Multistep resonant excitation of Erbium Ions in Thin Silicon Oxide Layers
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Multistep resonant excitation of Erbium Ions in Thin Silicon Oxide Layers Z. Fleischman1, Chr. Sandmann1, V. Dierolf1, M. White2, Y. Zhao2, J. Michel3, M.A. Stolfi3, L. Dal Negro3 1 Physics Department, Lehigh University, Bethlehem, PA 18015 2 Electrical and Computer Engineering Department, Lehigh University, Bethlehem, PA 18015 3
Materials Science, MIT, Cambridge, MA 02139
ABSTRACT Using the site-selective technique of combined excitation emission spectroscopy, we have studied a variety of Er doped silicon oxide layers with and without silicon nanocrystals. This technique allows clear distinction of cluster defect sites that are created during thermal annealing and become dominant for higher Er ion concentration and are suppressed by the presence of nanocrystals. In several samples, we were able to observe fluorescence line narrowing under resonant excitation allowing defect-selective excitation.
INTRODUCTION Following recent reports from Coffa et al.[1] that claim rather high quantum efficiencies, erbium (Er) doped silicon films within MOS structures have gained renewed attention as a possible pathway to obtain silicon based light emitters. Due to the inhomogeneous nature of the oxide films and the lack of lattice sites that are charge neutral, the Er-ion is incorporated in various environments (“sites”) leading in regular photoluminescence measurements to a significant line broadening. In films that have been prepared such that they contain silicon nanocrystals, the number of different Er-ion sites is even higher and it has been reported that the emission efficiency is increased [2-4]. The large number of sites makes site-specific statements about important quantities, such as emission lifetimes and excitation efficiencies, very difficult. A common problem in incorporation the erbium ions into the oxide layers is the limited solubility of the ions in the matrix such that clustering and emission quenching occurs already at fairly low concentrations. In this contribution, we use site-selective spectroscopy to spectrally identify particular Er environments (“sites”) such as clusters and perturbation by Si-nanocrystals. Using this identification, we address the following questions: • Under what annealing conditions are clusters formed preferentially? • How do nanocrystals influence clustering? • How do growth conditions and annealing influence the Er-sites? • Can nanocrystal related sites be excited resonantly and selectively? SAMPLE PREPARATION We investigated samples from two different sources that have been produced in different ways:
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(1) At Lehigh, we prepared samples by thermally growing 200nm-thick SiO2 layers on top of a Si(100) substrate. For a subset of the samples, the layers were then implanted with Si+ ions at the three energies of 30, 60, and 85 keV to doses of 1.2, 2.5, and 2x1016 cm-2, respectively. This "box implantation" yields a nearly flat profile with ~7% excess Si concentration. The samples were then annealed at 1100 oC in flowing nitrogen gas for 5 hrs to facilitate the growt
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