Transparent and low surface roughness HfO₂: Tb³⁺, Eu³⁺ luminescent thin films deposited by USP technique

In this work, the optical and morphological properties of HfO₂: Tb³⁺, Eu³⁺ thin films deposited by the Ultrasonic Spray Pyrolysis technique from metal-organic precursors are reported. The films showed optical average transmittance values in the visible region greater than 90%, with surface roughness lower than 3.9 nm. The films deposited at 500 °C showed the lowest average roughness with a value of 0.9 nm, and the smallest thickness was 35 nm for sample deposited at 500 °C during 45 s. XRD measurements indicate a hafnium oxide monoclinic phase for films deposited at substrate temperatures higher than 500 °C. All films deposited showed the luminescent emissions (PL and CL) characteristic of Tb³⁺ and Eu³⁺ ions. A luminescence concentration quenching was observed for both Tb³⁺ and Eu³⁺ ions. The HfO₂: Tb³⁺ (5 at%) and HfO₂: Eu³⁺ (10 at%) films deposited at 500 °C, showed the highest PL and CL emission intensity. Quantum Efficiency measurements (up to about 35%) were measured for these films which have a refractive index between 1.97 and 2.04 and band gap of 5.4 eV. The chemical composition of the films as measured by XPS is also reported. In addition, decay time measurements were performed on some HfO₂: Tb³⁺, Eu³⁺ samples.