Hydrogen-enhanced recrystallization in N<Superscript>+</Superscript>-implanted GaAs

The hydrogen-enhanced recrystallization during thermal annealing in N⁺-implanted GaAs has been studied by combinatorial implantation process. Raman spectroscopy was used to study the crystallization properties of a set of hydrogenated cells on the N⁺-implanted GaAs wafer. A whole competitive process between H⁺ implantation-induced damage and recovery in the regrowth process of amorphous GaAs was observed within the proton dose region of 1.6×10¹⁵ to 1.1×10¹⁷ cm^-2. In H⁺ dose region of 2.1×10¹⁶ to 5.4×10¹⁶ cm^-2, H-enhanced recovery of crystal dominates the regrowth process. The crystal quality is better than that of unhydrogenated cell of N⁺-implanted GaAs in the H⁺ dose range from 4.7×10¹⁶ to 8.1×10¹⁶ cm^-2. It is suggested that the vacancy supersaturation produced during hydrogen irradiation is dominantly responsible for the enhancement of thermal regrowth in the N⁺-implanted GaAs. Both the crystallization and amorphization process are clearly observed in different proton implantation dose regions. PACS 61.72.Vv; 63.20.Dj; 81.05.Ea     

Fabrication and microstructure analysis of SeO<Subscript>2</Subscript> nanowires

Selenium dioxide nanowires were fabricated as the co-products of ZnSe nanostructures. The SeO₂ nanowires have diameters between 2070 nm and lengths up to several hundred micrometers. The morphology and microstructure of the nanowires were analyzed using TEM, and the growth mechanism of the SeO₂ nanowires was explained under the framework of a vapor-solid model, in which structure defects may play a very important role in the nanowire growth. The nanostructured SeO₂ materials may find application in both catalytic and biological fields. PACS 81.07.-b; 81.16.-c; 81.07.Nb; 81.05.-t; 68.37.Lp     

Synthesis and the magnetoresistance effect of layered-perovskite La<Subscript>2.5-x</Subscript>K<Subscript>0.5+x</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7+δ</Subscript> (0<<Emphasis Type="Italic">x</Emphasis><0.5) polycrystals

Polycrystalline layered-perovskites La_2.5-xK_0.5+x·Mn₂O₇₊ (0<x<0.5) have been prepared by a modified sol-gel method and their structures, and magnetic and electric properties have been studied. Experimental results show that these materials exhibit ferromagnetic-to-paramagnetic phase transitions at 200, 225, 235, 247, and 253 K for x=0.05, 0.15, 0.25, 0.35, and 0.45, respectively. A large deviation between the metal-insulator transition temperature (T) and the magnetic transition temperature (T_C) is observed, and a large magnetoresistance (MR) effect with /₀ of 40% at 12 kOe is obtained over a wide temperature range. These behaviors are quite different from those observed in the well-knownABO₃-type perovskite manganites. PACS 75.30.Gw; 75.30.Et; 81.20.-n     

C and Si ion implantation and the origins of yellow luminescence in GaN

We have studied the influence of C and Si ion implantation with different implantation doses on yellow luminescence (YL) from GaN. Three kinds of GaN samples were used. In their as-grown states, #1 samples had strong YL, #2 samples had no YL, while #3 samples had weak YL. Our experimental results show that: (i) after annealing at 950 °C, the YL intensity for Si ion implanted #1 sample decreased with increasing implantation dose, while that for C ion implanted #1 sample exhibited a reverse rule; (ii) for #2 samples, C ion implantation produced much stronger YL than Si ion implantation did after annealing at 950 °C; (iii) the YL intensity sequence for Si ion implanted and 950 °C annealed #1, #2, and #3 samples was consistent with that for the unimplanted #1, #2, and #3 samples. However, the YL intensity sequence for the C ion implanted and 950 °C annealed #1, #2, and #3 samples reversed with that for the unimplanted ones. In order to explain all these phenomena, we suggested a physical model which claims that the deep C center is another important origin of YL in GaN, and during C ion implantation, the C ion prefers to combine with a V_Ga to form a C_Ga. PACS 78.55.-m, 85.40.Ry, 68.55.Ln     

Diffusion-limited photopolymerization in scanning micro-stereolithography

The trade-off between process speed and resolution in microstereolithography (SL) roots on the diffusion-limited kinetics of photopolymerization. Using a numerical model, we have investigated the influence of diffusion dominant effect under high photon flux. Radical depletion turned out to limit the smallest feature achievable to the order of 10 m under high process speed. A solution of pulsed laser curing is proposed in order to realize sub-micron resolution in high speed SL process. PACS 66.30.Ny; 85.85.+j; 85.40.Ux     

