The thermal annealing effects on the optical transparency and luminescent characteristics of the Eu-doped Y₂O₃ thin films have been investigated. The as-deposited Y₂O₃:Eu films exhibited an optical band gap of 5.78 eV with a transparency of 89 % at a wavelength of 550 nm. As the annealing temperature increased from 1000 to 1300 °C, the optical band gap and transparency of the films decreased from 5.77 to 4.91 eV and from 86.8 to 64.5 % at 550 nm, respectively. The crystalline quality of the films was improved with increasing annealing temperature. The annealed Y₂O₃:Eu films emitted a red-color photoluminescence (PL) with the highest emission peak near 612 nm. The PL intensity was increased with increasing annealing temperature to 1200 °C, resulting from the improvement in the crystalline quality of the films. The PL intensity was decreased with further increasing temperature above 1200 °C due to the formation of Y₂SiO₅ phase by the reaction of the film with the quartz substrate.

Keywords: Transparency, Optical band gap, Yttrium oxide, Thermal annealing

Lanthanide (Ln)-doped oxides are one of the most important class of luminescent materials which have attracted intense attention. These materials have been widely used in optical applications due to the unique luminescent features of Ln ions [1-3] The light emission from the Ln ions originates primarily from the 4fⁿ intrashell transitions. The partially filled 4f inner shell in the Ln ions is shielded from the surroundings by the completely filled 5s² and 5p⁶ outer shells. Therefore, the emission spectra of Ln ions generally appear as very narrow and sharp peaks at wavelengths which are weakly dependent on the coordination environment or the crystal field [3].

Among various Ln-doped oxides, europium-doped yttrium oxide (Y₂O₃:Eu) has been intensively studied owing to its red luminescence with high efficiency and good color purity [4-6]. Y₂O₃:Eu has been practically used as a red-emitting cathodoluminescent and photoluminescent material for various types of display devices, such as field emission displays (FEDs), vacuum fluorescent displays (VFDs), and plasma panel displays (PDPs) [7-10]. Y₂O₃:Eu is typically prepared in powdered form by using various synthesis techniques [4-12]. Special attention also needs to be paid to thin film form of Y₂O₃:Eu because it has diverse potential applications [13]. In particular, thin Y₂O₃:Eu films with a high transparency in the visible wavelength region could be used as a red light emission layer in transparent display devices [14]. In this study, optically transparent and luminescent thin films of Y₂O₃:Eu were deposited by radio frequency (RF) magnetron sputtering. This work primarily aimed to clarify the effects of thermal annealing treatment on the optical transparency and the luminescent characteristics of the Y₂O₃:Eu films. The relationship between the structure and luminescence of the Y₂O₃:Eu films was also examined.

The optically transparent Y₂O₃:Eu thin films were deposited by using an RF magnetron sputtering technique. Sputtering was carried out on fused quartz substrates using an Eu-doped Y₂O₃ target in an Ar gas atmosphere. Prior to deposition of the films, the sputtering chamber was pumped by a turbomolecular pump until a base pressure of less than 1 ´ 10^-6 Torr was achieved. The deposition was performed with no substrate heating. The gas pressure and the RF power density were held at 2 ´ 10^-2 Torr and 5.92 Wcm^-2, respectively, during deposition. The film thickness was maintained at approximately 500 nm by adjusting the deposition time. The deposited films were thermally annealed for 1 hour in air ambient at temperatures from 1000 to 1300 °C.

The optical transmission spectra were measured for the Y₂O₃:Eu films and are presented in Fig. 1 with the variation of annealing temperatures. The as-deposited films exhibit an optical absorption edge in ultraviolet region and a high transparency in visible and near-infrared regions. It can be seen that the transparency is reduced as the annealing temperature increases. The as-deposited films have an optical transmittance of 89% at a wavelength of 550 nm. The transmittance reduces to 86.8% after annealing the films at 1000 °C, and then to 64.5% after annealing at 1300 °C. The optical band gap, E_g of the Y₂O₃:Eu films can be estimated using the following equation [15]:

(α hn )² = A(hn - E_g)                                        (1)

where α is the optical absorption coefficient, hν is the photon energy, and A is the constant. The obtained E_g values are presented in Fig. 2 as a function of annealing temperature. The E_g is found to be 5.78 eV for the as-deposited films, which is comparable with the value reported for yttrium oxide [16]. The E_g slightly decreases as the annealing temperature increases to 1200 °C. A significant reduction in E_g for the films annealed at 1300 °C is considered to be due to the formation of Y₂SiO₅ phase, as will be discussed below with the X-ray diffraction data.

