Nanocrystals and Quantum Dots Formed by High-Dose Ion Implantation
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chosen to give an overlap of the individual profiles in order to form compounds or alloys. Following implantation, samples were annealed in flowing Ar + 4% H2 to induce precipitation and nanocrystal formation. Characterization was carried out using a variety of techniques including x-ray diffraction, Rutherford backscattering ion channeling (2.3 MeV He ions), crosssection transmission electron microscopy, Raman spectroscopy, and various types of optical measurements (absorption, PL, infrared reflectivity). RESULTS Figure 1 shows the high density of Au nanocrystals that can be formed in fused silica by the implantation of Au (2.75 MeV) with the substrate at a temperature of 600'C. Individual nanocrystals are spherical crystals which are oriented at random relative to each other The average diameter in this case is -130 A. The size and size distribution can be changed considerably by changes in the implantation and annealing conditions as shown in Fig. 2. The average size can be changed from -25 A to -125 A by changing the implantation or annealing conditions as indicated. An unexpected finding is that Au nanocrystals in Si0 2 produced by annealing in a reducing environment (Ar + 4% H2 ) are considerably smaller than those produced by annealing in an oxidizing environment. 40 *
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1. Cross section micrograph showing Au nanocrystals formed in fused silica by the implantation of Au (2.75 MeV, 1.5 x 1013/cm 2 ) at a temperature of 600°C.
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Fig. 2. Size distributions for Au nanocrystals in fused silica following Au (2.75 MeV, 1.5 x 1017/cm 2 ) implantation. The top distribution was measured following implantation at 600'C. The other three distributions were measured after annealing samples implanted at room temperature.
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The presence of these Au nanocrystals in the near-surface region gives rise to a refractive index which is dependent on light intensity and a very fast response time, which makes these
depends materials interesting candidates for nonlinear optical applications.2 The nonlinear9 index on the volume fraction occupied by the nanocrystals and is of the order of -10- cm 2/watt (near wavelength) for volume fractions of -10%, which is easily achieved by the plasmon resonance implantation. 3 The nonlinear response time for Au nanocrystals in glass is in the picosecond time regime. 17 Si and Ge nanocrystals can be synthesized in both SiO27 12 and A1203. 13 In SiO2, the nanoparticles are very small, and their presence at high densities can give rise to strong PL. Figure 3 compares PL from porous Si with that arising from the implantation of Si (400 keV, 6 x 1017/cm 2) into Si0 2 followed by annealing at 1100*C to form silicon nanocrystals. The PL from the sample containing the silicon nanocrystals is comparable in intensity to that measured from porous silicon, but shifted in wavelength. PL from the sample with silicon nanocrystals i
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