1. Introduction

Efficient light emission and amplification in 1.5 µm telecom wavelength range have been crucial for silicon-based monolithic integration photonic circuit [1–4]. Erbium-containing materials are of great interest because of the ability to emit at 1.5 µm via the intra-4f transition. These materials include Er-doped materials (e.g. Er-doped silicon [5], silica [6]) and Er compounds (e.g. Er₂O₃ [7], Er₂Si₂O₇ [8], Er₃Cl(SiO₄)₂ [9]). However, Er-doped materials produce insufficient gain for chip-scale integration due to the low erbium concentration limited by solid solubility and segregation [10]. Crystalline Er compounds with periodical arrangement of erbium ions have a much higher erbium concentration up to 10²² cm⁻³ and no erbium segregation. Furthermore, researchers have found that nearly all erbium ions in Er silicate are optically active [10,11]. Recent study has reported giant optical gain excess 100 dB/cm in single-crystal erbium chloride silicate [12]. The outstanding work by C. Z. Ning’s group has revealed that it is extremely important to maintain long PL lifetime at 1.5 µm and high Er density at the same time. They proposed a concept of lifetime-density product (LDP) [13] which can be a convenient evaluation for erbium-containing materials.

Although Er compounds have relatively high erbium concentrations, their LDPs are normally low owing to the short lifetime about tens microseconds order [14]. There are mainly two reasons why the lifetime of Er compounds cannot be millisecond order as Er-doped materials. Firstly, these Er compounds are typically in the form of polycrystalline film with poor crystal quality and high defect density. Secondly, the short mean Er-Er distance owing to the high erbium concentration dramatically induces strong Er-Er interactions. The excitation energy can migrate along the sample through energy transfer from one excited Er³⁺ to a nearby Er³⁺ in ground state, and eventually will be lost in the defects acted as quenching centers. Moreover, the Er-Er ion interaction also causes strong up-conversion, leading to the energy loss of 1.5 µm transition.

Therefore, to achieve long lifetime, Er compound need both low defect density and weak Er-Er ion interactions. To data, very little attention has been paid to reduce Er-Er interactions except for diluted with other REs at the expense of lower erbium concentrations [15]. Here, we reported the largest LDP about 1.3 × 10¹⁹ s·cm⁻³ Er silicate material with 844 µs lifetime and erbium concentration up to 1.5×10²² cm⁻³. According to some studies [16], there are notable confinement effects on electron-phonon interaction in a nano-sized crystal because of the reduced and discrete phonon density of states (PDOS). Therefore, Er-Er energy transfer process in nano-sized materials will be confined by altered PDOS. The long lifetime α-Er₂Si₂O₇ nanowires embedded in silicon oxide films have been synthesized, which have low defect density, and simultaneously hold suppressed Er-Er interactions due to size-dependent PDOS.

2. Material and methods

FOx-16 (purchased from Dow Corning) is a liquid mixture of hydrogen silsesquioxane (HSQ) and solvent. Tris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium (III) (Er(thd)₃, purchased from Strem Chemicals) has a good solubility in FOx-16. All the chemical reagents in our experiments were used without any purification. Er-doped silicon oxide films were prepared by a modified sol-gel process and spin-coating technique. In our case, 20 mol % Er(thd)₃ (the molar ratio of Er to Si) was added to FOx-16 and agitated vigorously until the solids were dissolved completely. Then, the as-prepared sol was spin-coated on a clean Si substrate followed by volatilizing solvent completely. Subsequently, the samples were annealed for 1 h at 950 to 1200 °C in N₂ or O₂ atmosphere. Whereas the bulk α-Er₂Si₂O₇ particles (∼1 µm) for reference were synthesized by annealing ErCl₃-SiO₂ (Er: Si = 1:1) powder at 1200 °C in oxygen.

X-ray diffraction (XRD) data were collected on a Rigaku D/max-2550pc X-ray diffractometer by Cu Kα radiation with the grazing angle of 0.5° or 1°. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained on a Tecnai F20G2 microscope. Both steady-state and transient photoluminescence measurements were recorded on an Edinburgh Instruments FLS-920 fluorescence spectrophotometer with a 980 nm laser as the light source. Bonding configurations were studied by Fourier Transform Infrared (FTIR) spectra (IFS 66 V/S spectrometer).

