A stable Eu3+→Eu2+ reduction is accomplished by thermal annealing in N2 ambient of Eu2O3 films deposited by magnetron sputtering on Si substrates. Transmission electron microscopy and x-ray diffraction measurements demonstrate the occurrence of a complex reactivity at the Eu2O3/Si interface, leading to the formation of Eu2+ silicates, characterized by a very strong (the measured external quantum efficiency is about 10%) and broad room temperature photoluminescence (PL) peak centered at 590 nm. This signal is much more efficient than the Eu3+ emission, mainly consisting of a sharp PL peak at 622 nm, observed in O2-annealed films, where the presence of a SiO2 layer at the Eu2O3/Si interface prevents Eu2+ formation.
© 2012 OSA
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 , 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 , the achieving of optical gain in Er/Y silicate waveguides  and the observation of a very efficient Er/Yb coupling in mixed silicates . Furthermore, photon cutting phenomena, being of great relevance for photovoltaic applications of these materials, have been demonstrated in Er/Y silicates .
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  or Eu compound thin films , and the Eu emission properties have been exploited in light emitting devices based on Eu-doped SiO2 [11,12] or Eu silicates .
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.
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  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 LII,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 NIV edge) and by using the 3-windows method . 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 . 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 . 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 , 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  are found, while peaks corresponding to Eu2O3 disappeared.
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 . Eu3+ → Eu2+ reduction in glasses is accomplished by exploiting gaseous reactants such as H2  or CO ; 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 5DJ→7FJ 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.
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.
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].
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.
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.
The authors thank the CNR-IMM (Catania, Italy) for the use of their TEM facilities.
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