Change in yhe Optical Properties of Sapphire Induced by Ion Implantation
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polishing with diamond, and a final polishing with colloidal silica. Final thinning was done by Ar ion milling (4.5 keV, 180, sector milling) for five minutes per side. The TEM specimen preparation sequence preserved the original implanted surface for depth measurements. The average radius of metal nanoclusters is small compared with the wavelength of light and is determined [7] from the resonance optical absorption spectrum according to the equation r = (vf/Aco,/ 2), where vf is the Fermi velocity of metal and Aco112 is the full width at half maximum of the absorption band due to the plasmon resonance in small metal particles. The value of the FWHM is determined from the absorption band (Ao,1/ 2 = 2itc AM/XP 2) where A) is the full width at half maximum wavelength of the plasmon band and XP represents the observed peak wavelength of the plasmon band. RESULTS and DISCUSSIONS The structure of the implanted sapphire is complex and the amount and the type of damage at various depths strongly depends on the orientation of the ion beam relative to the crystallographic axes [8,9]. The radiation induced damage was observed by RBS channeling, TEM and by absorption photospectrometry. At higher annealing temperatures the radiation induced damage was reduced except in the layer where the density of the clustered nanocrystals was peaked. In that layer the (006) lattice spacing is highly distorted, measuring approximately 5% larger than for the single crystal sapphire substrate. Figure 1 compares the optical absorption spectra of four sapphire crystals implanted either with gold, silver, copper, or tin after subsequent heat treatment at 1323°K for Au implanted, 773°K for Ag implanted, 673°K for Cu implanted, and at 773°K for Sn implanted crystals. The Energy, eV 6
1.6 -
5
4
_..
160 keV Sn
3
2
1.4 1.2
2.0 MeV Au
•
396
1.5 MeVAg
• 0.8 0.8
".
561
10kVC
0.6 S0.4 0.2 0.0 200
300
400
500
600
700
800
Wavelength, nm Figure 1, the absorption optical density for sapphire crystals implanted with Ag at 8 x 1016/cm 2, Au at 12 x 1016/cm 2 , Cu at 20 x 1016/cm 2 and Sn at 8 x 1016/cm 2 after heat treatment.
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absorption spectra for sapphire implanted with each ion species follows the Mie's theory predicted shape [10]. To observe the effects of the MeV ion beam induced changes in accordance with the shape predicted by the Mie theory in the optical absorption spectra of sapphire, we bombarded a c-cut pristine sapphire with 3.0 MeV alpha particles at 1.0 x 1016 /cm 2 and 20 x 1016/cm 2 . These crystals were optically studied before and after annealing at temperatures between 673 0K to 11730K. Typically, the innert ion bombarded sapphire crystals showed absorption bands at 257 nm, 230 nm, and 203 nm. These bands were reduced to one absorption band at 203 nm with a 50% reduction in its absorption optical density after annealing at 673°K, and further reduced an additional 50% when heat treated at 11 73°K. The information obtained from alpha particle bombarded sapphire crystals helped us to distinguish between optical ab
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