Optical Properties of Si Nanocrystals Formed in SiO 2 by Ion Implantation
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Fig. 1. Cross-section micrographs showing Si nanocrystals formed in SiO 2 when excess Si concentrations of (a) 5 x 102'/cm 3 and (b) 2 x 1022/cm 3 are annealed (1100 0C/l h/Ar + 4% H2). EXPERIMENTAL RESULTS Examples of Si nanocrystals formed in SiO 2 are shown in the micrographs of Fig. 1. Multiple energy implants were used to produce flat profiles (to -900 nm) with excess Si concentrations of 2 x 1022/cm&and 5 x 1021/cm 3 . After annealing to cause precipitation, the average nanocrystal size is somewhat greater than 6 nm (with a few as large as 10 nm) in the sample with the higher concentration, whereas the average diameter is 3 nm in the sample with a concentration of 5 x 10 21/cm 3 . Consequently, the dose (or concentration) can be used to control the nanocrystal size.
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Fig. 2. PL spectra from Si (400 keV, 1.5 x 10'/cme, RT) implanted SiOC in the as-implanted state and after annealing 0 at 9500 C and I I WC (I b/Ar + 4% H).
Fig. 3. PL spctra from Si (400 keV, 1.5 x 1017/cm 2) implanted SiO 2 after annealing for 1 h at 1 00°C in (a) Ar + H2 (b) pure Ar.
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Fig. 4. Dose dependence of PL from Si (400 keV) implanted SiO 2 after annealing (1100 0C/1 h/Ar + H2).
Fig. 5. Optical transmission through Si (400 keV) implanted Si0 2 to the indicated dose after annealing (1 100°C/1 h/Ar + H2).
Figure 2 shows PL spectra (excited at 488 nm) from fused silica implanted by Si (400 keV, 1.5 x l0' 7 /cm 2, RT). The peak excess Si concentration is -5 x 1021/cm 3. In the as-implanted state, luminescence peaked at 650 nm is observed, which probably arises from defects introduced into the matrix during implantation.[2] Annealing at 950'C greatly reduces the defect luminescence. Annealing at 1100TC causes precipitation and Si nanocrystal formation, and after annealing an intense PL band peaked at -750 nm is observed. The intensity of this band is strongly dependent on the presence of H2 in the annealing environment as demonstrated in Fig. 3. Annealing in Ar + 4% H2 results in a very intense PL spectra, whereas annealing in pure argon gives rise to a PL intensity decr
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