were investigated on the meso- and macroscopic scale by performing compression tests and simultaneously monitoring the strain development at (001)_Nb and crystal faces. The full-field optical strain measurements (FFOM) revealed that the strain is localized at the interfaces when observed at the (001)_Nb face while it is along the maximum shear directions of 36–54° inclined to the interface when observed at the face. The strain localization along a specific maximum shear direction results in the cleavage of Al₂O₃, always initiating from the interface possessing the and orientation relationship.     

Optical and structural properties of Mg-ion implanted GaN nanowires

Mg⁺ ions (60 keV) were implanted into GaN nanowires (NWs) with total fluxes of 5 × 10¹²-5 × 10¹⁴ cm⁻² followed by thermal annealing at 700 °C in N₂ ambient. Transmission electron microscopic images showed amorphous layer formation and defect accumulation in the higher dose Mg-implanted GaN NWs after annealing. Photoluminescence spectra (300 K) of the annealed Mg-implanted GaN NWs exhibited near-band-edge (NBE) emission, donor-acceptor pair (DAP) emission, and defect-related yellow luminescence. With increasing dose, the NBE and DAP emissions are red shifted. Similar phenomena were observed in samples implanted with Ar to produce similar amounts of lattice disorder. The NWs show a much higher sensitivity to defect accumulation than GaN thin films.     

Magnetic susceptibility and effective magnetic moment of the Nd3+ and Co3+ ions in NdCo1 ? x Ga x O3

The magnetic susceptibility of NdCo_{1 − x} Ga _x O₃ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.8, 0.9, 1) has been measured at temperatures from 80 to 950 K. The effective magnetic moments (μ_eff) due to the magnetic moments of the Co³⁺ and Nd³⁺ ions have been determined in the temperature ranges of Curie-Weiss behavior, 130–370 and 600–940 K, and have then been used, together with the effective magnetic moment of Nd³⁺ (3.62μ_B or 4.20μ_B), to evaluate the effective magnetic moment of Co³⁺ in NdCo_{1 − x} Ga _x O₃. For the solid solutions with < 2.83μ_B, we have determined the fractions of intermediate-and low-spin Co³⁺ ions. In the range 2.83μ_B < < 4.90μ_B, we have determined the fraction of high-spin Co³⁺ ions. The results indicate that, in the temperature range 130–370 K, the Co³⁺ ions in NdCo_{1 − x} Ga _x O₃ with x = 0, 0.5, 0.8, and 0.9 are in the intermediate-and high-spin states, and the fraction of high-spin Co³⁺ ions gradually increases from 10% at x = 0 to 67% at x = 0.9. In the solid solutions with x = 0.1, 0.2, 0.3, and 0.7, more than half of the Co³⁺ ions are in the low-spin state, and the rest are in the intermediate-spin state. In the temperature range 600–940 K, the Nd³⁺ ions are in the ground and excited states, with theoretically predicted of of 3.62μ_B and 5.52μ_B, respectively. Because of the significant uncertainty in in this temperature range, has been determined less accurately compared to the range 130–370 K. Original Russian Text ? N.N. Lubinskii, L.A. Bashkirov, A.I. Galyas, S.V. Shevchenko, G.S. Petrov, I.M. Sirota, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 9, pp. 1137–1143.     

Facile synthesis of ultra-long α-MnO2 nanowires and their microwave absorption properties

Well crystallized α-MnO₂ nanowires (NWs) with an average diameter of about 40 nm and an average length of about 30 μm were successfully synthesized by hydrothermal method. The complex permittivity and permeability of α-MnO₂ NWs/paraffin composites with 20 vol.% α-MnO₂ NWs were measured in a frequency region from 0.1 to 13 GHz. The value of maximum reflection loss of the composites with 20 vol.% α-MnO₂ NWs is approximately − 35 dB at 3.13 GHz with a thickness of 3.6 mm, and the bandwidth corresponding to reflection loss below − 10 dB is higher than 1.8 GHz with a lower thickness of 1.2 mm.     

Density Measurement of Molten CaF2 by an Electrostatic Levitator

   High-quality single crystalline calcium fluoride (CaF₂) has been of considerable practical concern as an optical lens to an excimer laser stepper in the deep-ultraviolet region due to its excellent transparency. Highly accurate thermophysical properties of molten CaF₂ are essential as input data for a numerical simulation of the crystal growth process. The density of molten CaF₂ has been successfully determined in the stable and undercooled liquid states (1600–1820 K) with an electrostatic levitator. The temperature dependence of the density of molten CaF₂ is given by

Some multiple comparisons using sample quasi ranges on censored data

We considerk(k)≥2 independent populations (treatments or systems) and an solutely continuous member of location-scale family of distributions, index by the location parameter μ _i (-∞ < μ _i < ∞) and scale parameter θ _i (θ _i > 0), is used to model the observations from the ith population,i=1,...k. The problem of simultaneous selection of two subsets, one containing population associated with the smallest ϕ-value and other containing population with the largest ϕ-value with probability at least a pre-specified value is considered when the data are censored. We also construct 100P ^*% simultaneous upper and two-sided confidence intervals for where θ_[1] ≤ ... ≤ θ_[k] denotes the ordered values of ϕs. The proposed procedures, based on sample quasi ranges, are useful when the experimenter has smaller samples or censored samples or there is suspicion of outliers in the samples. The results are applied to exponential populations model and, for thes casi: (i) the constants have been computed to apply the proposed multiple comparisons; (ii) two members of the proposed class have been compared with the existing procedure. A numerical example is also given.     

