New Insights on Atomic-Resolution Frequency-Modulation Kelvin-Probe Force-Microscopy Imaging of Semiconductors Sascha Sadewasser, 1 Pavel Jelinek, 2 Chung-Kai Fang, 3 Oscar Custance, 3, * Yusaku Yamada, 4 Yoshiaki Sugimoto, 4 Masayuki Abe, 4 and Seizo Morita 4 1 Helmholtz Zentrum Berlin fu¨r Materialien und Energie, Hahn-Meitner-Platz 1, Berlin, Germany 2 Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague, Czech Rebublic 3 National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki, Japan 4 Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka, Japan (Received 9 September 2009; published 28 December 2009) 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 semi- conductor 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. DOI: 10.1103/PhysRevLett.103.266103 PACS numbers: 68.35.bg, 07.05.Tp, 68.37.Ps The work function is a property of solids with funda- mental relevance, among others, in photovoltaic energy conversion technology [1]. This magnitude describes the energy required to extract an electron from a material and bring it to infinity [2,3], and in the case of semiconductors it is generally regarded as the energy difference between the Fermi level and the local vacuum level [1]. The work function can be measured by thermionic emission, photo- emission spectroscopy or Kelvin-probe force microscopy (KPFM) [4]. The latter technique measures variations of the contact potential difference (CPD) between a surface and the probe of an atomic force microscope (AFM), which originates from a disparity in their respective work func- tions [5]. KPFM is widely applied to characterize local variations of the work function on a wide variety of mate- rial surfaces [6–11] down to the nanometer scale [12], and very recently also to characterize the charge state of indi- vidual atoms [13]. Although the work function is considered a macroscopic concept, founded on the crystalline arrangement, the elec- tronic properties, local structure, and composition of the surface, several authors have reported KPFM images showing variations of the CPD at atomic scale on semi- conductors [14–17] and insulators [18]. The origin of this atomic contrast in the local contact potential difference (LCPD) [19] is still not fully understood, existing a strong controversy between several hypotheses. In the case of ionic crystals, an origin based on short-range electrostatic forces due to the variations of the Madelung surface po- tential has been suggested [18], yet an induced polarization of the ions at the tip-surface interface due to the bias voltage modulation applied in KPFM may be an alternative contrast mechanism [20]. In the case of semiconductor surfaces, some authors attribute atomic resolution in KPFM images to possible artifacts [16,21]. In this Letter, we provide new evidence on the phenome- nology of atomic-scale KPFM imaging on semiconductor surfaces by means of a thoughtful set of experiments performed over the same surface area with identical tip- apex termination. By combining this rich experimental information with first-principles calculations, we propose an alternative mechanism for atomic-scale contrast in LCPD images on semiconductors. The experiments were performed with an ultrahigh- vacuum low-temperature AFM operated under the fre- quency modulation detection method [22], keeping the cantilever oscillation amplitude constant, and using a fully digital AFM controller [23]. The measurements were car- ried out at a tip and sample temperature of 77 K on a Sið111Þ-ð7  7Þ surface with a low concentration of substi- tutional Pb atoms. For details about tip and sample prepa- ration, see [24]. For topographic imaging, the electrostatic long-range interaction was minimized by compensating the CPD at a tip-surface separation of 5 nm [25]. Spec- troscopy acquisition and tip-surface short-range (SR) force quantification [26,27] is described in [24]. Density func- tional theory (DFT) simulations of vertical scans of a well- tested Si tip model [28] over the atoms of a Pb=Sið111Þ- ð7  7Þ surface were performed using the FIREBALL code [29]. The surface was modeled by a slab of 7 Si layers with H saturating the deeper Si layer. At each step of the simulation, the atoms were allowed to relax to minimize the total energy of the system with convergence criteria in energy and force of 10 6 eV and 0:05 eV=  A, respectively. The surface Brillouin zone was sampled with the  k point. For more details about the simulations, see [24]. PRL 103, 266103 (2009) PHYSICAL REVIEW LETTERS week ending 31 DECEMBER 2009 0031-9007= 09=103(26)=266103(4) 266103-1 Ó 2009 The American Physical Society