Cuprous oxide (Cu 2 O) is a semiconductor with large exciton binding energy and significant technological importance in applications such as photovoltaics and solar water splitting. It is also a superior material system for quantum optics that enabled the observation of intriguing phenomena, such as Rydberg excitons as solid-state analogue to highly-excited atomic states. Previous experiments related to excitonic properties focused on natural bulk crystals due to major difficulties in growing high-quality synthetic samples. Here, the growth of Cu 2 O microcrystals with excellent optical material quality and very low point defect levels is presented. A scalable thermal oxidation process is used that is ideally suited for integration on silicon, demonstrated by on-chip waveguide-coupled Cu 2 O microcrystals. Moreover, Rydberg excitons in site-controlled Cu 2 O microstructures are shown, relevant for applications in quantum photonics. This work paves the way for the widespread use of Cu 2 O in optoe-lectronics and for the development of novel device technologies.