Antirefletive Characteristics of Disordered GaAs Subwavelength Structures Realized By Metal-Assisted Chemical Etching Using Thermally Dewetted Au Catalytic Nanoparticles
Authors: Song Yunwon , Choi Keorock , Oh Jungwoo 
Published: February 2020

Surface antireflection techniques have been widely used to improve the performance of many optical and optoelectronic devices such as solar cells, optical sensors and light emitting diodes by eliminating unwanted surface reflection [1-3]. Among various types of antireflective coatings, subwavelength structures that have surface nanostructures smaller than the wavelength of light are the most appropriate type because they produce the same effects as multilayer thin film antireflective coatings [4]. Many works have addressed Si-based solar cells by applying subwavelength structures to improve device performance. GaAs is also a crucial semiconductor material in high efficiency solar cell applications, especially in single junction solar cells because it has the optimum bandgap energy for absorbing light. However, subwavelength structures with GaAs have not been intensively investigated. The addition of subwavelength structures on GaAs is expected to suppress surface reflection and provide a significantly higher quantum efficiency.
Common fabrication methods for subwavelength structures are based on dry etching on nano-scale etch mask patterns formed by e-beam or nanoimprint lithography. However, these fabrication methods are expensive and complicated. In addition, dry etch is not suitable for III-V semiconductor materials such as GaAs because high energy plasma ions induce incurable surface charging and damage the crystal structure causing carrier recombination sites. In this work, instead of a dry etch, a metal-assisted chemical etching technique is used to fabricate subwavelength GaAs surface structures. It involves a simple wet-based anisotropic etch process using a metal catalyst and redox solution [5-6]. There are no charging and crystal structure damages induced by dry etch because it doesn’t use the plasma ions in the etch process. Instead, thermally dewetted Au nanoparticles are used as etch catalysts.
The Au films of 5, 10 and 15nm thickness were thermally evaporated on GaAs substrates. Before evaporation, native oxide was removed using a dilute HCl solution. To form the catalytic Au nanoparticles, the samples were agglomerated by rapid thermal annealing at various temperatures of 400, 600 and 800°C for 100s in a nitrogen environment. Using these catalytic Au nanoparticles, GaAs subwavelength structures were fabricated by metal-assisted chemical etching, which was prepared by dissolving KMnO4
with DI and HF for various etch times of 3, 6, 9 and 12min. After etching, the reflectance property of <~5% in the wavelength range of 200-800nm was evaluated using an UV-VIS-VIR spectrophotometer.

Acknowledgements

This research was supported by the Ministry of Science, ICT and Future Planning (MSIP), Korea, under the IT Consilience Creative Program (NIPA-2014-H0201-14-1002) supervised by the National IT Industry Promotion Agency (NIPA). This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-8-0884).

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