A New Pathway for Si Nanocrystal Formation: Oxi-Reduction Induced by Impurity Implantation
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A New Pathway for Si Nanocrystal Formation: Oxi-Reduction Induced by Impurity Implantation L.G. Jacobsohn1*, A.R. Zanatta2, J.K. Lee1, D.W. Cooke1, B.L. Bennett1, C.J. Wetteland1, J.R. Tesmer1 and M. Nastasi1 1 Materials Science & Technology Division, Los Alamos National Laboratory MST-8 G755, P.O. Box 1663, Los Alamos, NM 87545, USA 2 Instituto de Física de São Carlos, Universidade de São Paulo Caixa Postal 369, 13560-250 São Carlos, SP, Brazil * [email protected]
ABSTRACT In this work we show the feasibility of producing silicon nanocrystals by means of a new method based on the oxi-reduction of silicon dioxide induced by the presence of an impurity and annealing. The choice of magnesium as the impurity relies on its chemical properties of oxireducing the SiO2 matrix while avoiding the formation of Si-based compounds. The samples were obtained by 3x1016 and 1x1017 at/cm2 ion implantation into fused silicon dioxide followed by annealing in vacuum at 900 oC for 2 or 10 h. Rutherford backscattering spectrometry (RBS) characterized the chemical content and the Mg depth distribution. In all cases, photoluminescence measurements that showed a broad band starting around 1.8 eV with increasing emission intensity for lower energies, indicating the presence of Si nanocrystals. The analysis of the photoluminescence data in the framework of the quantum confinement theory suggests the existence of relatively large Si nanocrystals. The presence of these nanocrystals was also confirmed by Raman spectroscopy. INTRODUCTION Light-emitting semiconductor nanostructures have the potential of total integration of optical and electronic devices with expected great impact in telecommunication systems. Among the candidate materials, silicon nanostructures have been investigated both on their synthesis as well as on their properties. In 1990, visible photoluminescence (PL) at room temperature from anodically etched Si, the so-called “porous silicon”, was first reported [1]. A few years later, Si+ implantation into fused silica followed by annealing was shown to produce intense PL output [2,3]. The presence of an emission band at 1.7 eV that appeared only after high temperature annealing was ascribed to the presence of Si nanocrystals. Moreover, it has been observed that the PL emission from Si nanocrystals shifts to lower energies for larger nanocrystals, and this behavior has been explained by some authors as due to the quantum confinement of the electron wavefunction [1-8]. Several other models have been proposed to explain this light emission, such as interface states, defects, siloxene species, etc. In addition to these approaches, several other procedures were employed to obtain Si nanocrystals, among them sputtering deposition, plasma enhanced chemical vapor deposition, laser ablation and molecular beam epitaxy. It is the aim of this work to show the feasibility of using the oxi-reduction method to the synthesis of Si nanocrystals. In this method, the implanted species reduce the matrix releasing its non-oxygen component that
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