Er3+-activated SiO2-based glasses and glass - CiteSeerX

Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, August 2007, 48 (4), 229–234

Er3+-activated SiO2-based glasses and glassceramics: from structure to optimisation N. D. Afify, G. Dalba1 Dipartimento di Fisica dell’Universit`a di Trento, Via Sommarive 14, I-38050 Povo (Trento), Italy

F. Rocca & M. Ferrari Istituto di Fotonica e Nanotecnologie del Consiglio Nazionale delle Ricerche, Sezione CeFSA di Trento - Via Sommarive 14, I-38050 Povo (Trento), Italy This paper describes and compares the local environments of Er3+ ions in pure SiO2, SiO2–Al2O3, SiO2–TiO2, and SiO2– HfO2 glasses, and in SiO2–HfO2 glass-ceramic waveguiding systems. This structural information is useful for selecting waveguides with optimised functional properties. This comparison has emphasised the peculiarity of the SiO2–HfO2 glass, in which the Er3+ ions are incorporated in HfO2-rich regions. By appropriate heat treatment, a transformation of SiO2–HfO2 system from glass to glass-ceramic occurs, where Er3+ remains incorporated in HfO2 nanocrystals as substituent of Hf4+. The spectroscopic interpretations are coherent with the present structural results.

1. Introduction Er -activated SiO 2-based materials are used in photonics devices such as Er3+-doped fibre amplifiers (EDFAs) and Er3+-doped waveguide amplifiers (EDWAs). The choice of Er3+ as the rare earth (RE) ion, and SiO2 glass as the basic host of Er3+ is due to the coincidence between the standard telecommunications wavelength 1·55 μm of the light emitted by the stimulated 4I13/2Æ4I15/2 transition of Er3+ and the lowest loss window in the absorption spectra of SiO2 glass based optical fibres.(1) Nowadays, the development of novel and customised materials based on SiO2 glass, where RE ions can be ‘efficiently hosted’, is an emerging field in photonics research. Although these materials can be empirically evaluated by resorting to the performance of resulting devices, the knowledge of the local environment of the RE ions in these materials remains the main driving force for better understanding, and consequently further improving the functional properties of photonics devices. One of the few tools able to monitor local environments of a selected atomic species is the extended x-ray absorption fine structure (EXAFS) technique. For Er3+-doped planar waveguide optical amplifiers (EDPWOAs), on which active and passive optical components are also integrated,(2) the device dimensions should be as small as few centimetres to comply with integrated optics (IO) technology. To achieve high optical amplification gain in such compact devices, large concentrations of Er3+ ions should be homogeneously (or randomly) incorporated into the 3+

Corresponding author. Email [email protected] Proceedings of the Eighth International Otto Schott Colloquium, held in Jena, Germany on 23–27 July 2006.

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host glass.(3) Unfortunately, above certain doping levels (the so called concentration quenching onsets), RE ions tend to form chemical clusters (at short distances of few angstroms) and/or interaction (or ‘physical’) clusters (at large distances of about 20–100 Å).(4) The chemical clustering results from the competition among Er3+ ions to gain oxygen atoms. This bonding to oxygen is necessary to compensate for the trivalent positive charges of erbium ions. When incorporated in SiO2 glass, Er3+ ions are bounded to nonbridging oxygen (NBO) atoms. Nevertheless, SiO2 glass shows a quite low solubility for Er3+ ions. Consequently, at least at higher Er3+ concentrations, at least chemical clustering of Er3+ ions occurs. This gives rise to a quenching of the luminescence intensity by means of ion–ion interaction mechanisms.(4) Co-doping SiO 2 glass with some oxides may improve the host solubility for Er3+ ions, for instance by enhancing the number of available NBO atoms. Additionally, co-doping may also modify the local environment and, accordingly, the crystal field strength of the optically active Er3+ ions. In this way, important spectroscopic parameters, such as optical transition probability, can be enhanced. In this paper, we summarise our previous results on the local structure around Er3+ ions incorporated in pure SiO2, SiO2–Al2O3, SiO2–TiO2, and SiO2–HfO2 glassy hosts.(5) The roles of the different co-dopant oxides are shortly discussed, and the particularity of HfO2 co-dopant oxide is emphasised. Following a quite recent approach, we will present samples where Er3+ ions were embedded in nanocrystals of co-dopant oxides, that were themselves dispersed in

