Spectral reflectance of core-shell GaN-(Al/Hf)O_x nanowires within adapted effective medium approximation

Gallium nitride-based nanowires (NWs) overcome heteroepitaxy limits, enabling GaN-on-silicon devices, and offer high sensitivity for detection, sensing, and photocatalysis. Additional nanowire coating enhances their performance, protects against photoadsorption, and enables control over structural and optical properties. In this work, we investigate core-shell GaN-(Al/Hf)O_x nanowires, which meet the aforementioned expectations. We focus on reflectance, a fundamental optical parameter indicating structure applicability. We identified the proper dependence between reflectance spectra and shell composition and thickness, based on microscopic imaging, and adapted effective medium approximation. Finally, we expand upon a previous model, achieving great agreement between the simulation and the experimental data. Thus, we demonstrate how the description of complex nanostructures can be simplified and the composition of the nanowire ensemble can be estimated.