ИСТИНА

Since 1988, when quantum size effect in hydrogenated microcrystalline silicon containing small crystalline silicon particles was first shown [1], Si quantum dots attract a lot of attention due to their outstanding optical properties. The brightest luminescence is observed from Si nanocrystals, which can be formed by high temperature annealing of amorphous Si quantum dots. After a great number of publications and research efforts in this field, optical gain in the ensemble of silicon nanocrystals was demonstrated by Pavesi et al. in 2000 [2]. At the same time, much less interest is focused on optical properties of amorphous Si nanoclusters (a-SiNCs), which might be explained by their weak quantum yield. However, as it was recently shown, in particular cases, like photosensibilization of Er3+ ions, a-SiNCs can act as more efficient sensitizers in comparison with Si nanocrystals despite the great difference in intrinsic photoluminescence intensity [3]. This means that amorphous Si nanoclusters also possess the potential for optical applications which can be realized by deep understanding of their quantum nature. The goal of the present study is to investigate photoluminescence (PL) properties of ensembles of amorphous silicon nanoclusters in silicon oxide matrix, produced by PECVD method. The samples are consisted of 20 pairs of SiO/SiO2 layers deposited onto Si wafer and thermally annealed in N2 atmosphere at temperatures in the range 500 – 1000 oC in order to form amorphous Si nanoclusters within SiO layers. To control the size of a-SiNCs the thickness of SiO layers was varied from 1 to 5 nm, while the thickness of SiO2 layers was kept at 4 nm. Under UV laser excitation a broad PL band (FWHM is equal to 200 - 300 nm) was observed with maximum in the range from 540 to 910 nm, which position depends directly on SiO layers thickness, i.e. on a-SiNCs size: the lower the SiO thickness, the shorter the PL peak position wavelength. This fact points out the presence of quantum size effect for a-SiNCs ensembles, though it was reported earlier that no significant shift of PL spectrum was observed for amorphous porous nanostructures with 3-5 nm mean size [4]. The PL lifetime measured at 750 nm is in microseconds range and it decreases with SiO layer thickness increase, which is a hint for a possible energy transfer in a-SiNCs ensemble. A nonmonotonic temperature dependence of PL peak position was observed during low temperature measurements in the range from 5 to 300 K. Such behavior differs totally from the corresponding temperature dependence for Si nanocrystals – this point has to be explained in future. This work was supported by the Russian Foundation for Basic Research (grant No. 11-02-01342-a). [1] S. Furukawa and T. Miyasato, “Quantum size effects on the optical band gap of microcrystalline Si:H”, Phys. Rev. B 38, 5726 (1998). [2] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, and F. Priolo, “Optical gain in Si nanocrystals”, Nature 408, 440 (2000). [3] G. Franzo, S. Boninelli, D. Pacifici, F. Priolo, F. Iacona and C. Bongiorno, “Sensitizing properties of amorphous Si clusters on the 1.54-µm luminescence of Er in Si-rich SiO2”, Appl. Phys. Lett. 82, 3871 (2003). [4] R.B. Wehrspohna, J.-N. Chazalviel, F. Ozanam, and I. Solomon, “Spatial versus quantum confinement in porous amorphous silicon nanostructures”, Eur. Phys. J. B 8, 179 (1999).