Growth of nano-crystalline metal dots on the Si(1 1 1)-7 × 7 surface saturated with C2H5OH

doi:10.1016/j.susc.2007.04.191

Surface Science

Volume 601, Issue 22, 15 November 2007, Pages 5093-5097

https://doi.org/10.1016/j.susc.2007.04.191 Get rights and content

Abstract

Metal atom on the Si(1 1 1)-7 × 7 surface undergoes migration by hopping among Si-adatom and Si-rest atom. If the hopping migration is prohibited, how change the deposited metals? In this paper, we studied the deposition of metals on the Si(1 1 1)-7 × 7 surface saturated with C₂H₅OH, on which the whole Si-rest atoms are changed to Si–H so that the hoping migration of metals will be prohibited. We found the growth of ca. 5 nm of crystalline dots by the deposition of Sn, Zn and Ag. Interestingly, Ag dots undergo layer-by-layer growth so that the surface is covered with 5 nm size dots with uniform height. When the hopping migration is prohibited, growth of dots is controlled by the kinetics of precursor state atoms instead of the lattice energy relating to lattice matching or strain.

Introduction

Adsorbed atoms and/or molecules detected by the STM (scanning tunneling microscopy), which are either migrating or still, are the final state of adsorption. However, the final state is not directly formed by the collision of atom or molecule to the surface but is provided via precursor state atom or molecule. Precursor state is like a two-dimensional gas which can diffuse freely over the surface. As it will be discussed in this paper, the role of precursor state may be indispensable to the epitaxial growth of surface, the formation of nano-wire or nano-dots, and the catalysis. On the other hand, the migration of metal atoms adsorbed on the Si(1 1 1)-7 × 7 surface occurs by hopping among the Si-adatom and Si-rest atom, which is different from the two-dimensional diffusion of precursor state. If all Si-rest atoms are reacted with H atom, the hopping migration of metal atom on the surface will be stopped although the diffusion of precursor state will be influenced little. If this is truth, an interest question may arise that when the hopping migration is prohibited how change the growth mode of metal dots? From this point of view, Zn, Sn and Ag atoms were deposited on the Si(1 1 1)-7 × 7 surface saturated with C₂H₅OH (hereafter describe as Si(1 1 1)-7 × 7–C₂H₅OH), because C₂H₅OH dissociates at the Si-adatom-rest atom pair sites so that three Si-rest atoms (whole rest atoms) and three Si-adatoms (a half of six adatoms) are changed to Si–H and Si–OC₂H₅, and three intact Si-adatoms are remained in every half unit cell on the Si(1 1 1)-7 × 7–C₂H₅OH surface.

Precursor state is like a two-dimensional gas, which is different from “meta-stable adsorption”. Such precursor state is indispensable to understand the growth of dot (zero dimensional), wire (one dimensional), layer (two-dimensional), and crystal (three dimension). A good example is the sticking probability of alcohols and NO molecule via precursor state and its behavior like a gas phase molecules expressed by partial pressure [1], [2], [3], [4].

Section snippets

Experimental

The STM experiments were performed in an UHV chamber with a base pressure of 1.2 × 10⁻⁸ Pa. A rectangular Si(1 1 1) wafer (n-type, ∼0.1 Ω cm) of 1 mm × 7 mm × 0.3 mm was degassed at about 450 °C (723 °K) in the STM chamber by direct current through the sample. After then, the degassed sample was repeatedly flashed up to ca. 1250 °C (1523 °K), and a clean well-ordered Si(1 1 1)-7 × 7 surface was obtained. The clean Si(1 1 1)-7 × 7 surface was exposed to C₂H₅OH gas of 1.0 × 10⁻⁶ Pa for 100 s in the STM chamber, which is

Results and discussion

The adsorption observed in the STM image is not given by direct collision of molecule or atom to the surface but if formed via precursor state.

When the hopping migration of the adsorbed atoms is prohibited, we can catch a glimpse of the influence of precursor state on the growth of dots. Fig. 1a and b refer the STM images of Sn and Zn atoms on the clean Si(1 1 1)-7 × 7 surface [5], [6]. The noisy triangle (indicated with arrows) observed in Fig. 1a reflects rapid migration of Sn atom in a half unit

Acknowledgements

This work has been supported by the High-Tech Research Center of Saitama Institute of Technology, and K. Tanaka expresses his thanks to Mr. Mitsushi Umino of Astech Co. for his stimulating support of our research.

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Growth of nano-crystalline metal dots on the Si(1 1 1)-7 × 7 surface saturated with C2H5OH

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Acknowledgements

Surf. Sci.

Surf. Sci.

Surf. Sci.

Phys. Rev. B

J. Chem. Phys.

J. Chem. Phys.

Phys. Rev. B

Phys. Rev. B

Phys. Rev. Lett.

Growth of nano-crystalline metal dots on the Si(1 1 1)-7 × 7 surface saturated with C₂H₅OH