Abstract:
The phase velocity of Surface Acoustic waves exhibits no dispersion in homogeneous materials. However, normal or anomalous dispersion takes place in layered systems. This dispersion effect has been used to determine the elastic constants of thin films. However, gradients in materials properties in the near surface region lead to dispersion effects also and therefore can be easily detected by surface acoustic waves (SAWs). We have used this technique to study changes in shear elastic constants of diamond irradiated with carbon ions in the energy range typically used in doping semiconductor materials. The range of the radiation damage extends from the surface to a depth of ≈0.3 μm. The fluence used for implantation was chosen so that the damaged diamond layer would not reform after thermal annealing at ambient pressure. In this study we generated and detected SAWs on the implanted and annealed diamond face after deposition of a very thin Pt film of thickness of ≈20 nm to enhance surface reflectivity. With increasing dispersion (k∥d), the Rayleigh wave velocity falls gradually from that of crystalline diamond to that of annealed amorphous layer. At critical values of product of parallel wave vector and film thickness (k∥d), Sezawa modes split off from the bulk wave threshold of the substrate (transonic state). These are guided modes with the displacement field having an oscillating component in the layer, and falling off exponentially in the substrate. In combination with the velocity of the longitudinal bulk waves, the Poisson ratio and Young's modulus of the implanted region were calculated.
