**1. Introduction**

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 [1–5]. Di fferent dimensions from zero to three-dimensional ZnO nanostructures have been synthesized using various precursors. These nanostructures are particularly important for realizing many applications, such as electronic devices, catalysis, and biomedical and sensing usage, especially, visible ultraviolet optical devices [6–11].

Especially, 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 [12–14]. According to e ffective medium approximation (EMA), the e ffective refractive index (*<sup>n</sup>*eff) of ZnO (ZnO film: *n* = 2.1 at visible wavelengths) decreases when it is converted into nanostructures [15–18].

For achieving a higher photon extraction e fficiency (PEE), a material with a refractive index lower than that of ITO (2.1 at visible wavelengths) is required to reduce the total internal reflection (TIR) in a conventional photonic emitter (C-emitter) and increase its outward light emission, that is, in air (*n* = 1). Although 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].

Therefore, alternative materials and structures are required for e ffective photon extraction from a GaN-based photonic emitter to the outside by matching the refractive indices and for realizing exceptional photon emission from surface nanostructures.

Recently, hierarchical and core–shell nanostructures that provide graded refractive index changes have been applied to achieve high PEE in photonic emitters [11,21–25]. However, for the realization of hierarchical nanostructures, a separate seed layer deposition, high cost vacuum systems, and complicate fabrication processes are required [24,26,27]. To solve these problems, rapid manufacturing techniques are required.

In this study, we demonstrate self-aligned hierarchical ZnO nanorod (ZNR)/NiO nanosheet (NNS) arrays to realize the high PEE of a GaN-based C-emitter. These hierarchical nanostructures are synthesized through a two-step hydrothermal process at low temperatures. The optical output power of the as-obtained C-emitter is approximately 17% and it is two times higher than that of the C-emitter with ZNRs and C-emitter without nanostructures. This increase can be ascribed to a graded change in the refractive index between the GaN layer and the device exterior, as well as a decrease in the TIR and Fresnel reflection of the photonic emitter.

#### **2. Materials and Methods**

#### *2.1. Device Fabrication*

Epilayers were grown on a sapphire substrate by metal–organic chemical vapor deposition. The photonic emitter (chip size: 350 × 350 μm2) with a 430 nm wavelength consisted of a 0.12 μm-thick *p*-type GaN:Mg (*n* = 3 × 10<sup>17</sup> cm<sup>−</sup>3) layer, a 0.08 μm active layer, a 2.5 μm-thick undoped GaN layer, and a 4.0 μm-thick *n*-type GaN:Si (*n* = 5 × 10<sup>18</sup> cm<sup>−</sup>3) layer on the sapphire substrate. All emitter samples were ultrasonically degreased with acetone, methanol, deionized (DI) water, and a mixture of sulfuric acid and hydrogen peroxide (3:1) for 5 min in each step to remove organic and inorganic contaminants. Then, to fabricate the *n*-electrode, the epilayers were partially etched until the *n*-type GaN layer was exposed. The 200 nm thick ITO layer was deposited using an electron-beam evaporator on the remaining parts of the *p*-type GaN layer and annealed at 600 ◦C in O2 atmosphere for 1 min using the rapid thermal annealing. The Ti/Al (50/200 nm) layers were deposited as an *n*-electrode. Finally, the Cr/Al (30/200 nm) layers were deposited on the *p*- and *n*-electrodes and annealed at 300 ◦C for 1 min.

#### *2.2. ZNRs Synthesis*

A ZnO seed layer was formed on the selectively deposited ITO by a simple dipping process as follows. First, 105 mM zinc acetate (Zn(C2H3O2)2) dissolved in DI water was synthesized at 90 ◦C for 1 h. Then, ZNRs were grown using 37.5 mM zinc nitrate hexahydrate (Zn(NO3)2\*6H2O) and 75 mM hexamethylenetetramine (C6H12 N4) dissolved in 300 mL of DI water at 90 ◦C for 6 h.

#### *2.3. Hierarchical ZNR*/*NNS Arrays Synthesis*

Nickel nitrate hexahydrate (Ni(NO3)2\*6H2O) (10.4 mg) was dissolved in DI water (50 mL) and stirred for 30 min. Then, this solution was used to synthesis NNSs on the ZNRs at 90 ◦C for 1 h.
