Background

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 [16]. 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.

Methods

The Eu2O3 /Si multilayer films with five periods were grown on Si substrates at 400°C by RF magnetron sputtering. The thin films were deposited in 3.0-mTorr Ar atmosphere. The Eu2O3 layer and Si layer were prepared by alternately sputtering the Eu2O3 target and Si target. The thickness of Eu2O3 layers was kept the same in all samples, while the thickness of Si layers was varied in different samples, as shown in Table 1. After deposition, the samples were thermally treated at 1,000°C for 30 s in N2 ambient by rapid thermal annealing. Transmission electron microscopy (TEM, Tecnai G2 F20 S-Twin, FEI Company, Hillsboro, OR, USA) was conducted to investigate the samples' morphology. The distribution of elements in the film was detected by scanning TEM (STEM), and crystallization was demonstrated by selected area electron diffraction pattern (SAED). Rutherford backscattering spectrometry (RBS) was carried out to investigate the film composition. The samples' crystalline phases were identified by X-ray diffraction (XRD, D/max 2400, Rigaku Corporation, Tokyo, Japan) measurements. RT photoluminescence (PL) and photoluminescence excitation (PLE) measurements were performed by using a spectrofluorometer (Nano Log, HORIBA Ltd., Minami-Ku, Kyoto, Japan) equipped with a 450-W Xe lamp.

Table 1 Eu 2 O 3 /Si multilayer structure
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Results and discussion

The cross-sectional TEM images of as-deposited sample are shown in Figure 1a,b. The film thickness is about 150 nm, with 5 nm in the Eu2O3 layer and 25 nm in the Si layer in one period. The interface between Eu2O3 and Si is very sharp and clear. Moreover, multicrystalline Si has formed in Si layers in the as-deposited sample, which has also been confirmed by SAED, as shown in Figure 1c. The interplanar spacing (d) is about 3.11 Å from the radius of the primary diffraction ring, which agrees with the d of the Si (111) plane. We think that the high substrate temperature and the Eu2O3 layer may induce Si crystallization.

Figure 1
figure 1

Cross-sectional TEM images of as-deposited sample 3. (a) Full view of the film, (b) partial enlarged view of the film, and (c) the SAED image of the film.

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Figure 2a,b shows the TEM cross section of the sample with a Si layer thickness of about 25 nm after annealing at 1,000°C for 30 s in N2 ambient. The interfaces between Eu2O3 layers and Si layers became blurry. This indicates that the strong reaction between Eu2O3 and Si has happened. Moreover, the crystalline europium silicate had been formed through solid-state reaction, as proven by SAED, which shows a multicrystalline ring in Figure 2c. From the SAED figures, the annealed film gave a totally different pattern compared with the as-deposited film. A lot of diffraction spots were distributed randomly, which may be ascribed to the different crystalline structures of europium silicate. In order to investigate the element distribution after the annealing process, STEM measurements were also carried out. As shown in Figure 3, Si, Eu, and O are distributed homogeneously along the thickness, suggesting that Eu2O3 and Si reacted completely in each layer.

Figure 2
figure 2

Cross-sectional TEM images of the annealed sample 3. (a) Full view of the film, (b) partial enlarged view of the film, and (c) the SAED image of the film.

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Figure 3
figure 3

The spectra of Eu, Si, and O distribution with thickness.

