Clarification of the Reaction Mechanism of the Photoassisted Anodic Oxidation of SiC in a Newly Developed Inverted Rotating Disk Electrode Cell Setup

Abstract

Due to the distinctive chemical and physical properties of silicon carbide (SiC), interest in the material as a substrate for semiconductor industry has risen significantly during the past two decades. As a wide bandgap semiconductor SiC is known to be highly chemically inert. Still, under specific conditions SiC takes part in a number of electrochemical reactions that lead to a rapid oxidation of the surface. The goal of this work is to understand the fundamental factors driving this process for n-type SiC crystals in aqueous electrolytes.In alkaline solutions, the photoelectrochemical behaviour of n-type SiC is determined by two partial reactions (oxidation/dissolution) 1 :(1a) SiC + 6h+ (hν) + 6OH- → SiO2 + CO + 3H2O(1b) SiC + 8h+ (hν) + 8OH- → SiO2 + CO2 + 4H2O(2) SiO2 + 2OH- → [Si(OH)2O2]2- For the investigation of such photoassisted or photoelectrochemical reaction mechanisms involving mass transfer, we specifically developed a new inverted rotating disk electrode (IRDE) cell setup 2 that allows to control the most essential experimental parameters (i.e. potential, diffusion conditions and light intensity).In this setup, rotation of the disk electrode is achieved by magnetic coupling between a driver magnet (i.e. a magnetic stirrer) and the follower magnet included in the sample holder. The cell allows a homogeneous illumination and removal of gaseous reaction products, while still providing leak tightness. The surface of the semiconductor electrode is illuminated with UV light (λ = 365 nm) from a high-power LED. The distance between light source and sample is minimized to achieve maximum surface illumination of the sample.To investigate mass transport phenomena, electrolytes with relatively low solubility of SiO2 are preferable. Therefore, we used potassium hydroxide solutions in a concentration range where mass transport of hydroxide ions plays a major role.Performing cyclic voltammetry in this newly developed IRDE cell allows to distinguish different reaction regimes that are either limited by mass transport, photon flux or hole transport. For low light intensities (either through a large distance between the sample and the LED or through low driving currents through the LED) the number of photon-induced holes is limiting. At low electrolyte concentrations, the transport of hydroxyl ions and the dissolution of the oxidation products determine the reaction rate. At low potentials, hole transport limits the overall current flow. Each of these reaction regimes was studied individually to conduct a potential reaction mechanism.[1] D. H. van Dorp und J. J. Kelly, „Photoelectrochemistry of 4H–SiC in KOH solutions,“ Journal of Electroanalytical Chemistry, Bd. 599, Nr. 2, pp. 260-266, 2007.[2] K. Mairhofer, P. Mayr, S. Radl, M. Nelhiebel, S. Larisegger, G. Fafilek, „An Inherently Leakage-Free Inverted Rotating Disk Electrode Setup“, 2022. Figure 1