Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Collapse of nanocavities studied by ion-channeling and Raman spectroscopy
Introduction
Nanocavities are suitable gettering sites for the removal of small concentrations of metal impurities [1]. Recently, they have been found to be unstable in amorphous Si (a-Si) [2]. On irradiation the cavities shrink and, for doses slightly higher than those needed to form a continuous amorphous layer, completely disappear. Since nanocavities act as preferential nucleation sites for ion beam induced damage, it has been suggested that they collapse as the surrounding a-Si structure expands [3]. An understanding of the parameters which govern the disappearance of nanocavities is crucial if they are to be used in practice. Their influence on damage accumulation in crystalline Si (c-Si) as manifested by changes in Rutherford backscattering and ion-channeling (RBS–C) spectra and Raman spectra is therefore examined in this study. RBS–C can routinely be used to determine the amount of disorder in c-Si by extracting the yield due to dechanneling [4]. Raman spectroscopy can also be used as it is sensitive to damage accumulation and phase transformations.
Section snippets
Experiment
Implants were performed on p-type, 1–10 cm, Si(1 0 0) Czochralski grown wafers using a 1.7 MV NEC tandem ion implanter. During implantation, substrates were affixed to the implanter stage with Ag paste to ensure good thermal contact and were tilted 7° off the incident beam axis to avoid channeling.
The nanocavities were produced by first implanting 20 keV H into c-Si to a dose of 3×1016 cm−2 at room temperature. This resulted in a H layer centered at ∼2850 Å below the surface. An anneal was then
Rutherford backscattering and ion-channeling
Fig. 1 shows the RBS–C spectra for samples implanted into a region which overlapped an area containing nanocavities and a c-Si region with 245 keV Si to a dose of 5×1014 cm−2 at room temperature. The random (solid curve) and channeled (open triangles) yields from an unimplanted region are also shown for comparison. The RBS–C yield was found to be greater in implanted Si containing nanocavities (open circles) than for implanted pristine Si (squares) at all implant doses and temperatures until
Conclusions
We have shown that RBS–C can be used to determine the increase in dechanneling and direct scattering events in the presence of nanocavities which are found to have an effect on the rate of damage accumulation in c-Si during implantation. These studies may lead to a better understanding of the defects involved in nanocavity collapse. Changes in the Raman spectra coincided with critical points in the RBS–C spectra and suggest that implanted Si with nanocavities may decrease compressive strain in
Acknowledgements
This work is supported by a grant from the Australian Research Council. The Department of Electronic Materials Engineering at the Australian National University is acknowledged for their support by providing access to ion implanting facilities. We also thank Dr. Richard Brown for writing and letting us use his Nd program.
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