Exciton photoluminescence and energy transfer in nanocrystalline Si/Si dioxide superlattice structures
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Exciton photoluminescence and energy transfer in nanocrystalline Si/ Si dioxide superlattice structures V. Yu. Timoshenko (a), O. A. Shalygina (a), M. G. Lisachenko (a), P. K. Kashkarov (a), D. Kovalev (b), J. Heitmann (c), M. Zacharias (c), B. V. Kamenev (d), L. Tsybeskov (d) (a) Moscow State M.V. Lomonosov University, Physics Department, 119992 Moscow, Russia (b) Munich Technical University, Physics Department E16, 85747 Garching, Germany (c) Max-Planck-Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany (d) Electrical and Computer Engineering Department, New Jersey Institute of Technology, University Heights, Newark NJ 07102
ABSTRACT Photoluminescence (PL) of nanocrystalline Si (nc-Si) assemblies formed by thermal crystallization of amorphous Si/SiO2 and SiO/SiO2 superlattices (SLs) has been investigated at different temperatures and excitation conditions. The low temperature resonant PL spectroscopy reveals phonon-assisted excitonic recombination. At room temperature the samples formed from a-SiO/SiO2 SLs possess relatively high PL quantum yield (∼ 1%). The PL transients have nonexponential decay, which indicates the exciton energy transfer in nc-Si ensembles. The excitonic energy of Er-doped nc-Si SL structures can be almost completely transferred to Er ions incorporated in SiO2 matrix that results in a strong emission line at 0.81 eV. INTRODUCTION Nanocrystalline Si (nc-Si) structures fabricated by controlled crystallization of amorphous Si/SiO2 [1] and SiO/SiO2 [2] superlattices (SLs) are attractive for future electronic and optical applications because of their well defined structure and full compatibility with the standard Si technology. The structures formed from a-Si/SiO2 SLs have a well-defined order in the direction perpendicular to the layer, but are disordered laterally due to the grain boundaries [1]. The electrical transport in such structures at low temperatures has been shown to be governed by resonant hole tunneling via quantized valence band states in nc-Si [3]. The nc-Si assemblies formed from a-SiO/SiO2 SLs consist of 3D-arranged size-controlled Si nanocrystals, which exhibit rather strong room temperature photoluminescence (PL) [2]. The Er doping of nc-Si SL structures results in a material which can emit efficiently at 1.5 µm [4]. In the present work, the PL properties of the both types of nc-Si structures are investigated under different temperatures and excitation conditions. We demonstrate that the excitonic energy transfer influences the photoexcited electron-hole recombination in nc-Si ensembles.
EXPERIMENTAL DETAILS Amorphous SiO/SiO2 SLs were prepared by alternating reactive evaporation of SiO powder in vacuum or in oxygen atmosphere on c-Si substrate (see for details [2]). The thickness of SiO
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layers dSiO was varied from 2 to 6 nm, while the SiO2 layer thickness dSiO2 was 2-4 nm. The number of SiO/SiO2 periods was typically 30-70. The samples were subjected to a conventional furnace annealing at 1100oC in N2-atmosphere for 1 hour. Some part of
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