Two excitation pathways of Pr³⁺ion emission in HfO₂:Si:Pr films depending on crystalline phase transformations in annealing

HfO₂ based materials offer high refractive index and high transparency in UV-NIR spectral ranges that are promising for photonic applications. For this purpose, the stabilization of HfO₂structure is required to minimize the optical losses in the HfO₂ based optical materials. It is known that stabilization of tetragonal/cubic structure can be achieved via HfO₂doping with trivalent atoms, such as rare-earth (RE) elements. In this case, electrical neutrality is achieved via formation of oxygen vacancies, i.e. one vacancy for two dopant atoms. At the same time, the interaction between different ions, the RE ion excitation mechanism and their redistribution upon high temperature annealing were not well addressed.<br/>In this report the impact of annealing on the emission and transformation of the crystalline phases in Si rich HfO₂:Pr films was investigated by analyzing the morphology, chemical composition, structure, and light emitting characteristics. The films were grown on Si substrates by radio frequency magnetron sputtering in argon plasma and annealed at T_A=1000 or 1100^oC for t_A=15-60 min in inert atmosphere. The transformation of film properties was studied by means of SEM, EDS, XRD, XPS, and photoluminescence (PL) techniques.<br/>Heat treatment at 1000^oC for 30 min stimulates phase transformation together with the appearance of a tetragonal HfO₂ phase and Si quantum dot (QDs). For HfO₂ films doped with Pr ions, the stabilization of the tetragonal HfO₂phase in annealed films was observed contrary to the monoclinic structure of pure HfO₂ films. The main reason responsible for this structural phenomenon is the formation of oxygen vacancies (V_O). The formation of Si QDs and oxygen vacancies is accompanied by the appearance of emission of rare earth (RE) ions Pr³⁺ related to the transitions in the 4f energy levels. The shape of RE-related PL spectra followed the structural transformation. Narrow RE-related PL peaks were detected in the samples annealed for 30-60 min that confirms the location of RE ions in the tetragonal phase with a high crystal field.<br/>Annealing for 60 min stimulates the complete oxidation of the Si QDs with the formation of the tetragonal SiO₂ phase along with partial destruction of a tetragonal HfO₂ phase. This last process is accompanied by the significant increase of the intensity of Pr³⁺ ion emission. Two forms of luminescence excitation in 4f energy levels of Pr³⁺ ions are discussed, related to energy transfer to Pr³⁺ ions, first from Si QDs and then from host V_O defects in HfO₂. These changes in the excitation pathways of Pr³⁺ion emissions are stimulated by the transformations of the crystalline phases in the thermal treatment together with the generation of host HfO₂ defects. Hafnia-based materials doped with RE elements are interesting for telecommunication technology and applications in waveguides and optoelectronic devices.