Thermal Oxidation of Si Nanoparticles Grown by Plasma-Enhanced CVD
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Thermal Oxidation of Si Nanoparticles Grown by Plasma-Enhanced CVD Debabrata Das, Jordi Farjas, Josep Costa, Pere Roura, Gregorio Viera1 and Enric Bertran1 GRM, Departament de Física, Universitat de Girona, Campus Montilivi, E17071 Girona, Catalonia, Spain. 1 FEMAN, Departament de Física Aplicada i Optica, Universitat de Barcelona, Av. Diagonal 647, 4ª planta, E08028 Barcelona, Catalonia, Spain. ABSTRACT Nanoparticles of amorphous silicon were grown by plasma-enhanced CVD. Their high hydrogen content has a great influence on the oxidation kinetics. Oxidation experiments done in a thermo-balance show that during a heating ramp under oxygen the onset of oxidation shifts to higher temperatures when hydrogen content is reduced by annealing. Apparently this higher oxidation rate of the as-grown particles is due to the great density of dangling bonds that are left behind just after the hydrogen is desorpted. This fact is supported by analyzing the oxidation dynamics under isothermal conditions. At lower temperature, when oxidation takes place just after hydrogen desorption, the oxidation transient begins with a finite slope which indicates that oxidation is not diffusion controlled. On the other hand, at higher temperatures, activation energy of the parabolic rate constant indicates that the oxide layer formed is less protective than the oxide layer formed on crystalline silicon. INTRODUCTION The growth of nanostructured thin films of amorphous silicon has a potential interest due to its enhanced properties for photovoltaic conversion[1]. They are currently grown by chemical vapor deposition (CVD) techniques in which the process parameters are chosen in such a way that first, small particles that grow in the plasma are deposited onto the substrate and second, they are covered by amorphous silicon. Consequently, after a great number of cycles, the final structure consists of amorphous or crystalline nanoparticles embedded in an amorphous matrix[2]. A common characteristic of these materials is their high hydrogen content that, besides passivating Si dangling bonds, plays a major role in the oxidation processes. The present work is devoted to analyze the oxidation of amorphous Si nanoparticles. Presumably, in view of its structure, oxidation of nanostructured silicon follows similar mechanisms. The main advantage of working with nanoparticles is their enormous specific surface that makes it possible to detect the very early stages of oxidation. As it is shown in this report, kinetics of the process can be easily followed by measuring mass gain down to oxide layer thicknesses well below 1 nm. Our main concern will be the role of hydrogen in the oxidation process. Previous work has shown that the presence of hydrogen in this kind of nanoparticles makes them very sensitive to oxidation in air at lower temperature (the case of Si) or even at room temperature (SiC or SiCN nanoparticles)[3]. However, the reason of this major influence was not analyzed.
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EXPERIMENTAL DETAILS Silicon nanoparticles were grown at room tempe
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