3. Results and discussion

The Er/Si molar ratio of the general Er silicates Er₂SiO₅ and Er₂Si₂O₇ are 2:1 and 1:1 respectively. Here, Er/Si in our sample was chosen to be 0.2:1 with the aim of producing nanocrystalline Er silicate embedded in silicon oxide instead of bulk Er silicate, which was smaller than ideal stoichiometric of Er silicate. Er silicate crystalline were not obtained when the Er/Si was too low, for example 0.15:1 in our system. This result is similar to that found in Er-doped SiO₂ film fabricated by magnetron sputtering [14].

Figure 1 presents the XRD spectra of films annealed at different temperatures in N₂ or O₂ atmosphere. XRD analysis demonstrated the amorphous nature of the films annealed up to 950 °C. Crystalline structures appear in the films treated at 975 °C both in N₂ or O₂ ambient. Unfortunately, there are no powder diffraction files in JCPDS and literature data of any known Er silicates correspond to the XRD spectra exactly. The very close ionic radius of Er³⁺ and Tm³⁺ (0.88 and 0.89 Å) makes it rational to use α-Tm₂Si₂O₇ (space group P-1, JCPDS file number 31-1994) as references since the crystalline structure of the rare earth disilicates strongly depends on the rare earth ionic radius. The XRD patterns in Fig. 1 are very close to that of the α-Tm₂Si₂O₇ phase. Therefore, the peaks are all attributed to triclinic α-Er₂Si₂O₇, which was in accordance with the Er₂Si₂O₇ films deposited by electron beam evaporation [17]. It is clear that the samples have only one phase after the high temperature process in various ambient whereas the mixed phases of Er silicate often formed in Er-doped SiO₂ film reported before [14]. The single phase makes it accurate to compare the properties of samples after different annealing processes directly, which is difficult for those composed of mixture of various Er silicate phases.

 

Fig. 1. XRD spectra of films with the same synthesis procedure annealed at different temperatures in N₂ or O₂ ambient.

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It is well known that hydroxyl (OH) groups are extremely effective in quenching excited Er³⁺. The quenching effect will become severe in high Er concentration materials because of the longer distance energy migration [18]. Typical precursors for sol-gel silicate contain water, in which large number of OH groups will remain. Although residual OH groups are gradually reduced with increasing annealing temperatures, Slooff et al. found that merely ∼10⁻⁴ mol% OH levels could affect Er³⁺ emission and lifetime readily [19]. Consequently, we compared the corresponding FTIR spectra of the films without annealing and after different annealing processes, as shown in Fig. 2. Surprisingly, the broad OH group vibrational band centered at 3400 cm⁻¹ was absent even in the as-prepared film. The films show no existence of OH group because the precursors do not contain any OH group and no OH group will be released upon the formation of siloxane network structures. Therefore, absolutely no OH groups in our films is critical to acquire efficient emission and long lifetime of erbium ions.

 

Fig. 2. (a) FTIR spectra of sample without annealing and treated at different temperatures. (b) Enlarged spectra in 880-1000 cm⁻¹ range of (a).

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The bands at 2960, 1140 cm⁻¹ and several peaks around 1500 cm⁻¹ of film without annealing showed the C-H stretching, C-O plane and C = O stretching modes arising from β-diketonate ligand of Er(thd)₃ [20]. The peak at ∼2255 cm⁻¹ of sample without annealing was attributed to O₃SiH vibrational mode of HSQ molecules [21]. The three bands at 1080, 840, and 460 cm⁻¹ of all samples were assigned, respectively, to the asymmetric stretching, symmetric stretching, and bending vibrations of the Si–O-Si bonds. Also, the peak at ∼2360 cm⁻¹ of all samples may be assigned to the SiH_x groups [22]. In Fig. 2(b), it should be noted that three bands at ∼906, 942 and 970 cm⁻¹ appeared in the samples annealed at above 975 °C, which were ascribed to Si-O-Er vibrations modes of crystalline α-Er₂Si₂O₇ [23]. These results are like that found in the XRD analysis.

A further study of α-Er₂Si₂O₇ structure was carried out by using TEM and HRTEM technique. As shown in Figs. 3(a), 3(d) and 3(g), the structures of dark area with nanowire form embedded in silicon oxide were emerged in the films. Figures 3(b), 3(e), 3(h) and 3(c), 3(f), 3(i) display the HRTEM images and corresponding FFT (fast Fourier transform) patterns of the dark area nanowires. The nanowires have lattice spacing of 5.80 Å, 5.98 Å and 4.58 Å, consistent with (10-1), (002) and (110) plane of α-Er₂Si₂O₇ respectively.

