Specific Crystal Orientation Research Articles

Mass-transport effect on InP corrugation shape control of DFB-LDs under atmospheric pressure MOVPE

The mass-transport effect is very important to control the corrugation shape of distributed feedback laser diodes (DFB-LDs). Quite uniform InGaAsP epitaxial growth has been achieved by atmospheric-pressure MOVPE(AP-MOVPE) with dual-fluid-layer structure reactor. However, mass-transport mechanism for AP-MOVPE has not yet been investigated. This paper discusses mass-transport effect under group V atmosphere using AP-MOVPE, for the first time. The mass-transported layer was found to be piled with the specific crystal orientation, unlike low-pressure MOVPE case, in the concave-region of the corrugation during heating time to the growth temperature. The piling rate for several temperatures was calculated from holding-time-dependence of the quantity of the mass-transported layer. So, it was derived that the piling rate at 625°C was three times larger than that at 575°C. Then, threshold temperature is estimated to be 550°C, where mass-transport starts to occur. Based on the above consideration, quick heating sequence (less than 15 s from 550°C to the growth temperature of 625°C) was adopted to realize well-controlled corrugation shape for 1.3 μm strained MQW-DFB-LDs, and excellent device characteristics, such as I th=24.7 mA and η s=0.36 W/A at 70°C, are attained.

A general setting for halo theory

We describe a general framework for systematically treating halos that are due to refraction in preferentially oriented ice wedges, and we construct an atlas of such halos. Initially we are constrained neither by the interfacial angles nor the orientations of real ice crystals. Instead we consider “all possible” refraction halos. We therefore make no assumption regarding the wedge angle, and only a weak assumption regarding the allowable wedge orientations. The atlas is thus a very general collection of refraction halos that includes known halos as a small fraction. Each halo in the atlas is characterized by three parameters: the wedge angle, the zenith angle of the spin vector, and the spin vector expressed in the wedge frame. Together with the sun elevation, the three parameter values for a halo not only permit calculation of the halo shape, they also give much information about the halo without extensive calculation, so that often a crude estimate of the halo’s appearance is possible merely from inspection of its parameters. As a result, the theory reveals order in what seems initially to be a staggering variety of halo shapes, and in particular it explains why halos look the way they do. Having constructed and studied the atlas, we then see where real or conceivable refraction halos, arising in specific crystal shapes and crystal orientations, fit into the atlas. Although our main goal is to understand halos arising in pyramidal crystals, the results also clarify and unify the classical halos arising in hexagonal prismatic crystals.

Atomic Layer Epitaxy

Atomic layer epitaxy (ALE) is a relatively new and generally poorly understood thin film growth technique which is likely to find application in high technology industries during the next decade. It can be used to grow high quality thin solid films with specific crystal structures or orientations and with very fine control of filn1 thicknesses to one atomic layer. Most thin film deposition techniques have, so far, simply involved material transfer from a suitable source to a suitable substrate via the vapour phase, e.g. vacuum evaporation, sputtering, or chemical vapour deposition. Nucleation takes place on the substrate surface, the molecules seeking the most energetically favourable position relative to one another and fonning n1icrocrystals that in later phases control thin film growth. This results in uncontrolled crystal boundaries, producing films whose average density can be considerably below that of the bulk crystal. Annealing is often employed to improve the quality of the deposited film. In ALE the compound thin film immediately achieves its final crystal form through sequential surface reactions in which one element of the con1pound reacts with the growing surface. This n1ethod is a self controlled process that automatically accepts only one atomic layer at a time.

High-pressure single-crystal neutron diffraction (to 20 kbar) using a pulsed source: preliminary investigation of Tl3PSe4

A new technique is described for performing high-pressure single-crystal neutron diffraction [up to 20 kbar (2 GPa) at room temperature], using a BeCu pressure cell, an area detector and the Los Alamos National Laboratory pulsed neutron source. Success of this method depends on the increase in information available with a multi-wavelength pulse neutron source, a novel orientation of a cylindrically symmetric pressure cell with its axis coincident with the neutron beam and a specific crystal orientation within the pressure cell. Bragg scattering from the pressure cell is avoided and background for a given 2θ is constant. For a crystal of orthorhombic or higher symmetry oriented with the incident beam passing midway between the major lattice vectors, it will be possible to refine a complete three-dimensional structure with data collected from only one pressure loading. Preliminary investigations of Tl3PSe4 lattice parameters (space group Pcmn) at 15(1)kbar yielded linear compressibilities (× 1000 in kbar−1) of Ka = 1.05(8), Kb = 1.50(10), Kc = 1.20(8). The anisotropic compressibility is explained by examination of the ambient-pressure room-temperature structure.

Interactive Computer-Aided Analysis for Bulk Acoustic Waves in Materials of Arbitrary Anisotropy and Piezoelectricity

Absfmcf-A computer-aided analysis program is described which makes it possible to calculate interactively, on a personal computer, the main properties of bulk acoustic waves in materials of arbitrary anisotropy and piezoelectricity. Curves of the slownesses, the electromechanical coupling constants, and the power flow angles can be simultaneously displayed for propagation in an arbitrary plane. Numerical data for propagation in a particular direction is displayed by moving an electronic cursor to the desired direction. Examples are given illustrating the usefulness of the program in the design of bulk wave transducers, the calculation of acoustooptic figures of merit, and the calculation of cutoff velocities for surface waves on layered media. The program formulation uses the material constants of a rotated coordinate system which, while it may not be the most convenient approach for some specific crystal orientation, has the pedagogical advantages of yielding simple expressions for the group velocities, power-flow angles, and electromechanical coupling constants. This paper, although it does not contain any original material, brings together in one work results which come from many sources.

