Twin boundary formation in Pb thin film under conditions of the quantum confinement effect

Pb growth on a clean vicinal Si(557) surface at room temperature was studied using Scanning Tunneling Microscopy. The Pb film growth occurred in accordance with the Stranski-Krastanov scenario. The anisotropic wedge-shaped Pb-islands were observed on the top of a wetting layer. DFT simulations revealed the electron energy oscillations as a function of the island thickness agrees with the electronic growth model. The out-of-plane (111) Pb island consisted of stacked 2 nm thick layers. Based on the DFT simulations and proposed one-dimensional model, it was shown that the layers were separated by the twin boundaries. The energy of formation of twin boundary between the 2 nm layers exceeded the energy gain due to the quantum confinement effect. However, the electron standing wave at the Fermi level in the 2 nm layer made the hcp position of the Pb adatom on the Pb(111) surface favorable. The seed of the twin boundary formation was realized via occupation of the hcp position by the Pb adatom and dimers of adatoms on the Pb(111) surface. The adatom separation in dimers was controlled by an indirect interaction through conductive electrons at the Fermi level of the 2 nm layer of Pb. The completion of the Pb(111) atomic layer growth was achieved by an unusual collective superdiffusive mechanism in the wetting layer and on the top of the Pb nanoisland surface. A new mechanism of twinning boundary formation based on quantum effects in a system of conducive electrons was proposed.