Towards Tm²⁺ in sialon thin films grown by sputtering using the gradient deposition

Abstract

The valence stability parameter (E_Ff), defined as the difference between the charge transfer energy to the host intrinsic Fermi energy, was used as criterion to analyze the capability of different host materials within the SiAlON class to stabilize divalent thulium. Available data on charge transfer energies and optical bandgap values are reviewed for Si₃N₄, SiO₂, AlN, and Al₂O₃. In addition, new data on thin films, collected by our gradient sputter deposition and characterization method on silicon and aluminum nitrides (Si_0.75xAl_1-xN), are reported. These data are sufficient to show that, at least in the nitride subsection of the SiAlON class, divalent thulium is not expected to be stable due to the presence of high E_Ff values. The use of sub-stoichiometric silicon nitride and oxide is also briefly considered.