We demonstrate a direct atomically resolved visualization and quantification of the impact of inhomogeneities in the dopant distribution on the nanoscale potential fluctuations in a two-dimensional semiconducting ͱ 3 ϫ ͱ 3 Ga overlayer on Si͑111͒ using scanning tunneling microscopy. By a quantitative analysis, two regimes of the potential at nanometer scale are found, which arise from the local distribution of charge carriers in the bands and from electron-electron interactions.With the ongoing push for smaller semiconductor devices, the feature size implemented in current commercial manufacturing processes is reaching dimensions, where ultimately the device operation depends on a single electron and traditional device concepts break down. In such conditions, statistical fluctuations in the dopant distribution become particularly important. 1,2 Inhomogeneities in the dopant distribution 3 may cause nanoscale and atomic-scale fluctuations in the potential, 4 which in turn would lead to a statistical lowering of the threshold voltages. 1 However, in other cases, no effect of the spatial positions of charged defects on the local potential was found. 5 This inconsistency is essentially due to the lack of solid experimentally proven facts about the effect of nanoscale dopant inhomogeneities on the local nanoscale and atomic-scale potential. It is still unclear which physical mechanisms determine the nano-and atomicscale potential and which quantitative models can be used for accurate simulations of the potential in spatially ͑and/or dimensionally͒ reduced semiconductor structures. Therefore, we illustrate here a direct atomically resolved visualization and quantification how dopant atoms and their statistical distribution affect the local nanoscale and atomicscale potential using scanning tunneling microscopy ͑STM͒. We identify two different origins of the nanoscale potential and derive a quantitative physical understanding of dopantinduced potential fluctuations at nanometer and subnanometer scales. As a model system, we utilize a two-dimensional ͑2D͒ ͱ 3 ϫ ͱ 3 Ga on Si͑111͒ structure, where we can tune the dopant concentration over a wide range by suitable adjustment of the deposition parameters. Figure 1͑a͒ shows an atomically resolved constant-current STM image of our model system: each maximum in the empty state STM image corresponds to one empty dangling bond above a Ga adatom. The weaker maxima ͑marked D͒, whose concentration decreases with increasing Ga deposition, arise from Si atoms located on ͱ 3 ϫ ͱ 3 Ga sites. 6 These Si atoms act as donors and provide the free electrons. 7 The resulting positive charges of the Si dopants induce a redistribution of the free charge carriers and thereby a potential change, which gives rise to the surrounding bright contrast on which the atomic corrugation is superimposed. 8 The local potential change also shows up in the tunneling spectra: the valence ͑E V ͒ and conduction band ͑CB͒ edges ͑E C ͒ shift Ϸ0.15 eV to higher energies with increasing spatial separation from the d...
In a classical view, abrupt dopant profiles in semiconductors tend to be smoothed out by diffusion due to concentration gradients and repulsive screened Coulomb interactions between the charged dopants. We demonstrate, however, using cross-sectional scanning tunneling microscopy and secondary ion mass spectroscopy, that charged Be dopant atoms in GaAs p-n superlattices spontaneously accumulate and form two-dimensional dopant layers. These are stabilized by reduced repulsive screened Coulomb interactions between the charged dopants arising from the two-dimensional quantum mechanical confinement of charge carriers.
Using atomically and momentum resolved scanning tunneling microscopy and spectroscopy, we demonstrate that a two-dimensional (2D) √3 × √3 semiconducting Ga-Si single atomic alloy layer exhibits an electronic structure with atomic localization and which is different at the Si and Ga atom sites. No indication of an interaction or an electronic intermixing and formation of a new alloy band structure is present, as if no alloying happened. The electronic localization is traced back to the lack of intra alloy bonds due to the 2D atomically confined structure of the alloy overlayer.
The effect of counterdoping on the Be dopant distribution in delta (d)-doped layers embedded in Si-doped and intrinsic GaAs is investigated by cross-sectional scanning tunneling microscopy. d-doped layers in intrinsic GaAs exhibit a large spreading, whereas those surrounded by Si-doped GaAs remain spatially localized. The different spreading is explained by the Fermi-level pinning at the growth surface, which leads to an increased Ga vacancies concentration with increasing Si counterdoping. The Ga vacancies act as sinks for the diffusing Be dopant atoms, hence retarding the spreading. V
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