Optical characterization using ellipsometry of Si nanocrystal thin layers embedded in silicon oxide
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Optical characterization using ellipsometry of Si nanocrystal thin layers embedded in silicon oxide E. Agocs1,2, P. Petrik1, M. Fried1, A. G. Nassiopoulou3 1 Research Institute for Technical Physics and Material Science, 1121 Budapest, Konkoly Thege u. 29-33, Hungary. 2 University of Pannonia, 8200 Veszprem, Egyetem u. 10, Hungary 3 Institute of Microelectronics (IMEL), NCSR Demokritos, Aghia Paraskevi, 153 10 Athens, Greece. ABSTRACT We have developed optical models for the characterization of grain size in nanocrystal thin films embedded in SiO2 and fabricated using low pressure chemical vapor deposition of Si from silane on a quartz substrate, followed by thermal oxidation. The as-grown nanocrystals thin film on quartz was composed of a two-dimensional array of Si nanocrystals (Si-NC) showing columnar structure in the z-direction and touching each other in the x-y plane. The nanocrystal size in the z-direction was equal to the Si nanocrystal film thickness, changing by the deposition time, while their x-y size was almost equal in all the samples, with small size dispersion. After high temperature thermal oxidation, a thin silicon oxide film was formed on top of the nanocrystals layer. The aim of this work was to measure the grain size and the nanocrystallinity of the Si nanocrystal thin films, a quantity related to the change of the dielectric function. We used a definition for the nanorcystallinity that is related to the effective medium analysis (EMA) of the material. The optical technique used for the investigations was spectroscopic ellipsometry. To measure the above sample properties the thickness and composition of several layers on a quartz substrate had to be determined by proper modeling of this complex system. We found that the nanocrystallinity (defined as the ratio of nc-Si/(c-Si+nc-Si) decreases systematically with increasing the Si-NC layer thickness. Using this approach we are sensitive to the lifetime broadening of electrons caused by the scattering on the grain boundaries, and not to the shift of the direct interband transition energies due to quantum confinement. INTRODUCTION Si nanocrystals (Si-NCs) embedded in dielectrics have been widely studied recently for use in non-volatile memory devices as charge storage medium, in Si-based light emitting diodes, and sensors [1-4]. Due to the quantum-confinement effect, the band structure of Si-NCs is different from that of bulk silicon and shows discrete energy levels in the conduction and valence bands that are strongly dependent on the Si-NC size. In a system containing a large number of Si-NCs, those with sizes larger than a few nanometers show a bandgap similar to that of bulk Si, while those with sizes below ~3-5 nm show a size dependent bandgap. Therefore the electric, the transport, and the charging properties change, and they are dependent hard on grain sizes in this range [5,6]. In this study we have investigated Si nanocrystal (Si-NC) thin layers in 5 different grain sizes embedded in silicon oxide with spectroscopic ellipsometry (SE). The pr
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