We report on the sensitized photoluminescence (PL) of Er silicates in a-Si-embedded Er silicate films. A two-step annealing process is utilized, where the first step determines the distribution and the sizes of the a-Si embedded structure and the second step modifies the crystal quality and the phase composition of Er silicates as well as the concentration of sensitizers. The determination of the annealing temperatures and the annealing time for each step requires an overall consideration of these factors. Optimized PL from Er silicates sensitized by luminescent centers (LCs) such as neutral oxygen vacancy (NOV) or non-bridging oxygen hole center (NBOHC) have been achieved in the film annealed at 1000 °C for 30 min followed by 1100 °C for 30 min, which is composed of well-crystallized y-Er2Si2O7 embedded with a-Si clusters.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Er-doped Si-based materials have been attracting considerable attention during the last decades because the emission of Er ions at 1.54 µm matches well with the low-loss window of standard silica based optical fibers. [1–8] Nevertheless, the Er concentrations in bulk Si and silica are limited to 1018 cm−3 and 1020 cm−3, respectively, which makes the optical gain in these materials far from practical applications. [9,10] Er compounds, in which Er is a major component rather than a dopant, can have Er concentrations as high as 1022 cm−3. [11–14] Therefore, Er compounds have been considered as a promising high gain medium for optical amplification in the communication band. In spite of the high Er concentrations, the small excitation cross section of Er ions in Er compounds also limits the optical gain. Incorporating sensitizers such as Si nanocrystals (Si-NCs) [15–18], luminescent centers (LCs) [19–21] into these materials can induce an electron-hole-mediated excitation process of Er, which increases the effective excitation cross section of Er. [21–23] It has been reported that an optical gain of 4 dB cm−1 was achieved in erbium-doped Si nanocluster sensitized waveguide.  Therefore, the combination of Er compounds and sensitizers with efficient energy transfer between them is a promising method to realize high optical gain in Er-doped Si-based materials. We have incorporated amorphous Si clusters into crystalline Er silicate layers in the previous paper. In order to achieve efficient sensitized PL of Er silicates in this system, some parameters of the film need to be carefully designed. First, the film structure should be as fine as possible to increase the interaction between sensitizers and Er silicates. Second, the concentration of the sensitizers should be very high to sensitize as many Er ions as possible. Third, different polymorphs of Er silicates have different optical properties [14,25], and therefore the polymorphs with stronger PL intensities should be selected preferentially. Fourth, the crystal quality of Er silicates should be improved to reduce the defect concentrations.
In the previous paper, we have investigated the formation of the a-Si-embedded Er silicate films. In this paper, we report on the sensitized PL of Er silicates from a-Si-embedded Er silicate films. A two-step annealing process is used. The influence of the annealing process of each step on the structure, the crystal quality and the concentration of LCs is investigated and an optimized sensitized PL of Er silicates is achieved.
Er-Si-O films about 550 nm thick were prepared on Si (100) substrates by reactive magnetron co-sputtering using a pure Si target with the radio frequency (rf) power of 130 W and a pure Er target with the rf power of 30 W in an Ar and O2 mixed atmosphere. The substrate was rotating and heated at 300 °C during the deposition. The as-deposited films were treated with different annealing processes in a nitrogen atmosphere using a tubular furnace. Photoluminescence (PL) in the visible range was measured by pumping with a He-Cd laser with the wavelength of 325 nm and detected by a charge-coupled device (PIXIS:100BR, Princeton Instruments, Trenton, America). PL in the near-infrared range was detected by an InGaAs photomultiplier tube (PMT, Hamamatsu R5509, Iwata City, Japan), where a 473 nm-laser and a 980 nm-laser were employed as the excitation light sources. A microsecond lamp was used in the transient PL measurements with an overall time resolution of about 2µs. All the PL measurements were performed at room temperature.
3. Results and discussion
It has been reported in the previous paper that an a-Si embedded Er silicate film was formed after the annealing of a Si-rich SiO2 film heavily doped with Er (Si, 38 at%; O, 48 at%; and Er, 14 at%). In order to achieve efficient sensitized PL of Er silicates, the microstructure of the Er silicate layer requires modifying. As demonstrated above, four factors need to be considered, which are structure size, polymorph and crystal quality of Er silicates, and concentration of sensitizers. We have demonstrated in the previous paper that there is a contradiction between the requirements for smaller structure size and higher crystal quality, which can be solved by a two-step annealing process, where the first step is aimed at reducing the structure size of the a-Si embedded Er silicate layer and the annealing temperature of the first step should be as low as possible as long as Er silicate is able to crystallize. Therefore, the annealing temperature of 1000 °C is chosen.
The aim of the second annealing step is to modify the crystal quality, the phase composition of Er silicates and the concentration of sensitizers. Since all these factors change simultaneously with annealing, we have to obtain an optimized annealing procedure with an overall consideration of these factors instead of modifying them separately.
The phase compositions of the Er-Si-O films with different annealing processes have been determined by XRD analysis in the first paper and are summarized in Table 1. The films are labeled as sample 1 to 4. Sample 1 to sample 3 are mostly composed of y-Er2Si2O7 while sample 4 is composed of only α-Er2Si2O7.
