1. Introduction

Rare earth compounds are currently attracting a growing interest in the field of photonics. Indeed, while the rare earth doping of suitable insulating or semiconducting host matrices has been considered for a long time the best approach to introduce light emitting impurities in Si-based materials [1], recently it has been demonstrated that the compound approach is able to overcome dopant clustering and precipitation, and therefore may open new and interesting perspectives for rare earth applications in photonics. Most of the recent efforts on these compounds have been focused on Er silicate thin films; relevant results include the observation of energy transfer between Si nanostructures and Er ions in Er silicate [2], the achieving of optical gain in Er/Y silicate waveguides [3] and the observation of a very efficient Er/Yb coupling in mixed silicates [4]. Furthermore, photon cutting phenomena, being of great relevance for photovoltaic applications of these materials, have been demonstrated in Er/Y silicates [5].

Among the various rare earths, a considerable attention is devoted to Eu, that presents an intense and stable emission in the visible region and has been successfully applied in phosphors and fiber amplifiers technologies [6–8]. Eu is stable in both its divalent (Eu²⁺) and trivalent (Eu³⁺) oxidation states. The photoluminescence (PL) spectrum of Eu³⁺ is characterized by several sharp lines at around 600 nm; on the other hand, Eu²⁺ emission is much stronger, being dipole-allowed, and is characterized by a broad peak, centered in the wavelength range 450-600 nm. These peculiar optical properties suggest for Eu-based materials interesting applications in Si photonics, provided that Eu-containing films can be synthesized and processed by using CMOS-compatible technologies. Several papers studied the optical properties of Eu-doped Si oxide [9] or Eu compound thin films [10], and the Eu emission properties have been exploited in light emitting devices based on Eu-doped SiO₂ [11,12] or Eu silicates [13].

In this work, we report an investigation on the properties of Eu₂O₃ films deposited on Si substrates. Annealing treatments in oxidizing and inert ambient allowed us to demonstrate a correlation between the structural and optical properties, and, in particular, to establish the conditions leading to a material exhibiting a very efficient room temperature (RT) light emission associated to the formation of Eu²⁺ ions.

2. Experimental section

Eu₂O₃ thin films were grown on (100) Si substrates heated at 400 °C by using an ultrahigh vacuum magnetron sputtering system. The base pressure was about 1 × 10⁻⁹ mbar. The deposition has been obtained by rf sputtering of a 4 inches diameter water-cooled Eu₂O₃ target (99.9% purity). The deposition was carried out with a sputter up configuration in a 5 × 10⁻³ mbar Ar atmosphere and a rf power of 200 W for 1 h. After deposition samples were thermally treated in O₂ or N₂ ambient by using a rapid thermal annealer. Transmission electron microscopy (TEM) analysis in bright field (BF) mode, performed by a 200 kV Jeol 2010 microscope, was used to investigate the sample structure, while a 200 kV Jeol 2010F microscope, equipped with a Gatan Image Filter, was used to investigate the chemical composition by energy filtered TEM (EFTEM) analyses. Crystalline phases were identified by x-ray diffraction (XRD) measurements, performed with a Bruker-AXS D5005 diffractometer using a Cu Kα radiation with a grazing incidence angle of 1.0°. RT PL measurements were obtained by using a 325 nm He-Cd laser chopped through an acousto-optic modulator at a frequency of 55 Hz. The PL signal was analyzed by a single grating monochromator and detected by a Hamamatsu photomultiplier tube in the visible range. Photoluminescence excitation (PLE) measurements in the 250-500 nm range were performed by using a FluoroMax spectrofluorometer by Horiba.

3. Results and discussion

Figure 1(a) reports the BF cross-sectional TEM images of an as-deposited Eu₂O₃ film.

 

Fig. 1 (Color online) Cross-sectional TEM images of as-deposited and annealed Eu₂O₃ films. (a) BF image of an as-deposited film; (b) BF image of a film treated at 1000 °C for 30 s in O₂; (c) BF image of a film treated at 1000 °C for 30 s in N₂; (d) EFTEM maps of Si (on the left), O (in the middle) and Eu (on the right) of a film treated at 1000 °C for 30 s in N₂. EFTEM images refer to the same region of the sample. The arrows highlight the correspondence between the A and B sublayers in the TEM image shown in (c) and the regions with a different composition in the EFTEM images in (d).

