Sanghamitra Neogi1

1, University of Colorado-Boulder, Boulder, Colorado, United States

Heat management in modern electronic devices is becoming increasingly important with escalating computing demands for fast data processing in a broad range of applications ranging from embedded smart cameras to artificial retinas. In order to optimize energy transport in multi-component devices, it is fundamental to characterize the impact of energy dissipation near interfaces to global transport characteristics. When the device size reaches the nanoscale, scattering at interfaces dictate the device performance and the functionality, since the characteristic dimensions of the devices approach electron and/or phonon mean free paths. Additionally, dimensional reduction significantly modifies the phonons in the nanostructure inducing dramatic changes in their dispersion relation and altering density of states. Bulk mode based description often fails to explain observed heat transport properties in nanostructures [1]. Thus, a complete treatment of thermal transport in a multi-component system requires solving the complex interplay between dimensional confinement and interface scattering. In this work, we investigate phonon transport properties of layered Si/Ge superlattice (SL) configurations with imperfect interfaces. The existence of a secondary periodicity in SLs suggests that bulk-like phonons will not exist in short-period superlattices. Instead, phonons related to the secondary periodicity are the vibrational mode of interest [2]. We employ classical molecular dynamics (MD) and lattice dynamics techniques to analyze superlattice phonons that develop in the transition from an isolated interface to periodic superlattices. We employ non-equilibrium MD to investigate phonon mean free paths in the Si and Ge subsystems and characterize the effect of dimensional confinement on phonon propagation [3]. A similar numerical method yields thermal conductivities of Si and Ge subsystems as well as the interface thermal conductance [4]. Knowledge of phonon MFPs combined with the thermal conductance values quantifies the impact of interface on phonon transport. We determine the extent of disruption of the superlattice phonons due to interfacial imperfections by investigating imperfect interfaces that contain defects, such as vacancies and interstitials. Interfacial defects can have a strong influence on both the electronic structure and charge-carrier scattering near imperfect interfaces. We investigate the effect of interstitial defects on electronic transport in Si/Ge SL by employing first principle DFT calculations with semi-classical Boltzmann transport theory. Our work illustrates the aspect of carrier size effects in multilayered systems and highlights the effect of interfacial structural characteristics on global energy transport in multi-component systems.
1. S. Kwon et al, Nanoscale, 8 (27) 13155 (2016). 2. S. C. Huberman et al, Phys. Rev. B, 88, 155311 (2013). 3. K. Sääskilahti et al, 90, 134312 (2014). 4. P. K. Schelling et al, 65, 144306 (2002).