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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.

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.

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.