Synthesis, Optical Properties, and Microstructure of Semiconductor Nanocrystals Formed by Ion Implantation
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displays the semiconductor nanocrystals which we have synthesized at ORNL by ion implantation into three technologically-important substrate materials. Table 1. Semiconductor nanocrystals grown by ion implantation. Y indicates nanocrystals formed. N indicates nanocrystals not observed. No symbol indicates systems not yet investigated.
it
Nanocrystal Si Ge SiGe GaAs GaP GaN InP InAs CdS CdSe CdSeo.5So.5 CdTe CdSeo.5 Teo. 5 ZnS ZnSe
IL
SUBSTRATE Si0
A12 0 3
2
Y Y Y Y
Y Y Y Y Y N Y Y Y Y Y
Y
Y Y
J
Si
y Y N Y Y Y Y Y Y
Y Y
Ion implantation offers several advantages, as well as challenges, as a technique for synthesizing nanocrystals. First, ion implantation is versatile, since almost any ion can be implanted into any solid substrate with extreme chemical purity, including isotopic discrimination. In some cases, this versatility may in fact provide a unique route for incorporating precipitates in substrates with particular chemical sensitivity or extreme melting temperatures. Second, ions are embedded directly in the matrix by implantation and hence the crucial steps of nanocrystal encapsulation or passivation are typically integral parts of the synthesis. Third, implantation can be carried out through masks or with focused beams, and hence is capable of creating well-defined, high-resolution patterns of regions containing quantum dots. Fourth, the quenching of the kinetic energy for implanted ions is an intrinsically metastable process and hence presents opportunities for unique kinetic routes to nonequilibrium structures. Finally, since ion implantation represents an integral part of existing semiconductor fabrication technology, improvements in equipment capabilities will continue and it will be relatively straightforward to incorporate quantum dots created by ion implantation into electronic devices. Considering the challenges, ion implantation often introduces structural damage into the substrate. As will be demonstrated in this paper, the ion damage
90
itself sometimes provides useful changes in the host material. However in many cases, it must be controlled by high-temperature implantations or subsequent thermal annealing. In addition, since the implantation approach for creating nanocrystals relies on nucleation and Ostwald ripening of precipitates, the particles possess a range of sizes. Thus, applications requiring greater monodispersity will involve more complex thermal processing steps or advanced techniques such as writing individual quantum dots with finely focused ion beams [14]. EXPERIMENTAL Semiconductor nanocrystals were formed by implantation of the constituent ions into several different host materials, including fused silica (Corning 7940 glass), silica grown by thermal oxidation on silicon wafers (Si0 2 /Si), a-A12 0 3 (0001), and Si(001) wafers. Doses ranged up to -101 7 /cm 2 for each species, and the ion energies were chosen such that the ion concentration profiles were superimposed after implantation. As expected, the size of the nanocrystals increased when the substrate temp
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