1. Introduction

Rare earth Er³⁺ ions have unique optical properties that can be used in many kinds of applications such as lighting, displays, and optical communications [1–4]. More interestingly, the transition of Er³⁺ ions from energy level ⁴I_13/2→⁴I_15/2 produces near-infrared (NIR) luminescence at a wavelength of 1550 nm, which corresponds exactly to the minimum loss window range of silica optical waveguides [5–8]. Therefore, the study on stable and reliable Er³⁺ ions doped silicon dioxide NIR electroluminescent devices to realize silicon-based monolithic optoelectronic integrated interconnects has attracted much attention [9–12]. Usually, Er³⁺ ions are placed in a silica package to obtain NIR emission at 1550 nm. However, the luminescence intensity and efficiency are limited by the doping concentration of Er³⁺ ions, and excessive doping will cause concentration quenching effect [13]. Moreover, the realization of highly efficient silicon-based electroluminescent devices is also limited by the electrical insulating properties of the silica matrix and the extremely low absorption cross-section of Er³⁺ ions [14,15].

Co-doping semiconductor nanocrystals (NCs) such as Si, TiO₂, In₂O₃, and SnO₂ NCs with Er³⁺ ions can effectively improve NIR luminescence, because nanocrystals can quickly transfer energy to Er³⁺ ions for sensitization [4,16–19]. Compared with Si NCs, the wide bandgap and large absorption cross-section of oxide semiconductor can better sensitize Er³⁺ ions and improve the luminescent performance. For example, SnO₂ has a wide bandgap (300 K) of 3.59 eV [20], and the emission spectra of its defect state overlap with the characteristic absorption spectra of Er³⁺ ions, which is conducive to the energy transfer to Er³⁺ ions and promote its photoluminescence (PL). In our previous work, the introduction of size-tunable SnO₂ NCs into SiO₂:Er³⁺ films achieved a three-order of magnitude increase in NIR PL of Er³⁺ ions [21]. To further enhance the luminescence of Er³⁺ ions, metal ions such as Mg²⁺, Ca²⁺, and Ba²⁺ can be introduced. The introduction of Ba²⁺ doping into the SiO₂-SnO₂:Er³⁺ films resulted in a 12-fold enhancement of the NIR PL of Er³⁺ ions [8]. Despite the fact that NIR PL of Er³⁺ ions can be enhanced by introducing wide bandgap SnO₂ NCs and metal ions, there have been few reports on relevant EL devices. In our present work, high-quality Ca²⁺-doped SiO₂-SnO₂: Er³⁺ thin films have been fabricated via a sol-gel method to improve the NIR PL performance of Er³⁺ ions. The concentration of Sn was adjusted to increase the conductivity of the silica matrix. At appropriate Sn doping ratios, our EL devices produced significant NIR electroluminescence (EL). After operating the device for 180 minutes at room temperature, the NIR EL intensity of the device hardly decreases. The optimal EL device achieves the optical power density of 13.47 mW/cm², with the power efficiency of 1.04 ${\times} $ 10⁻².