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The film is about 120 nm thick and exhibits a smooth interface with the Si substrate, where a thin SiO₂ layer (about 2 nm thick) is present. Figure 1(b) shows the BF cross-sectional TEM images of an Eu₂O₃ film annealed in O₂ at 1000 °C for 30 s. The process in oxidizing ambient increases the thickness of the interfacial SiO₂ layer up to about 15 nm and densifies the Eu₂O₃ film, the actual thickness being about 110 nm. EFTEM confirmed the composition of the layers shown in Figs. 1(a) and 1(b). Replacing O₂ with N₂ drastically changes the thermally-induced structural evolution of the film. Indeed, the BF TEM cross-section in Fig. 1(c) shows that, after a process performed at 1000 °C for 30 s in N₂, the Eu₂O₃ film becomes thicker (about 170 nm), and two sublayers, labeled in the figure as A (surface sublayer) and B (interfacial sublayer), can be distinctly recognized. EFTEM measurements indicate that no one of the two sublayers is pure SiO₂. In the absence of any interfacial SiO₂ layer, acting as diffusion barrier, the strong reactivity between Si and rare earth oxides is activated [14] and Si atoms can diffuse inside the film. This is demonstrated in Fig. 1(d), showing the Si, O and Eu chemical maps of the N₂-treated film, obtained by EFTEM. The images were taken by selecting electrons at 99 eV in the case of Si (corresponding to the L_II,III edge), at 532 eV in the case of O (corresponding to the K edge) and at 132 eV in the case of Eu (corresponding to the N_IV edge) and by using the 3-windows method [15]. EFTEM data demonstrate that the interface between sublayers A and B evidenced by TEM corresponds to a quite abrupt variation of the concentration of the three elements. In particular, sublayer B is characterized by an appreciable Si concentration and has a higher O content and a lower Eu content than sublayer A.

The crystalline structure of the films was studied by XRD. As shown in Fig. 2 , the as-deposited film exhibits a diffraction spectrum mainly consisting of peaks belonging to the monoclinic Eu₂O₃ phase [16]. However, also a few peaks belonging to the sub-stoichiometric oxide Eu₃O₄, accounting for the oxygen deficiency which characterizes oxide films grown by sputtering, are identified [17]. The XRD spectrum of Eu₂O₃ annealed at 1000 °C for 30 s in O₂ shows only peaks corresponding to the Eu₂O₃ phase, suggesting the presence of a more ordered crystalline structure, due to a reduced defect density and to the restoration of a correct stoichiometry. The peaks are sharper than the ones of the as-deposited film, indicating an increased grain size. No Eu₃O₄-related peaks are found anymore. Figure 2 also reports the XRD spectrum of a film treated at 1000 °C for 30 s in N₂; peaks assigned to Eu₂O₃ are still present, but new peaks, corresponding to the monoclinic orthosilicate Eu₂SiO₄ [18], appear. The presence in the XRD spectrum of two different crystalline compounds (Eu₂O₃ and Eu₂SiO₄), coupled with the bilayered structure shown by TEM, suggests that the surface sublayer A mainly contains unreacted Eu₂O₃, since no appreciable Si diffusion occurs in this region. In the interfacial sublayer B the high Si concentration determines a reaction with Eu₂O₃ to form Eu orthosilicate. By increasing the annealing time, the Si diffusion length into the Eu₂O₃ film increases, and Si can therefore more extensively react with Eu₂O₃, as demonstrated by the XRD spectrum of the sample treated at 1000 °C for 300 s in N₂. Eu₂SiO₄-related peaks are still present, but also peaks corresponding to the metasilicate EuSiO₃ [19] are found, while peaks corresponding to Eu₂O₃ disappeared.

 

Fig. 2 XRD spectra of Eu₂O₃ films as-deposited, annealed at 1000 °C for 30 s in O₂, annealed at 1000 °C for 30 s in N₂ and annealed at 1000 °C for 300 s in N₂.

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Since EFTEM measurements (not shown) do not detect any sublayer in this sample, we can reasonably hypothesize the presence of grains of both silicates randomly distributed inside the film. It is strongly remarkable that, in both the silicates detected by XRD (Eu₂SiO₄ and EuSiO₃), Eu is in its divalent state, Eu²⁺, while in the as-deposited film, according to the target composition (Eu₂O₃), almost only Eu³⁺ ions are present. The formation of Eu₂SiO₄ and EuSiO₃ is in agreement with the EuO/SiO₂ phase diagram [20]. Eu³⁺ → Eu²⁺ reduction in glasses is accomplished by exploiting gaseous reactants such as H₂ [21] or CO [22]; in our case, Eu³⁺ reduction is reasonably due to a solid-state reaction involving the elemental Si (which, in turn, is oxidized) diffusing inside the Eu₂O₃ film.

Figure 3 shows the RT PL spectrum of an as-deposited Eu₂O₃ film; according to literature [6,7,23,24], the spectrum consists of a sharp (full width at half maximum (FWHM) of about 10 nm) main peak at 622 nm, corresponding to the ⁵D₀→⁷F₂ transition, and of less intense peaks centered at 595, 654 and 709 nm, corresponding to ⁵D₀→⁷F₁, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ transitions of Eu³⁺ ions, respectively. Figure 3 also demonstrates that an annealing process at 1000 °C for 30 s in O₂ increases by about a factor of 2 the PL signal attributed to the ⁵D_J→⁷F_J transitions, due to the presence of a more ordered crystalline environment for the Eu³⁺ ions and to an optimized stoichiometry. No relevant changes of the PL peak shape and intensity occurs by varying the temperature (up to 1000 °C) of the annealing process in O₂ ambient.

