We present quantitative measurements of the work function of semiconductor and metal surfaces prepared in ultrahigh vacuum (UHV) using a combination of UHV noncontact atomic force microscopy and Kelvin probe force microscopy. High energetic and lateral resolution is achieved by using the second resonance frequency of the cantilever to measure the electrostatic forces, while the first resonance frequency is used to simultaneously obtain topographic images by the frequency modulation technique. Spatially resolved work-function measurements reveal a reduced work function in the vicinity of steps on highly oriented pyrolytic graphite. On the GaAs(110) surface it could be demonstrated that defect states in the forbidden band gap cause a local pinning of the Fermi level along monolayer steps. On p-WSe2(0001) work-function variations due to the Coulomb potential of single dopant sites were resolved.
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.
Abstract. The van-der-Waals surfaces (0001) of the layered structure semiconductors WS 2 and WSe 2 are known to be free of intrinsic surface states. Therefore, they provide an ideal system for investigations of the influence of individual dopants on the local electronic properties, which can be measured by scanning tunneling microscopy (STM). Individual dopant sites were resolved as topographic depressions superimposed on the atomically resolved lattice. The apparent depth of these depressions showed a discrete statistical distribution and was attributed to the spatial depth of the dopant site. Using an STM-induced electrochemical process, we could locally expose the first and second sub-surface layer to correlate the previously recorded topographic contrast to the location of buried dopants. To our knowledge this is the first direct proof of the capability of STM to detect individual sub-surface dopants. An interpretation of the contrast mechanism is given in terms of tip-induced band-bending effects and current transport mechanisms involving minority charge carrier injection and majority charge carrier extraction.
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