Quantum Size Effect on the Diffusion Barriers and Growth Morphology ofPb/Si(111)

Abstract

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.

Experimental observation of quantum oscillation of surface chemical reactivities

“…In theory, the QSE could affect both surface thermodynamics and surface kinetics. In the latter case, it could result in, for example, an oscillation in the Pb adatom diffusion barrier between even and odd layers (31). In the present case of oxygen, however, because there is a fixed areal density ratio between the adsorbed oxygen at low temperature and oxide nanoparticles at high temperature for all film thicknesses, we expect the oxygen coverage shown in Fig.…”

Self-organized transient facilitated atomic transport in Pt∕Al(111)

“…In the bulk, only few light elements show ultrafast interstitial diffusion 4,29,31 . However, the quantum tunneling diffusion of heavy adatoms on various substrate surfaces have also been observed recently 32,33 . Quantum diffusion (QD) have not been observed yet for heavy elements in the bulk although the de Broglie wavelength could be in the range of tunneling distance which allows QD on the surface 32 .…”

Adsorbate-induced restructuring of Pb mesas grown on vicinal Si(111) in the quantum regime

“…This contrasting behavior suggests that the Cs step decoration plays an important role in the formation of Pb nano-islands which preferably reside on the quantum unstable layers. We next discuss the role of kinetic processes such as thickness-dependent surface diffusion, island coarsening, or mesa edge barrier fluctuations on the observed bi-layer island density oscillation 17,24 . Surface diffusion of Pb adatoms is relatively slower on quantum stable regions 24 , therefore a higher island density is expected to be observed on quantum stable regions 17 .…”

“…We next discuss the role of kinetic processes such as thickness-dependent surface diffusion, island coarsening, or mesa edge barrier fluctuations on the observed bi-layer island density oscillation 17,24 . Surface diffusion of Pb adatoms is relatively slower on quantum stable regions 24 , therefore a higher island density is expected to be observed on quantum stable regions 17 . Furthermore, bilayer oscillations in Pb surface diffusion are small in energy suggesting that the observed nano-island distribution oscillations should be heavily temperature dependent.…”

“…However, for these specific cases, there is no experimental evidence showing that adsorbates induce modifications to the underlying quantum growth phenomena. Furthermore, this thickness-dependent modulation, where the interplay between classical surface energy (which favors a smooth (111) surface) and the quantum size effect (QSE) energy (which favors bi-layer stability), gives rise to rich phenomena in atomic processes of film growth as well as kinetic phenomena 19,20,21,22,23,24,25 .…”