Figure 1 presents the PL spectra and the PL decay curves of the Er-Si-O films. The excitation wavelength is 980 nm, which is in resonance with the 4I11/2 level of Er and allows direct excitation of Er. As shown in Fig. 1(b), the PL decay curve of sample 1 shows two different decay processes with the lifetimes of 5 µs and 378 µs. Er ions in sample 1 can be divided into two groups. One group exists in the form of Er silicates and the other exists in amorphous regions which have Er concentrations not high enough to form Er silicates. We have reported that the PL lifetimes of Er silicate films fabricated by RF sputtering are generally less than 30 µs.  Therefore, the faster decay is attributed to Er ions in y-Er2Si2O7 and the slower decay is attributed to Er ions in uncrystallized regions with lower Er concentrations. The fast PL decay of y-Er2Si2O7 and the small proportion of this fast decay process makes the PL from the uncrystallized regions more significant, and consequently the PL spectrum of this film does not show the PL characteristics of y-Er2Si2O7, as shown in Fig. 1(a).  Different from sample 1, the PL decay curve of sample 2, which is also mostly composed of y-Er2Si2O7, shows a single exponential decay with the lifetime of 26 µs, as shown in Fig. 1(c), and the shape of the PL spectrum corresponds well with that of y-Er2Si2O7. The disappearance of the slow decay process and the big increase in the PL lifetime of y-Er2Si2O7 indicate that the amount of crystallized Er silicate has greatly increased and the crystal quality has greatly improved after the second annealing step at 1100 °C. In addition, the integrated PL intensity of sample 2 is 2.5 times as large as that of sample 1 due to the improvement of the crystal quality of Er silicates. The phase composition of sample 3 is similar to that of the previous samples. However, the crystal quality is relatively lower due to the short annealing time, leading to a smaller PL lifetime of 16 µs. Sample 4 is composed of α-Er2Si2O7 alone. The decay curve shows a single exponential decay with the lifetime of 11 µs and the integrated PL intensity is only two fifths of that of sample 2, which is consistent with the previous report that y-Er2Si2O7 has stronger PL intensity and longer PL decay than α-Er2Si2O7.  From a comparison between sample 1 and the other samples, it is concluded that the amount of crystallized Er silicates and the crystal quality of Er silicates are both far from satisfactory when the film is annealed only at 1000 °C, which demonstrates that sample 1 is highly unsuitable for the sensitized PL of Er silicates and the second step of the annealing process is indispensable. Therefore, we only consider sample 2 to sample 4 in the following analysis.
It should be noted that the Er PL lifetimes of our Si-rich Er silicate films are almost the same with those reported in deposited Er silicate films without Si excess annealed under similar conditions [12,14] and those in Er silicates fabricated by sol-gel method . This indicates that the introduction of Si excess has not resulted in extra non-radiative paths, mostly defects, for the Er silicates. This is good news because a large Er PL lifetime is necessary for the realization of optical gain in this system.
In addition to the crystal quality and the phase composition of Er silicates, the concentration of sensitizers is also closely related to the second step of the annealing process. In order to determine the types and concentrations of sensitizers, PL measurements in the 400-1000 nm range were performed. Figure 2(a) shows the PL spectra of sample 2 to sample 4. Two PL bands can be resolved through Gaussian fitting for all the samples and the fitted peak positions of different samples are almost the same, which are at 495 nm and 645 nm. It indicates that the peak positions of the PL bands are annealing-independent. PL lifetime measurements were also performed on the samples. Figure 2(b) shows the PL decay curves of sample 2 detected at 495 nm and 645 nm. The PL decay curves at 495 nm and 645 nm both coincide with the instrument response curve, demonstrating that the decay time at these wavelengths is below the instrument response time, which is around 2 µs. We attribute the PL peak at 495 nm to neutral oxygen vacancies (NOVs) based on the emission wavelength and lifetime. [27–29] For the PL peak at 645 nm, alternative interpretations can be invoked to explain the origin. This PL band could be attributed to a-Si clusters or non-bridging oxygen hole centers (NBOHCs).  However, a-Si clusters have to be smaller than 2 nm to generate light emission at 645 nm [31,32] and the sizes of the a-Si clusters in our films are much larger, as shown in the TEM images in the previous paper. In addition, the peak position of this band does not change with the annealing temperatures, which is not the case in the PL of a-Si clusters due to the change of cluster sizes under different annealing temperatures. Therefore, we attribute the PL peak at 645 nm to NBOHCs. Although no PL bands associated with Si-NCs or a-Si clusters are observed in these samples, it has been reported that LCs can also transfer energy to Er ions [20,21], so NOVs and NBOHCs are considered to be the main sensitizers in these samples. As shown in Fig. 2, the integrated PL intensity of sample 4 is the largest, which is 2 times larger than that of sample 2 and 4.6 times larger than that of sample 3. The integrated PL intensity is not only determined by the concentrations of the LCs but also associated with the energy transfer efficiency between LCs and Er ions. Under the same LC concentrations, the larger the energy transfer efficiency, the smaller the PL intensity. However, the types of LCs, the distributions and the sizes of a-Si clusters and Er silicate crystallites are almost the same for different samples. In this case, we suppose that no significant difference in energy transfer efficiency exists among these films. Therefore, the concentrations of LCs are nearly proportional to the integrated PL intensities. From the comparison among the integrated PL intensities of different samples, it can be seen that the concentration of sensitizes is the largest for sample 2, followed by sample 4, and the smallest for sample 3.
