Precise Structural Tuning of Porous GaN Using Two-Step Anisotropic Etching for Optical and Photo-Electric Applications
Authors: Kumazaki Yusuke , Matsumoto Satoru , Sato Taketomo 
Published: February 2020

Gallium nitride (GaN) and its alloys are attractive materials for various functional devices, such as light-emitting diodes, laser diodes, solar cells, and so on. One promising approach for improving device performance is forming the nanostructures with unique electrical and optical properties. GaN nanostructures have been commonly formed using selective-area growth or dry etching processes, however, there are severe limitations on increasing the density of nanostructures. And when a dry etching process is involved, the etching damage induced by ion bombardment is not negligible and could significantly degrade the device performance. One alternative approach is an anodic formation of porous structures utilizing low-energy electrochemical (EC) reactions. This process has a big advantage for high-density formation over the conventional lithography process. However, the control of structural dimension such as a pore diameter and depth has not been achieved adequately for the device applications. In this study, we aimed to improve structural controllability of GaN porous structures by combining EC process with conventional chemical etching process.
We have developed the two-step etching process for a GaN (0001) substrate. EC etching was first performed in the dark condition using a standard electrochemical cell with three electrodes. Wet chemical etching was subsequently conducted in 25 % tetramethylammonium hydroxide (TMAH) at 90°C. In order to clarify the structural controllability, the observation using scanning electron microscopy (SEM) was conducted on the porous samples formed by changing the applied EC voltage, EC etching time and TMAH etching time. The optical properties were evaluated by photo-reflectance measurement and photo-electrochemical measurement.
Figures 1(a) and (b) show the top and cross-sectional SEM images of the GaN porous structure formed by the first EC etching. Straight pores oriented perpendicular to the top surface was observed. 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 [000-1] 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.
Figures 1(c) and (d) show the top and cross-sectional SEM images of the GaN porous structure formed by the two-step etching process with a TMAH treatment. The pore shape appeared at the surface drastically changed from circular shape to hexagonal shape. In addition, the pore diameter estimated from SEM observation increased linearly with the TMAH etching time, whereas the pore depth was unchanged. This result denotes that the etching rate for a (1-100) plane is lower than that for other crystal planes. From these results, we can conclude that the pore depth and diameter were independently controlled by the EC etching time and the TMAH etching time, respectively.
We investigated the correlation between the structural properties and the optical properties of the porous structures formed by the two-step etching process. The photo-reflectance of GaN surface decreased after the formation of porous structures. In addition, the reflectance spectra showed oscillation behavior as function of light wavelength, where the oscillation periods were changed with TMAH etching time. The oscillation of photo-reflectance can be explained by the interference of two kinds of light that was reflected at air/porous GaN interface and porous/bulk GaN interface. The results obtained here indicate that the refractive index was varied with the structural features such as a pore diameter that were changed by the TMAH etching time.
The photo-electrochemical measurements were conducted on two kinds of porous samples formed by the first EC etching and the two-step etching processes. Figure 2 compared the photocurrents of various samples obtained under the UV light with a wavelength of 350 nm and an intensity of 0.1 mW/cm2. The photocurrents of both porous samples increased as compared with those of planar sample, and especially the large increase by 180% was obtained by the two-step etching. From the systematic measurements, it was found that the increase and decrease of photocurrents were sensitively affected by the change of the pore diameter. These strongly suggest that the efficiency of the photo-electric conversion was determined by such factors as the intensity of the electric filed induced at the GaN/electrolyte interface and the width of the depletion layer formed at the pore wall. In conclusion, the two-step etching process is very powerful technique to control the optical properties of GaN porous structures on the basis of the precise structural tuning.

Figure 1