Tunneling spectroscopy on semiconductors with a low surface state density

Abstract

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.

Quantitative description of photoexcited scanning tunneling spectroscopy and its application to the GaAs(110) surface

“…The resulting total current is dominated by the photoinduced tunnel current and is more than one order of magnitude larger than the measured one. Note, the onset voltage for the negative branch is moved to a small positive voltage of 0.3 V. Electrons tunneling out of the sample already at positive voltages have been indeed experimentally observed previously for large laser irradiance [18,19].…”

“…Prins et al [18], Sommerhalter et al [19], and Vu et al [20] developed first approaches to the problem. Prins et al focused on pinned surfaces with a high density of surface gap states, but did not take into account tunneling into the conduction and out of the valence band.…”

Direct Probing of Transient Photocurrent Dynamics in p-WSe2by Time-Resolved Scanning Tunneling Microscopy

“…In addition to the current interest in device applications, for example, field-effect transistors, [13][14][15] layered materials can be used as a model system for the analysis of carrier dynamics because of their advantageous surfaces with minimal roughness, dangling bonds and defect states. Among the layered materials, the tunneling characteristics of photocurrent in the stationary state are well understood for WSe 2 , 16,17) which allows detailed analysis of the results obtained for transient properties. Figure 1 shows a typical STM image of WSe 2 .…”

“…These results are in good agreement with those reported in a previous paper. 16,17) No current is observed for a negative bias voltage under the dark condition, indicating that the photocurrent is directly detected as a signal via tunneling, making it suitable for SPPX-STM measurement and analysis.…”

Adsorption behavior and current–voltage characteristics of CdSe nanocrystals on hydrogen-passivated silicon

“…A similar model has previously successfully been used in Ref. 23 to describe STS data of a p-type WS 2 surface. Materials parameters for silicon are taken from Sze, 22 and we model the tip as a tungsten surface with an area of 1 nm 2 .…”