Gallium nitride (GaN), which is a III-V compound semiconductor, has wide direct bandgap (3.4 eV), high breakdown electric field (3.3x10⁶ V/cm) and high saturation electron velocity (2.7 x10⁷ cm/sec). On the basis of excellent optical and electrical properties, significant progress has been achieved in the GaN-based devices, such as ultraviolet laser diodes, white light-emitting diodes, high electron mobility transistors (HEMTs), and high-power FETs. In addition, GaN and its alloys are getting much attention as building block materials for electrochemical (EC) energy conversion systems such as chemical sensors, water splitting, and artificial photosynthesis due to their superior chemical stability, direct transition, and widely tunable bandgap by alloying. Among the various techniques for improving their conversion efficiency, porosification utilizing EC reactions is one of the most powerful because a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. In addition, this technique is performed at room temperature and does not require any complicated process such as lithography, indicating lower damage and higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth.

In this presentation, we review our recent work on the fabrication of porous GaN utilizing various EC techniques and their applications [1-3]. One example is the two-step process combining EC etching process with conventional chemical etching process. EC etching was first performed in the dark condition using an aqueous electrolyte consisting of H₂SO₄ and H₃PO₄. Wet chemical etching was subsequently conducted in 25 % tetramethylammonium hydroxide (TMAH) at 90°C.

The porous structure formed by the first EC etching has straight pores oriented perpendicular to the top surface. Pore depth linearly increased with EC etching time, indicating that superior depth control was achieved. This behavior can be explained as resulting from anisotropy of the EC etching. Namely, the etching reaction proceeded only to the direction belong to the high-electric field induced by the anodic voltage. On the other hand, the pore diameter kept a constant value throughout the EC etching and could not be controlled by the EC etching time. However, the pore diameter could be changed by wet chemical etching utilizing TMAH as subsequent treatment after the EC etching. By applying TMAH etching, pore shape changed from a round to a hexagonal shape surrounded with 6 facets of {1−100}. In addition, the pore diameter estimated from SEM observation increased linearly with the TMAH etching time, whereas the pore depth was unchanged. These results indicate that TMAH etching proceeds anisotropically in the horizontal direction, but not in the vertical direction of the substrate.

The optical and photoelectrochemical properties of porous GaN were very sensitive to the structural properties. From the photoluminescence (PL) measurements, the intensity of yellow luminescence (YL) related to high-density point defects was drastically decreased by the formation of porous structures, suggesting that gallium vacancy (V_Ga) related defects were preferentially removed by EC etching. Photoreflectance measurement revealed that porous sample had an effective refractive index that could be controlled by TMAH etching time. In photoelectrochemical measurement, the incident-photon-to-current conversion efficiency (IPCE) was dramatically enhanced by the formation of porous structures. A series of experimental results were consistently explained by the change of thickness of pore wall with a width of space charge region. These results suggest that precise structural controlling is crucially important to obtain superior capability for EC energy conversion systems, and the two-step process utilizing anisotropic EC etching and TMAH etching is very promising as a nanostructure fabrication technique.

Acknowledgement: This work was supported by JSPS KAKENHI - JP15K13937，JP16H06421，JP17H03224.

1. A. Watanabe et al., ECS Electrochem. Lett., 4, H11 (2015).
2. Y. Kumazaki et al., J. Electrochem. Soc., 164, H477 (2017).
3. M. Toguchi et al., 233^rd ECS Meeting, Z01-2550, Seattle, WA, 15 May (2018).