Tunneling microscopy and spectroscopy, in conjunction with tight-binding molecular dynamics, provide compelling evidence that the "missing As" defect on GaAs(l10) is indeed an As vacancy. Neighboring Ga atoms relax upward by about 0.7 A, but do not rebond. The defect is positively charged and most likely in a +2 state. Both the relaxation and the preponderance of As vacancies on p-GaAs are explained by the energetics of the defect levels. The essential features of the observations can be understood from qualitative arguments based on hybrid orbitals. PACS numbers: 61.16.Ch, 68.35.Bs, 68.35.Dv Atomic-scale studies of semiconductor surface defects, using scanning tunneling microscopy and spectroscopy (STM and STS) [1-9],have enhanced the prospects for a fundamental understanding of their role in growth nucleation, carrier recombination, Fermi-level pinning, and initiation of surface chemical reactions. Since STM probes only valence levels, however, there are often ambiguities in interpretation, leaving even the identity of a defect in doubt. For example, the "missing dimer" defects at the Si (100) surface [2] have been interpreted as subsurface interstitials [10] as well as divacancies [11],and other defects on this surface have yet to find definitive assignments. In addition, chemisorbed species can mimic vacancies [12] by suppressing the local state density near the Fermi level.Here we argue that the identity of a simple native defect at the GaAs(110) surface -the "missing As" defect -can be established through a combination of (a) high resolution atom-selective imaging, (b) local spectroscopy, (c) qualitative chemical arguments, and (d) molecular dynamics simulations. We determine the nature, charge state, geometry, and electronic structure of this defect, and also explain its abundance on degenerate p-type GaAs [9].Our p-GaAs samples were grown by the Bridgman technique, and Zn doped at 2&10' cm . A fresh (110) surface was exposed by cleaving (001)-oriented wafers in UHV (~5x10 " torr). The STM probe tips were mechanically cut from 0.1 mm Pt wire and conditioned in situ by field emitting to the sample. All scans were recorded using setpoint currents below 100 pA.The structural features of the missing As defects observed on p-GaAs are displayed in the upper panels of Fig. 1, where we present topographic images simultaneously acquired [4,13] with sample biases of -1.8 and +2.0 V. As seen in the left panel, there is a localized reduction in the filled-state density directly above an As site, suggesting that a single atom has been removed from the As sublattice. Arsenic atoms in the same [1101chain As sublattice Ga sublattice Composite FIG. 1. Simultaneously acquired filled-and empty-state images of the missing As defect on degenerate p-GaAs(110). Defect composite shows the registry of the As (black) and Ga (gray) sublattices. adjacent to this defect appear to be symmetrically depressed. In the corresponding Ga image, two atoms near the defect appear to rise out of the surface. The registry of the As and Ga subl...

The structure of point defects situated at nominal arsenic or gallium sites on the (110) cleavage face of GaAs has been studied by scanning tunneling microscopy (STM). Alternate-bias imaging was used to simultaneously resolve the arsenic and gallium sublattices corresponding to filled and empty states, respectively, on both p- and n-type material. There is an interesting symmetry in the characteristic features associated with the most commonly observed defect of each type. Both types appear in the STM images as a missing surface atom, or, more precisely, a highly localized reduction in the corresponding filled- or empty-state density. For both types of defect, nearest neighbors within the same zigzag chain appear to be raised out of the surface, while second nearest neighbors show a slight depression. No large lateral displacements are detected in either the nearest or second nearest neighbor atoms. Point defects with this structure are observed exclusively at arsenic sites on p-type material, and exclusively at gallium sites on n-type material. The band bending accompanying the defects reveals that the arsenic defect on p-type samples is positively charged whereas the gallium defect on n-type samples is negatively charged. Migration of these defects in the [11̄0] direction along the zigzag chains, as well as in the [001] direction perpendicular to them, is occasionally observed in the STM scans.

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