Yutaka Miyatake

Variable-temperature independently driven four-tip scanning tunneling microscope

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.

Application of atomic stereomicroscope to surface science

A stereograph of atomic arrangement was displayed directly on the screen of display-type spherical-mirror analyzer without any computer-aided conversion process for the first time. This stereoscopic photograph enables viewing three-dimensional atomic arrangement. This technique was realized taking advantage of the phenomenon of circular dichroism in photoelectron angular distribution. The azimuthal shifts of forward focusing peaks in a photoelectron angular distribution taken with left and right helicity light in a special arrangement are the same as the parallaxes in a stereo view of atoms. Hence a stereoscopic recognition of three-dimensional atomic arrangement is possible, when the left eye and the right eye respectively view the two images obtained by left and right helicity light simultaneously.

Demonstration of low-temperature atomic force microscope with atomic resolution using piezoresistive cantilevers

Compared to current optical-lever methods adopted in atomic force microscopes, nonoptical methods such as piezoresistive methods and quartz fork methods can be more advantageous due to their smaller installation size and the lack of electromagnetic effects from laser light during electrical conductivity experiments. As a technological demonstration of nonoptical methods, a low-temperature atomic force microscope using piezoresistive cantilevers was developed and operated at liquid-He4 temperatures (5K). The cantilever and sample can be transferred from atmosphere to the microscope head operating at low temperatures. Both contact mode and noncontact mode were used for testing the system while carrying out atomic resolution studies on clean Si(111) and clean Si(100) surfaces.

GaP reconstructed surface studied with STM and LEED

Abstract We have studied GaP ( 1 1 1 ) reconstructed clean surface with scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). We found that the surface consists of six equivalent mirror-symmetric domains and each domain has stripe structure. The stripe directions tilt from three 〈 1 1 2〉 directions clockwise and counterclockwise at the same angle, resulting in six domains. The stripe period and the tilt angle are 1.30 nm and 6.8°, respectively. Magnified STM images revealed that all stripes in one domain have the same protrusion unit along the stripe direction, and that the origin of the protrusion unit is arranged by two vectors for the inter-stripe direction. The same unit-vector in the stripe direction and the two unit-vectors in the inter-stripe direction constitute two different surface-reconstruction units, namely units 1 and 2. We assigned the reconstruction matrices of units 1 and 2 in one domain to 3 −1 2 5 and 4 1 2 5 , respectively. A trial structure model assuming the same elements located at the protrusions well explains observed LEED patterns.

Development of Scanning Probe Microscope for Auger Analysis

A combined system of the Auger electron analysis and the scanning tunneling microscope (STM) has been developed for both topographical observation and elemental analysis of surface. An STM tip works as a field-emission electron source inducing the Auger process. Conventionally used STM tip has a problem for the Auger measurements; Auger electrons from the sample surface are accelerated toward the sample in the strong electric field between the cathode tip and the sample. We constructed a new special tip with a shield electrode around the cathode tip to prevent the bending of the trajectories of Auger electrons. The field-emission electron was ejected from the cathode tip, and the special tip played a role of primary electron source for the Auger electron analysis. We detected silicon Auger electrons successfully from a clean Si(111) surface using a cylindrical mirror analyzer that located behind the special tip. This system works for both STM observation and Auger element analysis by exchanging the tip in vacuum.

TWO-DIMENSIONAL CIRCULARLY-POLARIZED-LIGHT PHOTOELECTRON DIFFRACTION FOR THE ANALYSIS OF MAGNETIC AND ELECTRONIC PROPERTIES ON SURFACES

Circular dichroism has been measured in the photoelectron diffraction of bulk W 4f photoelectrons from the W(110)(1×1) clean surface. The forward focusing peaks along the symmetric axis in the diffraction pattern showed an azimuthal rotation similar to those reported in a prior experiment on Si(001) and chemically shifted W 4f photoelectrons from the W(110)(1×1)-O surface. The emission angle dependence of the rotation angles has been measured and analyzed for the first time and the angles observed are in good agreement with those calculated using the formula Δ ϕ=m/kR sin2θ derived previously by Daimon et al. [Jpn. J. Phys.32, L1480 (1993)] considering the angular dependence of m. This property gives a basis for the analysis of structure or various magnetic and electronic properties on surfaces.

Development of Electron Source for Auger Electron Spectroscopy in Scanning Probe Microscope Systems

We have developed a scanning tunneling microscope (STM) combined with an Auger electron energy analyzer for both topography observation and element analysis on surfaces. A special STM tip was designed for a field-emission source that induces the Auger process. We found that a simple STM tip is not effective, since the trajectories of Auger electrons ejected from the surface are bent considerably by the high electric field between the surface and the tip. Instead, the shield electrode around the tip reduces the bending, and carbon Auger electrons (KLL) ejected from the highly oriented pyrolytic graphite were detected. In future, such an Auger-STM will enable both types of observation conveniently on the same region by changing modes between topography and Auger measurements.