Scattered surface charge density: A tool for surface characterization

We demonstrate the use of nonlocal scanning tunneling spectroscopic measurements to characterize the local structure of adspecies in their states where they are significantly less perturbed by the probe, which is accomplished by mapping the amplitude and phase of the scattered surface charge density. As an example, we study single-H-atom adsorption on the $n$-type Si(100)-(4 × 2) surface, and demonstrate the existence of two different configurations that are distinguishable using the nonlocal approach and successfully corroborated by density functional theory.

Figure 1

Schematics of the Si(100)-(4 × 2) surface with adsorbed single-hydrogen atoms are presented in (a) where the majority (H${}_{\mathrm{B}}$) and minority (H${}_{\mathrm{T}}$) configurations found in the experiment are marked. The minority adspecies concentration, when present (see Sec. 2), was always more than an order of magnitude smaller than the majority hemihydride concentration. STM topographic lines [dotted lines in (a)] taken along the middle of the reacted dimer rows with 20 pA and $+$0.5 V are plotted in (b), with blue for the majority and with red for the minority configuration and are also marked with dotted lines in (a). The differential conductance recorded simultaneously with the topographic lines in (b) are presented in (c) with corresponding colors.

Figure 2

Topographic images of a single-hydrogen atom chemisorbed on the $n$-type Si(100)-$c$(4 × 2) surface. Filled-state (bottom) and empty-state (top) images are presented in (a) and (d), simulated with sample biases of $-0.5$ and $+$0.5 V, respectively. The corresponding experimental constant current (20 pA) images are shown in (b) and (c), recorded with sample biases of $+$0.5 V (top) and $-1.0$ V (bottom). Sketches of the Si dimer atoms are shown in parallel with the topographs, where big and small circles correspond to upper and lower Si dimer atoms, respectively. Bright symbols are used for the atoms dominating the images. Arrows indicate the dimer buckling adjacent to the reacted dimers. The hydrogen atom (green circles) and the dangling bond (red ellipses) are kept with the same orientation in all panels. On the bottom of the figure, large-area topographic images (20 pA) including the (c) majority and (b) minority (vertically flipped) adspecies are presented.

Figure 3

Spatial LDOS maps of the π* standing waves along the Si dimer row with single-hydrogen occupied dimer at $x$ $=$ 0. Shaded and nonshaded areas highlight the local and nonlocal approach discussed in the text. The map in (c) was constructed from LDOS spectra recovered[6] from experimental $dI$/$dV$ measurements taken on each point of the topographic image (top of the figure) along the dimer row ($x$ ordinate). The map in (b) was constructed as in (c). The same intensity (color) scale as in (b) and (c) was applied for the corresponding simulated LDOS maps in (a) and (d). All panels correspond to the configurations in the respective panels in Fig. 2.

Figure 4

Fitting parameters for the standing waves. The energy dispersion $E$($k$) and the phase-shift dependences Θ($k$) obtained by fitting the results in Fig. 3 are shown in (a) and (b), respectively. In (a), the simulated band dispersion for the bare surface is also presented (dashed line). The theoretical spectra have been shifted to align the bottom of the conduction band at 0 eV. Black, red, green, and blue lines are used for the results obtained from the LDOS maps in Figs. 3c, 3b, 3d, and 3a, respectively.