2. Experimental section

In this work, spin-coating methods were utilized to create alkaline-earth metal Ca²⁺-doped SiO₂-SnO₂: Er³⁺ films using a sol-gel manufacturing process, followed by thermal evaporation and sputtering to prepare electrodes, which were finally prepared as electroluminescent devices. Tetraethyl orthosilicate (1 mL), deionized water (1 mL), and anhydrous ethanol (2 mL) were combined to produce a sol. The pH of the sol gradually approached 2.0 by adding a specific amount of HCl to generate the precursors. The desired precursor solutions were obtained by completely dissolving SnCl₄·5H₂O, CaCl₂·2H₂O (0.033 g), and Er(NO₃)₃·5H₂O (0.099 g) with uniform stirring in the precursors. In our situation, the doping concentration of Er³⁺ ions was set at 5 mol%, the doping concentration of Ca²⁺ ions was set at 5 mol%, and the doping ratios of the regulated Sn were 0, 20, 30, 40, 50, and 60 mol%, and the EL devices prepared in this way were also named as 0-LED, 20-LED, 30-LED, 40-LED, 50-LED, 60-LED devices. The prepared precursor solution was then heated to 60 °C with continuous stirring and maintained for 4 h. The precursor solution was subsequently aged for 24 h to obtain the desired gel. Next, we take a small amount of solution, which is spin-coated to a washed silicon wafer to form a uniform and flat silica film. The obtained films were annealed under an air atmosphere at 1000 °C for 1 hour to generate alkaline earth metal Ca²⁺-doped SiO₂-SnO₂: Er³⁺ films (∼180 nm). To achieve EL, the devices were prepared by sputtering dotted indium tin oxide (ITO) with diameters of 2 mm on the surface of SiO₂-SnO₂: Er³⁺/Ca²⁺ films as top electrodes (∼218 nm), depositing aluminum (Al) electrodes (∼180 nm) on the backside of P-type silicon wafers by thermal evaporation, and annealing in a nitrogen atmosphere.

The crystalline phases of Ca²⁺-doped SiO₂-SnO₂: Er³⁺ films of alkaline earth metals were characterized by x-ray diffraction (XRD, Bruker D8). X-ray photoelectron spectroscopy (XPS, SSI S-Probe XPS Spectrometer) was used to characterize the elemental composition and corresponding chemical state of the fabricated thin films. Thorlabs PM100D optical power meter and Thorlabs S401C photodetector are used to measure the optical power density of the LED (see Supplement 1). Utilizing the fluorescence spectrophotometer (Edinburgh Photonics, FLS980), we measured the PL, photoluminescence excitation spectra (PLE) of the manufactured films, and the EL of the related devices.

3. Results and discussion

Figure 1(a) depicts the XRD patterns of SiO₂-SnO₂: Er³⁺/Ca²⁺ films doped with varying content of Sn, where Sn is doped at 0, 20, 30, 40, 50, and 60 mol%. When the doping concentration of Sn is 0 mol%, there is no diffraction peak in the XRD pattern, which proves that the silica film is amorphous. The results show that distinct diffraction peaks appear when doped with Sn, with clear sharp diffraction peaks at 2θ = 26.61°, 33.89°, 37.95°, 51.78°, 54.76° corresponding to the SnO₂ (110), (101), (200), (211) and (220) crystal planes, indicating that the sample crystallized in the tetragonal rutile phase of SnO₂, which is compatible with JCPDS 41-1445 standard XRD patterns. With the different Sn-doped concentrations, the diffraction peak intensity signal is slightly different, which indicates that the crystallinity of SnO₂ will change with the doping concentration [9]. When the doping concentration of Sn is 40 mol%, the diffraction peak for SnO₂ crystal is strengthened, which indicates a better crystallinity.

 

Fig. 1. (a) XRD patterns of SiO₂-SnO₂: Er³⁺/Ca²⁺ thin films with varying Sn concentrations; (b)-(e) XPS spectra of SiO₂-SnO₂: Er³⁺/Ca²⁺ thin film with 40 mol% Sn.

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Figure 1(b)-(e) depicts the element composition and chemical state of SiO₂-SnO₂: Er³⁺/Ca²⁺ films doped 40mol%Sn. The XPS results showed that Sn, Er, Ca, and O elements were detected in the film doped with 40 mol% Sn, which was consistent with the designed elements of the film. The corresponding XPS binding energy of Sn 3d_5/2 and Sn 3d_3/2 are 486.7 eV and 495.0 eV, which proves that Sn exists in the +4 oxidation state in the film [8,22]. Er exists in the +3 oxidation state, and the XPS binding energy of Er 4d is detected at 169.4 eV [23]. The XPS peaks of Ca 2p correspond to 347.4 eV and 350.9 eV, which proves that the Ca²⁺ ions are successfully doped into the matrix [24]. In addition, the XPS peaks of O 1s can be deconvoluted into three main peaks. Si-O and Sn-O bonds are responsible for the 532.6 eV and 530.2 eV peaks, respectively. The peak at 531.8 eV is caused by oxygen defect within the matrix. The proportion of oxygen vacancy has increased significantly after the addition of Ca²⁺ ions [8,19].

