Hideaki Okado

Drastic Decrease in Dislocations during Liquid Phase Epitaxy Growth of GaN Single Crystals Using Na flux Method without Any Artificial Processes

The behavior of dislocations of GaN single crystals grown using the liquid phase epitaxy (LPE) technique in a Na flux was investigated using a transmission electron microscope (TEM). Almost all dislocations are concentrated around the interface between LPE-GaN and metal organic chemical vapor deposition (MOCVD)-GaN, which was used as a homoepitaxial substrate. It was revealed that the reduction of dislocation is due to an unusual phenomenon in which many (1011) facets are formed in the initial LPE growth period, followed by the development of the GaN(0001) face with further dislocation reduction. On the basis of our TEM observations and an investigation of the surface morphology at each growth stage, we propose a dislocation reduction mechanism during LPE growth.

Electrical Characterization of Metal-Coated Carbon Nanotube Tips

Electrical characteristics of bare and metal-coated carbon nanotube (CNT) tips were investigated with an independently driven four-tip scanning tunneling microscope (STM). The CNT was glued on a W tip apex and wholly coated ex situ by metal thin layers. The resistance between the CNT-tip end and the W supporting tip scattered very widely from ca. 50 kΩ to infinity for the bare tips, while coating the tip with a 6-nm-thick PtIr film stably reduced the resistance to less than approximately 10 kΩ. The W coating was also effective for stabilizing the resistance, although they showed slightly larger resistance (ca. 50 kΩ). The metal-coated tips kept their low resistance and flexibility even after 100 repeated contacts to an object for conductivity measurements. They are expected to be useful for nanometer-scale transport measurements with multiprobe STM as well as for conventional single-tip STM.

Exploiting Metal Coating of Carbon Nanotubes for Scanning Tunneling Microscopy Probes

By exploiting the metal coating of carbon nanotube (CNT) tips for a scanning tunneling microscope (STM), we demonstrated atomic imaging/spectroscopy and showed their potential for electrical nanoprobes. A CNT glued to a W tip was uniformly coated with a thin W layer 3–6 nm thick. Using this tip, stable atomic imaging and spectroscopy were carried out on clean Si(111)-7×7 and Si(100)-2×1 surfaces. The mechanical flexibility of the coated CNT was maintained by virtue of the thin-layer coating, enabling repeated direct contact to the sample surface. Two W-coated CNT tips were brought together within a distance of approximately 50 nm. These results indicate that the tips are useful for electronic transport measurements on a nanometer scale after installation into a multiprobe STM.

Formation of Palladium Silicide Films on Silicon(111)7×7 Surface at ∼150 K

The formation of palladium silicide on Si(111) at ~150 K was investigated in situ by Auger electron spectroscopy, reflection high-energy electron diffraction analysis, scanning tunneling microscopy, and ex situ transmission electron microscopy. It is demonstrated that silicide was formed spontaneously by Pd deposition even at ~150 K. The phase of the silicide was Pd2Si, and films of Pd2Si were grown epitaxially. The surface of the films was uneven. There were bumps of 1–5 nm in height and approximately 20 nm in diameter on the surface. The primarily formed silicide phase is discussed using an effective heat of formation model.

Scanning Tunneling Spectroscopy Study of the ZnO(0001)?Zn Surface

The electronic structure of the ZnO(0001)–Zn surface was studied using scanning tunneling spectroscopy (STS) and first principles molecular dynamics. The STS spectrum indicated that the clean surface is n-type semiconducting with a band gap of about 3.3 eV. The local density of states (LDOS) calculated using ZnO slab model was in qualitative agreement with the STS spectrum, and revealed that occupied and unoccupied peaks originate from O and Zn atoms at the top bilayer of the surface, respectively. From the contour plots of LDOS, it was found that Zn atoms dominantly contribute to both occupied and unoccupied LDOS distributions and their broadening on the surface, which prevents atom-resolved scanning tunneling microscopy imaging of ZnO(0001) surface.

Real-Time Scanning Tunneling Microscope Observations of High-Temperature Oxygen Reaction with the 6H-SiC(0001)√3×√3 Surface

The initial reaction of the 6H-SiC(0001)√3×√3 surface with O2 molecules at 500°C has been observed in real time using a variable temperature scanning tunneling microscope. When the surface was exposed to O2 molecules, many bright spike noises were observed, which can be attributed to migrating O2 molecules. With an increase in O2 exposure, dark lines were formed on the surface. These dark lines indicate anisotropic surface etching. The dark lines had a tendency to be formed in parallel in the vicinity of previously formed dark lines. It was observed that some parallel dark lines rearranged to form a triangular domain. These results revealed that the products formed by surface etching dynamically rearranged through the movement of Si adatoms.

Room Temperature Formation of Crystallized Palladium Silicide on Si(111) Observed by Ultrahigh Voltage Electron Microscopy

The structure of products formed by Pd deposition on a Si(111) substrate at room temperature has been investigated by cross-sectional ultrahigh voltage electron microscopy. Samples were prepared by cleavage so as to eliminate any artifacts accompanying a conventional atom thinning process. Thus we could carry out all procedures at room temperature. Crystalline islands were observed on a Si substrate, and the fringes in the islands suggest that the islands were Pd2Si. We found that the silicide tends to grow epitaxially; however, the silicide misoriented about 6° toward the direction of the Si substrate.