Transient thermoreflectance from thin metal films

Abstract

New exotic materials such as amorphous alloys and superlattices are often deposited as thin films onto substrates thereby complicating conventional thermal transport measurements. Transient thermoreflectance (TTR) allows measurements on thin films by the generation of ultrafast heating and time-resolved measurements of the thermally induced reflectivity change on a picosecond time scale. Typically, the penetration depth of visible light in a metal is ~20 nm, and thermal diffusion out of this region occurs in a few hundred picoseconds. Therefore, for film thicknesses of ~100 nm or greater, the TTR measurement can be completed before substrate effects become important. Since the heating depth is small compared to the diameter of the illuminated surface, a 1-D heat flow model can be fitted to our measurements with the thermal diffusivity as a free parameter. We have made TTR measurements on sputtered and evaporated nickel, iron, and zirconium films with thickness between 100 and 500 nm. Deposition-induced disorder decreases the thermal diffusivity of the Fe and Ni, in agreement with electrical conductivity measurements. The Zr film exhibits anomalous behavior possibly indicative of a breakdown in the Wiedemann-Franz law. The extension of this technique to composite films is also discussed.