Geometry and electronic structure of the arsenic vacancy on GaAs(110)

Abstract

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...

Hydrogen on III-V (110) surfaces: Charge accumulation and STM signatures

“…Again, this weakening is very clear in the H ad + images. These differences are not conclusive, and the experimental images themselves do vary somewhat, [45][46][47][48][49][50][51][52][53][54][55][56][57][58] with others looking more like the simulated vacancy images, but it does seem likely that at least some of the reported images are due to H adsorption, not anion evaporation.…”

“…(Indeed, very similar results have recently been obtained for adsorbed hydrogen on cerium dioxide, which looks in STM just like surface oxygen vacancies. 44 ) Here on the III-V (110) surfaces, the only case that looks significantly different is the (rarely considered experimentally [45][46][47][48][49][50][51][52][53][54][55][56][57][58] ) case of H ad − and V C under negative bias (filled states), where H ad − does not look like V C , but does look somewhat like a native (or other) adatom, Fig. 7.…”

“…59), in contrast to the standard simulated images of V A . The interpretation of these STM images was long controversial, since the experiments [45][46][47][48][49][50][51][52][53][54][55][56][57][58] show the vacancies as symmetric, but theoretical studies [59][60][61][62][63][64] always find them to be Jahn-Teller distorted. The accepted explanation 59 is that thermal flipping between two degenerate Jahn-Teller distorted structures averages out to the symmetric simulated STM image shown in the center-right in Fig.…”

Arrangement of nitrogen atoms in GaAsN alloys determined by scanning tunneling microscopy

“…The well known case of a surface vacancy produces a depression in filled state images [14], and we expect similar behavior for a N atom since it is so much smaller than the As atom it replaces. For the case of anion vacancies in deeper planes, the theory indicates that their appearance in STM images will be relatively small [13].…”

Cross-sectional imaging of sharp Si interlayers embedded in gallium arsenide

“…A few theoretical studies of Si-doped or otherwise defected GaAs (110) surfaces exist, 6,7,8,9,10,11 but to the best of our knowledge no attempts have been done so far to model extended self-compensating donor-acceptor configurations. The purpose of the present paper is to start filling this gap by providing the first theoretical study of XSTM images resulting from the cleavage of (001) Si interlayers embedded in GaAs.…”