Elastic properties of metastable Mo1 − xSix alloy thin films: A Brillouin light scattering study

Elastic properties of Mo1 − xSix alloy films with 0 ≤ x ≤ 1 have been investigated by the Brillouin light scattering technique thanks to the analysis of the surface acoustic waves in the film on substrate: the so-called Rayleigh surface wave and the Sezawa waves. A transition from crystalline to amorphous state is observed for a Si content, x = 0.19. This transition is accompanied by different modifications of the elastic constants, namely, C11, C33, C13 and C44 that are necessary with C12 to define the effective hexagonal symmetry for the textured <110> polycrystalline alloys. A pronounced softening of the shear elastic C44 constant is observed, from 110 to 60 GPa when x varies from 0 to the critical threshold composition x = 0.19. The longitudinal C33 constant estimated in this work from the BLS measurements shows also a softening from 425 to 315 GPa. An intrinsic consequence of the high supersaturation of MoSi alloys is the development of an important lattice instability. In the amorphous state, the evolution of the elastic properties, that can be defined by the two independent constants C11 and C44 = (C11 − C12)/2, exhibits two distinct behaviors depending on the electronic properties and the interatomic bonding of the amorphous alloys.