Nanocomposites and nanomaterials

Microstructure and cathodoluminescence of Y2O3:Eu i Gd2O3:Eu thin films
O.M. Bordun, I.Yo. Kukharskyy, I.M. Kofliuk
Ivan Franko National University of Lviv, Ukraine, 79005, Lviv, st. Drahomanova, 50; e-mail: bordun@electronics.lnu.edu.ua

Among the large number of nanomaterials for optoelectronics, a special place is occupied by luminescent materials, which are used to create displays, scintillators, devices for recording and visualizing information. One of the most effective phosphors with a linear dependence of the luminescence brightness on the excitation current density is Y2O3:Eu [1, 2]. In particular, it is the most efficient phosphor that emits in the red region of the spectrum. The combination of small crystal particle sizes and the presence of a dopant, luminescent center, and Eu3+ ion provides uniform screen coverage when depositing thin Y2O3:Eu3+ films, which improves the efficiency and stability of luminescence and expands potential applications.
In this work, we investigate the surface structure and spectral properties of cathodoluminescence (CL) of isostructural Y2O3:Eu and Gd2O3:Eu thin films obtained by radio-frequency (RF) ion-plasma sputtering. This method is optimal for obtaining the most homogeneous semiconductor and dielectric films [3]. The used method of local cathodoluminescence is characterized by high sensitivity to changes in the electronic structure of the material (impurity and structural defects) and makes it possible to study changes in the luminescent properties of structures and materials at a depth of 10-20 nm to several microns.
Thin films Y2O3:Eu and Gd2O3:Eu 0.2 - 1.0 μm thick obtained by RF ion-plasma sputtering in an argon atmosphere on fused silica υ-SiO2 substrates. The initial raw material was Y2O3 of the ИтО-И brand, Gd2O3 and Eu2O3 of the “ос. ч.” brand. The activator concentration was 1.0 mol%. After the deposition of the films, they were heat treated in air at a temperature of 950-1050С. X-ray diffraction studies have shown the presence of a similar polycrystalline structure of both types of films with a preferred orientation in the (222) plane.
Investigations of CL properties were carried out in the mode of pulsed electronic excitation.
During the study by atomic force microscopy (AFM), micrographs of the surface of the Y2O3:Eu and Gd2O3:Eu films were obtained. The characteristic structural parameters of the surface of the films under study are given in Table. 1. As can be seen from the results obtained, the Gd2O3:Eu films are formed from larger nanocrystalline grains than the Y2O3:Eu films and have a rougher surface. 
Table 1
Average grain sizes, film surface roughness and "asymmetric ratio" I612/I596 in Y2O3:Eu and Gd2O3:Eu thin films
Thin film	Average grain size, nm	Average surface roughness nm	"Asymmetric ratio" I612/I596	
Y2O3:Eu	15.9	5.3	11.43	
Gd2O3:Eu	31.3	12.7	6.66	

The study of the cathodoluminescent (CL) properties was carried out in the mode of pulsed electronic excitation. The results obtained show that the shape of the CL spectra for the emission of thin Y2O3:Eu and Gd2O3:Eu films in both types of films is practically similar. In the CL spectra of both films, there are narrow luminescence bands caused by intracentric transitions between the electron shells of the Eu3+ activator. When describing the luminescence spectrum of Y2O3:Eu, the so-called "asymmetric ratio" I612/I596 of intensities with the luminescence peak at 612 and 596 nm is often used, which corresponds to the ratio of the number of Eu3+ cations in the corresponding states of local symmetry NC2/NC3i. In thin Gd2O3:Eu  films, europium ions substitute more uniformly for gadolinium ions in the cubic Gd2O3 lattice, and also that, during annealing, oxygen ions diffuse more efficiently into the Gd2O3 lattice than in Y2O3. This situation is due to the fact that in the octahedral environment characteristic of Y2O3 and Gd2O3, the ionic radii of Eu3+ and Gd3+, which are 1.09 and 1.08 Å, respectively, are much closer to each other than the ionic radius of Y3, which is 1.04 Å [4 ]. As a result, the Eu3+ ions replacing the Gd3+ ions deform the Gd2O3 crystal lattice much less than when the Y3+ ions are substituted by the Eu3+ ions in the Y2O3 crystal lattice.

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