Raman Characterization of Silicon Nanoclusters Embedded in Fused Silica
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1055-GG12-05
Raman Characterization of Silicon Nanoclusters Embedded in Fused Silica David Barba, Milene Clavel, Francois Martin, and Guy Ross EMT, INRS, 1605 blvd. Lionel Boulet, Varennes, J3X 1S2, Canada ABSTRACT Silicon nanocrystals (Si-nc) and amorphous silicon (a−Si) aggregates either produced by silicon implantation in fused silica have been studied by micro-Raman spectroscopy. Under certain experimental conditions, relevant information regarding both the dimension and the size distribution of Si-nc can be extracted from the energy shifts and the spectral distortion of the phonon excitation observed near 520 cm-1. Results are presented for different sample annealing times and ion fluences. These data agree with the direct observation by TEM of non-uniformly depth-distributed silicon nanocrystals. Moreover, measurements recorded for different Raman probing depths give evidence of chemical composition changes within the fused silica matrix. The depth evolution of the phonon peak associated with the Raman signature of a-Si near 470 cm-1 shows the presence of amorphous silicon, whose concentration is higher in the region where the large Si-nc are located. INTRODUCTION The physical properties of nanocrystallites strongly depend on their dimensions. Direct measurements can be obtained by transmission electron microscopy, but this powerful technique requires a hard and fastidious sample preparation. To overcome this issue, many research groups use indirect investigation methods, based on non-destructive analysis such as measurements by X-ray diffraction (XRD) [1], photoluminescence (PL) [2], spectroscopic ellipsometry (SE) [3], and Raman scattering [4]. The Raman characterization of Si-nc consists in the spectral analysis of the phonon mode around 520 cm-1, associated with the optical excitation of Si-Si bonds. The shape of this phonon peak is affected by the shrinking of crystalline silicon to nanoscale dimensions, by inducing simultaneously an energy shift and an asymmetric spectral broadening. In this paper, spectral changes to the Si-nc Raman signature are presented for different thermal annealing time durations and different probing depths within fused silica samples containing Sinc. These Raman signatures are associated with changes to the average size of Si-nc, which are non-uniformly distributed in depth within the oxide matrix. The deconvolution of the spectral contributions to the Raman signal of both Si-nc size and size-dispersion effects can be achieved using the phenomenological model of the phonon confinement [5]. In this approach, the Raman intensity of the Si-nc phonon peak (I) is fitted as a function of the average diameter (D0) for a standard deviation (σ) and an instrument resolution Γ0 = 4 cm-1, using the relation I (ω , D0 , σ ) ∝ ∫
1
0
2
q 2 f 2 (q ) exp(−q 2 f 2 (q) D0 / 4)
[ω − ω (q)]2 + (Γ0 / 2)2
dq
(1)
where, f(q) =(1+q2σ2/4)1/2 and ω(q) is the phonon dispersion of bulk silicon over the entire Brillouin zone.
EXPERIMENT Fused silica slides were implanted using a Si+-ion beam of 50
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