Carbon diffusion and nanocrystalline diamond formation in carbon ion-implanted oxides studied by nuclear elastic scattering
Introduction
Many studies involving the synthesis and characterization of nanometer-sized clusters of atoms have been carried out in the last few years with a view to creating materials with properties which can be varied according to size. A perfect example is the observed variation in bandgap with cluster size [1], which allows for the tuning of luminescence wavelengths according to the desired application.
Recently, we succeeded in creating nanocrystalline diamond directly from carbon implantation of fused quartz followed by thermal annealing [2], [3]. The formation of diamond was found to be possible within a narrow range of doses and only when the samples were annealed in forming gas (4% hydrogen in argon) for 1 h at 1100°C. Using techniques such as absorption spectroscopy, transmission electron microscopy (TEM) and Raman spectroscopy [3], we determined that significant carbon loss occurred when samples were annealed in oxygen or argon but not in forming gas. In addition, sapphire showed less carbon loss compared to fused quartz for the same annealing conditions.
In this report, we use the backscattering technique, taking advantage of the large C(p,p)C scattering cross-section at around 1.73 MeV [4], to profile the implanted carbon and thus gain valuable information on the distribution of carbon and any loss thereof.
Section snippets
Experimental
The samples studied were optical grade fused quartz supplied by Esco Products and sapphire (α-Al2O3) from Crystal Systems. In both cases, samples were implanted with 1 MeV carbon ions at room temperature using the 1.7 MeV ion implanter located in the Research School of Physical Sciences, The Australian National University. The source material was compacted carbon powder, which was sputtered by cesium atoms and accelerated down a tandem accelerator into the specimen chamber through an aperture
Experimental results and discussion
Fig. 1 shows RBS spectra for fused quartz samples implanted to doses of 0.5, 5 and and annealed for various durations in argon, oxygen and forming gas annealing environments. Results for unimplanted fused quartz and the as-implanted samples are included for comparison. The peak observed at ∼1190 keV is due to the implanted carbon as confirmed by the non-Rutherford modified-RUMP [5] simulation using 1.78 MeV protons (smooth curve) and the nominal carbon depth scale shown, calculated
Conclusion
The substrate, annealing environment and annealing time all determine the amount of carbon retained. To maximize the formation of nanodiamond, all these parameters must be optimized. Because there is less carbon loss in sapphire, this material may be ideal for maximizing nanodiamond yield. We propose that one way to achieve higher yields would be to implant overlapping ranges of carbon and hydrogen. In this way, formation of nanocrystalline diamond would not depend on the inward diffusion of
Acknowledgements
We thank the department of Electronic Materials Engineering, Australian National University, for allowing us to access their ion implantation facilities.
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