Observation of near-surface damage by phonon scattering

We have investigated the feasibility of phonon-reflection techniques as nondestructive means to probe surface and/or near-surface damage in otherwise highly perfect crystals. An UHV liquid-helium stage, suitable for phonon-reflection measurements, was installed on a beam line of a tandem van de Graaff accelerator which was used to implant MeV ions into the substrate in order to modify the surface region in situ. Here, we report our investigation on the effects of 1-MeV Ar+ implantation in Al2O3 single crystals by monitoring the reflection of terahertz (THz) phonons (50-Å wavelength) from the implanted region. The results are compared to other surface techniques. Using a 15-kV ion gun on the same beam line, we have also bombarded Al2O3 crystals coated with thin films of gold. The effects of a 7.5-keV Ar+ beam on this Au-Al2O3 system are also discussed in this paper.