Electrochemical Tuning of GaN Porosity for Advanced Photonics Applications

Abstract

The nanoporous GaN from electrochemical process have mainly found roles as strain-free template for light-emitting diodes1, laser diodes2, Bragg reflectors3 and for photonic and optoelectronic applications4.Based on previous researches, a complete review exists on GaN electrochemical and photo-electrochemical etching5: it shows the impact of different parameter on pores morphology and its physical properties. However, according to our knowledge, no literature depicts a systemic pure electrochemical study enabling fine-tuning of porosification process.Our work proposes a three electrodes set up to study the porosification reactions by dynamic electrochemical methods such as cyclic voltammetry (CV) and chronoamperometric (CA) to understand and control the GaN anodic oxidation. It aims to highlight the influence of crucial parameters, relative to both electrochemical and semi-conductor contribution, on the electrochemical signal and on the physico-chemical characteristic of the obtained porous GaN. The main objective is to have a new diagnostic tool for GaN porosification mechanism to better tune the final porous structure.Therefore, the impact of dopant concentration and the influence of thickness of the doped GaN layer was studied by CV analyses and CA method. Coupling the electrochemical curves with SEM analyses enables to define four regimes in the electrochemical charge-transfer: pre-breakdown, porosification zone, a transition phase and electropolishing (figure 1).Moreover, this work show that higher is the doping rate, thinner is the space charges region (SCR): as a result, the conductivity of the n-GaN working electrode increases, and the passage of electrons through the SCR requires lower electric field. This lead to a shift of the CV curves to lower potentials. On the other hand, the range of porosification potentials reduces.The pore morphologies are directly related to the thickness of the space charge region (SCR) which depend on the doping rate and the applied potential:- The nucleation zone corresponds to low potentials region. In this range, dislocations and v-pits control the pores formation. At the tips of these impurities, the radius curvature becomes small enough that a low electric field is sufficient to move electrons through the SCR. In the non-peak areas of the defects, the thickness of SCR is larger and the electric field in the nucleation zone is not sufficient for the electrons passage. Hence the low porosity.- In porosification zone, the electric field is sufficient to accelerate the electrons passage through the SCR, which favors the propagation of nanopores in GaN. The result is the formation of pores on the entire surface, the higher the imposed potential, the higher the pore density and the porosity.The effect of the doped GaN layer thickness was also inspected with CV and CA. The CV curves show that the intensity of the current peaks increases with increasing doped layer thickness because the quantity of available charges is higher. The current drop results from charge consumption in the doped layer.The kinetics of the electrochemical charge transfer reaction between GaN and the electrolyte depends on several parameters. On the semiconductor side, it depends on the doping rate and the quantity of available charges; these two parameters control the creation of holes at the SC/electrolyte interface. On the electrolyte side, it depends on the concentration and the composition of the electrolyte, which control the diffusion of anions towards the SC/electrolyte interface.The cyclic voltammetry curves obtained on different types of electrolyte show that depending on the concentration and the nature of the electrolyte, the charge transfer regime could be diffusional which influences the rate of porosification and thus the pore morphology.The SEM images complete the electrochemical study and confirm that the porosification of GaN depends on:- The doping rate in the semiconductor, which controls the thickness of the SCR in the SC.- The concentration and nature of the electrolyte, which controls the diffusion of material from the electrolyte to the SC/electrolyte interface.- The applied potential, which controls the rate of the charge transfer reaction between the SC and the electrolyte. 1 K.J. Lee, S.-J. Kim, J.-J. Kim, K. Hwang, S.-T. Kim, and S.-J. Park, Optics Express 22, A1164 (2014). 2 S.-M. Lee, S.-H. Gong, J.-H. Kang, M. Ebaid, S.-W. Ryu, and Y.-H. Cho, Opt. Express, OE 23, 11023 (2015). 3 G.-Y. Shiu, K.-T. Chen, F.-H. Fan, K.-P. Huang, W.-J. Hsu, J.-J. Dai, C.-F. Lai, and C.-F. Lin, Scientific Reports 6, 29138 (2016). 4 S.H. Park, G. Yuan, D. Chen, K. Xiong, J. Song, B. Leung, and J. Han, Nano Lett. 14, 4293 (2014). 5 P.H. Griffin and R.A. Oliver, Journal of Physics D-Applied Physics 53, 383002 (2020). Figure 1