Brillouin scattering study of guided modes in TiN films on high-speed steel

Brillouin scattering measurements are presented of guided modes in TiN films on high-speed steel (HSS). Because of its relatively high elastic moduli, TiN has a stiffening effect on the surface wave velocity of HSS. With increasing k(parallel to)h (wavevector x film thickness), the Rayleigh surface wave of HSS increases in velocity and then merges into the bulk wave continuum and becomes a highly damped pseudo-surface acoustic wave (p-SAW). Its higher modes can also be observed. It is found that the elastic constants of the TiN film play a crucial role in the acoustic behavior of TiN/HSS. There is a discontinuity in the calculated dispersion curve for normal TiN-HSS - a strong stiffening system, but not for reduced elastic moduli TiN-HSS - a relatively soft stiffening system. However, the acoustic modes in both situations evolve into a 'quasi-Rayleigh' wave of the corresponding systems at large k(parallel to)h, which are very close to the Rayleigh waves of TiN. Comparison between the measured and calculated dispersion relations indicates that the effective elastic moduli of TiN for film thicknesses d<500 nm are about 25% smaller than those of bulk TiN. A possible explanation for the lowered elastic moduli of the thinner films could be composition variations at the TiN-HSS interface. (C) 1998 Elsevier Science B.V.