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the SiO2 glass. This method allows one to substantially improve the crystal field strength around Er3+, and the host solubility for RE ions. Moreover, we will show that under appropriate thermal annealing conditions, the SiO2–HfO2 glassy system transforms into a glass-ceramic. The glass-ceramic combines typical properties of glasses (e.g. transparency and brittleness) and the typical mechanical properties of ceramics (e.g. toughness). For an Er3+-doped glassceramic, given that nanocrystals are Er3+-activated, some new properties are expected:(3,6–10) (i) the crystalline environment of Er3+ ions will exhibit low energy phonons, reducing therefore losses due to nonradiative multiphonons de-excitation processes, (ii) the Er3+ optical transition cross section will increase, so enhancing the photoluminescence emission process, (iii) Er3+ ions might become constrained by a well defined symmetry typical for crystalline materials, possibly reducing therefore the probability of chemical clustering, and (iv) the nanocrystals should be small enough, so that the hosting material remains transparent, and losses due to scattering by particles (i.e. the Rayleigh scattering) are minimised.

2. Experimental Er3+-doped pure SiO2, SiO2–Al2O3, SiO2–TiO2, and SiO2–HfO2 glasses, with different co-dopant contents (1·2 and 1·8 mol% for Al2O3; 7, 12, and 15 mol% for TiO2; 10, 20, 30, 40, and 50 mol% for HfO2) and slightly different Er3+ concentrations, were prepared following the sol-gel route. The pure and Al2O3-doped SiO2 samples were studied in bulk format, while SiO2–TiO2 and SiO2–HfO2 ones were planar waveguides deposited on v-SiO2 substrates by the dip coating method.(11) Thermal annealing was carried out in air at a temperature of 900°C for a duration optimised for each sample, so to yield a full densification of the system, as it was verified by Raman measurements. As for the glass-ceramic system, 1 mol% Er3+doped 70SiO2–30HfO2 waveguides were prepared by sol-gel and dip coated on vitreous silica substrates. The waveguides were initially heat treated in air at 900°C for 5 min, yielding fully densified films. Each sample was successively heat treated for one time at a given temperature ranging from 900–1100°C for 30 min. The final thickness of the thin films resulted less than one micron. These samples are labelled E900S, E1000S, and E1100S, where the numbers stand for heat treatment temperatures and the letter ‘S’ stands for the short (30 min) duration of thermal annealing. Room temperature EXAFS measurements were carried out at the BM08-GILDA Beamline of the European Synchrotron Radiation Facility (ESRF).(12–14) For the Er3+-doped SiO2, SiO2–Al2O3, SiO2–TiO2, and SiO2–HfO2 glasses, EXAFS data were collected at the

Er L3-edge. For the 1 mol% Er3+-doped 70SiO2–30HfO2 glass-ceramic waveguides both Er and Hf L3-edges EXAFS data were collected. EXAFS spectra were collected in fluorescence mode, using the standard thirteen elements high purity Ge detector available on the Gilda Beamline. More spectra were collected and averaged, due to the low concentration of the Er3+ and, for the Hf L3-edge, to the low film thickness. Some reference materials, required for EXAFS data analysis, were measured in transmission mode. The x-ray absorption spectra were reduced using the EDAEES(15,16) and AUTOBK(17) codes. The resulting EXAFS signals k2χ(k) were modelled following the usual multi-shell fitting procedure. Data fitting was done using the EDAFIT code.(15,16) The Gaussian approximation, yielding the average interatomic distance R, the distribution width σ2, known as the EXAFS Debye–Waller disorder factor, and the coordination number N, was applied. The backscattering amplitudes and phase shifts for different atomic pairs, required for EXAFS data fitting, were either experimentally extracted resorting to the measured reference compounds, or theoretically calculated using the FEFF8 code.(18,19)

3. Results and discussions 3.1. Er3+-doped SiO2-based glassy hosts

Here, we report and compare the local structures around Er3+ ions in pure SiO2, SiO2–Al2O3, SiO2–TiO2, and SiO2–HfO2 glassy hosts, based on the EXAFS technique. In Figure 1, Fourier transform moduli of EXAFS signals k2χ(k) are reported. For all samples, two major contributions can be identified in Fourier transforms. The first peak is centered at about 1·7 Å, and the second one is located at about 2·9 Å. Basically, these structures can be attributed to first coordination shells around Er3+, as well as to multiple scattering (MS) effects. Our analysis has shown that MS contributions are negligible in these disordered systems. Qualitatively speaking, the first peak in the Fourier transforms is strongly similar for all samples, showing no significant dependence on either the codopant oxide or on the chemical composition of the binary matrices. On the contrary, the second peak in the Fourier transforms shows a strong dependence on the co-dopant oxide, and sometimes on its concentration too. The analysis of the first peak in Fourier transforms attributes the first coordination shell around Er3+ to oxygen atoms. This Er3+–O shell is the main common structural feature in all Er3+-doped SiO2-based hosts.(5,12–14,20,21) The attribution of the second coordination shell is more important, while in the same time it is not a trivial job. It is important since it can allow to determine the preferred co-dopant oxide host of Er3+ ions in such SiO2 based systems; it is a difficult

