Fixing the Energy Scale in Scanning Tunneling Microscopy on Semiconductor Surfaces (original) (raw)
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Physical Review B, 2007
The electronic properties of shallow acceptors in p-doped GaAs{110} are investigated with scanning tunneling microscopy at low temperature. Shallow acceptors are known to exhibit distinct triangular contrasts in Scanning tunneling microscopy images for certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to identify their energetic origin and behavior. A crucial parameter -the STM tip's work function -is determined experimentally. The voltage dependent potential configuration and band bending situation is derived. Ways to validate the calculations with the experiment are discussed. Differential conductivity maps reveal that the triangular contrasts are only observed with a depletion layer present under the STM tip. The tunnel process leading to the anisotropic contrasts calls for electrons to tunnel through vacuum gap and a finite region in the semiconductor.
Low-temperature scanning tunneling spectroscopy
Journal of Electron Spectroscopy and Related Phenomena, 2000
Low-temperature scanning tunneling spectroscopy measurements on semiconductor surface are described. We consider both surface which do not possess surface states within the bulk bandgap, such as GaAs , and surfaces which do have states within the gap, such as Ge and Ge(111)c(2×8). Band bending in the semiconductor due to the electric field in the vacuum penetrating the semiconductor is found to be a substantial effect in the former case. Transport limitations in the semiconductor give rise to additional voltage drops, which can be observed by making measurements over a wide range of tunnel current magnitudes.
Physical Review B
Ab initio pseudopotential total energy techniques are used to investigate the tip-surface interaction in atomic force microscopy on a GaAs͑110͒ surface with a Si tip. Our simulations show significant surface relaxation effects in the near contact region, which lead to a complicated behavior of the total energy and force curves. In particular, the tip-induced displacement of the Ga atoms can exceed 1 Å even in the attractive force region, leading to hysteresis in the energy and force curves. These large tip-induced relaxations of the surface Ga atoms provide a natural explanation to the simultaneous imaging of both anions and cations in recent nearcontact scanning tunneling microscopy experiments on this surface. We show that, for tip-surface distances where the surface topography remains unchanged and for a charge neutral Si tip, only the anion sublattice can be resolved in noncontact atomic force microscopy. Close to contact, our simulations prove that, even for atomically sharp tips ͑1͒ there is a significant contribution to the total interaction from tip atoms different from the apex atom; ͑2͒ large lateral ͑bonding͒ forces on the tip apex may develop and change the tip structure well before significant normal repulsive forces appear. ͓S0163-1829͑99͒09939-7͔
Applied Surface Science, 1998
We report bias voltage-dependent images of scanning tunneling microscopy taken on a GaAs 110 surface with small Ag Ž. clusters. The direction of the observed atom rows changes at certain negative and positive sample bias voltages V. Such s Ž. changes are attributed to the different atoms Ga or As in the case of V-0 and to the different surface states of Ga in the s case of V) 0. The images also show a change in contrast with the V. All of these results are explained by tip-induced and s s surface charge-induced band bendings in addition to the fundamental surface states.
Dual-probe scanning tunneling microscope for study of nanoscale metal-semiconductor interfaces
Review of Scientific Instruments, 2005
Using a dual-probe scanning tunneling microscope, we have performed three-terminal ballistic electron emission spectroscopy on Au/ GaAs͑100͒ by contacting the patterned metallic thin film with one tip and injecting ballistic electrons with another tip. The collector current spectra agree with a Monte-Carlo simulation based on modified planar tunneling theory. Our results suggest that it is possible to study nanoscale metal-semiconductor interfaces without the requirement of an externally-contacted continuous metal thin film.
Internal image potential in semiconductors: Effect on scanning tunneling microscopy
Physical Review B, 1993
The tunneling of electrons from a semiconductor surface to a metal tip, across a vacuum gap, is influenced by two image interactions: an attractive image potential in the vacuum region, which lowers the apparent tunneling barrier, and a repulsive image potential in the semiconductor interior, which raises it for conduction-band electrons. We report on detailed calculations of tunneling currents and apparent barrier heights for a model metal-vacuum-semiconductor junction which utilize semiclassical dielectric functions to compute the image potential in all three regions. The effect of image forces is found to be small compared to that of either the vacuum barrier or tip-induced band bending. In particular, the image-induced barrier in the semiconductor has only a minor influence on either the apparent barrier height or the shape of current-voltage characteristics, both of which are routinely measured in scanning-tunneling-microscopy experiments. This finding is explained by a qualitative WKB analysis and several simple arguments.
Physical Review B, 2002
Atomically resolved, voltage-dependent scanning tunneling microscopy ͑STM͒ images of GaAs͑110͒ are compared to the results of a one-dimensional model used to calculate the amount of tip-induced band bending for a tunneling junction between a metal and a semiconductor. The voltage-dependent changes in the morphology of the atomic lattice are caused by the four surface states of the GaAs͑110͒ surface contributing in varying relative amounts to the total tunneling current. Tip-induced band bending determines which of these states contributes to the total tunneling current at a given bias voltage, and thus has a profound influence on the voltage-dependent STM-images. It is shown that certain voltage regions exist, for which none of the surface states present at the GaAs͑110͒ surface can contribute to the tunneling current. For these voltages, tunneling occurs between the tip and bulk states of the sample through a surface depletion layer several nm wide. Nevertheless, we observe atomic, surface like corrugation for these circumstances.
Tunneling spectroscopy on semiconductors with a low surface state density
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1997
A detailed study of tunneling spectroscopy concerning semiconductors with a low surface state density is presented. For this purpose, I -V curves under dark conditions and under illumination were measured on the ͑0001͒ van der Waals surface of a p-type WS 2 single crystal, which is known to be free of intrinsic surface states. The measurements are interpreted by an analytical one-dimensional metal-insulator-semiconductor model, which shows that the presence of the finite tunneling current has to be considered in the calculation of the tip-induced bandbending. Rectification of the dark I -V curves is explained by the absence of an inversion layer at the semiconductor surface. In contrast, the I -V curves measured for different light intensities and tip-sample separations indicate the existence of an optically induced inversion layer. Since no surface recombination needs to be considered to model these spectra, we conclude that bulk recombination, diffusion and direct tunneling of photogenerated minority charge carriers are the dominant processes for semiconductors with a low density of surface states. In contrast to the standard interpretation of tunneling spectroscopy, which can be applied to semiconductors with a high surface state density, our results clearly show that in this case the normalized differential conductivity (dI/dU)/(I/U) cannot be used to determine the energetic distribution of the local surface state density.
Influence of surface states on tunneling spectra of n-type GaAs (110) surfaces
2009
We show that surface states within the conduction band of n-type GaAs͑110͒ surfaces play an important role in reducing the tunneling current out of an accumulation layer that forms due to an applied potential from a nearby probe tip. Numerical computation of the tunneling current combined with an electrostatic potential computation of the tip-induced band bending ͑TIBB͒ reveals that occupation of the surface states limits the TIBB, thus leading to the limitation of the accumulation. As a result, the tunneling current out of the accumulation layer is strongly suppressed, which is in quantitative agreement with the experiment.
Scanning tunneling microscopy of semiconductor surfaces
Surface Science Reports, 1996
This review describes advances in understanding the structural, electronic, and chemical properties of clean low-index semiconductor surfaces during the first decade following the advent of the scanning tunneling microscope (STM). The principles of STM are discussed together with the instrumentation required to perform STM measurements on semiconductor surfaces in ultrahigh vacuum. A comprehensive review of the structures of the clean, low-index surfaces of elemental and compound semiconductors is presented. These structures are discussed using the general physical principles that determine them.