Role of phonon anharmonicity in time-domain thermoreflectance measurements

Abstract

Comparison of the 3 ω method and time-domain thermoreflectance for measurements of the cross-plane thermal conductivity of epitaxial semiconductors

Anisotropic failure of Fourier theory in time-domain thermoreflectance experiments

“…Prior TDTR and FDTR studies have explained deviations between L A and L B by assuming phonons with MFPs longer than some experimental length scale are ballistic and do not contribute to L A (refs 1,6,11). This is equivalent to the explanation offered by our ballistic/diffusive model in the limit that TDTR is ARTICLE measuring K z (zoc), that is, the through-plane temperature profile length scale is less than c, and the heat carried by diffusive phonons is much greater than the heat carried by ballistic phonons, |L 0 rT|c|J NL |, where L 0 is the thermal conductivity of the high-wavevector diffusive phonons.…”

“…Our model's requirement that the vast majority of a solid's heat capacity is due to phonons with MFPs much shorter then temperature profile length scales limits its applicability to problems where the vast majority of thermally excited phonons do not require a nonlocal expression for the heat current, precluding our ballistic/diffusive model from making predictions at low temperatures. Our nonlocal expression for the heat current does not distinguish between elastic and inelastic scattering, which should have different effects on the nonequilibrium length scale 11 . Finally, another weakness in our model predictions is that to predict the thermal response of our samples at 1/f timescales, we used our ballistic/diffusive model to derive effective thermal properties, K z (z) and K r (z), as inputs for the heat diffusion equation.…”

“…The breakdown of Fourier theory at small length scales has implications for nanoelectronics 8 and nanostructured thermoelectric devices 9 and has received a great deal of recent attention [1][2][3][4][5][6][7][10][11][12][13][14] . For example, deviations between Fourier theory predictions and experimental data have been reported to depend on (1) the width of the nickel heater lines in nanofabricated Ni/sapphire samples 7 , (2) the diameter of the heating pump laser beam, 2w 0 , in time-domain thermoreflectance (TDTR) measurements of bulk Si between 30 and 100 K (ref.…”

Radial quasiballistic transport in time-domain thermoreflectance studied using Monte Carlo simulations

“…19 Quasiballistic transport has been studied using simulation with a variety of techniques. [20][21][22][23][24][25] Ezzahri and Shakouri used a Green's function formulation to examine electronic ballistic transport. 21 Cruz et al used ab-initio calculations in an attempt to explain a modulation frequency dependence of thermal conductivity in TDTR.…”

Length-dependent thermal transport and ballistic thermal conduction

“…While it is commonly assumed that the observed deviation to be due to ballistic phonons, ab-initio calculations suggest that agreements on Si are better if simply considering harmonic and anharmonic phonon channels but without directly incorporating ballistic phonons. 12 Thus the interpretation of the experimental data is model-dependent and nontrivial.…”