This article presents the technology for the formation of optical microcavities based on the GaP(NAs) semiconductor material system on silicon. For the first time, a plasma etching mode which ensures the achievement of an aspect ratio of 5:1 and low roughness of the side walls has been proposed in etching layers of III–V groups. A technological approach was also put forward to ensure optical separation of the microcavity with the Si substrate, that being important for efficient localization of light in the photonic structure. The optical studies and numerical calculation showed the presence of modulations in the micro-photoluminescence spectra of microstructures caused by the appearance of Fabry – Perot resonances. This research is an important step in the development of the technology of creation and application of combined structures with silicon-based optical waveguides.