Erbium environment on Er-doped silica and alumino-silicate glass films: An EXAFS study

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Abstract

Er-doped dielectric films are characterized by the emission of a photoluminescence signal at λ = 1.54 μm, the main used in the optical telecommunications. The efficiency of the radiative emission is strongly related to the characteristics of the Er3+ environment. Er-doped SiO2 films (synthesized by rf-magnetron co-sputtering) and 87SiO2:10Al2O3:3Na2O silicate glass films doped with 0.5 mol% of Er (prepared by sol–gel route and subsequently doped with silver by Ag+  Na+ field-assisted solid-state ion exchange) were studied by extended X-ray absorption fine structure spectroscopy performed at Er LIII-edge (Italian beamline GILDA of the ESRF). In the silica samples the Er coordinates about 4.5 O atoms at a short distance (R = 2.07–2.13 Å), similar to the one observed in Er-doped glasses when the preparation conditions are far from the thermodynamical equilibrium. In alumino-silicate samples the first shell of atoms is formed of 5.5–7.5 O atoms at a distance of about 2.31 Å, showing a local structure similar to other Er-doped sol–gel glasses and glass–ceramics. A comparison between the first shell structure around Er ions and the different intensity of the photoluminescence emission suggests that the increase of the radiative emission upon thermal annealing is mainly related to the decrease of the defects number in the glass structure as a consequence of the annealing.

Introduction

The increasing demand for telecommunications and broadband services can be achieved by using optical data transfer through optical fibers [1], for which the large bandwidth of optical communication enables the development of very high rate information transmission systems. While optical fibers and fiber amplifiers are usually used for long-distance communications, the development of the optical networks requires systems able to process optical signal on a local scale. The synthesis of new materials is therefore required for the development of planar integrated optics devices. In this framework, erbium-doped glasses are interesting materials due to the optical transition at 1.54 μm shown by the Er3+ ions [2], [3]: this radiative transition falls in the range of minimum transmission loss for silica-based optical fibers [4]. The performances of erbium-doped glass materials crucially depend on both the host matrix and the doping process, since they determine the atomic scale structure around the Er3+ ions on which the characteristics of the optical transition at 1.54 μm depend. Moreover, the contemporary presence in the glass matrix of other metallic species (such us gold or silver) in the form of dimers, trimers and/or nanoparticles can enhance the out-of-resonance Er3+ absorption cross-section, thus increasing the radiative decay emission at 1.54 μm [5], [6], [7].

In literature, several papers deal with the study of the erbium environment in different Er-doped matrices, such us silica (synthesized by means of ion implantation, sol–gel routes, chemical vapour deposition (PVD) techniques, see for instance [8], [9], [10], [11]), silicate glasses (obtained by ion implantation, melt, sol–gel, or ion exchange [8], [11], [12], [13], [14], [15]), phosphate glasses (obtained by melt [16]), zinc and lead chloro-tellurite glasses (obtained by melt [17]).

In this paper, Er:SiO2 thin films on silica were prepared by radiofrequency magnetron co-sputtering deposition followed by a suitable thermal treatment to optically activate the rare-earth. Moreover, Er:SiO2–Al2O3–Na2O silicate glass films were prepared by sol–gel route and subsequently doped with silver by Ag+  Na+ field-assisted solid-state ion exchange (FASSIE) [18]. The extended X-ray absorption fine structure (EXAFS) spectroscopy, performed at the Er LIII-edge, allowed to determine the local structure around the Er ions as a function of the synthesis route and to evidence possible structural modifications of the Er environment depending on the post-synthesis treatments.

Section snippets

Experimental

Erbium-doped silica films were synthesized by simultaneous deposition of silica and erbia on fused silica slides 25 × 75 mm2, 1 mm thick, in a radiofrequency magnetron sputtering deposition apparatus. Co-depositions were performed by means of two 13.56 MHz radiofrequency sources acting in a neutral atmosphere (pure Ar), at a pressure of 0.40 Pa. The rf-power to the 2 in. diameter targets was fixed at 150 and 12 W for silica and erbia, respectively. The 75 (or 225) min co-deposition of silica and erbia

Results and discussion

In Table 1 the synthesis parameter used are reported for the two series of samples. The values of the Er concentration (not higher than 1.4 × 1020 atoms/cm3) are below the solubility limit of Er in glass: this allows to avoid concentration quenching effects for the photoluminescence signal [8], [9]. The total amount of Er was measured by Rutherford backscattering spectrometry (RBS); in all samples the rare-earth showed an almost uniform in-depth distribution. In the samples deposited by sputtering

Conclusion

Er:glass systems, synthesized with different methods and showing 1.54 μm photoluminescence emission were investigated with EXAFS to obtain information about the Er environment. In the silica samples deposited by sputtering, the Er coordinates about 4.5 O atoms at a short distance (R = 2.07–2.13 Å): this site is similar to the one observed in Er-doped glasses when the preparation conditions are far from the thermodynamical equilibrium. In alumino-silicate samples produced by sol–gel route, the first

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      Being well-known that the local order around the erbium ions influences the optical emission properties (in primis number and distance of the first-neighbor oxygen atoms; probably also the disposition of the second- and third-shell atoms) [10,21,29–33], another possible explanation of the PL intensity behavior in Fig. 4(a) is that the Er3+ environment could be slightly different from sample to sample. A preliminary investigation of the Er first-shell environment performed with EXAFS [34] on two of our Er-doped silica films showed that, in spite of the strong difference between the PL signal intensity before and after the thermal activation, the similarity between the Er first-shell environment in the samples suggests the possibility that important mechanisms of non-radiative emission take place in the as-deposited samples (much probably related to the presence of defects in the as-deposited silica structure), or that the PL signal could be strongly influenced by Er site difference over the first-shell. The samples in which the energy was delivered to the growing film contemporaneously by heating and by ion bombardment show high values of the 1.54 μm PL signal also by using strong bias negative voltage.

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