Influence of quantum size effects on Pb island growth and diffusion barrier oscillations

Quantum size effects are successfully exploited in manipulating the growth of (111) oriented Pb islands on Si(111) substrate with a scanning tunneling microscope. The growth dynamics and morphology displayed can be well controlled through the quantum size effects defined by the island thicknesses and the interplay with the classical forces. The transition of growth modes from quantum to classical regime and the quantum beating in morphological dynamics are directly identified in real space and quantitatively analyzed. Atomic diffusion barriers, an important parameter in the thin film growth process, are also demonstrated to be modified by quantum size effects, and oscillate with a two-monolayer periodicity.

Figure 1

(Color) A sequence of STM images $(1000\mathrm{nm}\times 1000\mathrm{nm})$ recorded at room temperature showing the evolution of a Pb island on Si(111). (a) Original Pb island before STM manipulation. The thickness of Pb layers on top of each Si terrace increases successively from $4.5\text{to}7.3\mathrm{nm}$, measured on top of the wetting layer. The arrow marks the position where the voltage pulse $(+5\mathrm{V})$ was applied to the STM tip. (b)–(d) Selective strip-flow growth turning odd-layer-number regions into even numbers of layers. (e)–(f) Double-layer strip growth maintaining the even number of layers. (g)–(h) Flat top recovery growth to reduce surface steps. (i)–(l) Side-view schematics showing the original island, selective strip-flow growth, double-layer strip growth, and flat-top-recovery growth, respectively. Blue indicates the QSE-favored even-layer-number regions, and red the QSE-unfavored-layer-number regions.

Figure 2

(Color) (a) STM image $(1000\mathrm{nm}\times 1000\mathrm{nm})$ illustrating the quantum-classical growth transition, the white arrow indicates the critical thickness of 26 ML. (b) STM image $(1300\mathrm{nm}\times 1300\mathrm{nm})$ illustrating quantum beating in morphological dynamics, the arrow indicates the critical thicknesses of 17 and 18 ML. The thickness increases from left to right for Pb islands in both (a) and (b). (c) and (d) Schematic diagrams illustrating the quantum and classical growth mode, respectively. Red and blue arrows in (c) indicate the growth directions of the selective strip and the following flat-top recovery respectively, and black arrows in (d) indicate the growth directions for classical growth.

Figure 3

(Color) Time evolution of four stacked Pb layers created by a voltage pulse of $+7\mathrm{V}$ applied at $t=0^{\prime }$ to the STM tip. STM scanning was performed at $\sim 240\mathrm{K}$. The first, second, third, and fourth layers are marked from bottom to up. Insets show the corresponding STM images $(300\mathrm{nm}\times 230\mathrm{nm})$ of the growth fronts. The thickness of the original Pb island increases from right to left.

Figure 4

(Color) (a) Arrhenius plots for the selective strip growth at 21 ML and flat-top-recovery growth at 22 ML. (b) Line rate versus thickness obtained at $\sim 300\mathrm{K}$. (c) Relative effective diffusion barrier versus thickness.