Far-field emission patterns of nanowire light-emitting diodes

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.