Nucleation of III nitride semiconductors in heteroepitaxy

The nucleation of III nitride semiconductors in heteroepitaxy is theoretically investigated using GaN nucleation on the AlN surface as an example. It is inferred that the mechanism of this process is determined by the temperature at the initial stage of the layer formation (T). At low temperatures (T<500°C), liquid gallium droplets appear and the chemical reaction between the Ga and N atoms results in the formation of GaN nuclei. At substrate temperatures T>650°C, there arise only GaN nuclei. It is revealed that the GaN nucleation is governed by the generalized diffusion coefficient of GaN, which is a combination of the diffusion coefficients for gallium and nitrogen atoms. It is shown that the generalized diffusion coefficient of GaN on the crystal surface increases by seven orders of magnitude as the growth temperature increases from 600 to 800°C. This is accompanied by a change in the growth mechanism of the III nitride semiconductor epitaxial layers.     

Anodic oxidation of Cr3+ ions in a chromium electroplating bath on Pt and composite TiO x /PtO y

The effect of various factors (anodic current density, composition of electrolyte, electrode material) on current efficiency of the anodic reaction of formation of Cr(VI) ions is established on Pt and composite TiO _x /PtO _y under electrolysis of solutions containing Cr³⁺ ions. It is shown that the current efficiency values of Cr(VI) on TiO _x /PtO _y decrease at an increase in the active layer thickness and increase in the electrode treatment temperature. It is found that the major part of Cr(VI) compounds formed on the anode is reduced on the cathode and as a result of homogeneous chemical reactions in the bulk of electrolyte.     

Thermoelectric properties of p-Bi2 ? xSbxTe3 solid solutions under pressure

This paper reports on a study of the Seebeck coefficient and power factor κ of p-Bi_{2 − x} Sb _x Te₃ solid solutions with different contents of antimony atoms in the bismuth sublattice for x = 0, 1.4, 1.5, and 1.6 under variation of pressure of up to 15 GPa. The magnitude of κ has been found to grow nonmonotonically within the pressure region of 2–4 GPa. The effective mass of the density of states m/m ₀ and the mobility μ₀ have been calculated with due account of degeneracy within the parabolic model of the energy spectrum assuming isotropic charge carrier scattering. It has been shown that application of pressure brings about a decrease of the effective mass m/m ₀ and an increase of carrier mobility. The power factor κ of the p-Bi_0.6Sb_1.4Te₃ composition exhibits at the pressure P ≈ 4 GPa the largest increase of the power factor κ as a result of a weak decrease of the effective mass m/m ₀ and an increase of carrier mobility as compared to the other solid solution compositions. The specific feature of the variation of the power factor κ with a change of the pressure in bismuth telluride near P ≈ 3 GPa, which is accompanied by formation of a knee in the m/m ₀ vs. P dependence, can be assigned to an electronic topological transition.     

In-situ mechanical characterization of wurtzite InAs nanowires

High aspect ratio vertical InAs nanowires were mechanically characterized in a scanning electron microscope equipped with two micromanipulators. One, equipped with a calibrated atomic force microscope probe, was used for in-situ static bending of single nanowires along the 〈11–20〉 crystallographic direction. The other one was equipped with a tungsten tip for dynamic resonance excitation of the same nanowires. This setup enabled a direct comparison between the two techniques. The crystal structure was analyzed using transmission electron microscopy, and for InAs nanowires with a hexagonal wutzite crystal structure, the bending modulus value was found to BM=43.5 GPa. This value is significantly lower than previously reported for both cubic zinc blende InAs bulk crystals and InAs nanowires. Besides, due to their high resonance quality factor (Q>1200), the wurtzite InAs nanowires are shown to be a promising candidate for sub-femtogram mass detectors.     

Field-programmable gate arrays and quantum Monte Carlo: Power efficient coprocessing for scalable high-performance computing

Massively parallel architectures offer the potential to significantly accelerate an application relative to their serial counterparts. However, not all applications exhibit an adequate level of data and/or task parallelism to exploit such platforms. Furthermore, the power consumption associated with these forms of computation renders “scaling out” for exascale levels of performance incompatible with modern sustainable energy policies. In this work, we investigate the potential for field-programmable gate arrays (FPGAs) to feature in future exascale platforms, and their capacity to improve performance per unit power measurements for the purposes of scientific computing. We have focused our efforts on variational Monte Carlo, and report on the benefits of coprocessing with a FPGA relative to a purely multicore system.     

SnS and SnS2 thin films deposited using a spin‐coating technique from intramolecularly coordinated organotin sulfides

Synthesis and applications of organotin(II) sulfide ({2,6‐(Me₂NCH₂)₂C₆H₃}Sn)₂(μ‐S) ( 1 ), organotin(II) thiophenolate {2,6‐(Me₂NCH₂)₂C₆H₃}Sn(SPh) ( 2 ) and organotin(IV) heptasulfide {2,6‐(Me₂NCH₂)₂C₆H₃}₂Sn₂S₇ ( 3 ) as potential single‐source precursors (SSPs) for the deposition of SnS or SnS₂ thin films using a spin‐coating method are reported. Compounds 1 , 2 and 3 differ either by tin oxidation state or by Sn:S ratio (Sn:S = 2:1 in 1 , 1:1 in 2 and 2:7 in 3 ). It is shown that compound 1 is not a suitable SSP for thin‐film fabrication using the spin‐coating process because of its incomplete decomposition at annealing temperature. However, compounds 2 and 3 seem to be promising SSPs for spin‐coating of amorphous semiconducting thin films of SnS and SnS₂, respectively. Copyright © 2015 John Wiley & Sons, Ltd.