A 325 nm He-Cd laser was used to excite the SiO₂-SnO₂: Er³⁺/Ca²⁺ thin film and the resultant NIR PL spectrum is displayed in Fig. 2(a). In the NIR band, the emission spectra with the central peak at 1532 nm can be attributed to the ⁴I_13/2 - ⁴I_15/2 transition of Er³⁺ ions. When the SiO₂-SnO₂: Er³⁺ films are doped with Ca²⁺ at the concentration of 5 mol%, the NIR PL intensity of the films is significantly enhanced. The enhanced luminescence can be attributed to the fact that the doping of Ca²⁺ ions not only improves the crystal quality of SnO₂ NCs (see Figure S2, Supplement 1), but also generates more oxygen vacancies by occupying part of the Sn⁴⁺ sites (see Figure S3, Supplement 1), which is beneficial for energy transfer to Er³⁺ ions to enhance the luminescence [25,26].

 

Fig. 2. (a) NIR PL spectra of SiO₂-SnO₂: Er³⁺ thin films with and without Ca²⁺ under 325 nm excitation; (b) NIR PL spectra of SiO₂-SnO₂: Er³⁺/Ca²⁺ thin films with different Sn concentrations under 325 nm excitation; (c) PLE spectra of SiO₂-SnO₂: Er³⁺/Ca²⁺ thin film with different Sn concentrations measured at 1532 nm; (d) The schematic diagram of energy levels in SiO₂-SnO₂: Er³⁺/Ca²⁺ thin film.

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The doping concentration of Sn not only affects the NIR PL intensity of SiO₂-SnO₂: Er³⁺/Ca²⁺ film, but also improves the conductivity of the film, which is very important for the preparation of EL devices. The results of the PL spectrum on the fabricated films with various Sn doping are given in Fig. 2(b). The NIR PL intensity is the strongest when the Sn doping concentration is 40 mol%, which can be ascribed to the improved crystal quality of SnO₂ NCs [8,27]. When the doping content of Sn exceeds 40 mol%, the NIR PL intensity begins to decrease. This phenomenon can be attributed to the reduced dot density and surface-to-volume ratio of SnO₂ NCs, which will lead to a weakened energy transfer process [19,21].

In order to explore the NIR PL mechanism of Er³⁺ ions, the photoluminescence excitation (PLE) spectrum was tested by monitoring at 1532 nm. The PLE spectra of SiO₂-SnO₂: Er³⁺/Ca²⁺ films doped with different Sn are shown in Fig. 2(c). The UV band of the PLE spectra can be considered as the band-to-band transition of SnO₂ NCs [19]. The transition of Er³⁺ ions from the ground state ⁴I_15/2 to the excited state ⁴G_11/2 and ²H_11/2 is thought to be responsible for the PLE peaks at 378 nm and 521 nm, respectively [21]. The probable PL mechanism of SiO₂-SnO₂: Er³⁺/Ca²⁺ film is postulated based on the foregoing results, as illustrated in Fig. 2(d). SnO₂ NCs absorb the energy required to excite electrons from the valence band to the conduction band under 325 nm laser stimulation. Subsequently, the excited electrons are captured by defect states through non-radiative relaxation. The defect state energy level just overlaps with the Er³⁺ ions excited state energy level, which helps to transfer energy to the surrounding Er³⁺ ions excited state energy levels, such as ²H_11/2, ⁴F_9/2, and ⁴I_13/2 energy levels [3,4,12]. The electrons at high excited-state energy levels can produce visible luminescence through radiative transitions, or non-radiatively relax to the ⁴I_13/2 energy level. In addition, the population of ⁴I_13/2 level of Er³⁺ ions can also be obtained through cross-relaxation processes (CR1 and CR2) between Er³⁺ ions [8]. The electrons on the ⁴I_13/2 energy level jump to the ⁴I_15/2 energy level to produce excellent NIR luminescence at 1532 nm [15,28].