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Figure 1. Fourier transform moduli of Er L3-edge EXAFS signals k2χ(k) of Er3+-doped pure SiO2, SiO2–Al2O3, SiO2– TiO2, and SiO2–HfO2 glasses. Spectra of samples with the extreme co-dopant oxides concentrations are shown, and mutually displaced for clarity and a nonroutine job since the contribution of the outer coordination shells around the central atom to EXAFS spectra in glassy systems is usually rather weak. Accurate cross analysis of EXAFS signals beyond the first coordination shell has yielded reliable and interesting results. As expected, for the basic pure SiO2 glassy host, Er3+ ions are bounded to silicon atoms. When adding Al2O3 oxide to SiO2 glass, modeling procedures attribute the second coordination shell to silicon and/or aluminium ions. EXAFS is not able to distinguish between these two light atomic species, since they are close neighbours in the periodic table. However, the presence of small amounts of Al2O3 introduces significant changes in the local structure of Er3+. On the contrary, for the SiO2–TiO2 system, the presence of TiO2 (from 7 to 15 mol%) does not change significantly the second coordination shell. Moreover, the second shell analysis was not able to detect Er–Ti correlations. This result fully agrees with the fact that in SiO2–TiO2 glass, TiO2 oxide tends to be phase separated from the SiO2 network, in which Er3+ ions are accommodated.(22) Co-doping with HfO2 oxide yields a very different situation. Our modelling attributes the second coordination shell around Er3+ to hafnium atoms, and not to the silicon ones. Moreover, changing the content of

HfO2 from 10 to 50 mol% has no visible effect on the local structure around Er3+, as it can be seen from the almost two identical Fourier transforms reported in Figure 1 (spectra G and K in Figure 1). More details on the quantitative EXAFS results of Al- and Ti-containing waveguides are reported in Affify et al.(5) The quantitative results on the Er3+-doped SiO2– HfO2 glassy waveguides show that the second coordination shell of Er3+ is composed by 4–5 hafnium atoms at an average distance of 3·51±0·02 Å. Furthermore, the quantitative content of this coordination shell does not change as a function of Er3+ or HfO2 molar concentrations. It is worth noting that the Er3+ second shell distance for this system is very short compared to the ones in glasses not co-doped with HfO2. On the basis of the presence of hafnium ions in the second coordination shell around Er3+, we assume that Er3+ ions are dispersed in amorphous HfO2-rich regions of the waveguide. The finding that the structural parameters of the Er3+–Hf coordination shell do not depend on Er3+ (0·3–0·5 mol%) or HfO2 (10–50 mol%) concentrations, reveals that even at the lowest HfO2 concentrations, Er3+ ions become preferentially dispersed in the regions rich of HfO2. This result agrees with an analogous finding that has been obtained for the same waveguides by Raman spectroscopy(23–25) and by optical spectroscopy.(26)

3.2. Er3+-doped SiO2–HfO2 glass-ceramic waveguides After having pointed out the particular property of the SiO2–HfO2 glassy waveguides, now we show how this system turns into a glass-ceramic through an appropriate thermal annealing. XRD measurements presented in Figure 2(a) show that the Er3+-doped sample thermally treated at 900°C for 30 min (E900S) is completely amorphous, and that the other two samples heated at 1000 and 1100°C (E1000S and E1100S) contain tetragonal HfO2 nanocrystals, with diameters 2·5 and 3·5 nm respectively. Refinement of lattice parameters of the tetragonal phase has yielded larger values for the samples doped with Er3+ with respect to pure 70SiO2–30HfO2 waveguides:(27) this has been interpreted as a substitution of hafnium by erbium in HfO2 nanocrystals. We report here Er and Hf L3-edges EXAFS results on waveguides with the composition 1 mol% Er3+doped 70SiO2–30HfO2, thermally treated at 900, 1000, and 1100°C for 30 min. The effect of longer annealing time will be discussed in a further paper. The refined coordination numbers N, average interatomic distances R, and the Debye–Waller disorder factors σ2 for the first two shells (Hf(Er)–O and Hf(Er)–Hf ) are compiled in Table 1. The first important information about the crystallisation of HfO2 can be obtained by looking the Fourier

