Quantum size effect on the diffusion barriers and growth morphology of Pb/Si(111)

An intriguing growth morphology of Pb islands on a Si(111) surface is observed in our STM experiments: the growth of a Pb layer on Pb islands with unstable heights starts from the periphery and moves towards the center, while the nucleation of the next layer on stable Pb islands starts away from the periphery. Using first-principles total energy calculations, we have studied the diffusion barriers of Pb adatoms on a freestanding Pb(111) film as a function of film thickness. The diffusion barriers are found to be very low (< 60 meV), and a bi-layer oscillation due to the quantum size effect (QSE) is observed, with a lower barrier on the odd-layered, relatively unstable Pb films. The diffusion barrier difference between the odd- and even-layered film is as large as 40 meV. The observed unusual growth can be attributed to this big difference in the diffusion barriers due to QSE.