The PhD thesis of Ana Meléndez, a researcher from the NANOPTO group at the Institute of Materials Science of Barcelona (ICMAB-CSIC), focuses on rare-earth mononitrides (RENs). These materials combine semiconducting and magnetic properties, making them promising candidates for spintronic and magneto-electronic applications.
The research addresses the challenges in the experimental investigation of RENs due to their propensity to form nitrogen vacancies and their extreme air sensitivity. Despite improvements in ultra-high-vacuum thin film growth techniques, comprehensive studies relating growth conditions to the properties of RENs remain limited.
The thesis presents a systematic investigation of the growth and characterization of epitaxial HoN, SmN, and YbN thin films grown by molecular beam epitaxy under moderate conditions. Film quality and physical properties were studied using in situ X-ray photoelectron spectroscopy (XPS), reflection high-energy electron diffraction (RHEED), and electrical transport measurements, complemented by ex situ X-ray diffraction (XRD) and spectroscopic ellipsometry (SE).
In-situ XPS monitoring confirmed the semiconducting character of all investigated materials. The influence of nitrogen vacancies on the electronic structure of SmN was investigated using X-ray absorption spectroscopy (XAS) and XPS. Magnetization measurements on optimized HoN films revealed a saturation moment lower than predicted by Hund's rules, attributed to intrinsic effects. The optical properties of SmN, HoN, and YbN were investigated by ex situ SE, determining band-gap energies, resistivities, and free carrier concentrations depending on nitrogen deficiency.
This work demonstrates that the structural, electronic, magnetic, and optical properties of rare-earth mononitrides are highly sensitive to growth conditions and stoichiometry. The developed methodologies provide a robust framework for the controlled synthesis and characterization of high-quality REN thin films, contributing to a deeper understanding of the relationship between defects, electronic structure, and functional properties.
The thesis defense will take place on Friday, November 27, 2026, at 10:00 AM in the Sala d'Actes Carles Miravitlles at ICMAB, and will also be available online.
