Si nanocrystals (Si nc) were formed by the implantation of Si+ into a SiO2 film on (100) Si, followed by high-temperature annealing. The microstructure of the Si nc produced by a high-dose (3 × 1017 cm-2) implantation has been extensively investigated using high-resolution transmission electron microscopy (HRTEM). For most of the Si nc (∼90%) with diameters larger than 6 nm, their configurations are characterized by the existence of nanotwins. The twinning structures include single twins, double twins, and multiple twins. The other planar defects such as stacking faults are also observed in some nanoparticles. However, in the Si nc smaller than 5 nm, no evident microstructural defects are observed. Possible reasons that no evident microstructural defects are found in the smaller Si nc are discussed. The microstructural defects inside the Si nc have a great influence on the optical properties.
The microstructure of the Si nanocrystals ͑Si nc͒ has been investigated using conventional and highresolution transmission electron microscopy ͑HRTEM͒. For most of the nanocrystals ͑Ͼ90% ͒ larger than 10 nm, HRTEM observations show that they are formed by the coalescence of smaller ones. Two kinds of coalescence, one being the preferential attachments of small particles to the ͕111͖ facets of a seed nanoparticle, and the other being an ordered combination of two or more small nanocrystals with appropriate orientations, have been observed.Si nanocrystals ͑Si nc͒ embedded in a SiO 2 matrix have attracted much interest as a candidate system to act as an efficient light emitter. Although the physical mechanism for the light emission remains unclear, a lot of progress has been made in both the preparation 1-4 and characterization 3-6 of the Si nc. In order to better understand and control the physical properties of the Si nc, it is fundamental and necessary to study the main factors that affect the crystal growth and microstructure development of these nanocrystals. For the nanocrystals embedded within a matrix, the coarsening is usually attributed to Ostwald ripening, 7 in which the crystal growth takes place by diffusion of atoms between neighboring nanocrystals. Recent studies 8-10 of TiO 2 and ZnS nanocrystals growing under hydrothermal conditions have shown that the oriented attachment or coalescence plays an important role in the coarsening of nanocrystals. In addition, the coalescence of small particles by twinning was also reported in FePt nanocrystals. 11,12 In the process of the oriented attachment or coalescence, the nanoparticles can themselves act as the building blocks for crystal growth. However, for the Si nc embedded in a matrix such as SiO 2 , coalescence or oriented attachment has not been observed using highresolution transmission electron microscopy ͑HRTEM͒.In this Rapid Communication, we report the conventional and HRTEM observations of the Si nc produced in a SiO 2 film by ion implantation and annealing. The Si nc range from 2 to 22 nm diam., and have a peculiar size distribution with the depth of the implanted layer. For most of the nanocrystals larger than 10 nm, HRTEM observations show that they are formed by the coalescence of smaller ones. Two kinds of coalescence, one being the preferential and ordered attachments of small particles to the ͕111͖ facets of a seed nanoparticle, and the other being an ordered combination ͑by ͕111͖ twinning͒ of two or more small nanocrystals with appropriate orientations in SiO 2 , have been observed. The high concentration of Si ions is essential for the coalescence.The Si nc were produced by a high-dose ͑3 ϫ 10 17 cm −2 ͒ implantation of Si + into SiO 2 film and annealing ͑1100°C͒. For a detailed experimental procedure, see Ref. 13. The specimens for TEM examination were prepared in a crosssectional orientation ͓͑011͔ zone axis for the Si substrate͒ using conventional techniques of mechanical polishing and ion thinning. Dark-field examination was carried out...
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