New Insights on Atomic-Resolution Frequency-Modulation Kelvin-Probe Force-Microscopy Imaging of Semiconductors

Sascha Sadewasser, Pavel Jelinek, Chung-Kai Fang, Oscar Custance, Yusaku Yamada, Yoshiaki Sugimoto, Masayuki Abe, and Seizo Morita

Phys. Rev. Lett. 103, 266103 – Published 28 December 2009

Abstract

We present dynamic force-microscopy experiments and first-principles simulations that contribute to clarify the origin of atomic-scale contrast in Kelvin-probe force-microscopy (KPFM) images of semiconductor surfaces. By combining KPFM and bias-spectroscopy imaging with force and bias-distance spectroscopy, we show a significant drop of the local contact potential difference (LCPD) that correlates with the development of the tip-surface interatomic forces over distinct atomic positions. We suggest that variations of this drop in the LCPD over the different atomic sites are responsible for the atomic contrast in both KPFM and bias-spectroscopy imaging. Our simulations point towards a relation of this drop in the LCPD to variations of the surface local electronic structure due to a charge polarization induced by the tip-surface interatomic interaction.

Received 9 September 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.266103

Authors & Affiliations

Sascha Sadewasser¹, Pavel Jelinek², Chung-Kai Fang³, Oscar Custance^3,*, Yusaku Yamada⁴, Yoshiaki Sugimoto⁴, Masayuki Abe⁴, and Seizo Morita⁴

¹Helmholtz Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, Berlin, Germany
²Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague, Czech Rebublic
³National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki, Japan
⁴Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka, Japan

^*CUSTANCE.Oscar@nims.go.jp

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Vol. 103, Iss. 26 — 31 December 2009

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Images

Figure 1
(a) Topographic image of an area of the $Si (111) − (7 \times 7)$ surface with substitutional Pb adatoms (brighter protrusions) measured on a $64 \times 64$ pixel grid, and (b) simultaneously obtained bias-spectroscopy image (see text for details). (c) Topography and (d) simultaneously measured KPFM image at the same surface location shown in (a) on a $256 \times 256$ pixel grid. The $Δ f_{SP}$ for (a) and (c) were $- 9.4$ and $- 8.6 Hz$ , respectively; both were measured at $V_{s} = 180 mV$ . Bright (dark) contrast in (b) and (d) correspond to 66.8 mV ( $- 470 mV$ ) and 55.5 mV ( $- 342.7 mV$ ), respectively. KPFM and AFM parameters were $V_{ac} = 0.5 V$ , $f_{ac} = 1 kHz$ , free-oscillation cantilever fundamental resonance frequency ( $f_{0}$ ) 161 372.0 Hz, cantilever oscillation amplitude ( $A$ ) 93 Å , and cantilever stiffness $30 N / m$ .Reuse & Permissions
Figure 2
(a) $Δ f (V_{s}, Z)$ map measured over the Si corner adatom pointed in the inset of Fig. 3a. (b) Bias-spectroscopy curves extracted from (a) at several tip-surface separations with parabolic fits (gray lines) to extract the bias voltage that minimizes the tip-surface electrostatic interaction ( $V^{*}$ ). Acquisition parameters are the same as in Fig. 1.Reuse & Permissions
Figure 3
(a) $Δ f (Z)$ curves extracted from $Δ f (V_{s}, Z)$ maps measured over the atomic positions marked with arrows in the inset topography image, quantifying the $Δ f$ at minimum electrostatic interaction for each tip-surface separation [ $Δ f (V_{s} = V^{*}, Z)$ ]. The color code for arrows and plot lines is the same. (b) Short-range interaction force obtained from the $Δ f (Z)$ curves in (a). (c) Variation of $V^{*}$ with the tip-surface separation obtained from the $Δ f (V_{s}, Z)$ maps mentioned in (a). (d) Variation of $V_{LCPD}$ with the tip-surface separation obtained from individual $Δ f (Z)$ curves acquired over the same atomic positions using an active KPFM feedback to minimize the electrostatic interaction at each point of the curve. Acquisition parameters are the same as in Fig. 1.Reuse & Permissions
Figure 4
Calculated differential charge density distribution ( $e / Å^{3}$ ) in the real space [24], projected on a plane crossing the tip model foremost Si atom and a Si corner adatom at two relevant tip-surface separations: (a) $Z = 6 Å$ and (b) $Z = 3.5 Å$ . Calculated (c) short-range force, (d) change of induced surface dipole moment, and (e) variation of the local chemical potential upon approaching the tip model over a substitutional Pb and a Si corner adatom of the $Pb / Si (111) − (7 \times 7)$ surface with respect to the absence of tip interaction. Here, the local chemical potential is defined as the difference between the vacuum level and the Fermi level of the system [2].Reuse & Permissions

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