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CdS nanocrystals formed in SiO2 substrate by ion implantation

Semiconductor crystallites buried in glass are potentially very interesting for applications in optical devices due to large non-linear optical properties and fast response times. In this work Cd and S ions were implanted in fused silica up to ion doses of 1017/cm2. Altogether 18 samples were then annealed in temperature range Ta = 300o to 900o C. The implanted and annealed layer was studied by X-ray Diffraction (XRD), Raman spectroscopy, and UV Transmittance and Reflectance measurements (energy range 1.4 - 6.5 eV). Upon annealing all methods proved the synthesis of CdS crystallites from starting components. In XRD the sharp lines, characteristic for the hexagonal CdS, emerged superimposed on the wide background, related to amorphous SiO2. In the transmittance measurements the absorption above 2.5 eV was observed, which exhibit a marked blue shift in Eo, especially for lower doses and lower Ta, indicating the confinement of charge carriers. An interesting finding was that the nanocrystal size strongly influences not only Eo but also E1. Consistent results were obtained in the reflectance measurements. In Raman spectra the formation of CdS nanocrystals resulted in the crystalline zone-center LO peak close to 300 cm-1 as well as in the appearance of low frequency Raman band. All of these features were strongly and consistently dependent on ion dose and annealing temperature. However, none of them appeared in control samples. These results support the conclusion that smaller CdS nanocrystals were obtained for lower doses and lower annealing temperatures.

nanocrystals; CdS; ion implantation

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