Fig. 3 presents the X-ray diffraction patterns of the as-deposited Y₂O₃:Eu films and the films annealed at different temperatures. It can be found in Fig. 3(a) that the as-deposited films had a mixed structure of cubic and monoclinic Y₂O₃ phases. The peaks marked with Miller indices in the pattern are those from the cubic Y₂O₃ phase [17]. The peaks marked with the symbol (○) correspond to the monoclinic Y₂O₃ phase [18]. After annealing treatment, the XRD patterns show much more intense and narrower peaks as compared to those of the as-deposited films, indicating that the crystalline quality of the films was significantly improved by the anneal. The annealed films show the peaks from the cubic Y₂O₃ phase with a preferential orientation along the (400) plane. After annealing at 1200 °C, as shown in Fig. 3(d), the diffraction peaks attributed to the Y₂SiO₅ phase start to appear. The peaks marked with the symbol (5) correspond to the Y₂SiO₅ phase [19]. At 1300 °C, as shown in Fig. 3(e), the diffraction pattern is dominated by the peaks from the Y₂SiO₅ phase as the reaction between the film and the quartz substrate was accelerated at this high temperature. Most of the peaks, except for the (211) and (400) peaks of the cubic Y₂O₃ phase, arose from the Y₂SiO₅ phase. In order to analyze the crystalline quality of the films, we measured the full width at half maximum (FWHM) of the (400) peak from the cubic Y₂O₃ phase. As shown in Table 1, the FHWM is 0.5415° for the as-deposited films and significantly reduces to 0.2274° by annealing at 1000 °C. The FWHM decreases with further increasing annealing temperature and reaches 0.1480 at 1300 °C. This clearly indicates the improved crystalline quality at higher annealing temperature.

Fig. 4 shows AFM images for the as-deposited and annealed Y₂O₃:Eu films. It can be found that no obvious change in surface morphology was observed after annealing at temperatures up to 1100 °C. The surface of the films became rougher as the annealing temperature was raised above 1200 °C, resulting from the reaction between the film and the quartz substrate. It should be mentioned that cracks appeared at the surface of the samples after the annealing treatment. It is believed that the cracks were caused during annealing, probably due to the difference of the thermal expansion coefficient between the Y₂O₃:Eu film and the quartz substrate.

Fig. 5 shows the PL emission spectra of the Y₂O₃:Eu films annealed at different temperatures. The as-deposited films did not show any PL emission because of their poor crystalline quality. As shown in Fig. 5, several PL emission peaks were detected from the annealed films, which originated from the f – f   transitions of Eu³⁺ ions. These PL spectra are similar to those observed from the Y₂O₃:Eu powders [8]. The highest emission peak near 612 nm in the red range is associated with the transition from the ⁵D₀ excited state to the ⁷F₂ level. The PL excitation spectra of the Y₂O₃:Eu films annealed at different temperatures are shown in Fig. 6. The excitation band below 219 nm is due to the absorption of Y₂O₃ host lattice. Additional bands observed in the range between 219 and 290 nm are related to the charge transfer between O^2- and Eu³⁺ ions [3]. As the annealing temperature increased, the PL emission and excitation intensity increased and showed a maximum at 1200 °C. This was attributed to the improvement in the crystalline quality of the films. The PL emission and excitation intensity decreased at 1300 °C which was due to the formation of Y₂SiO₅ phase by the reaction of the film with the quartz substrate.

Optically transparent Y₂O₃:Eu films were deposited on fused quartz substrates by RF magnetron sputtering. We have shown that the optical transparency, crystalline quality, and luminescent characteristics of the Y₂O₃:Eu films were strongly affected by the thermal annealing treatment. The as-deposited Y₂O₃:Eu films showed a transparency of 89% at 550 nm and an optical band gap of 5.78 eV. The transparency and optical band gap of the films were found to decrease with increasing annealing temperature. XRD analysis revealed that the crystalline quality of the films was significantly improved by the thermal annealing treatment. The annealed Y₂O₃:Eu films showed a PL emission with the highest emission peak near 612 nm. The PL intensity was increased with increasing annealing temperature due to the improvement in the crystalline quality of the films. The PL intensity decreased at 1300 °C resulting from the formation of Y₂SiO₅ phase.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1F1A1040480).