Lateral Composition Gradients Research Articles

Homogenous indium distribution in InGaN/GaN laser active structure grown byLP-MOCVD on bulk GaN crystal revealed by transmission electron microscopy and x-raydiffraction

We present transmission electron microscopy (TEM) and x-ray quantitative studies of the indium distribution inInxGa1−xN/GaN multiple quantumwells (MQWs) with x = 0.1 and 0.18. The quantum wells were grown by low-pressure metalorganic chemical vapourdeposition (LP-MOCVD) on a bulk, dislocation-free, mono-crystalline GaN substrate. Byusing the quantitative TEM methodology the absolute indium concentration wasdetermined from the 0002 lattice fringe images by the strain measurement coupled withfinite element (FE) simulations of surface relaxation of the TEM sample. In the x-raydiffraction (XRD) investigation, a new simulation program was applied to monitor theindium content and lateral composition gradients. We found a very high qualityof the multiple quantum wells with lateral indium fluctuations no higher thanΔxL = 0.025. The individual wells have very similar indium concentration and widths over the wholemultiple quantum well (MQW) stack. We also show that the formation of ‘false clusters’ isnot a limiting factor in indium distribution measurements. We interpreted the ‘falseclusters’ as small In-rich islands formed on a sample surface during electron-beam exposure.

Sharply defined lateral composition gradients of copper on gold by spatiotemporal control of the in-plane electrochemical potential distribution

Injecting an in-plane current by connecting the two ends of a thin Au film to different potentials relative to a common reference couple results in the formation of a linear potential gradient at the surface of the electrode. Coupling the surface potentials to a redox species in solution then causes spatially dependent redox chemistry, effectively mapping the electrochemical reaction onto the surface. Copper was selectively deposited to the surface of the Au film by selecting applied potentials that encompass E eq for the Cu 2+/Cu 0 reduction. Upon potential application copper deposits onto the area of the electrode for which V( x)< E eq, while the areas with local potentials positive of E eq contained no observable Cu. A transition region is established on the surface in an area corresponding to the anodic peak (strip-out) potential. Careful calibration of the surface electrochemical potential gradient indicates that the transition occurs at ca. 116 mV in agreement with an anodic peak potential centered at ca. 125 mV vs. Ag/AgCl from cyclic voltammetry. The width of the interfacial region analyzed by Auger electron spectroscopic line profiling and atomic force microscopy (AFM) is found to be dramatically sharper than expected. For a reversible two-electron process the development of surface composition based on a Nernstian local potential would yield a transition region ∼59/2 mV wide. Translated to the spatial domain the interfacial width should be ∼243 μm when a 123 mV/mm electrochemical potential gradient is imposed. Auger and AFM data confirm the interfacial width is strikingly smaller—by a factor of approximately 10—strongly suggesting that either the local electrochemical potential, V( x), is altered significantly upon Cu deposition, or V( x) alone is not responsible for the sharpness of the interface.

