Resonant excitation of Er 3+ by the energy transfer from Si nanocrystals
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Resonant excitation of Er3+ by the energy transfer from Si nanocrystals Kei Watanabe1, Minoru Fujii2 and Shinji Hayashi1,2 1
Graduate School of Science and Technology, Kobe University, Rokkodai, Nada, Kobe, Japan
2
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University,
Rokkodai, Nada, Kobe, Japan ABSTRACT Photoluminescence (PL) of SiO2 films co-doped with Si nanocrystals (nc-Si) and Er was studied. The average size of nc-Si was changed in a wide range in order to tune the exciton energy of nc-Si to the energy separations between the discrete electronic states of Er3+. PL from exciton recombination in nc-Si and the intra-4f shell transition of Er3+ were observed simultaneously. At low temperatures, periodic features were observed in the PL spectrum of nc-Si. The period agreed well with the optical phonon energy of Si. The appearance of the phonon structures implies that nc-Si which satisfy the energy conservation rule during the energy transfer process can resonantly excite Er3+. The effects of the quantum confinement of excitons in nc-Si on the energy transfer process are discussed. INTRODUCTION Recently, Er-doped Si has been attracting much interest, because Er3+ incorporated into Si emits light from the intra-4f shell transition (4I13/2 to 4I15/2) around 0.81eV, which corresponds to the absorption minimum in silica optical fibers [1]. Incorporation of Er into Si offers the possibility of realizing Si based optoelectronic integrated circuits. Since the bandgap of bulk Si is relatively small and the lifetime of the first state (4I13/2) is very long, the relaxation of excited Er3+ by the phonon-assisted re-excitation of host Si (energy back transfer [1,2]), is one of the major non-radiative relaxation processes at relatively high temperatures. Interest has also been focused on Er-doped Si nanostractures such as porous Si and Si nanocrystals (nc-Si) [3-7], which exhibit the small temperature quenching. The band gap widening of Si nanostructures arising from the confinement of an electron-hole pair in a small volume (quantum size effects) is considered to be responsible for the observed small temperature quenching. In previous work, we have demonstrated the direct evidences indicating the strong coupling between nc-Si and Er3+ [5-7]. However, the mechanism of the interaction between nc-Si and Er3+ has not been F14.28.1
understood. In this work, we studied PL of SiO2 films containing nc-Si and Er. The average size of nc-Si was changed in a wide range in order to tune the exciton energy of nc-Si to the energy separation between the discrete electronic states of Er3+. Since the electronic states of Er3+ are discrete and the bandgap of nc-Si changes with the size, nc-Si with some specific sizes are considered to be able to resonantly excite Er3+. Observation of such resonant nature offers useful information to understand the energy transfer mechanism. EXPERIMENTAL PROCEDURE SiO2 films containing nc-Si and Er were prepared by a cosputtering method. Details of the preparation pro
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