**4. Conclusions**

In conclusion, we successfully grew self-aligned hierarchical ZNR/NNS arrays on a C-emitter through a low-temperature two-step hydrothermal process. Compared to the device with only ZNRs, the device with the hierarchical ZNR/NNS arrays exhibited optimal refractive indices (GaN: 2.49, ITO: 2.1, ZNR: 1.47, NNS: 1.44) and antireflection properties as a result of the graded refractive index and waveguide e ffect. Therefore, at the injection current of 100 mA, its output power was approximately 17% higher than that of the C-emitter with ZNRs and twice as high as that of the device without nanostructures; in addition, there was no degradation of electrical properties. The proposed nanostructures can be used to realize various nanotechnology applications, such as photonic emitters, gas sensors, supercapacitors, electrochromic devices, and solar cells.

**Author Contributions:** W.-S.L., S.-H.K., and K.-K.K. conceived and designed the experiments; H.-J.C. and S.O. conducted the simulation; W.-S.L., S.-H.K., K.-K.K., and H.L. performed the experiments; W.L., S.-H.K., and H.-J.C. analyzed the data; W.-S.L., S.O., K.-G.I. and K.-K.K. wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partly supported by the MSIT(Ministry of Science and ICT), Korea, under the ITRC(Information Technology Research Center) support program(IITP-2020-2019-2018-0-01426) supervised by the IITP(Institute for Information & Communications Technology Planning & Evaluation, Korea Institute for Advancement of Technology (KIAT) gran<sup>t</sup> funded by the Korea Government(MOTIE) (P0008458, The Competency Development Program for Industry Specialist), and the work reported in this paper was conducted during the sabbatical year of Korea Polytechnic University in 2019.

**Conflicts of Interest:** The authors declare no conflict of interest.
