(Invited) Phonon Heat Transport Processes and Thermal Conductivity of II-Oxide and III-V Superlattices: Wave-like Coherent Transport in CaTiO3/SrTiO3 Superlattices and Ballistic-Diffusive Transport in GaAs/Alas Superlattices
Authors: Hopkins Patrick 
Published: February 2020

The thermal conductivity of superlattices is strongly reduced as compared to the thermal conductivity of the individual parent materials comprising these multilayered structures due to phonon-scattering and thermal boundary resistances at the superlattice period interfaces. However, given the relatively length scale of the superlattice periods compared to typical phonon wavelengths and mean free paths, the vibrational heat transport in superlattices can give rise to unique phonon scattering processes. For example, partially ballistic and partially diffusive phonon transport in superlattices in which long wavelength phonons effectively do not “see” the interfaces on the order of the superlattice period can occur if superlattice periodicity is short enough. As another example, under certain conditions in which phonon coherence is not destroyed at superlattice interfaces, the emergence of a “minimum” in the superlattice thermal conductivity as a function of interface density can be realized. In this work, we present a series of works on III-V (e.g., GaAs/AlAs) and II-oxide (CaTiO3/SrTiO3) superlattices in which we observe unique phonon thermal transport properties based on measurements of thermal conductivity of these superlattice thin films. First, we will discuss our work on CaTiO3/SrTiO3 superlattices in which we observe coherent phonon transport based on the observation of a minimum in thermal conductivity of superlattice thin films as a function of interface density. When the period thickness decreases to below the phonon coherence length of the phonons in CaTiO3 and SrTiO3, an increase in interface density leads to an increase in thermal conductivity, a phenomenon that indicates a “particle-wave-like” crossover and coherent phonon transport influencing the in phonon thermal conductivity in superlattices. This observation is enabled by the chemically and structurally abrupt nature of the CaTiO3/SrTiO3 interfaces in these superlattices. Next, we will discuss our work on GaAs/AlAs superlattices in which we observe a ballistic-diffusive cross over based on the total thickness of the superlattice. We report on the room temperature thermal conductivity of AlAs-GaAs superlattices (SLs), in which we systematically vary the period thickness and total thickness between 2–24 nm and 20.1–2,160 nm, respectively. The thermal conductivity increases with the SL thickness and plateaus at a thickness around 200 nm, showing a clear transition from a quasiballistic to a diffusive phonon transport regime. These results demonstrate the existence of classical size effects in SLs, even at the highest interface density samples. Our results reveal that the change in thermal conductivity with period thickness is dominated by incoherent (particlelike) phonons in these III-V superlattice films, whose properties are not dictated by changes in the AlAs or GaAs phonon dispersion relations. This work demonstrates the importance of studying both period and sample thickness dependencies of thermal conductivity to understand the relative contributions of coherent and incoherent phonon transport in the thermal conductivity in SLs.

“Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices,” Nature Materials, 13, 168–172 (2014).

“Interplay between total thickness and period thickness in the phonon thermal conductivity of superlattices from the nanoscale to the microscale: Coherent versus incoherent phonon transport,” Physical Review B, 97, 085306 (2018).