Combinatorial Fabrication and Study of Luminescent Nanocrystalline Si Particles Embedded in a SiO 2 Matrix
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Combinatorial Fabrication and Study of Luminescent Nanocrystalline Si Particles Embedded in a SiO2 Matrix L. F. Fonseca,1 O. Resto,1 S. Z. Weisz,1 and J. Shinar2 1 Department of Physics, University of Puerto Rico, Rio Piedras, PR 00931 2 Ames Laboratory – USDOE and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 ABSTRACT The combinatorial fabrication of nanocrystalline Si particles embedded in a SiO2 matrix (ncSi:SiO2) by RF co-sputtering of Si and SiO2 targets is described. The peak of the photoluminescence (PL) spectra of the films varies systematically from 760 to 600 nm, consistent with the presumed systematic variation in the size distribution of the embedded Si particles. The correlation between the optical properties of the samples and the formation parameters is also analyzed. INTRODUCTION Improvements in the room temperature photoluminescence (PL) quantum efficiency of nanocrystalline silicon (nc-Si) have stimulated extensive efforts to understand the basic mechanism of this PL as well as its potential applications in Si-based optoelectronic devices.1 Various models for visible light emission in Si have been proposed, notably quantum confinement effects2 and radiative states at the Si/SiO2 interface.3 The effect of carrier confinement and band gap upshift in nc-Si have been discussed intensively.1 The empirical pseudopotentials (EPS),4 third nearest-neighbor tight binding,5 time-dependent tight binding,6 and corrected ab initio local density approximation (LDA) methods give reliable predictions of the nc-Si band gap. However, the quasiparticle gaps and exciton binding energies require large corrections which are not included in the one electron theory.7 Despite the apparent disagreement in predicting the resulting peak PL energy, these calculations demonstrate that the PL energy increases as the nanocrystal size decreases and the dominant contribution to the visible light emission comes from nanocrystals smaller than 2 nm. Furthermore, the PL energies exhibit an important Stokes shift relative to the absorption edges of oxidized silicon nanocrystals, possibly due to self trapped excitons and surface defects.7 The Stokes shift also increases with decreasing nanocrystal size. This paper describes the combinatorial fabrication of arrays of Si nanocrystals embedded in SiO2 (nc-Si:SiO2) synthesized by rf co-sputtering of bulk Si and SiO2-targets. From the optical absorption edge, which varies systematically across the array, we obtain the nanocrystal size distribution, assuming spherical nanocrystallites. The dependence of the PL intensity peak emission wavelength on the concentration of the constituents, sputtering conditions, and annealing temperature, is also discussed. EXPERIMENTAL PROCEDURE The 13.56 MHz, 1100 V, 180 (2.5) W forward (reflected) RF power co-sputtering system used to prepare the nc-Si:SiO2 films consisted of a Varian system in a bottom target–top
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substrate vertical configuration. The 62 liter chamber was connected to a 2500 liter/se
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