(Invited) Photo-Electrochemical Etching and Porosification of III-Nitride Semiconductors
Authors: Sato Taketomo , Toguchi Masachika 
Published: December 2020

Gallium nitride (GaN), which is a III-nitride 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 and III-nitride based devices, such as ultraviolet laser diodes, white light-emitting diodes, and high-frequency power transistors. 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. For the fabrication of nitride semiconductor devices, the plasma-assisted dry etching process has commonly been used. However, various types of damage and defects induced by the bombardment of reactive ions and radicals leads to severe operational stability problems in optical and electrical devices.
Photo-electrochemical (PEC) etching probably solves these problems on nitride semiconductors. PEC etching possesses some big advantages such as low-damage process conducted at room temperature. In this presentation, we introduce our recent work on the PEC etching and porosification of III-nitride semiconductors. The etching features such as smooth and flat etching and porosification etching have been controlled by changing the PEC conditions [1,2]. The porous structure formed by the present method has attractive features such as a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. In addition, this technique does not require any complicated process such as lithography, indicating higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth.
One example is the formation of straight pores along with the electric field induced in the vertical to the GaN substrate. This kind of straight pores was formed in the dark condition under the high-electric filed applied. Here, we found that the pore diameter was enlarged by the irradiation of UV light to the GaN surface during the porosification. In the case of the irradiation of UV light with a photon energy (hv) above the bandgap energy (E
g), the pore diameter was enlarged only in the top region with a depth less than 1µm, where the photo-absorption occurred only near the surface. On the other hand, under the illumination with hvbelow E
g, the pore diameter was enlarged overall pore from the top to the bottom. This kind of photo-assisted etching can be explained by the sub-bandgap absorption during the porosification. Sub-bandgap light with a photon energy below the bandgap energy was transmitted through bulk GaN, but a certain type of photo-absorption additively occurred at the pore tip where high electric filed was concentrated. To confirm such specific absorption process, we systematically investigated the correlation between the photocurrents and photon energy by changing the light intensity. The photocurrents observed when the high-electric field was applied to n-GaN were well reproduced by calculation taking into account the Frantz-Keldysh effect [3]. The results suggest that the independent control of pore diameter and depth is possible by controlling the PEC conditions.
Acknowledgement: This work was supported by JSPS KAKENHI-JP16H06421, 20H02175.
[1] Y. Kumazaki et al., J. Appl. Phys., 121, 184501 (2017).
[2] Y. Kumazaki et al., J. Electrochem. Soc., 164, H477 (2017).
[3] M. Toguchi et al., J. Electrochem. Soc., 166, H510 (2019).

Figure 1