Synthesis and size control of Si nanocrystals by SiO/SiO 2 superlattices and Er doping
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Synthesis and size control of Si nanocrystals by SiO/SiO2 superlattices and Er doping J. Heitmann, D. Kovalev1 , M. Schmidt, L.X. Yi, R. Scholz, F. Eichhorn2 and M. Zacharias Max-Planck-Institut f¨ur Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany 1 Technische Universit¨at M¨unchen, Physik Department E16, D-85747 Garching, Germany 2 Forschungszentrum Rossendorf, Institut f¨ur Ionenstrahlphysik und Materialforschung, PO Box 510119, 01314 Dresden, Germany ABSTRACT The synthesis of nc-Si by reactive evaporation of SiO and subsequent thermal induced phase separation is reported. The size control of nc-Si is realized by evaporation of SiO/SiO2 superlattices. By this method an independent control of crystal size and density is possible. The phase separation of SiO into SiO2 and nc-Si in the limit of ultrathin layers is investigated. Different steps of this phase separation are characterized by photoluminescence, infrared absorption and transmission electron microscopy measurements. The strong room temperature luminescence of nc-Si shows a strong blueshift of the photoluminescence signal from 850 to 750 nm with decreasing crystal size. Several size dependent properties of this luminescence signal, like decreasing radiative lifetime and increasing no-phonon transition properties with decreasing crystal size are in good agreement with the quantum confinement model. Er doping of the nc-Si shows an enhancement of the Er luminescence at 1.54 µm by a factor of 5000 compared to doped SiO2 layers. The decreasing transfer time for the nc-Si to Er transition with decreasing crystal size can be understood as additional proof of increasing recombination probability within the nc-Si for decreasing crystal size. INTRODUCTION After initial reports on quantum confinement of porous Si [1, 2] which exhibits a photoluminescence (PL) signal in the red region of the spectrum at room temperature, interest in the optical properties of Si nanocrystals (nc-Si), has grown over the last decade [3, 4, 5]. Their compatibility with common microelectronic device fabrication materials and techniques make them attractive for potential applications in integrated optoelectronic devices. A debate as to whether this luminescence band is caused by quantum confinement [6], defects at the nanocluster surface [7], by excitation via the nanocluster-nanocluster interface [7], by Si-Si bonds within the nanocluster itself [8] or by oxide-related defect states [9] is still on-going. The influence of the wide variation in the synthesis processes might be one reason for the different explanations of the origin for the red luminescence of nc-Si systems. Different processes for the nc-Si synthesis like Si ion implantation into high quality oxides [7], sputtering of Si rich oxides [10] or reactive evaporation of Si rich oxides [11] are known. Within these methods the Si crystal size is controlled by the Si content in the SiO2 matrix. Therefore, crystal size and their concentration cannot be controlled independently. A lot of effort were put into the independe
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