Deformations of accreting neutron star crusts and gravitational wave emission

Motivated by the remarkably narrow range of measured spin frequencies of ∼20 accreting (and weakly magnetic) neutron stars in the Galaxy, Bildsten conjectured that their spin-up had been halted by the emission of gravitational waves. If so, then the brightest persistent X-ray source on the sky, Scorpius X-1, should be detected by gravitational wave interferometers within 10 years. Bildsten pointed out that small non-axisymmetric temperature variations in the accreted crust will lead to ‘wavy’ electron capture layers, and the resulting horizontal density variations near e− capture layers create a mass quadrupole moment. Neglecting the elastic response of the crust, Bildsten estimated that even e− capture layers in the thin outer crust can develop the quadrupole necessary to balance accretion torque with gravitational waves, for accretion rates We present a full calculation of the crust's elastic adjustment to the density perturbations induced by the temperature-sensitive e− capture reactions. We find that, due to the tendency of the denser material to sink rather than spread sideways, neglecting the elastic response of the crust overestimates, by a factor of 20–50, the Q22 that results from a wavy capture layer in the thin outer crust. However, we find that this basic picture, when applied to capture layers in the deep inner crust, can still generate Q22 in the necessary range, as long as there are ≲5 per cent lateral temperature variations at densities in excess of 1012 g cm−3, and as long as the crustal breaking strain is high enough. By calculating the thermal flow throughout the core and the crust, we find that temperature gradients this large are easily maintained by asymmetric heat sources or lateral composition gradients in the crust. If the composition or heating asymmetries are independent of the accretion rate, then for the induced quadrupole moments have approximately the same scaling, ∝M1/2, as that necessary to balance the accretion torque at the same spin frequency for all M. Temperature gradients in the deep crust lead to a modulation in the thermal emission from the surface of the star that is correlated with Q22. In addition, a ∼0.5 per cent lateral variation in the nuclear charge-to-mass ratio in the crust will also result in a Q22 sufficient to halt spin-up from accretion even in the absence of a lateral temperature gradient. We also derive a general relation between the stresses and strains in the crust and the maximum quadrupole moment they can generate. We show, under quite general conditions, that maintaining a Q22 of the magnitude necessary to balance the accretion torque requires a dimensionless strain at near-Eddington accretion rates, of order the breaking strain of conventional materials. This leads us to speculate that accreting neutron stars reach the same equilibrium spin because they all are driven to the maximum Q22 that the crust can sustain.

Effects of Electrodiffusion on the Pb-Sn Eutectic Growth

In the growth of a binary eutectic alloy, the formation of two new solid phases results in an unusual solute segregation process. In order for growth to happen, one of the solid phases must experience a decrease in the initial melt concentration of one of the alloy constituents and an increase in the amount of the other element. An analogous phenomenon occurs with respect to the other phase. The lateral composition gradient gives rise to mass transport parallel to the growth direction and across the solid/liquid interface, which controls the liquid interfacial composition field. The presence of an electric field during solidification can change the interfacial liquid composition distribution and thereby the growth condition. In this work, the Pb-Sn eutectic alloy was directionally solidified under the influence of an electric field normal to the growth direction. The results obtained show that electrodiffusion due to the electric field can change the eutectic lamellar spacing, the lamellar growth orientation and the eutectic microstructure regularity.

Compositional gradients in Ga xIn 1− xAs on patterned InP substrates grown by atmospheric-pressure metalorganic vapour phase epitaxy

The growth rate and composition of Ga xIn 1−xAs layers on nonplanar InP substrates deposited in the MOVPE process at atmospheric pressure using the precursors trimethylgallium-trimethylamine (TMGa-TMN) and trimethylindium-trimethylamine (TMIn-TMN) were investigated. Comparing the growth rates r on the different (001) areas of the nonplanar substrate, we found that r ridge > r planar > r groove. Studying the composition of the Ga xIn 1−xAs on planar substrates, we observed x Ga(001)> x Ga{111}A > x Ga{111)B. Similarly, on patterned samples the (001) plane is that with the highest gallium concentration. On the (001) plane of the nonplanar substrate, the tendency x Ga(ridge) x Ga(planar) x Ga(groove) was observed. Superimposed on this we observed, adjacent to the polar {111} planes, lateral composition gradients, which depend on the orientation of the sidewalls. The experimental results are discussed, using a stagnant boundary layer model. Different reactivities of the Ga- and In-organometal compounds are considered.

Influence de la forme de l'interface liquide-solide sur la diffusion chimique pendant une solidification eutectique lamellaire

In-situ observations have yet shown that the liquid-solid interface during eutectic growth can be markedly corrugated. The influence of the interface shape on the chemical diffusion process in the liquid is evaluated in this paper. The periodicity of the boundary conditions allows a Fourier analysis of the problem of the mass transfer in the liquid. It leads to a matricial equation that could be solved analytically. Computer calculations were carried out for the Pb-Sn eutectic system assuming circular shapes of the cross section of the interface between the liquid and each lamella. Even when the volume fraction satisfies the lever rule, the mean liquid composition along the interface can be strongly different from the eutectic composition for a corrugated interface. The discrepancy depends mainly on the bulk composition of the liquid and on the leading distance of one phase on the other. Moreover, the more convex the interface is, the lower are the lateral composition gradients.