1. Introduction

Silicon-based light sources with 1.55 µm near infrared (NIR) emission are mandatory for Si-based monolithic optoelectronic integrations [1,2]. Erbium ions doped Si-based materials have aroused great interest because the electronic transition ⁴I_13/2→⁴I_15/2 of Er³⁺ ions around ∼1.55 µm [3–5]. Co-doping semiconductor nanocrystals (NCs) and exploiting the resonance energy transfer mechanism between semiconductor NCs and Er³⁺ ions have been proven as an effective way to improve luminescence efficiency [6–8]. However, Er³⁺ ions have abundant meta-stable levels, which are suitable not only for NIR emission but also for visible light emissions. Therefore, it would be very interesting to study regulate energy transfer processes, for reducing the transfer probability of visible light emission bands and mainly enhance the NIR emission at 1.55 µm of Er³⁺ ions.

Silica materials have been proved to be good host materials for optical interconnection, for their specialty on high rare earth (Re) solubility, transparency and compatibility with Si-integrated technology [9]. Tin dioxide is a unique semiconductor with a wide band gap of 3.59 eV at 300 K and high ultraviolet light absorption coefficient [10]. In addition, it has been shown that the emission spectrum of SnO₂ defect states in SiO₂-SnO₂ thin films overlaps well with the excitation spectra of Er³⁺ ions. The matching of SnO₂ NCs defect states and the ²H_11/2/⁴S_3/2 levels of Er³⁺ ions can provide efficient energy transfer [11]. Yb³⁺ ion has a two-level structure and is often used as sensitizer in up-conversion fluorescent materials due to its large absorption cross-section near 980 nm, as well as its energy level matching with the low excited state levels of other Re³⁺ ions [12]. Co-doping Yb³⁺ ions in SiO₂-SnO₂:Er³⁺ would introduce a new energy transfer channel from SnO₂ NCs to Er³⁺ via Yb³⁺ intermediate state. Recently, Dung et al. prepared Er³⁺/Yb³⁺ co-doped SiO₂-SnO₂ glass ceramics and obtained enhanced Er³⁺ emission at 1.54 µm [13,14]. In SiO₂-SnO₂:Er³⁺/Yb³⁺ nanomaterials, the energy transfer efficiency from SnO₂ NCs to Re³⁺ is an important parameter. However, to our knowledge, there were few literature studies on it. In this work, we prepared Yb³⁺ ions co-doped SiO₂-SnO₂:Er³⁺ thin films by using sol-gel method. The effects of Yb³⁺ doping concentration on the photoluminescence (PL) characteristics under 325 nm excitation are investigated. By controlling the Yb³⁺ ions doping concentration, the increased energy transfer efficiency from SnO₂ NCs to Re³⁺ ions and the enhanced NIR emission of Er³⁺ ions are achieved.

2. Experimental section

Yb³⁺ ions co-doped SnO₂-SiO₂;Er³⁺ thin films were prepared by sol-gel method. In brief, tetraethyl orthosilicate (1 mL), deionized water (1 mL) and ethanol (2 mL) were mixed together via stirring, to form a transparent silica sol. As a catalyst, HCl was dropped into the precursor to adjust the pH valve close to 2.0. Subsequently, SnCl₄·5H₂O, Er(NO₃)₃·5H₂O and Yb(NO₃)₃·5H₂O were accurately weighted and dissolved in the precursor solution. The effects of Sn and Er³⁺ concentrations on fluorescence emission have been investigated in our previous work, and the optimal Sn and Er³⁺ concentrations are 20 mol% and 5 mol%, respectively [11]. Therefore, in this case, the amount of Sn and Er³⁺ were fixed at 20 mol% and 5 mol% of SiO₂ amount, while Yb³⁺ ratio was changed from 0 to 15 mol%. The mixed solution was sealed and vigorously stirred at 60 °C for 4 hours and aged at room temperature for 24 hours. Then, 30 µL of as-prepared gel was extracted and spin-coated onto clean Si substrates with a speed of 4000 rpm for 30 seconds to form silica thin films. After drying at 100 °C for 1 hour, the obtained thin films were annealed at 1000 °C in an air ambient to form SnO₂-SiO₂:Yb³⁺;Er³⁺ thin films.

