Nanoscale Laser Flash Measurements of Ballistic and Diffusive Heat Currents in Si, Diamond and AlN Thin-Films

Mean free path spectroscopy using standard time-domain thermoreflectance and frequency-domain thermoreflectance techniques relies on interpreting deviations of measured data from diffusive behavior when important experimental length scales are comparable to phonon mean free paths. Both these techniques measure the temperature rise at the surface of films in the same location that the films are heated. As a result, they do not directly observe the effect that ballistic transport and/or interfacial scattering have on the temperature vs. depth across the film. Here, we present an alternative approach designed to probe effect of ballistic heat currents, interfacial phonon scattering, and mean free path distributions on the temperature vs. depth in semiconductor thin films. We perform time-domain thermoreflectance in a laser-flash geometry where the heater and thermometer layer are spatially separated by the semiconductor film of interest. We measure the time-of-flight of heat across the thin-film, as well as the thermal response to harmonic heating. Time-of-flight measurements allow us to quantify non-diffusive heat transfer due to the difference in time-scales between diffusive and ballistic transport. We perform these nanoscale laser flash measurements of Si, diamond and AlN thin films as a function of temperature. We compare our experimental data to the predictions of a non-local model of ballistic transport, as well as diffusive theory. We observe clear deviations from diffusive theory when film thicknesses are less than phonon mean-free-paths responsible for thermal conduction.

Acknowledgement: This work was supported as part of ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0021230.