Atomic force microscopy and Raman spectroscopy of nanoscale Si/SiO 2 superlattices
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Atomic force microscopy and Raman spectroscopy of nanoscale Si/SiO2 superlattices. R. Krishnan1, G.F. Grom, P.M. Fauchet, and L. Tsybeskov Nanoscale Silicon Research Initiative, Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627 S.Papernov, Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627 G.I Sproule, D.J. Lockwood National Research Council, Institute of Microstructural Research, Ottawa, Canada
An atomic force microscope (AFM) has been used to optimize the radio frequency (rf) / radio frequency magnetron sputtering deposition conditions for obtaining atomically smooth, hydrogen free amorphous silicon (a-Si) and silicon dioxide (a-SiO2) thin films. Superlattices composed of periodically repeating units of nanoscale (a-Si/a-SiO2) units were fabricated at these optimized conditions and subsequently crystallized. The amorphous and crystallized superlattices were characterized by AFM and Raman spectroscopy. Raman spectroscopy of the superlattices was performed by enhancing the weak scattered signal and eliminating the silicon substrate signal by using either waveguiding, cross polarization or interferometric enhancement techniques. The enhanced Raman spectrum clearly indicates formation of nanocrystals after crystallization. (a) INTRODUCTION There is a considerable interest in the fabrication of silicon nanocrystals for quantum devices. One of the techniques compatible with the present integrated circuit fabrication lines is thermal crystallization of a-Si/SiO2 superlattices (SLs). This results in the self-organized growth of silicon nanocrystals with excellent size control in the growth direction [1,2,3]. The individual Si/SiO2 layers are only a few nanometers thick (typically, between 3 to 20 nm). For such small dimensions, the surface roughness of the film is very critical. We have used radio rf/rf magnetron sputtering as the deposition technique. While it is well documented that sputtered film quality generally depends on the sputtering conditions, specific references to their influence on the surface roughness of a-Si/SiO2 thin films are sparse. In this study, an AFM was used to study the influence of rf power, deposition temperature, and pressure on the morphology and surface roughness of a-Si, and a-SiO2 thin films. Amorphous SLs were deposited at the optimum conditions obtained from this study. They were subsequently crystallized and optically characterized by Raman spectroscopy. Due to the low volume of silicon present in the SL, the Raman scattered signal is weak in comparison to that obtainable in systems like porous silicon films or bulk silicon crystals. A further complication arises when crystalline silicon (c-Si) is used as the substrate. In this case, the much stronger substrate signal obscures the nanocrystal signal at and near the silicon TO phonon peak at 520 cm-1. Previously, these problems were overcome either by making a SL with a large number of periods or numerically subtracting the substrate signal from the total signal at
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