Geometry-related optical properties in vertically aligned GaAs nanowires arrays: A study of sizes and embedding medium effects

We investigate the effects of the geometrical sizes and surrounding media on the optical properties of a periodic square array of GaAs nanowires (NWs) as candidates for applications on photovoltaic solar cells as well as sensors. As filling media, different insulator materials such as PMMA, Polycarbonate, Polystyrene and PVP are considered in this study. The simulated system is made of three stacked layers along with two semi-infinite air spaces. The stacked layer system is defined by a transparent layer of Indium Tin Oxide (ITO), the active region of GaAs nanowires array embedded into the filling medium and a substrate of Silicon (Si). The multilayer absorbance is simulated by transfer matrix method (TMM); while the GaAs nanowires surrounded by filling medium is treated as an homogeneous layer whose effective dielectric function is described by Bruggeman effective medium theory. For both s- and p-polarizations at normal and oblique incidences, we observe a typical oscillating behavior in the absorbance/reflectance spectra, regardless the type of embedding medium and sizes investigated. For all investigated sizes, we obtain that the absorbance values decrease for longer wavelength. Greater D/P ratio for longer NWs benefit the light trapping with absorbance values up to 80 per cent in the VIS range. For small variations of the refractive index (dielectric constant) of the filling media surrounding GaAs NWs, we observed a perceptible change in their reflectance spectral values at specific wavelengths. On the other hand, the Brewster angle, calculated at the interface of the ITO layer and the effective embedded nanowires composite, is seen to be sensitive to the D/P ratio size, as far as the wavelength is varied. The above numerical results are in good agreement with those reported in the literature by experimental research and sophisticated FDTD simulations in III–V semiconductor nanowires, which demonstrates that our TMM simulations are adequate approximations to account on optical properties in multilayered structures when homogenization of multilayers is required.