Dynamics of femtosecond laser interactions with dielectrics

Femtosecond laser pulses appear as an emerging and promising tool for processing wide bandgap dielectric materials for a variety of applications. This article aims to provide an overview of recent progress in understanding the fundamental physics of femtosecond laser interactions with dielectrics that may have the potential for innovative materials applications. The focus of the overview is the dynamics of femtosecond laser-excited carriers and the propagation of femtosecond laser pulses inside dielectric materials. PACS 61.80.Ba; 52.38.Mf; 42.65.Jx; 78.47.+p; 71.35.-y     

Identification of pre-sliding friction dynamics

The hysteretic nonlinear dependence of pre-sliding friction force on displacement is modeled using different physics-based and black-box approaches including various Maxwell-slip models, NARX models, neural networks, nonparametric (local) models and dynamical networks. The efficiency and accuracy of these identification methods is compared for an experimental time series where the observed friction force is predicted from the measured displacement. All models, although varying in their degree of accuracy, show good prediction capability of pre-sliding friction. Finally, we show that even better results can be achieved by using an ensemble of the best models for prediction.     

Coaxial ZnO/SiO<Subscript>2</Subscript> nanocables fabricated by thermal evaporation/oxidation

ZnO/SiO₂ coaxial nanocables have been synthesized on silicon substrates by simply evaporating zinc powder under an argon and argon/oxygen mixed atmosphere sequentially. The diameters of these nanocables vary from 50 to 100 nm and the lengths up to several millimeters. Electron microscopy and chemical composition investigations reveal that the nanocable consists of a crystalline ZnO core surrounded by an amorphous silica sheath. The electron diffraction pattern proves that the long-axis direction of ZnO cores grows along the [0001] direction. Silica nanotubes with wall structures have been obtained by the selective dissolution of the cores with hydrochloric acid. PACS 81.10.Bk; 81.05.Hd     

Leakage current and pyroelectric properties of compositionally graded (Pb,Ca)TiO<Subscript>3</Subscript> films

Highly (100)-oriented, compositionally graded (Pb,Ca)TiO₃ (PCT) thin films with a Ca content from 0 to 24 mol% on Pt/Ti/SiO₂/Si substrates were prepared by a sol-gel process. The graded structure of the Au/PCT/Pt film capacitor showed a well-saturated hysteresis loop at an applied field of 500 kV/cm with remanent polarization (P_r), and coercive electric field (E_c) values of 9.35 C/cm² and 130 kV/cm, respectively. At 100 kHz, the dielectric constant and dielectric loss of the film were 129 and 0.024, respectively. The leakage current density of the graded PCT film was less than 1.0×10^-7 A/cm² over a voltage range from 0 to 4 V. The conduction current depended on the voltage polarity. At low electric field (110 and 180 kV/cm, respectively, for Pt and Au electrodes biased negatively), the Au/PCT and PCT/Pt interfaces form a Schottky barrier. At high electric field (>110 kV/cm), the Au electrode biased negatively shows space-charge-limited current (SCLC) behavior. The temperature dependencies of the pyroelectric coefficients of the graded PCT film were measured by a dynamic technique. From 20 to 82 °C, the pyroelectric coefficients of graded PCT film remain steady in the range 106 to 118 C/m²K. The detectivity figure of merit (F_D) of the graded PCT film was 6.7×10^-6 Pa^-0.5. PACS 77.80.-s; 77.70.+a; 77.22.-d; 51.50.+v; 68.37.-d     

Self-assembly of CuS nanoparticles to solid,hollow, spherical and tubular structures in a simple aqueous-phase reaction

We report fabrication of CuS particles with solid, hollow, spherical and tubular structures in a simple aqueous system under microwave irradiation, employing CuSO₄ and Na₂S₂O₃ as the starting materials without assistance of any surfactant or template. Energy-dispersive X-ray analysis and an X-ray powder diffraction pattern proved that the product is hexagonal CuS phase. The morphologies of the product were observed by scanning electron microscopy and transmission electron microscopy. Some factors affecting the morphologies of the product are discussed. PACS 81.05.Hd; 81.07.Bc; 81.16.Be; 81.16.Dn; 81.20.Ka