We report on the fabrication of GaAs hexagonal nanowires surrounded by ͕110͖ vertical facets on a GaAs ͑111͒ B substrate using selective-area ͑SA͒ metalorganic vapor-phase epitaxial ͑MOVPE͒ growth. The substrate for SA growth was partially covered with thin SiO 2 , and a circular mask opening with a diameter d 0 of 50-200 nm was defined. After SA-MOVPE, GaAs nanowires with a typical diameter d ranging from 50 to 200 nm and a height from 2 to 9 m were formed vertically on the substrate without any catalysts. The size of the nanowire depends on the growth conditions and the opening size of the masked substrate. A possible growth mechanism is also discussed.

We report on a catalyst-free approach for the growth of semiconductor nanowires which is attracting interest as building blocks for nanoscale electronics and circuits. Our approach is based on selective-area MOVPE and nanowires are grown from small circular openings of SiO 2 mask defined on (111)B-oriented substrates. At optimized conditions, extremely uniform array of GaAs and InGaAs nanowires with diameter of about 200 nm was grown on GaAs and InP substrates, respectively. The nanowires have hexagonal cross-section and are perpendicular to the substrates, indicating that they are surrounded by {110} facet sidewalls. By reducing the mask opening size, nanowires with diameter down to 50 nm and length more than 5µm were successfully formed. Photoluminescence and transmission electron microscopy characterization was also carried out. (This article is published in J.

We fabricated GaAs/ AlGaAs core-shell nanowires by using selective-area metalorganic vapor phase epitaxy. First, GaAs nanowires were selectively grown on partially masked GaAs ͑111͒B substrates; then AlGaAs was grown to form freestanding heterostructured nanowires. Investigation of nanowire diameter as a function of AlGaAs growth time suggested that the AlGaAs was grown on the sidewalls of the GaAs nanowires, forming GaAs/ AlGaAs core-shell structures. Microphotoluminescence measurements of GaAs and GaAs/ AlGaAs core-shell nanowires reveal an enhancement of photoluminescence intensity in GaAs/ AlGaAs core-shell structures. Based on these core-shell nanowires, AlGaAs nanotubes were formed by using anisotropic dry etching and wet chemical preferential etching to confirm the formation of a core-shell structure and to explore a new class of materials.

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We report on the growth of GaAs and GaAs/AlGaAs heterostructured hexagonal pillar structures using selective area (SA) metalorganic vapor phase epitaxy (MOVPE). By doing growth on SiO 2 masked (111)B GaAs substrates with circular or hexagonal hole openings, extremely uniform array of hexagonal GaAs/AlGaAs pillars consisting {110} vertical facets with their diameter of order of 100 nm were obtained. Unexpectingly strong intense light emission was observed for the room temperature photoluminescence measurement of the pillar arrays in triangular lattice, which is promising for the application to the photonic crystals to enhance the light extraction efficiency from the materials with high refractive index. Furthermore, it was also found that hexagonal pillars with size 60 nm and large aspect ratio (> 100) by reducing the size of initial hole size of mask, opening a possibility to grow nanowires using epitaxial growth. (This article is published in Physica E, 23, 3-4, pp. 298-304 (2004).)

Single InGaAs nanowire-top-gate metal-semiconductor field-effect transistors (MESFETs) were fabricated and characterized. Silicon-doped n-InGaAs nanowires (with a typical diameter of 100 nm) were grown by catalyst-free selective-area metalorganic vapor-phase epitaxy (SA-MOVPE). The FETs of single nanowires on SiO 2 -coated Si substrates were fabricated by defining metal contacts at both ends of the nanowires and the metal top gate between contacts. According to the measurements of drain current-voltage and gate transfer characteristics, the top-gate MESFETs exhibited significant enhancements in device performance characteristics compared with FETs under back-gate operation; that is, a peak transconductance of 33 mS/mm and a current on-off ratio of 10 3 were obtained. A possibility for further improvements in FET characteristics was also considered.