Er-Doped Alumina Crystalline Films Deposited by Radiofrequency Magnetron co-Sputtering

E. Cattaruzza1, G. Battaglin1, F. Gonella1, P. Riello1, E. Trave1, G. Mattei2, P. Mazzoldi2. 1 Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari, Venezia, Italy.

2 Dipartimento di Fisica dell'Università di Padova, Padova, Italy.

INTRODUCTION

The current development of high speed optical communication systems needs devices such as waveguide optical amplifiers [1]. Er-doped alumina can be used for obtaining waveguides to integrate in a glass fiber system. The sputtering deposition technique is very promising for the fabrication of Er-doped dielectric thin films, allowing a relatively fast and controlled deposition over a large substrate area with a low contaminants content. This technique usually originates amorphous structures [2]. People proposed to use a chromium oxide template layer for obtaining alpha alumina thin films by sputtering deposition at relatively low temperatures [3-6]. As a first attempt we also tried to synthesized alpha alumina starting from a crystalline chromium oxide layer, but after post-deposition annealing in air for 1 hour at temperature T > 500 °C we observed the diffusion of chromium atoms into the alumina film. This causes troubles, being a thermal treatment often necessary in Er-doped alumina films for the optimal optical activation of erbium. Thus, we used as a template a crystalline alumina films (also produced by sputtering deposition and subsequent thermal treatment) for obtaining Er-doped crystalline alumina films.

EXPERIMENTAL

α-alumina template film were synthesized at 500 °C with −200 V of substrate bias to induce ion bombardment during the film growth, and subsequent thermal treatment

ρ=(3.3 ± 0.3) g/cm3, in agreement with data reported in the

of substrate temperature and with no substrate bias, from two different rf sources equipped with targets of metallic

Ar+O2(10%) gas at a pressure of 0.5 Pa. The rf-power used was 250 W and 15 W for aluminum and erbia, respectively. The 60 min. co-deposition of alumina and erbia was followed by a single alumina deposition for 10 min, acting as a protective layer. The deposition chamber was cryogenically pumped, to avoid water contamination. The sample holder was rotated at 5 rpm to have a good homogeneity of the film composition and thickness. The thickness of the Er-doped films was about 0.5 μm. Samples were then annealed for 1 hour in air at

activation of the Er3+ ion PL emission.

The samples composition and concentration profiles of the different elements were obtained by Rutherford backscattering spectrometry (RBS), performed at LNL, using a 4He+ beam at the energy of 1.8 MeV, with the detector placed at 160° with respect to the incident beam direction. XRD patterns were recorded at RT, with a step size of 0.05°, on a 20°–50° 2θ range. The optical emission properties of the Er-doped films were investigated at room temperature by photoluminescence (PL) spectroscopy, under excitation given by a 25 mW Ar ion laser pumping at 488 nm.

RESULTS AND DISCUSSION

The XRD spectrum of the crystalline alumina template film is shown in figure 1, together with the one for the “crystalline alumina template film+Er-doped alumina film” system. The more intense peaks are due to the presence of α-alumina and the less intense ones are related to the presence of a small fraction of θ-alumina [9,10].

Fig. 1. XRD spectra for the alumina template film and for the system formed by the Er-doped alumina film deposited on the alumina template.

The growth of a crystalline Er-doped alumina film is evident from the intensity increase of the peaks characteristics of the α-alumina structure. The RBS spectrum of the final system is reported in figure 2: erbium atoms are distributed homogeneously inside the Er-doped alumina film and their concentration is (7.8 ± 0.4)×1019 atoms/cm3, namely slightly lower than 0.1 at.%. The O/Al atomic ratio is 1.50 ± 0.05.

in air for 2 hours at 1200 °C. The density of the films was

literature [7,8]. Onto these templates, a simultaneous co-

aluminum and erbia, working in a reactive atmosphere of

deposition of alumina and erbia was performed, at 500 °C

different temperatures for T > 500 °C, for a proper

LNL Annual Report 101 Applied, General and Interdisciplinary Physics

Fig. 2. RBS spectrum of the system formed by the Er-doped alumina film deposited on the alumina template film.

All the samples showed a PL activity, even without the final thermal treatment. Figure 3 displays the intensity of the 1.54 μm signal as a function of the annealing temperature.

Fig. 3. Er 1.54 μm PL emission intensity (and the corresponding lifetime in the inset) as a function of the annealing temperature. The lines are guide for the eye.

The best performing samples are those annealed at a

behaviour, as a function of the temperature annealing, is mainly related to the behaviour of the emission lifetime, reported in the inset in figure 3. All the experimental decay curves, detected for samples both as-deposited and annealed at different temperature, could be reasonably fitted by a single exponential function. The highest detected lifetime values are slightly lower than those

detected in Er:SiO2 systems synthesized by means of

the PL intensity starts decreasing, without a corresponding remarkable decrease of the lifetime: this suggests a reduction of the total number of the photoluminescent Er ions for high temperature treatments. The XRD spectra of the system “crystalline alumina template film+Er-doped alumina film” (not reported) showed that for annealing at

aluminates [9] such as ErAlO3 inside the matrix appear and their intensity increases for annealing at higher temperatures, suggesting the formation of erbium compounds that can determine a local increase of the Er concentration and consequent PL intensity reduction due to concentration quenching effect.

CONCLUSIONS

The radiofrequency magnetron co-sputtering deposition technique provides effective synthesis protocols for the preparation of both amorphous and crystalline Er-doped alumina thin films. The reported study is the first step towards the fabrication of a waveguiding structure. In this study, the Er-doped crystalline alumina films were grown on a template film made of crystalline alumina and obtained by sputtering deposition with thermal treatments during and after deposition. After a suitable post-synthesis thermal activation, Er−doped dielectric films showed an intense 1.54 μm photoluminescence emission. Even if the as-deposited films show a marked PL activity, a post-synthesis annealing is needed to obtain the best-performing samples: for 1 hour annealing in air, the best results are

temperatures erbium aluminates form, inducing a reduction of the overall PL emission due to the concentration quenching effects.

Ions Doped Glasses, (eds. N.S. Hussain and J.D. Santos), Trans Tech Publication Inc., Stafa-Zurich (2008), pp. 69–118.

[2] J.D.B. Bradley et al., Appl. Phys. B: Lasers Opt., 89 (2007) 311.

[3] A. Aryasomayajula et al., Thin Solid Films, 516 (2007) 397. [4] P. Jin et al., J. Vac. Sci. Technol., A 20 (2002) 2134. [5] P. Jin et al., Appl. Phys. Lett., 82 (2003) 1024. [6] J.M. Andersson et al., J. Vac. Sci. Technol., A 22 (2004) 8. [7] B.G. Segda et al., Vacuum, 62 (2001) 27. [8] N.R. Rajopadhyea et al., Thin Solid Films, 142 (1986) 127. [9] X.J. Wang et al., J. Mater. Res., 18 (2003) 2401. [10] ICDD, pdf 2 46-1212, 35–121. [11] E. Cattaruzza et al., J. Non-Cryst. Solids, 355 (2009) 112.

temperature in the 700–800 °C range. The PL intensity

[1] M. Ferrari et al., Physics and Chemistry of Rare-Earth

obtained in the temperature range 700–800 °C. At higher

800 °C, peaks attributed to the formation of erbium

similar rf sputtering deposition routes [11]. For T > 800 °C

LNL Annual Report 102 Applied, General and Interdisciplinary Physics