III-V compound semiconductors have attracted attention as next-generation materials and potential alternatives to silicon-based semiconductors because of their excellent properties including superior carrier mobility and direct band gap. In a previous study, we fabricated microbump arrays of InP [1] and line patterns and pillar arrays of GaAs using metal-assisted chemical etching [2]. However, the dimensions of the resultant patterns ranged from several micrometers to several tens of micrometers. To the best of our knowledge, no study has reported the formation of ordered GaAs nanostructures with submicron-scale or smaller periodicity using metal-assisted chemical etching. Thus, we attempted to clarify the etching mechanism of GaAs on the nanometer scale. In this study, we investigated the effects of etchant composition and temperature on the morphology of the etched GaAs during metal-assisted chemical etching using Au nanodot arrays as a catalyst. A through-hole porous alumina mask with an ordered array of openings was set on an n-type GaAs. Subsequently, a 20-30 nm thick Au layer was evaporated through the alumina mask using a mixed vacuum deposition system. The periodicity of Au nanodot arrays, which were formed on GaAs substrate, was 100 nm and the diameter of Au dot was 50-60 nm corresponding to the dimensions of anodic alumina mask. After the formation of Au dot arrays on GaAs substrates, the specimens were immersed in a mixed solution of HF and KMnO₄. When the specimen was etched in a HF containing relatively low concentration of KMnO₄ at high temperature, Au dots sank into the GaAs substrate. The positive holes (h⁺), which were generated by reduction of oxidant (i.e., KMnO₄) at Au surface, are thought to be consumed immediately at the Au/GaAs interface. As a result, Au dot arrays sank into the substrate, resulting in the formation of pore arrays. On the other hand, in the case of high concentration of KMnO₄, h⁺ diffused into the surrounding Au-coated GaAs; thus, site-selective etching occurred on the exposed GaAs surface. Attempts to clarify the effects of the concentration of oxidizing agent on the generation of h⁺ and morphology of etched are currently underway.

[1] H. Asoh, T. Yokoyama and S. Ono, Jpn. J. Appl. Phys., 49, 046505 (2010).

[2] H. Asoh, Y. Suzuki and S. Ono, Electrochim. Acta, 183, 8-14 (2015).