The microstructures of the thin films were investigated by X-ray diffraction (XRD, Rigaku Ultima III) and transmission electron microscopy (TEM, Tecnai G2 F20). Steady state PL, PL decay curves were measured by a fluorescence spectrophotometer (Edinburgh Photonics, FLS980). A UV-Visible-Infrared spectrophotometry (UV3600) was employed to record absorption spectra.

3. Results and discussion

Figure 1(a) shows a typical TEM image of 2 mol% Yb³⁺ ions co-doped SiO₂-SnO₂: 5 mol% Er³⁺ thin film annealed at 1000 °C for 1 hour in an air ambient. It can be clearly seen that the spherical morphology SnO₂ nanocrystals dispersed in amorphous SiO₂ matrix with an average diameter of about 9 nm. The inset in Fig. 1(a) is a high resolution TEM image. The well-distinguished lattice fringes in the high resolution TEM corresponding to the (110) plane of the tetragonal rutile phase SnO₂ with a d-spacing of 3.41 Å. The nanostructures are further confirmed by the XRD. Figure 1(b) displays the XRD patterns of SiO₂-SnO₂: 5 mol% Er³⁺ films without and with 2 mol% Yb³⁺ ions. The characteristic diffraction peaks at 26.6°, 33.9°, 37.9°, 51.8°, 54.8°, 58.1°, 61.9°, 64.8°, 65.9° can be assigned to the diffractions from (110), (101), (200), (211), (220), (002), (310), (112), and (301) crystal planes of the tetragonal rutile phase SnO₂ (JCPDS 05-0467). The diffraction peak (110) is consistent with the high resolution TEM observation.

 

Fig. 1. (a) TEM images of SiO₂-SnO₂:5 mol% Er³⁺/2 mol% Yb³⁺ ions. (b) XRD patterns of the SiO₂-SnO₂:5 mol% Er³⁺ films without and with 2 mol% Yb³⁺ ions

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The UV-Visible absorption spectra of SiO₂-SnO₂:Er³⁺ thin films doped with different Yb³⁺ concentrations are shown in Fig. 2. The absorption peak of ultraviolet band (<350 nm) is attributed to the band-gap absorption of SnO₂ [15]. It can also be found that the Yb³⁺/Er³⁺ doped thin films exhibit obvious blue shift, comparing to the un-doped thin film. The optical bandgap of the thin films can be estimated by Tauc equation:

 

Fig. 2. Absorption spectra of SiO₂-SnO₂:Er³⁺ thin films with varying Yb³⁺ concentrations.

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Room temperature PL spectra of SiO₂-SnO₂:Er³⁺ thin films co-doped with different Yb³⁺ concentrations are recorded by a fluorescence spectrophotometer under the excitation wavelength of 325 nm. As shown in Fig. 3(a), for the SiO₂-SnO₂ thin film with Re³⁺-free, a broad visible emission band centered at 530 nm can be observed, which is originated from the radiative recombination of the defect-states of SnO₂ nanocrystals [20]. It is very clear that co-doping with 5 mol% Er³⁺ ions attenuates the SnO₂ NCs signal and produces Er³⁺-related green emissions (525 nm, 549 nm and 565 nm), red emission (662 nm and 682 nm) and NIR emission (1532 nm) corresponding to the ²H_11/2/⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions of Er³⁺ ions, respectively. The result indicates that there is an efficient energy transfer process between SnO₂ NCs and Er³⁺ ions [11,21]. In addition, when Yb³⁺ ions are introduced into the thin films, a NIR emission band at 970 nm can be observed, which is assigned to the ²F_5/2→²F_7/2 transition of Yb³⁺ ions. It can be found that with the increase of Yb³⁺ concentration, the intensity of visible emission band gradually decreases, while the 1532 nm NIR emission of Er³⁺ ions increases first and then decreases. The integrated intensity of visible and 1532 nm NIR emissions are calculated respectively, and the data are plotted in Fig. 3(b). When co-doping with Yb³⁺ ions, the reduction in visible fluorescence can be attributed to the following factors. Co-doping Yb³⁺ ions into SiO₂-SnO₂:Er³⁺ thin films can introduce a new energy transfer channel from SnO₂ NCs to ²F_5/2 level of Yb³⁺ ions, so that part of the energy of SnO₂ NCs is transferred to Yb³⁺ ions, resulting in weakened visible light emissions [13]. Furthermore, increasing Yb³⁺ content will promote cross-relaxation (CR) process between Yb³⁺ and Er³⁺ ions: ²F_7/2(Yb³⁺)+⁴S_3/2(Er³⁺)→²F_5/2(Yb³⁺)+⁴I_13/2(Er³⁺), which can depopulate the ⁴S_3/2 levels of Er³⁺ ions and in turn attenuate the visible light emission of Er³⁺ ions. Concentration quenching effect at high Yb³⁺ content is also one of the reasons. For NIR emission of Er³⁺ ions, the intensity reaches its maximum when the Yb³⁺ ion concentration is about 2 mol%, and then gradually decreases. Yb³⁺ ion usually acts as a sensitizer, due to its simple energy levels structure, large absorption cross-section and efficient energy transfer to Re³⁺ ions. In Yb³⁺-Er³⁺ system, the ²F_5/2 level of Yb³⁺ matches well with the ⁴I_11/2 level of Er³⁺. As mentioned above, the establishment of energy transfer channel from SnO₂ NCs will populate ²F_5/2 level of Yb³⁺ and promote the efficient ET process from ²F_5/2 level of Yb³⁺ to ⁴I_11/2 level of Er³⁺, resulting in the enhancement of NIR emission of Er³⁺ ions. When the Yb³⁺ concentration increased higher than 2 mol%, the NIR emission of Er³⁺ ions gradually weakens, which can be attributed to the concentration quenching effect caused by high Yb³⁺ concentration.

