Effects of surface vibrations on interlayer mass transport: Ab initio molecular dynamics investigation of Ti adatom descent pathways and rates from TiN/TiN(001) islands

Anharmonic lattice vibrations, for which restoring forces are not linearly dependent on atomic displacements from local potential energy minima, are known to significantly affect the thermodynamic and kinetic properties of crystalline solids. We carry out density-functional ab initio molecular dynamics (AIMD) simulations of Ti adatom (Tiad) migration on, and descent from, TiN -faceted epitaxial islands on TiN(001) at temperatures T ranging from 1200 to 2400 K. Adatom-descent energy-barriers determined via ab initio nudged-elastic-band calculations at 0 Kelvin suggest that Ti interlayer transport on TiN(001) occurs essentially exclusively via direct hopping onto a lower layer. However, AIMD simulations reveal comparable rates for Tiad descent via direct-hopping vs. push-out/exchange with a Ti island edge atom for T >= 1500 K. We demonstrate that the effect is due to surface anharmonic vibrations, which yield considerably lower activation energies at finite temperatures by significantly modifying the adatom push/out-exchange reaction pathway.