Light Emission in Nd Doped Si-Rich HfO₂ Films Prepared by Magnetron Sputtering

Hafnium oxide films doped with Si and Nd atoms were produced by radio-frequency magnetron sputtering of a HfO₂ target topped with calibrated Si and Nd₂O₃ pellets in pure argon plasma followed by an annealing in nitrogen atmosphere during t_A = 15 min at different temperatures (T_A = 800–1100°C). The evolution of structural, chemical and luminescent properties of the films with T_A was studied by means of the scanning electronic microscopy (SEM), x-ray diffraction (XRD), Raman scattering, energy dispersive x-ray spectroscopy and photoluminescence (PL) methods. The SEM method revealed that the surface of as-deposited film consists of the grains with the mean size of 20–60 nm. Annealing treatment stimulated the growing of the grains (up to 100 nm in lateral size) and film densification. The presence of Si-rich phase was detected by Raman scattering spectra in as-deposited films and those annealed at low T_A. The T_A increase results in the phase separation process. For the films annealed at T_A > 950°C, the tetragonal HfO₂ and SiO₂ phases were clearly detected by the XRD method. PL spectra of the films were found to be complex. They demonstrated several PL bands in the visible (400–750 nm) and infrared (800–1430 nm) spectral ranges. Besides PL components caused by the recombination of carriers via host defects, the PL signal from Nd³⁺ ions due to the transition in the 4f inner electronic shell was observed. The highest Nd³⁺ related PL signal was observed for the films annealed at T_A = 950°C. Peculiarities of PL excitation and the mechanism of the phase separation are analysed and discussed.