Figure 3(a) displays the structure of the fabricated electroluminescent device. To enable achieve EL, the device was prepared by sputtering dotted ITO on the top of SiO₂-SnO₂: Er³⁺/Ca²⁺ film as the top electrode and depositing aluminum as the bottom electrode on the back of a P-type silicon wafer. Figure 3(b) shows the SEM image of the cross-section of the 40 mol% Sn-doped device, and the inset is the surface SEM image of the film. The SiO₂-SnO₂: Er³⁺/Ca²⁺ film has a relatively flat surface with a thickness of about 180 nm, and ITO is sputtered on top of the film with a thickness of about 218 nm.

 

Fig. 3. (a) The structure of SiO₂-SnO₂: Er³⁺/Ca²⁺ EL device; (b) The SEM image of the cross-section of SiO₂-SnO₂: Er³⁺/Ca²⁺ EL device (the inset shows the surface SEM image of the SiO₂-SnO₂: Er³⁺/Ca²⁺ film).

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The NIR EL spectra of EL devices with various Sn concentrations are depicted in Fig. 4(a). When voltage is applied, the 40 mol%, 50 mol% and 60 mol% Sn-doped devices have NIR spectra, as illustrated in Fig. 4(a). At the same injection current, it can be found that the 40-LED device exhibits a greater NIR EL intensity, which indicates that the 40-LED has better luminescence performance. The NIR EL intensity of the 50-LED and 60-LED gradually declines as Sn doping concentration increases. This problem may be due to the fact that the crystallinity of SnO₂ NCs decreases with increasing doping concentration, which leads to weakened NIR EL intensity [9,12]. Similarly, we prepared 20-LED and 30-LED devices (Sn doping concentration less than 40 mol%) but failed to detect NIR EL when voltage was applied. This problem might be related to the fact that doped SnO₂ NCs improve the conductivity of the film. When the doping concentration of Sn is lower than 40 mol%, the electrical insulation of the silica matrix makes it difficult to inject carriers [4,14], so NIR EL will be difficult to realize. It is worth mentioning that the optimal device exhibits lower threshold voltage and higher EQE compared to the EL device without Ca²⁺ ions doping [19]. The improved device performance can be attributed to the following factors: the improved conductivity [9,29], the better crystallinity after Ca²⁺ ions doping as supported by XRD results, more oxygen vacancies, and stronger CR processes [8] after doping with Ca²⁺ ions as discussed before. The NIR EL process of Er³⁺ ions may involve several kinds of routes. Without doping SnO₂ NCs, some electrons gain energy into the conduction band of silica under the applied operating voltage and have enough kinetic energy to excite Er³⁺ ions by collision, but this method is inefficient [30]. When we doped the silica matrix with large amounts of SnO₂ NCs, most electrons would be transported by the hopping process rather than enter the conduction band of the silica [12]. To prepare for recombination with electrons, holes from the P-Si substrate reach the valence band of SnO₂ NCs. Energy is transferred to excite Er³⁺ ions by the nonradiative recombination of the electron-hole pairs, which improves the EL efficiency.

 

Fig. 4. (a) NIR EL spectra from LED devices doped with various content of Sn; (b) The relation between current density and voltage of devices with varying content of Sn doping (the inset shows the relationship after log processing).

