We report flexible synthesis of group III–nitride nanowires and nanostructures by metalorganic chemical vapor deposition (MOCVD) via a catalytic vapor-liquid-solid (VLS) growth mechanism. Supersaturation and surface stoichiometry strongly influence the stability of liquid droplets and growth selectivity. To facilitate and sustain the VLS growth, indium catalyst is introduced based on thermodynamic consideration. The employment of mesoporous molecular sieves (MCM-41) helps to prevent the coalescence of catalyst droplets and to promote nucleation statistics. Both GaN and AlN nanowires have been synthesized using MOCVD. Three-dimensional AlN∕GaN trunk-branch nanostructures are reported to illustrate the versatility of incorporating the VLS mechanism into MOCVD process.
Growth of ternary AlGaN nanowires using metalorganic chemical vapor deposition is investigated. Structural, chemical, and optical characterization at nanoscopic scale is carried out by high resolution transmission electron microscopy, x-ray energy dispersive spectroscopy, and spatially resolved cathodoluminescence. Spontaneous formation of Al(Ga)N∕GaN coaxial nanowires with distinct emission at 370 nm is observed. It is identified that the interplay between surface kinetics and thermodynamics facilitates the catalytic growth of GaN core while a limited surface diffusion of Al adatoms leads to nonselective, vapor-solid growth of Al(Ga)N sheath. The observation points to a fundamental difference in nanosynthesis using near-equilibrium and nonequilibrium techniques.
The gamma-ray blazar 1611+343 was observed with polarization VLBI mode at 5 GHz in February 1999. The total intensity (I) VLBI image of the source shows a core-jet structure. The jet bends eastward at ∼ 3 mas south of the core. Four components have been detected from results of fitting, with apparent speeds estimated at 6.7 ± 0.7, 2.5 ± 0.3, 4.5 ± 0.5 h −1 c for three jet components (taking H 0 = 100 h km s −1 Mpc −1 , q 0 = 0.5). The polarization (P ) VLBI image of 1611+343 displays the polarized configuration in the jet. The mechanism of the curved jet is discussed.
Surface morphological changes in ZnSe-related II–VI epitaxial films grown by molecular beam epitaxy have been investigated by atomic force microscopy and transmission electron microscopy. We found that under group-II-rich conditions with c(2×2) surface reconstruction, the process of roughening gives rise to periodic elongated corrugations aligned in the [11̄0] direction. Under group-VI-rich conditions with (2×1) surface reconstruction, rounded grains form instead of corrugated structures. The surface morphology is dependent on the VI/II ratio and growth temperature, but is independent of the film strain. The observed morphological changes are mainly due to growth kinetics and are not stress driven. We propose a model to explain the changes in surface morphology under group-II-rich conditions and group-VI-rich conditions.
A very shallow p+n junction was formed by BF2
+ ion implantation and excimer laser annealing (ELA) with single pulse irradiation. A thin amorphous layer formed by low-energy implantation was recrystallized effectively by single-pulse irradiation of an excimer laser. A shallow junction with little redistribution and sheet resistance as low as 100 Ω\Box was obtained. The junctions formed by ELA were compared with those formed by conventional rapid thermal annealing (RTA). It was confirmed that ELA with single-pulse irradiation can suppress unintentional diffusion of the implanted boron, and is more suitable for controlling the impurity profile of very shallow p+n junctions than RTA.
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