In Situ Process Control during Wet-Chemical Etching of Heteroepitaxial III-V Layers Using Reflection Anisotropy Spectroscopy.

The preparation of semiconductor surfaces for renewable energy or other optoelectronic applications requires highly precise control of surface structure and composition, rendering vacuum-based techniques typically the method of choice. However, when precise process control is guaranteed, wet-chemical etching processes constitute a resource-efficient and scalable alternative. In this work, we show that reflection anisotropy spectroscopy (RAS)─a nondestructive, optical technique─offers a powerful in situ control of wet-chemical etching processes. Here, we employ RAS to determine the etching depth, rate, and time during the wet-chemical preparation of a photoelectrode for direct solar water splitting. This is achieved by observing and evaluating Fabry-Pérot-type oscillations of reflectance and RAS transients during the etching process. The occurrence of these oscillations confirms the uniform character of the investigated surface in a suitable etching parameter space and allows the determination of the etching depth with a precision of ±8 nm. A variation of the etching procedure demonstrates that RAS control can be used for the development of suitable etching routines and for in situ process control, as validated for two different types of photoelectrodes.