Strong Eu2+ light emission in Eu silicate through Eu3+ reduction in Eu2O3/Si multilayer deposited on Si substrates
© Li et al.; licensee Springer. 2013
Received: 16 March 2013
Accepted: 18 April 2013
Published: 26 April 2013
Eu2O3/Si multilayer nanostructured films are deposited on Si substrates by magnetron sputtering. Transmission electron microscopy and X-ray diffraction measurements demonstrate that multicrystalline Eu silicate is homogeneously distributed in the film after high-temperature treatment in N2. The Eu2+ silicate is formed by the reaction of Eu2O3 and Si layers, showing an intense and broad room-temperature photoluminescence peak centered at 610 nm. It is found that the Si layer thickness in nanostructures has great influence on Eu ion optical behavior by forming different Eu silicate crystalline phases. These findings open a promising way to prepare efficient Eu2+ materials for photonic application.
KeywordsEu2+ silicate Multilayer Photoluminescence Si-based photonics
Efficient light emission from Si-based structures and devices has drawn worldwide attention with the aim of developing an integrated optoelectronic platform on Si [1–6]. Such light emitters present an attractive application not only for inter-/intrachip optical interconnects but also, e.g., micro-displays and biological detection. Among the different approaches, rare-earth ion-based materials are very promising candidates due to their outstanding optical properties. Recently, it has been demonstrated that erbium silicate has one order of magnitude higher optically active rare-earth ions than those done through doping, without clustering or precipitation [7–10]. This may open new and interesting perspectives for rare-earth applications in photonics.
Among the various rare earths, Eu ions also have been attracting great interest in optoelectronic application because of its intense and stable emission in the visible region. Compared with other trivalent rare-earth ions, Eu2+ emission intensity is several orders stronger because of dipole-allowed transition. This makes for the successful application of Eu2+ in phosphors [11, 12], and electroluminescent devices, by incorporating Eu2+ (such as those doped in SiO2 and Eu silicate), have been demonstrated [13–15]. Bellocchi et al. have shown that the external quantum efficiency of Eu2SiO4 can be reached at about 10%, making Eu silicate of great interest for photonic application . However, in their work, Eu silicate was obtained through the complex reaction between the deposited Eu2O3 film and Si substrate, which would inevitably cause inhomogeneous distribution of Eu silicate.
In this paper, we show that Eu silicate can be fabricated by optimizing the Eu2O3/Si multilayer nanostructure deposited on Si substrates. Both the structural and optical properties of nanostructures are studied in detail. Through precisely controlling the thickness of Eu2O3 and Si layer at nanometer scale, the Eu silicate with highly efficient room-temperature (RT) light emission associated to Eu2+ ions is obtained after annealing in N2 atmosphere.
Eu 2 O 3 /Si multilayer structure
Thickness of Eu2O3layer (nm)
Thickness of Si layer (nm)
Results and discussion
The excitation property of sample 3 has been studied by PLE measurement from 300 to 450 nm and monitored at 610 nm. As shown in the left inset of Figure 5, the PLE spectrum exhibits a very intense and broad excitation band centered at about 395 nm, which is typical of Eu2+ 4f65d → 4f7 transition.
Indeed, we have also grown different Si contents of Si-rich Eu2O3 films without multilayer structure. However, no Eu2+ ions were found after the annealing process. This indicates that divalent Eu ions only appear in the Eu2O3/Si multilayer structure. We think that in Si-rich Eu2O3 films, the Eu ions are surrounded by the Si and O ions, and Eu3+ silicate is formed directly during deposition. Also, it may be very difficult to form divalent Eu ions in Eu3+ silicate without reducing gas, even if there is abundant Si. Compared with the work of Bellocchi et al, the thickness of Si layer can be precisely controlled in nanostructure instead of the Si substrate to avoid product uncertainty. Moreover, it is reported that in silicate compounds, Eu2SiO4 is a more efficient host for Eu2+ light emission than the other configurations . Although, in this work, the Eu trivalent state vanished in the nanostructure with increasing Si layer thickness, the divalent Eu ions exist both in Eu2SiO4 and EuSiO3 crystalline structures. Thus, the efficiency and intensity of Eu2+ light emission in Eu silicate will be further improved if the Eu2O3/Si nanostructure is optimized to prepare pure Eu2SiO4 phase.
In summary, Eu silicate films were prepared by the annealing Eu2O3/Si multilayer nanostructure in N2 ambient. The Eu2+ silicates were distributed uniformly along the thickness by the reaction between Eu2O3 and Si layers. Different crystalline structures were formed and identified by changing the Si layer thickness. Through precisely controlling the thickness of Si layer in Eu2O3/Si multilayer, we have obtained Eu2+ silicate films, characterized by an intense and broad PL peak that centered at 610 nm. Moreover, it suggests that Eu2SiO4 phase is an efficient light emission for Eu2+ by forming [SiO4]4− configuration. These results will have promising perspectives for Si-based photonic applications.
This work was supported by National Natural Science Foundation of China under grant numbers 61223005, 61036001, 51072194 and 61021003.
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