Based on these results, it is concluded that the required annealing processes to obtain an Er silicate polymorph with the best luminescent property, good crystal quality, and high concentration of sensitizers are different from each other. Therefore, an overall consideration of the three factors is needed. crystal quality and the phase composition of Er silicates, and the concentration of sensitizers. Each factor is closely related to the sensitized PL from the Er silicate layers embedded with a-Si clusters.
Figure 3(a) shows the Er PL spectra of sample 2 to sample 4. The excitation wavelength is 473 nm which is not in resonance with any Er levels, and therefore only electron-hole mediated processes are allowed. The mediated excitation of Er is also confirmed by PLE measurements. Figure 3(b) shows the PL intensity at 1535 nm as a function of the excitation wavelength in the 200-600 nm range of sample 2, where no direct excitation peaks of Er can be observed, indicating that the film can be efficiently excited within a wide range of wavelengths. It confirms that the Er ions in Er silicates can be excited via an energy-transfer process mediated by LCs in the film. As shown in Fig. 3(a), sample 2 shows the largest PL intensity, while the PL intensities of sample 3 and sample 4 are similar and are much weaker than that of sample 2. This can be explained on account of the three factors determined by the second annealing step, which are polymorph and crystal quality of Er silicates, and concentration of sensitizers. The largest PL intensity of sample 2 is due to the good optical performance of y-Er2Si2O7 and the good crystal quality of Er silicates. Sample 4 has a good crystal quality and a large concentration of sensitizers. However, the poor optical performance of α-Er2Si2O7 decreases the sensitized PL intensity of this film. For sample 3, the concentration of sensitizers becomes the limiting factor and the crystal quality is not as good as that of sample 2. Therefore, although the PL intensity of Er silicates in this film is much higher than that in sample 4, the sensitized PL intensities of the two films are similar.
Figure 3(c) shows the non-resonantly excited Er PL decay curves of sample 2 to sample 4, respectively. All the curves consist of two decay processes: a fast one and a slow one. The lifetimes of the two processes for each sample are listed in Table 2. It is observed that the lifetime of the fast processes are similar to but slightly larger than the PL lifetimes of the corresponding samples measured using a 980 nm-laser, which are 26 µs, 16 µs and 11 µs for sample 2, 3 and 4 respectively. We attribute the fast PL decay to Er ions in the Er silicates. The slight increase in the PL lifetime results from the energy transfer process from LCs to Er ions. The slow PL decay is attributed to the Er ions in the uncrystallized regions with lower Er concentrations. It should be noted that the slow decay process is barely seen when the films are resonantly excited with the wavelength of 980 nm, as shown in Fig. 1(c). It demonstrates that the amount of the uncrystallizaed regions is small, but many sensitizers are located in the uncrystallized regions and can transfer energy to nearby Er ions in these regions. In other words, the distribution of the sensitizers requires further improvement.
Although the sensitized PL from Er silicates has been improved after the modification of the structure size, the crystal quality and the phase composition of Er silicates and the concentration of sensitizers, some problems still remain. First, The average thickness of Er silicate shells is larger than 10 nm while the largest sensitizer/Er effective sensitization distance reported is only 1.5 nm. [33–35] Although a larger sensitization distance is expected in our film due to the strong Er-Er interaction induced by higher Er concentrations, the Er silicate utilization rate is unsatisfactory. Second, the distribution of LCs is difficult to control. Therefore, using LCs as sensitizers may not be the best choice. Nanoscale a-Si clusters should be a better option, but the sizes of a-Si clusters in our films are not small enough, resulting in poor light emission efficiency. Both of the two problems need to be solved by further reduction of structure sizes. However, the downscaling of the structure by modifying annealing processes has reached a limit due to the restrictions from crystallization temperatures. Therefore, alternative methods should be utilized. Further decrease in the size of a-Si clusters may be achieved by properly increasing Er concentrations and reducing Si concentrations. But the best option is to find an approach to reduce the crystallization temperature of Er silicates, under which we can simply reduce the structure sizes by decreasing the annealing temperature of the first step.
In conclusion, we have investigated sensitized PL of Er silicates in SiO2 films embedded with a-Si clusters. In order to optimize the sensitized PL, a two-step annealing process is utilized. The influence of the annealing process on the crystal quality and phase composition of Er silicates and the concentration of sensitizers is investigated. Optimized PL of Er silicates excited through an energy transfer process mediated by LCs is achieved in the film annealed at 1000 °C followed by 1100 °C with the film structure of well-crystallized y-Er2Si2O7 embedded with small-size a-Si clusters.
National Key R&D Program of China (2018YFB2200102); National Natural Science Foundation of China (61874095).
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