The excitation mechanism of rare-earth ions in silicon nanocrystals

A detailed investigation on the excitation mechanisms of rare-ear th (RE) i ons introduced in Si nanocrystals (nc) is reported. Silicon nanocrystals we re produced by high-dose 80-keV Si implantation in thermally grown SiO2 fol lowed by 1100 degrees 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 x 10(1 2)-3 x 10(15) /cm(2). 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 degrees C for 5 min was performed in order to eliminate the implantation da mage. These samples show intense room-temperature luminescence due to inter nal 4f shell transitions within the RE ions For instance, luminescence at 1 .54 mu m and 0.98 mu m is observed in Er-doped nc, at 0.98 mu m in Yb-doped nc at 0.92 mu m in Nd-doped ne and two lines at 0.78 mu m and 1.65 mu m in Tm-doped nc. Furthermore, these signals are much more intense than those o bserved when RE ions are introduced in pure SiO2 in the absence of nanocrys tals, demonstrating the important role of nanocrystals in efficiently excit ing the REs, It is shown that the intense nc-related luminescence at around 0.85 mu m decreases with increasing RE concentration and the energy is pre ferentially transferred from excitons in the nc to the RE ions which, subse quently, 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 phenom enological model for the energy transfer mechanism emerges, The pumping las er 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 mu m 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 t ransferred at a distance while they are embedded within SiO2. Rare-earth ex citation can quantitatively be described by an effective cross section sigm a(eff) taking into account all the intermediate steps leading to excitation . We have directly measured a,rf for Er in Si ne obtaining a value of appro ximate to 2 x 10(-17) cm(2). This value is much higher than the cross secti on for excitation through direct photon absorption (8 x 10(-21) cm(2)) demo nstrating that this process is extremely efficient. Furthermore, the non-ra diative decay processes typically limiting rare-earth luminescence in Si (n amely back-transfer and Auger) are demonstrated to be absent in Si ne furth er improving the overall efficiency of the process. These data are reported and their implications discussed.