 

Fig. 3. (a) PL spectra of SiO₂-SnO₂:Er³⁺ thin films with varying Yb³⁺ concentrations. (b) The normalized PL intensity versus the Yb³⁺ concentration. (c) Diagram of the energy levels in SiO₂-SnO₂:Er³⁺/Yb³⁺ thin film under 325 nm excitation.

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In order to investigate the energy transfer processes from the defect-states of SnO₂ nanocrystals to Re³⁺ ions, the PL decay curves of SnO₂ nanocrystals emission at 530 nm for SiO₂-SnO₂ thin films doped with different concentrations of Er³⁺/Yb³⁺ ions are measured by using a 375 nm picosecond laser (see Fig. 4(a)). It is clear that 530 nm emission from SiO₂-SnO₂:Er³⁺/Yb³⁺ decays faster than that of SiO₂-SnO₂:Er³⁺ thin film, and the lifetime of 530 nm emission of SnO₂ nanocrystals becomes shorter with the increase of Yb³⁺ concentration. The gradually declined lifetime can be attributed to the energy transfer process from SnO₂ nanocrystals to Yb³⁺ ions, which contributes to the increase of the nonradiative decay rate of SnO₂-related emission. The result indicates that introduction of Yb³⁺ ions does establish a new energy transfer channel from SnO₂ nanocrystals to Yb³⁺ ions. The average lifetime of the emission at 530 nm can be estimated by the following equation [11]:

 

Fig. 4. (a) PL decay curves of SnO₂ emission at 530 nm for the thin films with different Yb³⁺ concentrations. (b) Lifetime of SnO₂ emission and energy transfer efficiency as a function of Yb³⁺ concentration. (c) PL decay curve of SnO₂ emission for 15 mol% Yb³⁺ co-doped SiO₂-SnO₂:Er³⁺ thin film; the inset is the corresponding NIR PL spectrum.

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4. Conclusion

In summary, Yb³⁺ ions co-doped SiO₂-SnO₂:Er³⁺ thin films have been prepared by employing a sol-gel method with spin-coating technique. The introduction of Yb³⁺ ions can establish a new energy transfer channel from SnO₂ NCs to Er³⁺ ions via Yb³⁺ intermediate state, resulting in weakened visible light emission and enhanced Er³⁺-related NIR emission. By controlling Yb³⁺ ions concentration, the Er³⁺-related NIR emission can be increased by about 1.5-fold. The results of PL decay curves of SnO₂ NCs emission indicate that the ETE of SnO₂ NCs to Re³⁺ ions is increased gradually with the increase of Yb³⁺ concentration. For the thin film with 15 mol% Yb³⁺ ions, the estimated ETE is about 68.7%. The SiO₂-SnO₂:Er³⁺/Yb³⁺ thin films would be a definitely promising candidate for the electroluminescence devices.