 

Fig. 3. TEM images, HRTEM images and corresponding FFT patterns of the sample annealed at 975 °C in N₂ (a)(b)(c), the sample annealed at 1200 °C in N₂ (d)(e)(f), and the sample annealed at 1200 °C in O₂ (g)(h)(i).

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According to the research of X.Y. Chen et al., the PDOS of 20-40 nm diameter nanomaterials were significantly different from bulk materials, which decreased greatly and became discrete [24]. Additionally, the low-energy acoustic phonon modes were cut-off. The mean diameters of nanowires embedded in the films were about 20 nm, which were small enough to show distinct size restriction of electron-phonon interactions. Luminescence dynamics of Er³⁺ in α-Er₂Si₂O₇ nanowires, particularly, Er-Er resonant energy migration and nonradiative relaxation of Er³⁺, are expected to be confined compared with the bulk Er₂Si₂O₇. Moreover, the up-conversion emission that is dominantly induced by phonon-assisted energy transfer process may be restricted due to the change of PDOS. In other words, these confinement effects may significantly reduce the adverse effects introduced by high erbium concentration, such as short lifetime and unfavorable up-conversion luminescence. In the following section, we will discuss the impact of the confinement on optical properties of α-Er₂Si₂O₇ in detail.

It is also worth noting that the sizes of nanowires do not become larger with the increase of annealing temperatures. The fine nanowire structures can play a critical role in the maintenance of confined Er-Er interactions due to size restriction. This observation can be understood in context of the confined crystallization growth strategy which the growth and agglomeration of α-Er₂Si₂O₇ nanowires are confined by rigid Si-O network effectively. There are similarities between the growth mechanism of α-Er₂Si₂O₇ nanowires in this study and SnO₂ nanocrystal arrays in the previous report [25].

Optical properties of Er³⁺ including light emission and lifetime in infrared range are the most important aspect of Er silicate materials. The room temperature optical properties of samples with different annealing processes were studied by steady-state photoluminescence (PL) and time-resolved PL measurements. Figure 4(a) compares the PL spectra annealed at different temperatures in N₂ or O₂ ambient. The film annealed at 950 °C shows a broad and weak PL signal, which is typically observed in amorphous Er-containing materials. While for the films annealed at 975 °C or higher temperatures, two main peaks at 1529 and 1533 nm accompanied by several less intense peaks are observed. This fine structure is characteristic for transitions between the Stark splitting energy levels of Er³⁺ in a crystalline environment. Also the fine structure in the PL spectra corresponds well with that of α-phase rare earth disilicate reported in the literature [26]. The PL spectrum shapes that is determined by crystal field are consistent with the structure analysis mentioned before.

 

Fig. 4. (a) PL spectra of samples with N₂ or O₂ treatment. Decay curves of (b) N₂-treated films and (c) O₂-treated films detected at 1533 nm. And the green lines are single exponential fits. (d) Integrated room temperature PL intensity (open black symbols) and lifetime (full blue symbols) and ratio of integrated PL intensity to lifetime (I/τ, red cross symbols) as functions of annealing temperatures treated in N₂ (circles) or O₂ (triangles) ambient. And the lines are drawn to guide the eye.

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The annealing temperature dependence of integrated near infrared PL intensity is presented in Fig. 4(d) (left-hand scale). The PL intensities of films treated in N₂ and O₂ ambient increase by a factor of about 2.9 and 3.3 respectively with increasing the annealing temperature from 950 to 975 °C, which can be related to the crystallization of α-Er₂Si₂O₇. A continued increase of PL intensity with annealing temperatures can be mainly attributed to the improved crystal quality and decreased defect density. Furthermore, the emissions of O₂-treated films are always stronger than N₂-treated films. A possible explanation might be that O₂ treatment can reduce the defect density, especially oxygen vacancies more efficiently than N₂ treatment, as reported in the literature [27].

