We show how cross-sectional scanning tunneling microscopy may be used to reconstruct the Sb segregation profiles in GaInSb /InAs strained-layer superlattices. These profiles are accurately described by a one-dimensional model parametrizing the spatial evolution of an Sb seed at the InAs-on-GaInSb interface in terms of two-anion-layer exchange. We argue that the segregation seed, which decreases from 2 / 3 to 1 / 2 monolayer when growth conditions are made less anion rich, has its origin in the Sb-bilayer reconstruction maintained during GaInSb epitaxy.

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...

We describe how cross-sectional scanning tunneling microscopy (STM) may be used to image the interfacial bonding across the nearly lattice-matched, non-common-atom GaSb/InAs heterojunction with atomic-scale precision. The method, which takes advantage of the length difference between interfacial and bulk bonds, appears equally applicable to AlSb/InAs and suggests how one might recover the complete structure of either heterojunction from atomic-resolution STM data.

We consider the influence of tip-induced band bending on the apparent barrier height deduced from scanning tunneling microscopy (STM) experiments at unpinned semiconductor surfaces.Any voltage applied to a probe tip appears partly in the vacuum gap as an electric field at the semiconductor surface and partly in the semiconductor interior as band bending. The fraction appearing in each region is a function of gap spacing so that modulation of the tip-sample separation inevitably modulates the induced surface potential in the semiconductor.At finite temperature, the height and shape of this barrier determine the probability that an electron will reach the semiconductor surface where it can subsequently tunnel through the vacuum gap. Since the surface potential decreases with increasing tip-sample separation, STM measurements of the tunneling barrier at unpinned semiconductor surfaces will yield unusually low values. Detailed numerical calculations of the effect for passivated n type-Si (111) show it to be of observable magnitude.This mechanism may be distinguished from other recently proposed barrier-lowering mechanisms in that it is doping dependent, potentially long range, and possesses a unique voltage signature.

Atmospheric composition studies on weather and climate timescales require flexible, scalable models. The ICOsahedral Nonhydrostatic model with Aerosols and Reactive Trace gases (ICON-ART) provides such an environment. Here, we introduce the most up-to-date version of the flexible tracer framework for ICON-ART and explain its application in one numerical weather forecast and one climate related case study. We demonstrate the implementation of idealised tracers and chemistry tendencies of different complexity using the ART infrastructure. Using different ICON physics configurations for weather and climate with ART, we perform integrations on different timescales, illustrating the model's performance. First, we present a hindcast experiment for the 2002 ozone hole split with two different ozone chemistry schemes using the numerical weather prediction physics configuration. We compare the hindcast with observations and discuss the confinement of the vortex split using an idealised tracer diagnostic. Secondly, we study AMIPtype integrations using a simplified chemistry scheme in conjunction with the climate physics configuration. We use two different simulations: the interactive simulation, where modelled ozone is coupled back to the radiation scheme, and the non-interactive simulation that uses a default background climatology of ozone. Additionally, we introduce changes of water vapour by methane oxidation for the interactive sim-ulation. We discuss the impact of stratospheric ozone and water vapour variations in the interactive and non-interactive integrations on the water vapour tape recorder, as a measure of tropical upwelling changes. Additionally we explain the seasonal evolution and latitudinal distribution of the age of air. The age of air is a measure of the strength of the meridional overturning circulation with young air in the tropical upwelling region and older air in polar winter downwelling regions. We conclude that our flexible tracer framework allows for tailor-made configurations of ICON-ART in weather and climate applications that are easy to configure and run well.