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Figure 2. XRD and EXAFS data for the 1 mol % Er3+-doped 70SiO2–30HfO2 waveguides heat treated at 900 (sample E900S), 1000 (sample E1000S), and 1100°C (sample E1100S) for 30 min. For EXAFS results, observed (empty circles) and calculated (solid lines) Fourier transforms of EXAFS signals k2χ(k) are reported transform moduli of the Hf L3-edge EXAFS signals k2χ(k) reported in Figure 2(b) (solid lines). The main peak, centred at ~1·5 Å, is attributed to oxygen atoms as the first nearest neighbour of hafnium. The position of this peak (i.e. Hf–O distance) exhibits slight changes as a function of heat treatment. Contributions from the outer coordination shells are even more important in the case of a glass-ceramic, since they may enable us to distinguish the presence of the absorber atom (e.g. Hf) in different amorphous (disordered) and crystalline (ordered) environments, given that the respective distances are sufficiently different. The amorphous sample E900S shows a relatively broad and weak peak located at about ~2·8 Å. This peak corresponds to the second coordination shell Hf–Hf. It is evident that the second coordination shell peaks in the glass-ceramic samples E1000S and E1100s are very similar, and both are significantly different from that of the glassy waveguide. In fact, the Hf–Hf distances for the nanocrystalline waveguides are significantly shifted to larger values with respect to the one of the amorphous sample. In the quantitative analysis, two coordination shells were fitted: Hf–O and Hf–Hf (see Figure 2(b) and Table 1). For the sample E900S, the structural

parameters, in particular the short Hf–Hf distance (3·36 Å), are typical of hafnium in amorphous HfO2 environment. The glass-ceramics E1000S and E1100S demonstrate a modified local structure that resembles the one in tetragonal HfO2, in particular the long Hf–Hf distance (3·54 Å). Comparison between the samples E1000S and E1100S yields an increase in the interatomic distances, and a decrease in the Debye–Waller factors (increase of ordering around hafnium ions). This result is expected, since the sample E1100S has a larger fractional weight of HfO2 nanocrystals with respect to the sample E1000S. It may be useful also to know the effect of doping with Er3+ on the local environment of Hf: other EXAFS measurements on similar samples but without Er3+, not reported here for space limitation, have shown that the Hf–O and Hf–Hf distances are shorter than those in the case of Er3+-doped glass-ceramics.(27) This can be interpreted as a substitution of Hf4+ (ionic radius=0·71 Å) by Er3+ (ionic radius=0·881 Å) in the crystalline lattice, as it has been also found by XRD results.(28) Now we move the attention to the local structure around Er3+ ions. The effects of heat treatment on

Table 1. EXAFS local structures parameters around erbium and hafnium ions in 70SiO2–30HfO2 waveguides at different heat treatments. N, R, and σ2 are the coordination numbers, average interatomic distances, and Debye–Waller factors respectively. Numbers in parenthesis are uncertainties on last digits: they take into account the variance of the mean of different experiments on the same sample and of different data analysis procedures on each experiment Sample parameter

Er L3-edge First coordination shell Second coordination shell Er3 –O Er3 –Hf N (atom) R (Å) σ2 (Å2) N (atom) R (Å) σ2 (Å2)

Hf L3-edge First coordination shell Hf–O N (atom) R (Å) σ2 (Å2)

Second coordination shell Hf–Hf N (atom) R (Å) σ2 (Å2)

E900S E1000S E1100S

5·0(3) 5·3(9) 5·4(8)

5·0(1) 4·9(1) 4·8(1)

15(3) 12(2) 9(1)

2·32(3) 0·0130(1) 2·33(4) 0·0112(3) 2·31(3) 0·0108(3)

3·8(9) 8·3(8) 9(1)

3·46(3) 0·014(3) 3·58(1) 0·012(1) 3·57(1) 0·015(2)

2·10(2) 0·0109(4) 2·12(4) 0·0102(6) 2·13(4) 0·0096(6)

3·36(6) 0·033(2) 3·54(8) 0·023(2) 3·57(6) 0·015(1)