 

Fig. 3 Room temperature PL spectra obtained by exciting with the 325 nm line of a He-Cd laser Eu₂O₃ films as-deposited and annealed at 1000 °C for 30 s in O₂.

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On the other hand, N₂-treated samples show completely different optical properties. Figure 4 reports the RT PL spectra of Eu₂O₃ samples annealed at 1000 °C in N₂ ambient for times ranging from 1 to 300 s; a very bright emission, consisting of a broad band (FWHM of about 95 nm), having the maximum at about 590 nm, is observed. This emission is due to the 4f⁶5d→4f⁷ transitions of Eu²⁺ ions [9,22,23]. These transitions are dipole-allowed and produce a stronger PL intensity (by more than three orders of magnitude) if compared with the forbidden intra-4f transitions characterizing Eu³⁺ ions.

 

Fig. 4 Room temperature PL spectra obtained by exciting with the 325 nm line of a He-Cd laser Eu₂O₃ films annealed at 1000 °C in N₂ for times ranging from 1 to 300 s. The inset shows the dependence of the integrated PL intensity on the annealing temperature for processes performed for 30 s in N₂.

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The Eu²⁺-related PL intensity increases by increasing the annealing time. We have also explored the dependence of the PL intensity on the annealing temperature for a fixed annealing time of 30 s; the data are reported in the inset of the figure and demonstrate that 1000 °C is the temperature that maximize the PL emission efficiency. We strongly underline that the external quantum efficiency of the brightest N₂-treated Eu₂O₃ samples has been measured to be about 10%, which makes this material of great interest for photonic applications.

The excitation properties of Eu₂O₃ films have been investigated by PLE measurements reported in Fig. 5 . PLE spectra were obtained by integrating the PL peaks of the samples and changing the excitation wavelength in the 250-500 nm range. The PLE spectrum of the as-deposited sample exhibits an intense excitation band for wavelengths lower than 300 nm, due to charge transfer transitions [6,7,23]. These findings hold for all samples exhibiting an Eu³⁺ emission. On the other hand, N₂-treated samples (in particular refers to an annealing at 1000 °C for 300 s) exhibit the typical PLE spectrum of Eu²⁺, characterized by a very intense and broad excitation band centered at about 350 nm, due to 4f⁶5d→4f⁷ transitions [22,23].

 

Fig. 5 Room temperature PLE spectra of Eu₂O₃ films as-deposited and annealed at 1000 °C for 300 s in O₂. The spectra were taken by integrating the PL peaks obtained for different excitation wavelengths.

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The intriguing dependence on the annealing process of the PL properties of Eu₂O₃ can be fully explained by the above discussed TEM and XRD structural data. As-deposited and O₂-treated films are essentially pure Eu₂O₃ and, accordingly, exhibit the typical red emission of Eu³⁺ ions. On the other hand, the Eu²⁺ silicates formed in N₂-treated films exhibit a broader and much more intense PL signal. The possible co-existence of Eu²⁺ and Eu³⁺ in N₂-annealed films remains a partially open question. Residual Eu₂O₃ is still detected in the XRD spectrum of the film annealed for 30 s, but, given the extremely different PL efficiency of the two Eu species and the overlap of the corresponding PL spectra, it is not surprising the absence of any Eu³⁺ contribution to the PL spectrum which is covered by the overwhelming Eu²⁺ contribution. This is even more obvious in the case of the annealing for 300 s, where no XRD peak relative to Eu³⁺ was detected. We remark also that no Eu³⁺ PL signal has been obtained by using an excitation wavelength of 275 nm, corresponding to an enhanced excitation efficiency for Eu³⁺ by about a factor of 30. Therefore, although the occurrence of the almost complete Eu³⁺→Eu²⁺ reduction suggested by XRD is a reasonable hypothesis, the presence of crystalline Eu³⁺ compounds concentrations below the detection limit of XRD, as well as of amorphous Eu³⁺ compounds, cannot be excluded a priori, also in the absence of any optical evidence.

4. Conclusion

In conclusion, we have demonstrated that it is possible, through the control of the Eu oxidation state, to obtain a very strong RT PL emission from Eu-containing thin films deposited on Si substrates. In particular, a stable Eu³⁺→Eu²⁺ reduction has been simply accomplished, and a very high external quantum efficiency (about 10%) has been measured. This is due from one hand to the fact that Eu²⁺ is more easily and efficiently excitable than Eu³⁺ through a broad excitation band, from the other to the observation that Eu²⁺ radiative transitions are dipole-allowed. These properties open the way to promising perspectives for photonic applications of Eu-based materials.