Keiji Takata

Tunneling acoustic microscope

A new type of microscope, which is based upon both a scanning tunneling microscope (STM) and a technique for detecting acoustic waves, is described. An acoustic wave generated in a STM’s sample, by vibration of its tip, is detected by a piezoelectric transducer coupled to the sample. The amplitude of the acoustic wave corresponds to the strength of the force interaction between the tip and the sample, and is sensitive to tip‐sample spacing. We have been successful in keeping the spacing constant by using a new feedback loop that holds this amplitude constant without tunneling. This method enhances the features of the STM without reducing its functions and enables simultaneous use of both force interactions and tunneling current to investigate the properties of samples. Topographies taken by the new feedback system and tunneling current images are shown.

Observation of pn junctions on implanted silicon using a scanning tunneling microscope

Si pn junctions fabricated by photoresist masked As+ implantation were observed using current imaging tunneling spectroscopy (CITS) in a scanning tunneling microscope (STM). Using the CITS, a specific bias was chosen to define n‐type or p‐type areas according to whether or not current flowed. The pn junctions could be easily identified from the current image at this bias and in the STM topographic image. It also proved possible to find processing faults related to implantation. The STM images also identified the structure (corrugations) near the junctions, associated with volume expansion caused by implantation and annealing.

Au‐induced reconstructions of the Si(111) surface

Au‐adsorbed structures (∼1 ML) on the Si(111) surface are studied using a scanning tunneling microscope. In the initial stage of deposition, locally Au atoms are adsorbed onto the Si(111) surface with a 5×5 periodicity. At lower coverage, images showing a 5×2 periodicity are recorded, in which there are a dark line and two atomic rows running in the [101] direction in a five times period. In each atomic row, atoms are arranged in a two times period along the row. At higher coverage, images showing a (3)1/2×(3)1/2 structure are also recorded. In these images, in addition to the (3)1/2 ×(3)1/2 periodicity, there is an undulation which is explained as a phase shift of the (3)1/2 ×(3)1/2 structure.

STRAIN IMAGING OF A PB(ZR,TI)O3 THIN FILM

Strain‐imaging observation of a lead‐zirconate‐titanate film using a tunneling acoustic microscope (TAM) is described. This method detects fine strains in the piezoelectric film generated by the tip voltage and, therefore, measures and images the piezoelectric properties of the sample. The strains are detected either as a surface displacement of the sample by the feedback loop to keep the tip‐to‐sample spacing constant, or as a vibration generated by an alternating current tip voltage by the acoustic transducer in the TAM. Microscopic properties of a 70‐nm‐thick Pb(Zr0.5Ti0.5)O3 film grown by sputtering were investigated, and ferroelectricity was observed by applying voltages to areas of a couple of tens of nanometers in diameter. Local polarization control and its related space charge effect induced by the tip voltage are also shown.

Initial stage of Au adsorption onto a Si(111) surface studied by scanning tunneling microscopy

The initial stage of Au adsorption onto a Si(111) surface (<0.2 ML) is studied by scanning tunneling microscopy (STM). Au is deposited at room temperature, and then the sample is annealed at 700 °C. At the very early stage of Au adsorption, a Au‐adsorbed 5× structure is found not to break a 7×7 structure. By increasing the amount of adsorbed Au, the Si substrate itself shows a 5× structure. Rows of the Au‐adsorbed 5× structure are observed to have grown from the lower side of steps.

Scanning tunneling microscope with reliable coarse positioners

An inchworm system with a new type of clamping mechanism has been made for a coarse positioner in a scanning tunneling microscope. The clamping mechanism operates by means of springs and piezoelectric actuators. This system can move a probe tip transversely and longitudinally in the horizontal plane, and the sample vertically, in steps, with a range of 8 mm in each direction.

Fast scanning tunneling microscope for dynamic observation

A fast scanning tunneling microscope (FSTM) is developed using a new method of compensating for the probe tip servo error in constant current mode. The compensation value is derived from the ratio of tunnel current fluctuation to tunnel current (ΔI/I) in a differential tunnel current equation. Dynamic video images of Si(111) adatomic structures taken using the FSTM prove that this method is effective for fast scanning.

Strain imaging of a ferrite core head

This paper describes magnetic domain observation of a ferrite core head of a hard-disk drive (HDD) using a new method. The principle of the method, strain imaging, is detection and imaging of the strains generated in a magnetic material subjected to an external magnetic field, using a scanning probe microscope (SPM). Because the magnetic-field induced strains involve the factors depending on the magnetization of each domain, we can image domain structures by detecting the surface displacements induced by the strains, using the SPMs. The atomic resolution of the SPMs results in strain imaging with high resolution. We observed that domains aligned to the magnetic field increased with the coil current of the ferrite core head. Nothing, except for magnetic fields provided by the head coil itself, has any magnetic influence on the domain structures of the ferrite core. Thus, this method is suitable for domain observation of HDD heads made of soft materials.

Strain-imaging observation of the polarization freezing of the domains under the electrode of a Pb(Zr, Ti)O3 film

Strain imaging is used to measure and image piezoelectric properties by detecting electric-field-induced strains using a scanning probe microscope. In this letter, we present ferroelectric domain imaging under an electrode in a lead-zirconate-titanate (PZT) film. The imaging has been achieved by detecting the surface force modulation caused by the electrode displacements. Observation through the electrode suppresses the effect of space charges and enables us to investigate the PZT film in the same situation in devices. We observed the freezing of polarization of a PZT film deoxidized by atomic hydrogen produced by catalysis of platinum upper electrodes due to hydrogen annealing.

Tunneling barrier height imaging and polycrystalline Si surface observations

An influence of the dopant concentration on polycrystalline Si grain structure, electronic structure and dopant segregation in 800 °C and 30 min annealing is investigated. Barrier height images and topographies of arsenic implanted polycrystalline silicon surface are obtained using tunneling barrier height imaging (TBI) and scanning tunneling microscopy (STM) with the vacuum pressure less than 5×10−7 Torr. As results, a conduction band bending caused by the arsenic ion implantation can be observed by the TBI. A comparison of the TBI image with the STM image can identify whether region is implanted or nonimplanted. In addition, dopant segregation structures at the grain boundary can be found out. With dopant concentration, 1014 cm−2 As+ implanted polycrystalline silicon grain structures of 4–80 nm size grow large.