Nanocomposites and nanomaterials
Impact of rare-earth doping on crystalline phase and optical properties of HfO2 based thin films 
L. Khomenkova1,2, T. Torchynska3, N. Korsunska1, O. Melnichuk4,
X. Portier5, F. Gourbilleau5
1 V.Lashkaryov Institute of Semiconductor Physics at the NASU, 41 Pr. Nauky, Kyiv 03028, Ukraine; E-mail: khomen@ukr.net
2National University “Kyiv-Mohyla Academy”, 2 Skovorody str., Kyiv 04170, Ukraine
3Instituto Politécnico Nacional - IPN, ESFM, Mexico City, 07738, Mexico 
4Mykola Gogol State University of Nizhyn, 2 Hrafska Str., Nizhyn 16600, Ukraine
5 CIMAP Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, 6 Boulevard Maréchal Juin, 14000 Caen, France 

Last decades HfO2-based materials are considered mainly as alternative dielectrics to SiO2 due to their high dielectric constant, while optical and luminescent properties of such materials are addressed only in few reports. In this work, the impact of the doping with rare-earth (RE) elements (Er, Nd, Pr) on crystalline structure evolution and optical properties of HfO2 and HfSiOx films is reported. The films were grown on Si substrates by radio frequency magnetron sputtering in argon plasma and annealed at TA=800-1100C for tA=15-60 min in nitrogen atmosphere. The transformation of film properties was studied by means of SEM, EDX, XRD, TEM and photoluminescence (PL) techniques. 
For HfO2 films doped with RE ions, the stabilization of tetragonal HfO2 phase in annealed films was observed contrary to monoclinic structure of pure HfO2 films. The main reason responsible for this phenomenon is formation of oxygen vacancies. For RE-doped HfSiOx films, a phase separation between SiOx and HfO2 occurs upon annealing. For Nd or Er-doped films, the presence of RE ions was detected in SiOx and HfO2 phases, while for Pr ions they were found in HfO2 phase only. This latter was transform into cubic one up to 1050°C, while formation of monoclinic HfO2 phase was detected after annealing at 1100°C. 
The shape of RE-related PL spectra followed the structure transformation. Narrow RE-related PL peaks were detected in the samples annealed at 1000-1100°C that confirms the location of RE ions in the phase with high crystal field. The peculiarities of PL spectra and the mechanism of phase separation for different films are discussed.

This work was partly supported by the CONACYT Mexico (project 258224) and SIP-IPN Mexico (project 20180495) and the Ministry of Education and Science of Ukraine (project 89452).