Europium monoxide (EuO) is a well-known ferromagnetic semiconductor with single crystal band-gap similar to Si (1.12 eV) and is compatible with the actual semiconductor technology, and suited for the development of integrated spintronics components. However, very little is known of the optical properties of this notable material in thin film configuration, and especially nanostructured. Only very recently some evidence on possible quantum confinement in EuO nanostructures has been reported [1]. In part, this is because EuO high quality films and nanostructures are very difficult to obtain and preserve. Indeed the only data available on its dielectric function are for single crystal EuO and dates from the 70’s from reflectance measurements.

Recently we have been able to prepare high quality textured nanocrystalline EuOx films by pulsed laser deposition in vacuum at room temperature through a reduction process. Because the EuO films rapidly oxidize under exposure to ambient, we have encapsulated them with a 10 nm thick Al₂O₃ layer. This has allowed us to preserve the EuOx nanostructures and to study both their linear optical and luminescent properties. In this contribution, we will show the optical dielectric function of thin nanocrystalline EuOx films in the NUV-VIS-NIR, using spectroscopic ellipsometry measurements with perfect Kramers-Kronig consistency. The results show how tweaking the EuOx composition and nanostructure modifies its dielectric function, and therefore it enables the control of the excitonic response modifying the band-gap of the films. We will show the photoluminescent emission of these films, and discuss its excitonic or ionic (Eu²⁺) origin. We believe that the knowledge of the optical properties of this unique material will open a new route for the integration with commercial semiconductors (Si, GaAs, GaN) for novel spintronics and photonics combined devices.

[1] G.M. Prinz, T. Gerber, A. Lorke, M. Müller, Quantum confinement in EuO heterostructures, Appl. Phys. Lett. 109 (2016). doi:10.1063/1.4966223.