Comparison of damping in thin films of aluminum, TiN and ZnO using transient thermoreflection

Transient thermoreflectance (TTR) signal generated from delaminated films of Al, TiN and ZnO upon incidence of nanosecond pulsed laser beam and creation of oscillations is used to investigate the damping mechanism. The thermal and acoustic components of the TTR signal are evaluated and the decay in the acoustic component with time is related to the energy dissipated during damping. The damping of acoustic oscillations in MHz frequency range is explained by the dissipation of energy when the mobile dislocations oscillate in the presence of a drag force created by electron and phonon scattering. The variation of dislocation density with the size of the oscillating films in Al, TiN and ZnO is consistent with the Granato-Lṻcke theory of energy loss from mobile dislocations. TiN films with lower phonon scattering and higher transverse velocity of sound show lower energy loss during damping provided the dislocation density is not high. Al and ZnO films with higher phonon scattering and lower transverse velocity of sound show higher energy loss. The results indicate the necessity of maintaining lower dislocation density to minimize energy loss during damping.