Elsevier

Thin Solid Films

Volume 519, Issue 16, 1 June 2011, Pages 5376-5382
Thin Solid Films

Photoluminescence optimization of Er-doped SiO2 films synthesized by radiofrequency magnetron sputtering with energetic treatments during and after deposition

https://doi.org/10.1016/j.tsf.2011.02.043Get rights and content

Abstract

By radiofrequency magnetron sputtering co-deposition we synthesized Er:SiO2 film 0.5 μm thick on silica substrates, with Er content < 0.3 atomic %. By changing the preparation condition (during deposition we have used an additional negative bias voltage applied to the substrates for inducing a low-energy ion bombardment, with or without a contemporary heating) and by varying the thermal treatment after the synthesis (the best conditions were 1 h in the range 700–800 °C, in air) we have obtained an Er:SiO2 system with an intense photoluminescence emission at λ = 1.54 μm. The best-performing Er:SiO2 samples obtained by sputtering have shown a photoluminescence response comparable to that of the typical Er:SiO2 thin film systems obtained by conventional techniques used in applicative framework.

Introduction

As a consequence of the increasing demand for telecommunications, a strong impulse towards the synthesis and the development of photonic materials is constantly addressed. The optical communication technology has been largely attracted by materials doped with rare-earth (RE) elements such as erbium, to be used as active elements in photonic devices: actually, the Er3+ ions transition at 1.54 μm of wavelength falls in the range of minimum transmission loss for silica optical fibers [1]. As far as the optical data processing is concerned, the frontier is represented by planar integrated optical devices in which all the optical components are fabricated and directly integrated on a single chip, with the goal of obtaining an all optical communication network [2].

Different techniques are used for preparing Er-doped silica films [3], for instance Er-doping during the silica film synthesis by chemical vapor deposition (CVD) methods or Er implantation in silica substrates. The main drawbacks of ion implantation are the need of two or more implantation steps at different energies for obtaining the required Er-implanted concentration profile (for matching the distribution of the field profile of the optical mode in the waveguide), and of a post-implantation high-temperature annealing step to remove the damage caused by implantation. In the CVD synthesis, the presence of a considerable amount of hydroxyl groups in the films, that can act as quenching centers for the excited state energy, is an important drawback: high-temperature thermal treatments are consequently necessary to remove the undesired hydroxyl groups. In general, the need of high-temperature treatments can however prevent the integration with Si-based electronics.

The sputtering technique is a synthesis approach that allows a production of glass films at relatively low temperature, with a small amount of hydroxyl contaminants; post-synthesis thermal treatments at lower temperature are possible for activating the Er photoemission; the erbium is expected to be homogeneously dispersed within the host material, and its local concentration can be easily changed by acting (in the case of a multisources configuration) on the Er source power to obtain the desired depth profile. With a multisources configuration for co-sputtering and by changing the substrate temperature and/or pressure and composition of the working gas and/or by applying a bias potential to the substrate during deposition, mixed compounds can be produced and different microstructures of the growing host matrix can be obtained.

In literature several investigations of Er:SiO2 systems obtained by sputtering deposition are present (see for instance Refs. [4], [5]). Usually the syntheses adopted are characterized by the following two peculiarities: i) the use of only one rf source for the deposition, with a SiO2 target covered by small pieces of Er or Er2O3, and ii) the choice of suitable working conditions in order to have substoichiometric silica film as host matrix for erbium (this is done for having, after the post-synthesis thermal annealing, a stoichiometric host silica matrix containing also Si aggregates able to sensitize the erbium ions). The latter characteristic interferes with the complete optimization of the erbium ion sites needed to maximize the Er3+ photoluminescence intensity, being the photoluminescence strongly influenced by the presence of sensitizing species inside the host matrix.

In the present study we report on the synthesis of Er:SiO2 films by rf magnetron sputtering co-deposition, investigating the effects of several preparation parameters in order to optimize the photoluminescence performances of the Er3+ ions embedded in a stoichiometric silica matrix. The effects of a simple thermal treatment during the film deposition were recently investigated for this kind of systems [6]. Here, we examine in particular the possibility to improve both the intensity of the emitted 1.54 μm signal and the figure of merit of the material by means of different energetic treatments performed on the growing Er-doped films, namely thermal heating and/or low-energy ion bombardment. These energetic treatments allow exploring different microstructural growing condition for the deposited material, thus possibly permitting an improvement of the final properties in general, and of the photoluminescence optical performances in our special case.

Section snippets

Experimental details

Erbium-doped silica films were synthesized by simultaneous deposition of silica and erbia on fused silica slides of 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 working pressure of 0.40 Pa. The magnetron sources were tilted offline to adjust the deposition focal point on the silica substrates. During depositions, the sample holder was

Results and analysis

Er:SiO2 films were first deposited at room temperature, RT (i.e. without intentional heating of the substrate during the film deposition), with no rf substrate biasing (0 V sample) and with different negative mean voltages due to rf bias (− 50 V, −75 V, −150 V, and − 300 V samples), obtained with 5 W, 6 W, 13 W, and 42 W of power, respectively. A random variation of about ± 5 V was observed for the self-bias values. RBS measurements allow determining in any sample the concentration of erbium atoms in a very

Discussion

The main limitation of the SiO2 as host material is its low erbium solubility (of the order of ~ 1020 atoms/cm3) that sets a concentration limit to avoid clustering effects and related reduction of the 1.54 μm emission efficiency: a much higher local erbium density can originate concentration quenching effects, like Er3+ cooperative up-conversion, detrimental for the full activation of the 1.54 μm fluorescence process [9 and Refs. therein, 10]. Table 1, Table 2 show that with the rf source power

Conclusions

By using radiofrequency-assisted magnetron sputtering co-deposition we were able to synthesize Er:SiO2 film 0.5 μm thick on silica substrates. By changing the preparation condition (during deposition we used an additional negative bias voltage applied to the substrates with or without a contemporary heating of the substrates themselves) and by varying the thermal treatment after the synthesis (the best conditions were 1 h in the range 700–800 °C, in air) we obtained an Er:SiO2 system with an

Acknowledgements

One of us (E.C.) thanks sincerely Dr. V. Santon (University of Trieste, Italy) for the helpful discussions.

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