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If a current of electrons flows through a normal conductor (in contrast to a superconductor), it is impeded by local scattering at defects as well as phonon scattering. Both effects contribute to the voltage drop observed for a macroscopic complex system as described by Ohm's law. Although this concept is well established, it has not yet been measured around individual defects on the atomic scale. We have measured the voltage drop at a monatomic step in real space by restricting the current to a surface layer. For the Si(111)-( [see text]3 x [see text]3)-Ag surface a monotonous transition with a width below 1 nm was found. A numerical analysis of the data maps the current flow through the complex network and the interplay between defect-free terraces and monatomic steps.

Scanning tunneling spectroscopy (STS) at 8 K is used to study single shallow acceptors embedded near {110}-surfaces in gallium arsenide (GaAs). At appropriate bias voltages the circularly symmetric contrast normally observed for charged defects evolves into a pronounced triangular shaped protrusion. Comparing dopants at different depths under the surface, we find a linear shift of the associated conductivity maximum along directions. Comparative studies of Carbon and Zinc acceptors in a modulation-doped heterostructure reveal that both dopants act similarly. The experimental findings suggest that the highly anisotropic features induced by acceptors resemble a bulk property of the GaAs crystal prominently demonstrating its Zincblende symmetry.

We investigate low temperature grown, abrupt, epitaxial, nonintermixed, defect-free n-type and p-type Fe/GaAs(110) interfaces by cross-sectional scanning tunneling microscopy and spectroscopy with atomic resolution. The probed local density of states shows that a model of the ideal metal-semiconductor interface requires a combination of metal-induced gap states and bond polarization at the interface which is nicely corroborated by density functional calculations. A three-dimensional finite element model of the space charge region yields a precise value for the Schottky barrier height.

A combined study of the optical and structural properties of $\mathrm{Al}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum wells is presented. Microphotoluminescence experiments, magnetomicrophotoluminescence, and atomically resolved cross-sectional scanning tunneling microscopy were performed on the same quantum well sample. Constant-current topographs with aluminum and/or gallium sensitivity are used to directly extract disorder potentials. Using these potentials, exciton absorption spectra, microphotoluminescence spectra, and diamagnetic shifts of individual exciton states are calculated in an envelope function approximation. Very good agreement between the theoretical and experimental results is found.

Tunneling transport through the depletion layer under a GaAs ${110}$ surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.

We present a cross‐sectional STM study of delta‐doped GaAs on the nanoscale. GaAs (001) layers grown by MBE with a series of embedded carbon delta‐layers having nominal sheet densities in the range 3×1011/cm2 to 1×1014/cm2 were investigated. The dopant concentrations were checked with SIMS. The carbon concentration profiles in growth direction were determined, and found to be atomically sharp and symmetric up to the highest concentrations, with a FWHM of 0.85nm. This shows that even at the elevated growth temperature of 860K and up to 1×1014/cm2 density neither segregation nor diffusion plays a significant role in carbon delta‐doping of GaAs. The observed residual spreading of the profile is attributed to the generic surface roughness during epitaxial growth. In a second step we analyzed in detail the spectra in the delta‐layer regions. In the vicinity of near surface acceptors we found a nonvanishing conductivity throughout the band gap energy range. This is markedly different from the discrete LDOS peak found in the band gap for isolated acceptors in bulk samples, and it indicates metallic conductivity. Inhomogeneities of the metallic conductance are found to be strongly correlated with the arrangement of dopant atoms in the delta‐layer.

Gold contacts on $n$-type $\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}(110)$ have been investigated using scanning tunneling microscopy and spectroscopy in cross-sectional configuration. In spatially resolved current voltage spectroscopy the Schottky barrier potential is visible. We find signatures of delocalized gap states at the interface decaying into the semiconductor and observe a defect density at the interface below $3\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$. Both findings support that the Fermi level pinning at the $\mathrm{A}\mathrm{u}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}(110)$ interface is dominated by metal-induced gap states.

Gold contacts on n-type GaAs(110) have been investigated using scanning tunneling microscopy and spectroscopy in cross-sectional configuration. In spatially resolved current voltage spectroscopy the Schottky barrier potential is visible. We find signatures of delocalized gap states at the interface decaying into the semiconductor and observe a defect density at the interface below 3 x 10(13) cm(-2). Both findings support that the Fermi level pinning at the Au/GaAs(110) interface is dominated by metal-induced gap states.

Fe films of up to 10 ML thickness were grown on cleaved GaAs(110) in a two-step process that associates low-temperature deposition at 130K with a subsequent annealing to room temperature. Low-energy electron diffraction, scanning tunneling microscopy, and in situ magneto-optical Kerr effect were combined to study these films. The observed magnetic and structural properties are distinctly different from the characteristics of conventionally grown Fe films on GaAs. We found no indication of interface compound formation. The applied two-step growth process is a promising technique to further increase spin-injection and detection efficiencies of Fe∕GaAs hybrid structures.

We have investigated Au∕GaAs(110) Schottky contacts with scanning tunneling spectroscopy in cross-sectional configuration. The tunneling spectra show that both band edges are shifted and modified by the electrostatic potential of the space charge layer. The shifts were extracted and compared with simulations of the tunneling current including a local potential in the semiconductor. The resulting potential landscape is mapped on a near-atomic scale in the plane perpendicular to the metal-semiconductor interface.

Delta doped carbon acceptors in molecular-beam-epitaxy-grown GaAs were investigated on the atomic scale with scanning-tunneling microscopy in cross-sectional configuration. Monolayer-sharp spatial distributions were found at the intended positions of the delta layers in a wide range of dopant concentrations from 3×1012∕cm2 up to 1×1014∕cm2. The carbon concentrations were checked independently with secondary-ion-mass spectroscopy. All distributions had full widths at half maximum around 0.85nm and were symmetric up to the highest concentration investigated. These results point out that even at the elevated growth temperature of 590°C, and for very high dopant concentrations, neither segregation nor diffusion plays an important role in delta doping of GaAs with carbon acceptors. The observed small spreading of the dopant distribution is attributed to the generic surface roughness during growth.

Coordinators of Illinois supported employment programs were surveyed for information regarding the demographics and reasons for job turnover of employment specialists. In addition, the employment specialist position was compared with that of direct service personnel in local rehabilitation agencies in terms of salary, benefits, and turnover. Survey results showed that although 58% of the programs responding required employment specialist applicants to hold a bachelor's degree, only 34% of those employment specialists actually hired held a bachelor's degree in a related field and an additional 10% held a degree in an unrelated field. Approximately two individuals were hired for every available employment specialist position in the Illinois supported employment program, with salary identified as the primary reason for the turnover. Although the nature of the direct service role fulfilled by the employment specialist in supported employment differs greatly from that of traditional direct service personnel within the local rehabilitation agency (e.g., workshop floor supervisors, day activity trainers), 55% of the programs responding stated that the salary levels for the two groups were comparable.