Dynamic crack propagation and unloading behavior of brittle polymers

The method of caustics in combination with a Cranz–Schardin high-speed camera was utilized to study dynamic crack propagation and unloading behavior of epoxy, PMMA and Homalite-100 specimens. Dynamic stress intensity factor K _ID and crack velocity were evaluated in the course of crack propagation. Caustic patterns at the loading points were also recorded to estimate load P applied to the specimen. Unloading rate , the time derivative of P, was determined as a function of time t, and its time correlation with K _ID or was examined. The findings showed that the change in was qualitatively in accord with the change in K _ID or . However, there existed slight differences among the values of t giving the maximum , and K _ID, so that their order was , and K _ID. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spin-orbit interaction in the field of an optical vortex of a few-mode fiber

A quantum-mechanical analog of the spin-orbit interaction operator is constructed for the fields of optical CV vortices and TE and TM modes in an optical fiber. It is shown that the polarization correction δβ to the propagation constant, which is the mean value of this operator, is a measure of the “level splitting” of the propagation constant in the scalar case. The difference in the operation of the individual parts of the operator on the fields of CV vortices and on the fields of TM and TE modes is indicative of the presence of two different physical processes — circular and linear birefringence in the locally isotropic optical fiber. The conversion of the “scalar” field to a vector field e ₁ as a consequence of the spin-orbit interaction operator can be regarded as resulting from a re-radiation of the additional field e ₁ by the vortex field , which rotates around the optical axis of the fiber. In this picture the additional field e ₁ can be regarded as being a “relativistic” correction to the vortex field for the distortions of the main field and arising as a result of the rotation of the field of the optical vortex in the medium of the few-mode fiber. Zh. Tekh. Fiz. 24, 87–93 (October 26, 1998)     

On the growth mechanism of nickel and cobalt nanowires and comparison of their magnetic properties

Magnetic nanowires (NWs) are ideal materials for the fabrication of various multifunctional nanostructures which can be manipulated by an external magnetic field. Highly crystalline and textured nanowires of nickel (Ni NWs) and cobalt (Co NWs) with high aspect ratio (∼330) and high coercivity have been synthesized by electrodeposition using nickel sulphate hexahydrate (NiSO₄·6H₂O) and cobalt sulphate heptahydrate (CoSO₄·7H₂O) respectively on nanoporous alumina membranes. They exhibit a preferential growth along 〈110〉. A general mobility assisted growth mechanism for the formation of Ni and Co NWs is proposed. The role of the hydration layer on the resulting one-dimensional geometry in the case of potentiostatic electrodeposition is verified. A very high interwire interaction resulting from magnetostatic dipolar interactions between the nanowires is observed. An unusual low-temperature magnetisation switching for field parallel to the wire axis is evident from the peculiar high field M(T) curve.      

Ammonothermal synthesis of thick gallium nitride film employing acidic mineralizers

Fabrication of wurtzite-type gallium nitride (GaN) thick films on HPVE-grown {0001} GaN substrates under moderate ammonothermal conditions is reported. Supercritical ammonia (NH₃) as solvent and the mineralizer ammonium chloride (NH₄Cl) is employed for temperature and pressure conditions of 400–550 °C and ≤135 MPa, respectively. Growth rates of 30 μm per day over long-term growth runs were obtained. The effect of surface morphology of the substrate on homoepitaxial nucleation of GaN films prepared from ammonoacid solutions is investigated. Two-dimensional nucleation is obtained for substrates etched by hot concentrated KOH prior film growth. In this case the interface between film and the ( ) substrate does not show any signs of voids or island nucleation. Cracking pattern reveals similar mechanical-elastical properties for film and substrate.     

AC long-range phase-coherent effects of S-N-S junctions in the dirty limit

The conductance of a normal metal film (N) in contact with two superconductors (S) is calculated in the presence of a constant and oscillating V_ωcosωt bias voltage between N and S. It is shown that the conductance as a function of has a Shapiro-like step at = ħω/2e. This exists in the case when the spacing between two superconductors largely exceeds the coherence length. It is also shown that the Shapiro step height decreases rapidly when the junction size exceeds the phase breaking length.     

