Direct Observation of Atomic Processes: Silicon Adatoms on Tungsten Surfaces

Abstract

A wide variety of the behaviors of single silicon adatoms on tungsten surfaces have been studied in the field ion microscope (FIM). Existing works are summarized, and some new results are presented. Diffusion parameters for single silicon atoms on the W{110} plane were found to be Ed=0.70±0.07eV and Do = 3.1 × 10−4 × 10±1.28 cm2 s−1, where Ed is the activation energy of diffusion and Do the diffusivity. Although chain-form atomic clusters were often formed when more than one adatom was present on a plane, the clusters were not stable during diffusion. Silicon atom transport on the W{110} plane is thus achieved dominantly by migration of single silicon atoms. The desorption field as a function of radial distance from the W{ 110} plane center was measured, and the data were related to the field distribution on the plane at a given applied voltage. The binding energy of silicon adatoms on the W{l10} plane was estimated from the desorption field. Relative pair energies for the Si-Si interaction on the W{110} plane for several bond lengths were obtained from the relative frequencies of observing these bonds in the FIM. The maximum binding occurred at a separation √2a, with the bond aligned in the [110] direction, the maximum antibinding occurred at a distance a in the [001] direction, and a weak binding occurred at 2a along the [001] direction; a was the substratelattice constant. Using the pair energy data, the relative binding energy in an adlayer for various adlayer structures was calculated. The[2√2/-√3×/√3]R35.26° superstructure, which is conventionally expressed as p(2 × 1), had the lowest energy. Our subsequent thermal equilibration experiments with deposited silicon adatoms at about 300 K confirmed the existence of such a structure. Since further heatings at about 300 K changed only the adlayer shape and composites, but did not change the structure of the superlattice, the observed superstructure was the equilibrium adlayer structure at a low degree of coverage at about 300 K. Monte Carlo computer simulations using the measured pair energies predicted the same equilibrium superstructure. In addition, many growth defects of the two-dimensional lattice, such as antiphase domains and stacking faults, were observed. The computer simulations also showed these and other defects. On the W{112} plane, no cooperative walk of silicon adatoms similar to that of metal adatoms was found. However, the staggered and straight bond configurations were observed much more often than that expected from a random distribution. When two silicon adatoms were deposited onto the same surface channel, they were mobile although the ,√3a bond separation seemed to occur more often. In addition, interactions of plane edges with silicon adatoms and some other observations are discussed.