Figures 4(b) and 4(c) present the PL decay curves detected at 1533 nm of films annealed in N₂ and O₂ ambient respectively. Values of lifetimes τ are estimated from a single exponential fitting. The well-fitting by single exponential indicates that nearly all Er ions are in the same environment, in other words, the vast majority Er ions are participated in the formation of α-Er₂Si₂O₇ rather than remained in the amorphous oxide. As shown in Fig. 4(d) (right-hand blue scale), the lifetime increases with increasing annealing temperatures. Surprisingly, the lifetime of the best performance films reaches to 844 µs that is the longest lifetime of Er silicate films reported so far. At the same time, the largest LDP about 1.3 × 10¹⁹ s·cm⁻³ is acquired.

However, lifetimes of bulk Er silicate are generally in the range of a few tens of microseconds. In terms of the long lifetime observed in our samples, there are many possible explanations. In general, the surface quenching effect in nano-sized materials is severe due to the large surface-to-volume ratio. Therefore, the lifetime of luminescence centers in nano-sized materials is often short. For instance, C.Z. Ning’s group found that the erbium chloride silicate nanowires with larger diameters show longer lifetimes [12]. Given the long lifetime, we can conclude that the surface traps and dangling bonds of α-Er₂Si₂O₇ nanowires were passivated well owing to the coating of amorphous silicon oxide.

In a linear excitation regime, the PL intensity can be described as the following equation:

It is also worth noting that the 844 µs lifetime is even longer than the single-crystal erbium chloride silicate [13]. Anyway, it is not possible that the defect densities in our films are much lower than the single crystalline materials although the defect densities have reduced effectively through high temperature treatment. Therefore, there must be another factor plays a critical role in obtaining such the long lifetime. As described in the TEM analysis, it is difficult to ignore the existence of confinement effect in α-Er₂Si₂O₇ nanowires. Here, the 844 µs long lifetime provides evidence for the reduced detrimental non-radiative channels in nanowires, which were efficiently suppressed by the restricted Er-Er energy migration and Er³⁺ multi-phonon relaxation process. Further evidence for size confinement effect is provided by the analysis of the up-conversion emissions of films.

It is well known that up-conversion in higher Er concentration materials is mainly originated from phonon-assisted energy transfer between Er ions. The up-conversion process of α-Er₂Si₂O₇ nanowires should be weaker compared with bulk α-Er₂Si₂O₇ because of the lack of phonon modes. Figure 5(a) presents the PL spectra in visible and near-infrared range of films annealed in O₂ ambient and reference bulk sample. Obviously, there are three visible peaks at ∼525, 550 and 660 nm and the visible up-conversion PL intensities share the same trends with the near infrared PL intensities, as shown in Fig. 5(b) (left-hand scale).

 

Fig. 5. (a) PL spectra in visible and near infrared range excited by 980 nm laser, the up-conversion emission of Ref is divided 3 times for clarity. The Ref corresponds to a bulk α-Er₂Si₂O₇ sample. (b) The visible (black column) and near infrared band integrated intensity (red column) and the ratio of integrated visible range to near infrared range (VTN, right blue scale).

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With regard to a much larger number Er ions of the reference bulk sample than our films, the concept of relative up-conversion defined as the ratio of integrated total visible range to near infrared range (VTN) is introduced here for exact comparation. As shown in Fig. 5(b) (right-hand blue scale), the VTNs of the films are indeed smaller than the reference bulk sample. The result confirms that the up-conversion emissions in α-Er₂Si₂O₇ nanowires are restricted owing to the phonon nanoconfinement effects. Meanwhile, it is no doubt that up-conversion emissions are the main limitation of the near infrared PL efficiency enhancement in high Er concentration system. In a word, this nanoconfinement effect can also play an important role in increasing 1.5 µm emission through the restriction of up-conversion emission.

4. Conclusion

In summary, about 20 nm diameter α-Er₂Si₂O₇ nanowires embedded in silicon oxide films have been prepared via sol-gel method without any OH group. With increasing annealing temperatures and substituting N₂ with O₂ ambient, we obtain an 844 µs lifetime of α-Er₂Si₂O₇ annealed at 1200 °C in O₂ ambient, which is the longest in Er compound materials recorded so far. The excellent optical properties are attributed to the combined action of low defect density and the confined Er-Er interaction. Furthermore, the annoying up-conversion emission which limits the near infrared luminescence is suppressed effectively owing to the nanoconfinement effect. The α-Er₂Si₂O₇ nanowires embedded in oxide films with long lifetime and high Er concentration is expected to have the potential in on-chip photonic integration applications, such as silicon-based waveguide amplifiers and light source.