Tetsuya Narushima

Surface stress in thin oxide layer made by plasma oxidation with applying positive bias

Abstract The time evolution of the surface stress during plasma oxidation of Si(100) at a very early stage (oxide thickness

Electron-stimulated surface stress relaxation of Si

We have observed the nonthermal relaxation of surface stress in Si induced by electron irradiation at room temperature. An atomically thin disordered layer was introduced by Ar ion bombardment. The surface stress change during ion bombardment and the following electron irradiation of Si(100) was measured by means of an optical microcantilever technique. We have found that the compressive stress in the Si surface due to disorder induced by ion bombardment was completely relaxed by electron irradiation at low energy. The criterion for complete relaxation is found not to be total energy deposition, but the number of irradiated electrons.

Sub-20-fs Time-Resolved Measurements in an Apertured Near-Field Optical Microscope Combined with a Pulse-Shaping Technique

We have constructed an ultrafast near-field optical microscope system with near-field pump/probe pulses with a 17 fs duration at the probe tip. The dispersion effects arising from the optical components, especially from the optical fiber, were removed by a pulse-shaping system that consists of passive and adaptive group-velocity dispersion compensators. With this apparatus, we succeeded in measuring dephasing in a single gold nanostructure with a time constant on the order of 10 fs.

Effects of surface disorder on the surface stress of Si 100 during oxidation

We have studied the effects of disorder on surface stress during oxidation. The surface stress change during ion bombardment and the following plasma oxidation on Si(100) was measured by means of an optical microcantilever technique. We have found compressive stress on Si surface due to disorder induced by ion bombardment and determined it quantitatively in terms of the number of defects. This disorder-induced compressive stress was completely relaxed by the plasma oxidation processes. The initial evolution of the surface stress during oxidation on bombarded surfaces is quite different from that on unbombarded Si(100) surfaces.

Relationship between Ozone Oxidation and Stress Evolution on an H-Terminated Si Surface

We observed the stress evolution of an H-terminated Si (100) surface during ozone oxidation and the ozone oxidation on a partially H-terminated Si surface of Si (100) and Si (111) to study the surface orientation effect on ozone oxidation. The evolution of stress on the H-terminated Si (100) surface was observed in real-time by an optical micro-mechanical cantilever method. The results show that the stress evolution on the H-terminated Si surface was unexpectedly large when considering that H-termination reduced the sticking amount of oxygen. Both the Si (111) and Si (100) surfaces showed that the rate of ozone oxidation was reduced as hydrogen covered the surfaces. However, at high-H coverage the H-terminated Si (111) surface showed a greater increase of sticking rate than the H-terminated Si (100) surface. The relationship between the oxidation-induced stress and oxidation rate is discussed.

Low-energy ion-induced tensile stress of self-assembled alkanethiol monolayers

Monolayers of alkanethiols on gold have been exposed to low energy Ar ion. A micromechanical cantilever sensor technique was used to determine in situ the influence of the ion dose on the surface stress in the monolayers. In contrast to compressive surface stress during self-assembled monolayer growth, a strong tensile surface stress of about −0.7 N/m was found when the sensor is exposed to Ar ions. This value is 3–4 times larger than the compressive surface stress. We attribute this stress to a reaction between the alkyl chains in the molecules of the alkanethiol monolayer.

Circular Dichroism Microscopy Free from Commingling Linear Dichroism via Discretely Modulated Circular Polarization

In this work, we developed a circular dichroism (CD) imaging microscope with a device to suppress the commingling of linear birefringence (LB) and linear dichroism (LD) signals. CD signals are, in principle, free from the commingling influence of LD and LB if the sample is illuminated with pure circularly polarized light, with no linear polarization contribution. Based on this idea, we here propose a novel circular polarization modulation method to suppress the contribution of linear polarization, which enables high-sensitivity CD detection (10−4 level in optical density unit or mdeg level in ellipticity) for microscopic imaging at a nearly diffraction limited spatial resolution (sub-μm level). The highly sensitive, diffraction-limited local CD detection will make direct analyses of chiral structures and spatial mappings of optical activity feasible for μm- to sub-μm-sized materials and may yield a number of applications as a unique optical imaging method.

Optical Activity Governed by Local Chiral Structures in Two‐Dimensional Curved Metallic Nanostructures

Chiral nanostructures show macroscopic optical activity. Local optical activity and its handedness are not uniform in the nanostructure, and are spatially distributed depending on the shape of the nanostructure. In this study we fabricated curved chain nanostructures made of gold by connecting linearly two or more arc structures in a two-dimensional plane. Spatial features of local optical activity in the chain structures were evaluated with near-field circular dichroism (CD) imaging, and analyzed with the aid of classical electromagnetic simulation. The electromagnetic simulation predicted that local optical activity appears at inflection points where arc structures are connected. The handedness of the local optical activity was dependent on the handedness of the local chirality at the inflection point. Chiral chain structures have odd inflection points and the local optical activity distributed symmetrically with respect to structural centers. In contrast, achiral chain structures have even inflection points and showed antisymmetric distribution. In the near-field CD images of fabricated chain nanostructures, the symmetric and antisymmetric distributions of local CD were observed for chiral and achiral chain structures, respectively, consistent with the simulated results. The handedness of the local optical activity was found to be determined by the handedness of the inflection point, for the fabricated chain structures having two or more inflection points. The local optical activity was thus governed primarily by the local chirality of the inflection points for the gold chain structures. The total effect of all the inflection points in the chain structure is considered to be a predominant factor that determines the macroscopic optical activity. Chirality 28:540-544, 2016.

Stress inversion from initial tensile to compressive side during ultrathin oxide growth of the Si(100) surface

We report the real-time observation of the stress change during sub-nanometer oxide growth on the Si(100) surface. Oxidation initially induced a rapid buildup of tensile stress up to -1.9 × 10(8) N m(-2) with an oxide thickness of 0.25 nm, followed by gradual compensation by a compressive stress. The compressive stress saturated at 5 × 10(7) N m(-2) for an oxide thickness of 1.2 nm. The analysis, assisted by theoretical study, indicates that the observed initial tensile stress is caused by oxygen bridge-bonding between the Si dimers. Atomistic model calculations considering mutually orthogonal orientations of the Si(100) surface structure reproduce the stress inversion from the tensile to the compressive side.

Simultaneous scanning tunneling microscopy and stress measurements to elucidate the origins of surface forces

We have developed a new combined measurement system to investigate the underlying origins of forces on solid state surfaces from the viewpoint of atomic surface morphology. This system consists of two main parts: the measurements of force based on displacements and detailed atomic resolution observations of the surface morphology. The former involves a large sample cantilever and a capacitive detection method that provide sufficient resolution to detect changes of a few meV/atom or pN/atom at surfaces. For the latter, a scanning tunneling microscope was incorporated to observe structural changes occurring on the surface of the cantilever sample. Although this combined observation is not trivial, it was accomplished by carefully designing sample dimensions while suppressing the self-oscillation of the cantilever. To demonstrate the performance of this system a preliminary study of the room temperature adsorption of Br(2) on the clean Si(111)-7x7 surface is presented.