Si Nanocrystals as Sensitizers for Er PL in SiO 2
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Si nanocrystals as sensitizers for Er PL in SiO2 M. Forcales1, M. Wojdak1, M.A.J. Klik1, T. Gregorkiewicz1, O. B. Gusev2, G. Franzò3, D. Pacifici3, F. Priolo3, F. Iacona4 1 Van der Waals - Zeeman Institute, University of Amsterdam, Valckenierstraat 65, NL-1018 XE Amsterdam, The Netherlands 2 A.F. Ioffe Physicotechnical Institute, Russian Academy of Sciences, 194021 St.Petersburg, Russia 3 INFM and Dipartimento di Fisica e Astronomia, Università di Catania, Via Santa Sofia 64, I95123 Catania, Italy 4 CNR-IMM, Stradale Primosole 50, I-95121 Catania, Italy
ABSTRACT Sensitization of Er3+ photoluminescence in SiO2 layers by silicon nanocrystals has been investigated under resonant and nonresonant pulsed optical pumping. We observed that the very efficient channel of erbium excitation introduced by the nanocrystals can be saturated when very high pulsed photon flux is used. While this channel is saturated, tuning to the resonant excitation wavelength results in an increase of erbium-related photoluminescence, indicating that direct and indirect excitations can take place simultaneously. INTRODUCTION Due to a transition at λ=1.54µm, which coincides with a minimum of losses in optical fibers, Er3+ ion is the optical dopant of choice for optoelectronic devices. The choice of a host material poses contradictory demands: it should provide efficient excitation of the dopant, but should not disturb its emission. It was shown that excitation cross section of erbium photoluminescence (PL) in crystalline silicon is several orders of magnitude larger than in a dielectric matrix like SiO2 [1]. But non-radiative de-excitation processes characteristic for Si, lead to thermal quenching of erbium emission [1, 2]. On the other hand, a dielectric provides good thermal stability of Er3+ PL, but its excitation is inefficient and possible only when photon energy is resonant with the electron transitions in the 4f-shell of erbium ion. The advantages of both hosts can be combined in erbium doped SiO2 matrix with a high concentration of silicon nanocrystals (Si-nc’s) [3-6]. The Si-nc’s efficiently absorb light in a broad spectral range and transfer the excitation to Er3+ ions located preferentially outside Si-nc, where the large SiO2 energy bandgap provides thermal stability of emission. The Si-nc’s mediated energy transfer offers an effective excitation cross section of ~10-16 cm2 [5, 6], much higher than the ~10-20 cm2 excitation cross section for Er3+ in SiO2 [7]. In this way, by doping with Si-nc’s an efficient channel for nonresonant excitation of temperature stable emission from Er3+ ions is realized. In this work we report on photoluminescence excitation measurements under high power pulsed pumping. We observe full saturation of erbium emission in Si-nc doped SiO2 sample, which indicates that sensitization with Si-nc’s could have certain limits.
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