Size control of gas phase grown silicon nanoparticles by varying the plasma OFF time in silane pulsed plasma
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Size control of gas phase grown silicon nanoparticles by varying the plasma OFF time in silane pulsed plasma A. Mohan1, I. Poulios1, R.E.I. Schropp2 and J.K. Rath1 1
Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science-Physics of Devices, High Tech Campus 21, 5656 AE Eindhoven, The Netherlands 2
Energy research Center of the Netherlands (ECN), Solar Energy, High Tech Campus 21, 5656 AE Eindhoven; and Eindhoven University of Technology (TU/e), Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven ABSTRACT Silicon nanoparticles are synthesized by very high frequency Plasma Enhanced Chemical Vapor Deposition (vhf-PECVD) in the gas phase. Pulsed plasmas are used to obtain particles with a narrow size distribution. The role of plasma OFF times is studied to tailor the size of the silicon nanoparticles. Various plasma OFF times are chosen, both longer- and shorter -than the residence time of the gases in the discharge. Time resolved optical emission spectroscopy (TROES) studies provide additional information about the growth precursor dynamics during plasma modulation. The size and the size distribution studies of the particles are done with transmission electron microscopy (TEM). These studies reveal that a plasma OFF time longer than the residence time is favorable for the formation of quantum sized silicon particles. INTRODUCTION Silicon is the backbone material in the modern semiconductor era owing to its abundance and non-toxicity. However bulk c-Si is an indirect band gap material, hence, a thick material (a couple of hundred micron range) is needed for solar cells, due to inefficient absorption. By downscaling bulk silicon to sizes less than the electron-Bohr radius (4.2 nm) quantum confinement effects can be exploited and band gaps can be tailored by varying the size of the nanocrystal. Quantum dot not only allows to tailor the band gap, but also makes it an efficient absorber (increased transition probability), thus exploiting the advantage of thin film technology. The use of pulsed plasmas has shown to be very effective in obtaining precise size control of monodisperse particles [1]. Growth time of the nanoparticles is determined by the plasma ON times (ton). By reducing the duration of a pulse ton, growth times of the particles and hence, the size of the nanoparticles can be reduced. Longer pulse ON times resemble a continuous plasma condition and lead to the saturation of particle sizes. Nanoparticle growth can be interrupted by turning OFF the plasma. Plasma OFF times (toff) play a very important role in disrupting the growth of particles by refreshing the process gases. In this study, we vary toff, hence change the density of growth precursors to control their size and size distribution. EXPERIMENT
The particles reported here are grown in a dedicated very high frequency Plasma Enhanced Chemical Vapor Deposition (vhf-PECVD) reactor for a single step synthesis of silicon nanoparticles in the gas phase. This nanodot reactor has two paralle