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Figure 4(b) depicts the current density-voltage connection of LED doped with varying Sn concentrations. When the same forward voltage is provided, the current density of the EL device increases with Sn doping concentration is raised. This indicates that doping with Sn improves the conductivity of the silica film, which facilitates carrier injection [4,9]. Our device might conform to the space charge limited current model(SCLC), according to the analysis of the current density and voltage relationship after log processing, and its expression is as follows [31]:

The charge and mobility associated with electrons are represented by $\mu $ and e in the relation between current density and voltage in the SCLC model. ${N_0}$, ${\varepsilon _0}$, and $\varepsilon $ are all constants, representing the distribution trap density, absolute dielectric, and relative dielectric respectively. ${N_c}$, T, and s are the density of states in the dielectric conduction band, the temperature defining the model, the thickness of silica film, respectively. t is a constant with a value higher than one. In this case, the J-V equation can be equivalent to J ∝V^m (m = t + 1). The J-V curve fits J ∝V^m (m > 2) well, as seen in the inset of Fig. 4(b), which demonstrates that our LED device matches the exponential trap model of the SCLC. With the change of Sn doping concentration, the carrier transport mechanism of the LED device remains unchanged. From the EL results (Fig. 4(a)), the 40-LED device generated the highest NIR EL intensity at the same input current, indicating that it had the optimum luminous efficiency.

Figure 5(a) shows the NIR EL spectra of the 40-LED device. In the NIR band, the luminescence peak at 1532 nm can be considered as the transition of Er³⁺ ions from the ⁴I_13/2 energy level to the ⁴I_15/2 energy level. As shown in Fig. 5(b), the normalized integrated EL intensity of the 1532 nm peak increases rapidly with increasing voltage. This indicates that more carriers are injected into the SiO₂-SnO₂: Er³⁺/Ca²⁺ films, which excite the luminescent center Er³⁺ ions more effectively through the energy transfer process. The NIR luminous intensity is significantly enhanced.

 

Fig. 5. (a) NIR EL spectra of 40-LED; (b) Normalized EL intensity of 40-LED versus the voltage; (c) Optical power density versus current density; (d) NIR EL intensity of 40-LED devices varies with operating time.

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Figure 5(c) exhibits the optical power density of the 40-LED device at various current densities. As the current density increases, the output optical power density of the 40-LED device also increases. Wang et al. fabricated Er/SnO₂-doped silica film device and exhibit the power efficiency of 1.5 ${\times} $ 10⁻³ [19]. Yang et al. used nanolaminate Al₂O₃: Yb, Er films to achieve EL and present the power efficiency of 2.8 ${\times} $ 10⁻³ [7]. Our optimal 40-LED device exhibits good performance, achieving the optical power density of 13.47 mW/cm² with the power efficiency of 1.04 ${\times} $ 10⁻². Similarly, we estimate the external quantum efficiency of the optimal 40-LED device. When the input current density of the device is 31.85 mA/cm², the external quantum efficiency of the 40-LED device exceeds 10%.

Figure 5(d) shows the operational stability test of an unencapsulated 40-LED device at room temperature. The 40-LED device was tested continuously for 180 minutes at an applied operating voltage of 35 V, and the NIR EL spectra were tested at 5-minute intervals. It is found that the NIR EL intensity of the 40-LED device does not decay after 180 minutes of continuous operation, indicating that our prepared LED devices have good stability, which is important for practical applications.

4. Conclusion

In summary, alkaline earth metal Ca²⁺ doped SiO₂-SnO₂: Er³⁺ thin films were prepared by a low-cost sol-gel method combined with spin-coating technology. 40mol%Sn-doped SiO₂-SnO₂: Er³⁺/Ca²⁺ film has the best PL performance due to its good crystallinity and oxygen vacancies introduced by Ca²⁺ doping. The introduction of SnO₂ NCs not only improves the conductivity of the silica matrix but also enhances the luminescence of Er³⁺ ions through the energy transfer process. The optimal EL device achieves the optical power density of 13.47 mW/cm², with the power efficiency of 1.04 ${\times} $ 10⁻². Under the working voltage of 35 V, the NIR EL intensity of the 40-LED hardly decreases after 180 minutes of operation. This work may contribute to the development of stable silicon-based light sources for photoelectric integration.