Defect Mediated and Resonant Optical Excitation of Er 3+ Ions in Silicon-rich Silicon Oxide
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Defect mediated and resonant optical excitation of Er3+ ions in silicon-rich silicon oxide D. Kuritsyn, A. Kozaneckia), and H. Przybylińska Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland, W. Jantsch Institut für Halbleiter- und Festkörperphysik, Johannes Kepler Universität, Linz A-4040, Austria. PACS No. 61.72.Ww, 78.55.Hx ABSTRACT We study sensitization of the 4I13/2 – 4I15/2 Er3+ luminescence at 1.54 µm in silicon-rich silicon oxide (SRSO) in the whole range of blue-green pump wavelengths. We show that, in general, defects due to excess Si in silica act as luminescence sensitizers. They cause a wide excitation band with a long wave length limit of 600 nm. For monochromatic excitation the maximum luminescence yield is still smaller than for resonant excitation within the 4f shell of Er without additional Si. The large enhancement of the excitation cross section for electrical excitation reported in the literature is plausible, however, as the wide excitation band makes good use of the wide energy distribution of hot carriers. We suggest that the dominant factors, which limit the excitation efficiency of Er3+, are distance dependence of the transfer rate and little spectral overlap of the interacting states. INTRODUCTION Erbium doped silicon-based semiconductors are attractive for applications in optoelectronic devices as they may potentially allow to design all-silicon optoelectronic integrated circuits operating at 1.5 µm.[1] By now, it is widely accepted that for efficient Er- luminescence at room temperature (RT), wide band-gap host materials are necessary,[2] such as silicon-rich Si-oxide (SRSO).[3-6] Among SRSO materials nanocrystalline silicon (nc-Si) doped with Er is currently considered as most promising and numerous reports claim enhancement of the Er emission intensity by orders of magnitude in comparison with Er-doped silica at optical pumping in the bluegreen range of wavelengths.[6-10] Silicon nanocrystals in SRSO:Er play the role of luminescence sensitizers which efficiently absorb the pump light and then transfer energy to Er ions.[9,11] Kik and Polman concluded that one nanocrystal can excite 1-2 Er ions only.[9] This is a serious limitation, however, for the emission intensity of Er3+, as the number of excitable Er ions is only a small fraction of the total Er concentration. Discussing the excitation efficiency we find it necessary to consider details of the excitation path from nc-Si to the 4I13/2 luminescent level. Owing to the size- (and thus the band gap-) distribution of nc-Si in silica only excitons in a small fraction of nanocrystals have their energies resonant with one of the excited states of Er3+. Therefore most energy transfer events will require phonon assistance and/or energy migration within the nc-system making the transfer less probable than at resonance. In this contribution we report results of photoluminescence excitation spectroscopy of the 4 I13/2 → 4I15/2 Er3+ photoluminescence (PL) at 1.54 µm in SRSO performed in the whol
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