We investigated far-field (FF) emission patterns of nanowire light-emitting diodes (NW-LEDs). NW-LEDs were fabricated using vertical InP-NW arrays with axial pn-junctions grown on InP (111)A substrates, and the emission intensity of NW-LEDs was measured as a function of view angle θ, where θ = 0° indicates the direction normal to the substrate or that along the NWs. For NW arrays with pitch a of around 1 μm, we found a clear dip in the emission intensity at θ = 0°, which was explained by an analogy with dipole antenna, or a smaller contribution of the lowest order guided modes for emission as compared with higher order guided and free-space radiation modes. Results of the simulation of radiation patterns by the finite-difference time-domain method and near-field to far-field transformation are also described. They also confirm that the dip at θ = 0° is specific to light emission from NWs. We also investigated the dependence of the FF pattern on the pitch of the NW array, and the observation was qualitatively explained by the relative contribution of the guided and free-space radiation modes.
Control of the diameter and pitch of InGaAs nanowire arrays in selective-area metalorganic vapor-phase epitaxy was investigated. It was found that their nucleation was strongly dependent on the geometry of the mask, resulting in the difficulty of nucleation for a larger mask pitch, particularly for an opening diameter of less than 50 nm. Precise adjustment of the V/III ratio enabled us to control the nucleation independently of the mask pitch for smaller openings, and we successfully obtained 30-nm-diameter InGaAs nanowires independently of the mask pitch by the proposing V/III-ratio-controlled two-step growth method. #
Indium-rich InGaAs nanowires were grown on an InP(111)B substrate by catalyst-free selective-area metal-organic vapor phase epitaxy, and its growth-temperature dependence of growth rate and composition was studied. In particular, nanowire growth rate rapidly decreases as growth temperature increases. This tendency is opposite (for a similar temperature range) to that found in a previous study on selective-area growth of gallium-rich InGaAs nanowires. This difference between indium-rich and gallium-rich nanowires suggests that the influence of growth temperature on the growth of InGaAs nanowires is dependent on the group-III supply ratio. On the basis of previous experimental observations in InAs and GaAs nanowires, temperature dependence of nanowire growth rate and its dependence on group-III supply ratio are predicted. A guideline to determine the optimum growth conditions of InGaAs nanowires is also discussed.
Keywords:A1. Nanostructures
We attempted the selective-area metalorganic vapor-phase epitaxial (SA-MOVPE) growth of InAs nanowires (NWs) using a tungsten/dielectric composite mask and fabricated nanowire vertical surrounding-gate field-effect transistors (NW-VSG-FETs), where tungsten served as both the mask in SA-MOVPE growth and the bottom electrode of the FET. The growth of NWs with diameters as low as 100 nm was demonstrated using the composite mask. The fabricated NW-VSG-FET exhibited improved drain current density as compared with our previously reported NW-VSG-FETs, and a larger on/off ratio as compared with previously reported NW-VSG-FETs having similar electrodes at the bottoms of NWs.
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