Proton Magnetic Relaxation Study of Oxalic Acid Dihydrate

Abstract Proton magnetic relaxation times T1, T1ρ, and T1D were measured on a single crystal of α-form oxalic acid dihydrate in two modes: temperature dependence at one or two specific crystal orientations, and angular dependence at several temperatures. Results were compared with the calculation based on an isolated three proton system for the assumed modes of motions. The temperature dependence of T1 and that of T1ρ and T1D in the low temperature range are reasonably accounted for by the 180°-flip of the water molecule. The activation energy for the motion of 43.6 kJ mol−1 is obtained. T1ρ and T1D above about 35 °C decreased as the temperature was raised. The comparison of the observed results in this temperature range with the calculated ones suggests that there is exchange motion among the three spins. An activation energy for this motion of 80.8 kj mol−1 is obtained. This three spin exchange motion may be considered as a combination of a jump of carboxyl proton to a higher potential trough corresponding to the ionic configuration and the three-fold reorientation of the hydronium ion thus formed.

Superconducting Properties and Anisotropic Electrical Resistivities of Pure and Cadmium-Doped Tin

Approximately 60 cylindrically shaped single crystals of pure and cadmium-doped (%1 Cd) tin in the tetragonal phase were prepared. Electrical-resistivity determinations were made at 373, 273, 77, and 4.2\ifmmode^\circ\else\textdegree\fi{}K. An investigation was made of the variation of the superconducting critical-field parameters with impurity content. The anisotropy of the temperature-dependent electrical resistivity $\ensuremath{\rho}(T)$ for pure tin was determined. $\frac{{\ensuremath{\rho}}_{\mathrm{II}}(T)}{{\ensuremath{\rho}}_{\ensuremath{\perp}}(T)}$ is found to be 1.53\ifmmode\pm\else\textpm\fi{}0.01 at 373\ifmmode^\circ\else\textdegree\fi{}K, 1.555\ifmmode\pm\else\textpm\fi{}0.009 at 273\ifmmode^\circ\else\textdegree\fi{}K, and 1.684\ifmmode\pm\else\textpm\fi{}0.010 at 77\ifmmode^\circ\else\textdegree\fi{}K. ${\ensuremath{\rho}}_{\ensuremath{\perp}}(T)$ is 13.25\ifmmode\pm\else\textpm\fi{}0.05, 9.05\ifmmode\pm\else\textpm\fi{}0.03, and 1.772\ifmmode\pm\else\textpm\fi{}0.006 \ensuremath{\mu}\ensuremath{\Omega}-cm at 373, 272, and 77\ifmmode^\circ\else\textdegree\fi{}K respectively. The anisotropy of the residual resistivity ${\ensuremath{\rho}}_{0}$ for cadmium impurity was determined at 4.2\ifmmode^\circ\else\textdegree\fi{}K by two different experimental methods in different regions of impurity concentration $x$. $\frac{{\ensuremath{\rho}}_{0\mathrm{II}}}{{\ensuremath{\rho}}_{0\ensuremath{\perp}}}$ is found to be 1.6\ifmmode\pm\else\textpm\fi{}0.1 in both regions. The residual resistivity for any specific crystal orientation is found to vary linearly with $x$ ($\frac{{\ensuremath{\rho}}_{0\ensuremath{\perp}}}{x}=1.39$ \ensuremath{\mu}\ensuremath{\Omega} cm/at.%). Deviations from Matthiessen's rule are found to vary linearly [$\ensuremath{\rho}{(T)}_{\mathrm{impure}}\ensuremath{-}\ensuremath{\rho}{(T)}_{\mathrm{pure}}=0.16{\ensuremath{\rho}}_{0\ensuremath{\perp}}$] with impurity concentration and appear to be temperature-independent for $77\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}\ensuremath{\le}T\ensuremath{\le}373\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}$. The general features of the data relating the change in superconducting transition temperature (${T}_{c}$) with doping are in good agreement with the data of Lynton, Serin, and Zucker. However, there is evidence for anomalous ${T}_{c}$ behavior in the region of composition ($x\ensuremath{\approx}0.3%$) at which a lattice-spacing anomaly was reported by Lee and Raynor. The development of vacant sites as reported by Lee and Raynor in this region is not observed in plots of ${\ensuremath{\rho}}_{0\ensuremath{\perp}}$ versus $x$, however. The theory of Markowitz and Kadanoff is applied to the data relating ${T}_{c}$ to impurity concentration. Reasonable agreement between the theory and experiment is found. The mean-squared relative anisotropy of the superconducting energy gap parameter $〈{a}^{2}〉$, is found from this analysis to be 0.023. A recent theoretical treatment due to Clem which describes the change in the critical-field parameters with doping is compared to our experimental results. The so-called "similarity conditions" are found not to be satisfied. The shape of the reduced critical-field curve is found to be a function of impurity concentration. The value of $〈{a}^{2}〉$ obtained from our data employing Clem's equations is 0.022, in good agreement with the value obtained from the ${T}_{c}$ data alone. The data describing the reduced critical-field curve of pure tin is in good agreement with the results od Shaw et al. The change in shape of the reduced critical-field curve can be understood both qualitatively and quantitatively in terms of the reduction of the effects of $〈{a}^{2}〉$ with doping as given by Clem's theoretical treatment.