Fabrication of hexagonal GaN on the surface of β-Ga2O3 single crystal by nitridation with NH3

The fabrication of GaN on the surface of a bulk β-Ga₂O₃ single crystal by nitridation with NH₃ was investigated for the purpose of using it as a substrate for GaN epitaxial growth. A β-Ga₂O₃ single crystal was prepared using a floating zone furnace with double ellipsoidal mirrors, and its polished (100) plane was nitridated in NH₃ atmosphere at 850 °C for 5 h. It was found that hexagonal GaN with preferred in-plane orientation was produced on the surface of β-Ga₂O₃, and the thickness of nitride layers was approximately 50 nm. High resolution transmission electron microscopic observation indicated that the synthesized GaN was composed of the aggregation with single crystalline GaN particles, whose size ranged from ∼ 5 nm to ∼ 50 nm, and dislocation or defect was not observed in a GaN particle. This method could be expected as a new route to fabricate a substrate for epitaxial growth of III-nitride materials instead of using a bulk GaN single crystal.     

Growth of La0.7Sr0.3MnO3 films on Si(0 0 1) using SrMnO3 template layer

Using RF magnetron sputtering, we have successfully grown (1 1 0) orientated La_0.7Sr_0.3MnO₃ (LSMO) films on Si(0 0 1) wafers using SrMnO₃ (SMO) as a template layer. The X-ray diffraction (XRD) patterns of the SMO/Si heterostructures indicate that SMO grows along the (1 1 0) orientation, the orientation relationship between the SMO thin film and the Si (0 0 1) substrate being given by (0 1 1)_SMO∣∣(0 0 1)_Si and [01]_SMO∣∣[0 1 0]_Si. From the XRD patterns of the LSMO/SMO/Si heterostructures, we find that with an increase of substrate temperature, the required thickness of SMO, which plays an effective role of tuning the preferential orientation of LSMO, will decrease at first and then increase. It is thought that this originates from the fact that the crystallization of SMO is not perfect at low temperatures whereas too high a temperature results in reaction and diffusion at the interface of the two layers.     

Defect structure in GaN pyramids

High-quality GaN/AlN layers grown on (111) Si substrates have been used as the seeding layer for lateral epitactic overgrowth of GaN. The selective overgrowth was controlled by depositing a Si₃N₄ mask on the GaN seed layer. Growth of additional GaN resulted in the formation of GaN pyramids above the apertures in the patterned Si₃N₄ mask. Transmission electron microscopy showed that the GaN pyramids, the GaN seed layer, and the AlN buffer layer in the samples have the following epitactic relationship with respect to the silicon substrate: and . The pyramids were found to consist of a defective core region and a nearly defect-free outer region. In the core of the pyramid (at, or above, the aperture in the mask), numerous dislocations thread through the pyramid perpendicular to the interface plane. Some of these threading dislocations, which originated from the GaN/AlN seed layer, bend abruptly through 90° at the edge of this core region. In the outer part of the GaN pyramid, the density of vertically propagating dislocations was much lower. Most of the dislocations in this region are closely parallel to the original (0001) substrate plane. The top few microns of material are found to be essentially defect-free. The growth mechanism of the GaN pyramids is discussed in light of this defect structure.     

Growth of colorless transparent GaN single crystals on prismatic GaN seeds using a Ga melt and Na vapor

Growth of GaN on seeds of GaN prismatic single crystals was carried out at 900 °C and N₂ pressure (PN₂) of 0.8-7.0 MPa for 72 h by the Na flux method using premixed Na-Ga melts or Ga melt and Na vapor. Black GaN crystals, having some pits and striations on the facets, grew on the seeds when the premixed Na-Ga melts were used. A full-width at half maximum (FWHM) of the X-ray rocking curve measured for the (m plane) of the grown crystals was over 360 arcsec. Colorless and transparent GaN crystals with smooth facets were grown on the m plane of the seed crystals by using a Ga melt and Na vapor. The FWHM measured for the m plane of the colorless crystals was 112-204 arcsec. Cathodoluminescence (CL) spectra from the m plane of the crystals were measured at room temperature. Besides a near-band-edge (NBE) emission at 361-363 nm, the specimens grown with Ga melt and Na vapor at higher PN₂ had a broad deep emission peak at 617 nm, while the specimens grown at lower PN₂ had a shallow-level emission peak at 380 nm and a broad deep emission peak at 550 nm.     

Stability analysis of a low energy vortex configuration

Recent work using the London theory of anisotropic superconductivity has shown that, for small values ofB and large values of the anisotropy, a lattice of vortex lines parallel to is not necessarily the minimum energy configuration. When is not parallel to a symmetry axis, the “double lattice”—a lattice of lines parallel to plus a lattice of lines parallel to —can have a lower energy. The question arises, however, of whether some other configuration can have a yet lower energy. To investigate this question, we perform a stability analysis of the double lattice. We find that its energy can indeed be lowered by certain deformations of the vortex lines.