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the local environment of Er3+ are documented in Figure 2(c). Solid lines are the Fourier transforms of the Er L3-edge EXAFS signals k2χ(k) for the samples E900S, E1000S, and E1100S. The transform for the amorphous sample E900S contains two main peaks, centred at ~1·7 and ~2·9 Å. These two coordination shells are attributed to Er3+–O and Er3+–Hf shell correlations respectively. For the glass-ceramic samples E1000S and E1100S, a third peak centred at ~3·8 Å is present. Modelling procedures attribute this peak to the coordination of Er3+ ions with oxygen atoms at larger distances. The most significant change is the shift of the Er3+–Hf second shell distances in the glass-ceramic samples with respect to the one of the purely amorphous waveguide. Figure 2(c) (empty circles) reports also the best-fit calculated complex Fourier transforms Er L 3-edges EXAFS signals k 2χ(k). Er L3-edge EXAFS signals were fitted to two coordination shells (Er3+–O and Er3+–Hf) for the sample E900S, and to three coordination shells (Er3+–O, Er3+–Hf, and Er3+–O at larger distances) for the glass-ceramic samples. From the structural parameters reported in Table 1, it is evident that this local environment undergoes significant changes going from the amorphous waveguide to the one containing HfO2 nanocrystals. The most important changes are the ones exhibited by the second coordination shell around Er3+ ions. In fact, going from the amorphous sample (E900S) to the nanocrystalline waveguide (E1000S), the Er3+–Hf distance becomes much longer and the coordination number becomes larger. It can be noticed also that both coordination number and average interatomic distance of this shell are comparable to the values of the Hf–Hf co-ordination shell. This result once again suggests the substitution of hafnium by erbium ions in the formed HfO2 nanocrystals. It is useful to compare the Er3+ local environment in the two glassceramics E1000S and E1100S. From the slightly different structural parameters, it turns out the possibility of tuning the local environment by controlling the heat treatment of HfO2 nanocrystals, it means that the crystal-field applied to erbium may be changed by just tuning the microstructure.

4. Conclusions In this paper, the local structures around Er3+ ions in pure SiO2, SiO2–Al2O3, SiO2–TiO2, and SiO2–HfO2 glasses, and in SiO2–HfO2 glass-ceramic waveguides have been detailed and compared. For the various glasses and glass-ceramic hosts, the first coordination shell of Er3+ is composed by oxygen atoms, with no significant co-dopant oxide or chemical composition dependencies. On the contrary, the second coordination shell of Er3+ shows significant differences. No direct Er3+–Er3+ correlation has been detected by

EXAFS either in the glassy or in the glass-ceramic waveguides, ruling out the formation of any Er3+-rich crystalline phases. For pure SiO2, SiO2–Al2O3, and SiO2–TiO2 glassy hosts, the second coordination shell is composed mainly of silicon atoms: Al2O3 doping (less than 2 mol%) induces an ordering and elongation of the second coordination shell; for TiO2 doping (7–15 mol%), the second shell is very similar to the pure SiO2 case, and titanium has not been detected in the local environment of erbium. The structural results obtained on glass-ceramic waveguides are able to explain the spectroscopic observations. In Jestin et al,(3) we have reported room temperature photoluminescence (PL) spectra of the 4I13/2Æ4I15/2 Er3+ transition of Er3+ for the samples E900S, E1000S, and E1100S. There, it has been shown the improvement of spectroscopic properties of the Er3+-doped SiO2–HfO2 system, going from glass to glass-ceramic regime. The PL spectra of the glass-ceramics E1000S and E1100S have shown well resolved Stark components, due to the incorporation of Er3+ ions in crystalline environment. Furthermore, both the lifetime of the metastable level 4I13/2 and the waveguide loss have been found to be dependent on the size of HfO2 nanocrystals. The main message of this paper concerns the particularity of the SiO2–HfO2 system among the different glassy hosts. For HfO2 concentrations from 10–50 mol%, there is a very clear evidence of Er–Hf coordination, with Er3+ ions dispersed in the HfO2–rich regions of the amorphous waveguide. Moreover, through a controlled heat treatment, the 1 mol% Er3+-doped 70SiO2–30HfO2 glassy waveguide turns into glass-ceramic, where erbium ions are incorporated in HfO2 nanocrystals.

Acknowledgments This work was partially supported by MIUR, Italy, through the FIRB project ‘Sistemi Miniaturizzati per Elettronica e Fotonica’. We are grateful to Mrs C. Armellini, Dr L. Zampedri, and to Dr Y. Jestin for significant technological and scientific assistance in waveguides fabrication and optical characterisation. N. D. Afify wishes to thank Prof. M. Montagna and Dr A. Kuzmin for very useful discussions. The support by ESRF (F) and CNR-INFM (I) is acknowledged. Authors are grateful to the staff of the BM08-GILDA Beamline for their kind assistance during EXAFS measurements.

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