Hydrogen Pairing on Si(100)−(2×1): A Site-Blocking Study

The thermal desorption of hydrogen from a Si(100)−(2×l) monohydride surface has been reported previously to follow first-order kinetics. Pairing of hydrogen atoms on a Si-Si dimer prior to desorption is most likely the cause of this unusual behavior. To examine the degree of hydrogen pairing at various surface temperatures, this study examines the blocking of adsorption sites by hydrogen for subsequent acetylene adsorption. Temperature programmed desorption and Auger electron spectroscopy were used to measure the absolute saturation coverage of acetylene for various coverages of preadsorbed atomic deuterium. The observed linear decrease in saturation coverage of acetylene with deuterium coverage, for atomic deuterium adsorption between 550 and 150 K, indicates that deuterium is paired on Si-Si dimers. The observation that pairing occurs even at 150 K suggests that the diffusion of chemisorbed hydrogen is not responsible for pairing.