Toshihiro Uchiyama

Theory of scanning tunneling microscopy and spectroscopy of the hydrogen-terminated Si(001) surface

Abstract The hydrogen(H)-terminated Si (001)-2 × 1 : H and 3 × 1 : H surfaces are theoretically investigated. For this purpose, the electronic states are calculated in the density functional approach with the DV (discrete variation)-Xα-LCAO method. We also simulate the scanning tunneling microscopy (STM) images and the scanning tunneling spectroscopy (STS) spectra in a first-principles approach. Our results reproduce well the observed features of the images, especially those of the monohydride units appearing on the surfaces. It is found that the ball-like spots forming a bean shape indicate the H atoms of the units. In this case, the atomic geometry effect predominates over the tunneling current of the STM on the surfaces. In the STS spectra, however, we cannot find any characteristic structure due to the H-Si anti-bonding state. This result seems to contradict the STS observation on the initial hydrogenated Si(001) surface. Finally, we discuss on a possible origin of this discrepancy.

Step-induced state of a Si(001) reconstructed surface

Abstract We calculate the electronic structure of a Si(001) stepped surface, applying the DV-Xα-LCAO method in the density functional approach. For the surface, a slab model which consists of 4 Si-layers, including two different types of single-layer steps is applied. The dimers of the outermost Si atoms are assumed to take an asymmetric geometry. A localized state is found at the dimer on the lower terrace just below the S A step. We also simulate scanning tunneling microscopy (STM) images and scanning tunneling spectroscopy (STS) spectra in the first-principles approach. In the STS spectra, the localized state at the S A step gives a sharp peak, such as observed in the experiment by Iwawaki et al. [J. Vac. Sci. Technol. B 9 (1991) 711]. However, the up- and down-atoms of the dimer at the step edge of the upper terrace do not show any significant difference in the spectra. In the STM images, the dimer seems buckled no more than the geometry. Furthermore, to clarify the origin of the localized state, we analyze a phenomenological tight-binding model for the π-band of dimer rows. One of the π-states localizes to the dimer site just below the S A step, due to a potential induced at the step edge.

Theory of scanning tunneling microscopy and spectroscopy on hydrogen‐adsorbed Si(100) surface

The hydrogen (H) termination of the 2×1 and 3×1 structure and the initial stage of H adsorption on the Si(100) surface are theoretically investigated. For this study, the electronic states induced by the H atoms are calculated in the first‐principles local density functional method. The scanning tunneling microscopy (STM) images and the scanning tunneling spectroscopy (STS) spectra are also simulated. The results clarify what the STM depicts on the H‐adsorbed Si(100) surfaces. On the termination, two ball‐like spots each corresponding to the H atom form a beam shape. At the initial adsorption, the bright spot observed in the images is assigned to the remaining Si atom of the reacted dimers. However, the observed STS spectra of both the surfaces cannot be reproduced. The physical reasoning to amend this discrepancy in the spectra is discussed.

Novel features of surface electronic structure revealed by the theoretical simulation of scanning tunneling microscopy/spectroscopy

Theoretical simulation based on the first-principles local density functional approximation (LDA) calculation reveals the mechanism of STM as well as significant effects of microscopic states of the tip on experimental data. Moreover, various exotic properties of clean or chemisorbed surfaces can be explained with the theory of STM/STS. How the tip influences images of STM is discussed by case studies with a numerical simulation for Si(100) dimer rows and step structures and Si(111)3 × 3Ag and Ag(110)-O chemisorption surfaces.

Atomic and electronic structure of the Si(001) surface induced by hydrogen-adsorption

Abstract The initial stage of hydrogen (H) adsorption on the Si(001) surface is theoretically investigated to clarify the atomic and electronic structure induced by the adsorption. For this purpose, the electronic states are calculated in the density functional approach with the DV(discretized variation)-Xα-LCAO method. We also simulate the scanning tunneling microscopy (STM) image and the scanning tunneling spectroscopy (STS) spectrum in the first-principles approach. Our results of the STM image do not support the asymmetric dimer structure of the substrate with a H atom bonded to the upper Si atom. They conclude that the bright ball-like spot in the observed images comes from the free dangling bond induced on the remaining Si atom of the reacted dimer. However, the single particle picture cannot reproduce the observed features of the STS spectrum at the spot. We discuss that all the features can be well explained by the Coulomb blockade effect due to the electron correlation in the dangling bond state.