Abstract
Advancements in nanotechnology have facilitated the increased use of ZnO nanostructures. In particular, hierarchical and core–shell nanostructures, providing a graded refractive index change, have recently been applied to enhance the photon extraction efficiency of photonic emitters. In this study, we demonstrate self-aligned hierarchical ZnO nanorod (ZNR)/NiO nanosheet arrays on a conventional photonic emitter (C-emitter) with a wavelength of 430 nm. These hierarchical nanostructures were synthesized through a two-step hydrothermal process at low temperature, and their optical output power was approximately 17% higher than that of ZNR arrays on a C-emitter and two times higher than that of a C-emitter. These results are due to the graded index change in refractive index from the GaN layer inside the device toward the outside as well as decreases in the total internal reflection and Fresnel reflection of the photonic emitter.
Highlights
The advancements in nanotechnology have facilitated the increased use of ZnO nanostructures that, for example, are widely utilized in photonic devices because of their peculiar chemical and physical properties [1,2,3,4,5]
The ZnO nanostructures can partially mitigate the abrupt change of refractive indices between p-type GaN and air, total internal reflection (TIR) and Fresnel reflection losses occur at the ZnO/air interface [19,20]
We demonstrate self-aligned hierarchical ZnO nanorod (ZNR)/NiO nanosheet (NNS) arrays to realize the high photon extraction efficiency (PEE) of a GaN-based C-emitter
Summary
The advancements in nanotechnology have facilitated the increased use of ZnO nanostructures that, for example, are widely utilized in photonic devices because of their peculiar chemical and physical properties [1,2,3,4,5]. Different dimensions from zero to three-dimensional ZnO nanostructures have been synthesized using various precursors. These nanostructures are important for realizing many applications, such as electronic devices, catalysis, and biomedical and sensing usage, especially, visible ultraviolet optical devices [6,7,8,9,10,11]. One-dimensional (1D) ZnO nanostructures can be widely used in photonic emitters and photodetectors because of their easy refractive index control, transparency in the visible light range, high photoreactivity, and light waveguide properties [12,13,14]. The ZnO nanostructures can partially mitigate the abrupt change of refractive indices between p-type GaN and air, TIR and Fresnel reflection losses occur at the ZnO/air interface [19,20]
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