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We have developed a method to fabricate GaN planar nanowires and cavities by combination of Focused Ion Beam (FIB) patterning of the substrate followed by Metal Organic Vapor Phase Epitaxy (MOVPE). The method includes depositing a silicon nitride mask on a sapphire substrate, etching of the trenches in the mask by FIB with a diameter of 40 nm with subsequent MOVPE growth of GaN within trenches. It was observed that the growth rate of GaN is substantially increased due to enhanced bulk diffusion of the growth precursor therefore the model for analysis of the growth rate was developed. The GaN strips fabricated by this method demonstrate effective luminescence properties. The structures demonstrate enhancement of spontaneous emission via formation of Fabry-Perot modes.

GaN planar nanowires (NWs) are fabricated by selective area metal-organic vapor phase epitaxy using focused ion beam etching of trench pattern in the Si 3 N 4 mask. Two crystallographic orientations of NWs along 11 20 ½ and 10 10 ½ directions are investigated. The coherent growth is confirmed for both directions; however, the best morphology, crystalline and optical properties are found in the GaN planar NWs fabricated along the 10 10 ½ axis. Cathodoluminescence (CL) at 5 K reveals a presence of Fabry-Perot modes in the region of 1.8-2.5 eV for the NWs fabricated in the 10 10 ½ direction. The position and intensity of the Fabry-Perot peaks vary depending on measured point within the NW, which is explained by the model based on the Purcell coefficient calculations. It is shown that small fluctuations in the NW thickness cause a noticeable shift of the Fabry-Perot modes energies, while the enhancement or reduction of the emission intensity for the Fabry-Perot peaks depend on the position of the emitter inside the planar NW.

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1 Introduction Metal-dielectric nanocomposites (wire metamaterial) based on porous alumina matrices fabricated by electrochemical eatching attract increasing attention for many reasons, ranging from fundamental research problems (study of nonlinear properties [1]) to applications in many different fields (biosensors [2], optical metamaterials [3], nanophotonics [4], etc.). Other materials which have similar structural parameters (e.g. aspect ratio) and optical properties are metallic nanotubes based on dielectric [5] and semiconductors matrices [6,7]. However, as is shown in the theoretical [8] and experimental [9] works, the medium with nanotubes is not a suitable material to realise the properties of wire metamaterials because the latter require the thickness of the tubes to be greater than the skin depth of a metal for proper operation (at least 15-20 nm for operation in the visible domain). The investigation and application of the unique properties of these materials face significant challenges, such as small values of the aspect ratio (less than 100) and low values of electric permittivity, ε, in dielectric matrices. Another material in which porous matrices can be created by an anodic electrochemical etching [10,11] is an A III B V semiconductor. In this case the nanoporous matrices give a unique set of parameters: the aspect ratio greater than 10000, the thickness of porous matrices up to 150 μm and more [12]. Using different orientations of the original semiconductor crystal, etching regimes and types of electrolyte, one can control the param-