Phonon localization and resonance in thermal transport of pillar-based GaAs nanowires.

Exploring the possibility of nanostructures to modulate thermal conductivity (TC) contributes to promote a deeper comprehension of phonon diffusion and transport processes with the design of thermally insulated devices with high ZT values, and the GaAs nanowires (NWs) widely used in optoelectronic and microelectronic devices exhibit nondiffusive phonon thermal transport phenomena attributed to size effects, while ignoring the wave effects of phonons. Here, we simulate the TC of pillar-based GaAs NWs using non-equilibrium molecular dynamics and Monte Carlo simulations. The spatial distribution of density of states, temperature and heat flow distribution clouds, phonon participation rate, dispersion curves and phonon transmittance of atoms were calculated to investigate the phonon thermal transport processes in pillar-based NWs. The calculation results show that the pillar-based surface reduce the TC by 16%, the TC of pristine NW increases with axial and equivalent diameter, and the TC of pillar-based NW increases nonlinearly with axial length and increases with radial length. The phonon-surface scattering intensity is enhanced by the perturbation introduced by the pillared surface with a substantial decrease in phonon transmission capacity and a break in long-wavelength phonon transport even annihilated, which leads to surface phonon localization. Nanopillars not only enhance the phonon-surface scattering intensity at low frequencies, but also reconfigure the dispersion curve to reduce the group velocity. A series of flat resonance phonon modes are generated throughout the whole spectrum due to the hybridization between the local resonance phonon modes of the nanopillar and the phonon modes of the substrate NWs, resulting in the phonon modes shifting to lower frequencies. The pillar-based surface induced surface phonon localization and local resonance phenomenon contributes to the modulation of phonon thermal transport in GaAs-based field-effect transistors.