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The crystalline structure of the annealed films with different Si layer thicknesses was performed using XRD measurements, as shown in Figure 4. The XRD spectrum of the sample with 8-nm Si layer shows that Eu2O3, Eu2SiO5, Eu2SiO4, and EuSiO3 are mixed in the film after the annealing process. The corresponding JCPDS card numbers are 43-1008 (Eu2O3), 43-1009 (Eu2O3), 40-0286 (Eu2SiO5), 22-0286 (Eu2SiO4), and 35-0297 (EuSiO3). Eu2O3 peaks are stronger and sharper than the other peaks, suggesting that Eu2O3 is the major phase in the film due to the lack of Si. For the sample with a thicker Si layer, the XRD pattern was similar, but the Eu2O3 peak intensity had decreased. This is because more Eu3+ ions were involved in the reaction with increasing Si layer thickness. The sample with 25-nm Si layer exhibited different XRD patterns compared with the first two samples. The peaks corresponding to Eu2O3 and Eu2SiO5 (Eu3+) nearly disappeared, while the peaks corresponding to Eu2SiO4 became stronger. This indicates that Eu2SiO4 is the major phase in the film now. Moreover, through RBS measurements, the atomic concentrations of Eu, Si, and O were about 28, 14, and 58 at.% in the annealed film, which are very close to stoichiometric value of Eu2SiO4, which is consistent with the XRD results. This is interesting since the tetrahedron structure [SiO4]4− can prevent Eu2+ oxidation and energy transfer among the Eu2+ ions by isolating the Eu2+ ions with [SiO4]4−. Thus, Eu2+ in [SiO4]4− can exhibit longer stabilization and higher efficiency, which is already used in commercial phosphor such as Eu-doped silicate. By further increasing the Si layer thickness to 42 nm, Eu2O3 reacted with Si totally, and the Eu2O3-related peaks disappeared completely, as demonstrated by the XRD spectrum. Now, the film is mainly composed of Eu2SiO4 and EuSiO3 (Eu2+). This is consistent with Bellocchi's work where abundant Si may cause the formation of EuSiO3[16]. Finally, it is noticeable that most Eu ions in the sample with Si layer thicknesses between 25 and 42 nm became divalent (Eu2SiO4 and EuSiO3) after thermal treatments. This suggests that Eu2+ silicate can be achieved by precisely controlling the Eu2O3 and Si layer thicknesses.

Figure 4
figure 4

XRD patterns of the annealed samples.

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Figure 5 shows the RT PL spectra of the annealed samples, excited by 365-nm light. The intensity of the emission peak from sample 1 (with 8-nm Si layer thickness) was very weak. The spectrum had a sharp main peak centered at 616 nm with full width at half maximum (FWHM) of about 10 nm, corresponding to the 5D0 → 7F2 transition of Eu3+ ions; the other weak peaks centered at 579, 592, 653, and 703 nm, corresponding to the 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F3, and 5D0 → 7F4 transitions of Eu3+ ions, respectively. This indicates that most Eu ions are still trivalent in sample 1, which agrees with the XRD results. Compared to sample 1, other samples exhibited different PL spectra. They showed strong and broad band emissions, having the maximum peak at about 610 nm and FWHM at about 130 nm, which are typical dipole-allowed 4f65d → 4f7 transitions of Eu2+ ions in Eu2+ silicate [16]. The red shift emission was possibly due to the fact that in Eu2+ silicate the Madelung potential of the negative anions around Eu2+ is felt less by the 5d electron, leading to a lowering of energy [17]. The emission peaks of Eu3+ disappeared in the PL spectrum of sample 2 (with 17-nm Si layer thickness ) probably because more Eu3+ ions in Eu2O3 layers had been deoxidized by Si, and the emission peaks of Eu3+ were submerged in the PL spectrum of Eu2+. As shown in Figure 5, the sample with 25-nm Si layer thickness has the highest PL intensity among all the samples. The integrated PL intensity of sample 3 is more than two orders higher than that of sample 1, by forming Eu2SiO4 and EuSiO3 through reaction with Si layer, as demonstrated in the XRD tests. However, with further increase of the Si layer thickness, the PL intensity decreased. This may be due to the formation of EuSiO3 crystalline structure and the residual Si.

Figure 5
figure 5

RT PL spectra of the annealed samples. Excitation was 365 nm, and it was obtained by HORIBA Nano Log equipped with a 450-W Xe lamp. The spectrum of sample 1 is magnified tenfold. The top left inset shows the integrated intensity of the samples. The left inset shows the PLE spectrum of annealed sample 3 monitored at 610 nm.

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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 [18]. 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.

Conclusions

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.