The excitation mechanism of rare-earth ions in silicon nanocrystals |
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Authors: | G Franzò V Vinciguerra F Priolo |
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Affiliation: | (1) CNR-IMETEM, Stradale Primosole 50, I-95121 Catania, Italy (Fax: 39-095/713-9154, E-mail: franzo@imetem.ct.cnr.it), IT;(2) INFM and Dipartimento di Fisica dell’Università, Corso Italia 57, I-95129 Catania, Italy (Fax: 39-095/383-023, E-mail: priolo@ct.infn.it), IT |
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Abstract: | A detailed investigation on the excitation mechanisms of rare-earth (RE) ions introduced in Si nanocrystals (nc) is reported.
Silicon nanocrystals were produced by high-dose 80-keV Si implantation in thermally grown SiO2 followed by 1100 °C annealing for 1 h. Subsequently some of the samples were implanted by 300-keV Er, Yb, Nd, or Tm at doses
in the range 2×1012–3×1015 /cm2. The energy was chosen in such a way to locate the RE ions at the same depth where nanocrystals are. Finally an annealing
at 900 °C for 5 min was performed in order to eliminate the implantation damage. These samples show intense room-temperature
luminescence due to internal 4f shell transitions within the RE ions. For instance, luminescence at 1.54 μm and 0.98 μm is
observed in Er-doped nc, at 0.98 μm in Yb-doped nc, at 0.92 μm in nc and two lines at 0.78 μm and 1.65 μm in Tm-doped nc.
Furthermore, these signals are much more intense than those observed when RE ions are introduced in pure SiO2 in the absence of nanocrystals, demonstrating the important role of nanocrystals in efficiently exciting the REs. It is shown
that the intense nc-related luminescence at around 0.85 μm decreases with increasing RE concentration and the energy is preferentially
transferred from excitons in the nc to the RE ions which, subsequently, emit radiatively. The exact mechanism of energy transfer
has been studied in detail by excitation spectroscopy measurements and time-resolved photoluminescence. On the basis of the
obtained results a plausible phenomenological model for the energy transfer mechanism emerges. The pumping laser generates
excitons within the Si nanocrystals. Excitons confined in the nc can either give their energy to an intrinsic luminescent
center emitting at around 0.85 μm nor pass this energy to the RE 4f shell, thus exciting the ion. The shape of the luminescence
spectra suggests that excited rare-earth ions are not incorporated within the nanocrystals and the energy is transferred at
a distance while they are embedded within SiO2. Rare-earth excitation can quantitatively be described by an effective cross section σeff taking into account all the intermediate steps leading to excitation. We have directly measured σeff for Er in Si nc obtaining a value of ≈2×10−17 cm2. This value is much higher than the cross section for excitation through direct photon absorption (8×10−21 cm2) demonstrating that this process is extremely efficient. Furthermore, the non-radiative decay processes typically limiting
rare-earth luminescence in Si (namely back-transfer and Auger) are demonstrated to be absent in Si nc further improving the
overall efficiency of the process. These data are reported and their implications.
Received: 9 April 1999 / Accepted: 10 April 1999 / Published online: 2 June 1999 |
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Keywords: | PACS: 76 30 Kg 7 55 -m 61 72 Tt |
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