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Scanning tunneling microscopy (STM) has been used to determine the surface structure of the &3 X &3 reconstruction induced by Bi on the Si(111)surface. The STM images show that there are two distinct ordered surfaces with &3 X &3 periodicity. At -,-monolayer Bi coverage, the &3 X &3 phase consists of one Bi adatom per unit cell adsorbed at T4 sites on a Si(111) bilayer. At nearly one monolayer of Bi coverage, STM images show protrusions with a complex shape dependent on bias voltage.These STM images are discussed and compared with structural models derived from previous experimental studies.The characterization of metal-induced reconstructions of semiconductor surfaces is a significant subject that pertains to the understanding of both metal-semiconductor interface formation and general growth processes on semiconductor surfaces. Ordered surface structures with +3 X +3 periodicity are commonly seen for metaldeposited Si(111) surfaces, including group-III metals, ' noble metals, and some group-V elements (Sb, Bi).Column III metals [Al, " Ga, In (Ref. 6)] in the &3 X v'3 structure have been well studied and consist of -, ' of a monolayer (ML) of metal adatoms bonded at T~s ites directly above second-layer Si atoms. However, the column V metals [Sb, Bi (Ref. 8)] are known to make another V3X&3 structure with 1 ML of metal atoms forming trimers on the surface. The currently accepted model, called the milkstool model, is derived from analysis of photoelectron and x-ray-diffraction data, ' as well as electronic structure calculations. ' A recent dynamical low-energy electron-diffraction (LEED) study has shown that there are two different &3XV 3 structures induced by Bi. " At less than -, '-ML coverage, Bi atoms adsorb as adatom monomers at the T4 site. This is basically the same structure as for the column III metals. At nearly 1 ML, Bi atoms form trimer clusters centered over the T4 site. The Bi atoms in the trimer are each bonded to the two other metal atoms in the trimer, and have one remaining bond to an underlying Si atom, forming milkstool structures. Scanning tunneling microscopy (STM) is a direct tool for studying both surface geometric and electronic structure. We have taken STM images of Bi deposited on the Si(111) surface at metal coverages up to 1 ML. We have imaged both the low coverage and high coverage &3X&3 reconstructions. The appearance of the STM images is discussed in terms of previously proposed structural models for these surfaces, '" and the bias dependence of the images is explained in terms of what is known about the surface electronic structure. 'All sample preparation and measurements were performed in an ultrahigh-vacuum (UHV) system having base pressure better than 5X10 "Torr. The Si(111)surfaces were cleaned by chemical cleaning, UV pretreatment, and annealing at 1150'C in vacuum better than 2 X 10 ' Torr. Bi was deposited from a heated tungsten filament and metal coverages were determined by timed exposure to an evaporant Aux previously calibrated by a quartz crystal microbalance. All s...

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