We present a theoretical analysis for a system composed of two mesoscopic tunnel junctions coupled in series. We show that the current-voltage characteristic for this system can be obtained analytically. The usefulness of the model is demonstrated through the At of experimental data acquired with a cryogenic (4.2 K) scanning tunneling microscope. A simple extension of the model predicts additional structure in the system characteristics when discrete middle electrode states are present.Recently, considerable interest has been directed toward tunnel-junction systems where the discreteness of

The I-V characteristics of two serially coupled small tunnel junctions (about 10~~1 8 -10~1 9 F capacitances) are measured at 4 K. The junctions are formed using a scanning tunneling microscope to probe a metal droplet deposited on an oxidized metal substrate. Sharply defined Coulomb steps due to singleelectron dynamics, oxide polarization, and nonlinear (voltage dependent) tunneling rates are observed. The results show very good quantitative agreement with theoretical calculations based on the semiclassical picture.

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...