Kohei Yamasue

Control of microcantilevers in dynamic force microscopy using time delayed feedback

It has been recently shown that microcantilever sensors in dynamic force microscopes possibly exhibit chaotic oscillations due to the nonlinear tip-sample interaction force. In this article, we propose elimination of the chaotic oscillations using the time delayed feedback control method, which has an ability to stabilize unstable periodic orbits embedded in chaotic attractors. An extended operating range of the microscopes is numerically estimated by stability analysis of the target periodic oscillation. We also discuss an improved transient response of oscillation, which allows us to accelerate the scanning rate of the microscopes without reducing their force sensitivity.

Lateral resolution improvement in scanning nonlinear dielectric microscopy by measuring super-higher-order nonlinear dielectric constants

Scanning nonlinear dielectric microscopy (SNDM) can be used to visualize polarization distributions in ferroelectric materials and dopant profiles in semiconductor devices. Without using a special sharp tip, we achieved an improved lateral resolution in SNDM through the measurement of super-higher-order nonlinearity up to the fourth order. We observed a multidomain single crystal congruent LiTaO3 (CLT) sample, and a cross section of a metal-oxide-semiconductor (MOS) field-effect-transistor (FET). The imaged domain boundaries of the CLT were narrower in the super-higher-order images than in the conventional image. Compared to the conventional method, the super-higher-order method resolved the more detailed structure of the MOSFET.

Scanning nonlinear dielectric potentiometry

Measuring spontaneous polarization and permanent dipoles on surfaces and interfaces on the nanoscale is difficult because the induced electrostatic fields and potentials are often influenced by other phenomena such as the existence of monopole fixed charges, screening charges, and contact potential differences. A method based on tip-sample capacitance detection and bias feedback is proposed which is only sensitive to polarization- or dipole-induced potentials, unlike Kelvin probe force microscopy. The feasibility of this method was demonstrated by simultaneously measuring topography and polarization-induced potentials on a reconstructed Si(111)-(7 × 7) surface with atomic resolution.

Atomic dipole moment distribution on a hydrogen-adsorbed Si(111)-(7 × 7) surface observed by noncontact scanning nonlinear dielectric microscopy

Noncontact scanning nonlinear dielectric microscopy (NC-SNDM) can atomically resolve the polarization distribution on material surfaces. We observed a hydrogen-adsorbed Si(111)–(7 × 7) surface using NC-SNDM and found that hydrogen-adsorbed Si adatoms had lower dipole moments than bare Si adatoms. We discuss the charge distribution around a hydrogen-adsorbed Si adatom in terms of its geometry and the electronegativities of hydrogen and silicon. Our model suggests that the charge distribution around a hydrogen-adsorbed adatom is nearly symmetric and it qualitatively explains the experimental results. We thus conclude that the hydrogen-adsorbed Si adatoms are both electrically and chemically passivated.

Improved study of electric dipoles on the Si(100)-2 × 1 surface by non-contact scanning nonlinear dielectric microscopy

We studied a Si(100)-2 × 1 surface by non-contact scanning nonlinear dielectric microscopy (NC-SNDM). Simultaneously taken images of the topography and electric dipole moment distribution show that negative electric dipole moments are locally formed on individual dimers on the surface. In addition, we obtained the dc bias voltage dependence of the elocal(3) signal on a specific dimer by using an atom-tracking technique with NC-SNDM. We observed that the electric dipole induced a surface potential of around −250 mV on the dimer.

Atomic-dipole-moment induced local surface potential on Si(111)-(7 × 7) surface studied by non-contact scanning nonlinear dielectric microscopy

We have performed the site-specific, quantitative measurement of a local surface potential induced by atomic dipoles on a Si(111)-(7 × 7) surface by non-contact scanning nonlinear dielectric microscopy (NC-SNDM) combined with an atom-tracking technique. The measured potentials were quantitatively consistent with those estimated by a simultaneous measurement of the tunneling current, which validates a previously proposed hypothetical mechanism that explains the unexpected resemblance between the dipole and time-averaged tunneling current images in NC-SNDM imaging. The results show that an asymmetry arising in the current-voltage characteristics within the tunneling regime is governed by the local surface potential induced by atomic dipoles.

Simultaneous measurement of tunneling current and atomic dipole moment on Si(111)-(7 × 7) surface by noncontact scanning nonlinear dielectric microscopy

Non-contact scanning nonlinear dielectric microscopy (NC-SNDM) can resolve the topography and dipole moment distribution of a Si(111)-(7 × 7) surface on an atomic level. We discuss the origin of the atomic contrast in dipole moment images based on simultaneously acquired time-averaged tunneling current images. The dipole moment images are found to have the same characteristics as the simultaneously acquired current images. This similarity between these two images does not originate from circuit crosstalk since it is suppressed by our circuit design that decouples the tip-sample capacitance and the tunneling current. Constant-height images also indicate that the atomic contrast is not artificially caused by feedback crosstalk. These results suggest that the atomic contrast is due to the variation in the tip-sample capacitance caused by modulation of atomic dipole moments by the local density of states of the surface.

Observation of Polarization Distribution on Si(111) Surface by Scanning Nonlinear Dielectric Microscopy

Noncontact scanning nonlinear dielectric microscopy (NC-SNDM) has the ability to simultaneously acquire the surface topography and polarization distribution at the nanoscale through the measurement of local nonlinear dielectric constants of materials. NC-SNDM was here applied to the observation of Si(111) reconstructed surfaces. Images of the polarization distribution clearly distinguished disordered regions of the surface, often called 1×1 regions, at the boundaries between the regular (7×7) domains. We acquired polarization images of surfaces with different sizes of 1×1 regions and show that NC-SNDM has the potential to image trapped charges of surfaces and interfaces.

Graphene on C-terminated face of 4H-SiC observed by noncontact scanning nonlinear dielectric potentiometry

We studied graphene synthesized on the C-terminated face (C-face) of a 4H-SiC substrate by noncontact scanning nonlinear dielectric potentiometry. As already reported by other researchers, multilayer graphene sheets with moire patterns were observed in our sample, which indicates the existence of rotational disorder between adjacent layers. We found that the potentials of graphene on the C-face are almost neutral and significantly smaller than those observed on the Si-terminated face (Si-face). In addition, the neutrality of potentials is not affected by various topographic features underlying the multilayer graphene sheets. These results indicate that graphene on the C-face of SiC is decoupled or screened from the underlying structures and substrate, unlike graphene on the Si-face.

Experimental study of electric dipoles on an oxygen-adsorbed Si(100)-2 × 1 surface by non-contact scanning nonlinear dielectric microscopy

Oxygen-adsorption on a Si(100)-2 × 1 surface is investigated by using non-contact scanning nonlinear dielectric microscopy (NC-SNDM). On the Si(100)-2 × 1 surface exposed to oxygen (O2) gas at room temperature, several variations in atomic configuration and electric dipole moment of dimers are observed. Models are proposed for oxygen adsorption which are consistent with the topographies and electric dipole moment distributions obtained by NC-SNDM.