Masahiko Tomitori

STM study of the Ge growth mode on Si(001) substrates

Abstract The Ge growth mode on a Si(001) substrate was examined by scanning tunneling microscopy (STM) on the atomic scale. Germanium overlayers on Si substrates exhibited the Stranski-Krastanov growth mode, where the deposited films grew layer-by-layer up to a few ML, followed by the several types of the three-dimensional islands with distinctive facets. The phase diagram of the Ge growth was constructed for the Ge coverages from less than 1 ML up to 8 ML at the growth temperatures of 300, 400 and 500°C, examining the STM images. Furthermore, annealing effects for the Ge overlayer were also studied to clarify the stability of the islands.

Layered heteroepitaxial growth of germanium on Si(015) observed by scanning tunneling microscopy

Abstract Germanium growth on a Si(015) substrate was examined by scanning tunneling microscopy (STM) on an atomic scale. The Ge deposition on a Si(001) substrate showed the Stranski-Krastanov growth mode, where the deposited films grew layer-by-layer up to a few ML followed by three-dimensional island growth with {015} facets. Thus the Ge films were deposited on the Si(015) substrate in this study. As expected, the deposited Ge layers exhibited the layered growth mode up to thicker than 10 ML at a growth temperature of 400°C. The surface and the step structures, which seemed deeply related to the layered growth mechanism, were observed with STM and discussed in detail.

Tip cleaning and sharpening processes for noncontact atomic force microscope in ultrahigh vacuum

Abstract Tip cleaning and sharpening processes for noncontact atomic force microscope (AFM) operated in ultrahigh vacuum (UHV) were carried out and evaluated by a scanning Auger microscope (SAM) with a field emission electron gun and a noncontact AFM in UHV combined with a scanning tunneling microscope and a field emission microscope. The cantilever used in this study was piezoresistive, which can be heated by passing a current through the resistive legs of the cantilever. As a pretreatment, the tip was irradiated with ultraviolet light in oxygen to remove carbon contaminants. It was heated at about 750°C to form a clean oxide layer in oxygen of 5×10−5 Torr in an SAM chamber. The desorption of the layer can make a remained tip apex sharper by heating under electron beam irradiation. A thermally oxidized layer was also eliminated by HF etching to sharpen the tip apex. The procedures are useful to obtain a well-defined Si tip suitable for a noncontact AFM.

STM study of initial stage of Ge epitaxy on Si(001)

Abstract A scanning tunneling microscope (STM) was employed to study the superstructures and the formation process of islands at the initial stage of epitaxial growth of Ge layers on the Si(001) surface. Amount of Ge deposition was varied from one half of a monolayer (ML) to 4 ML. At a low Ge coverage the Ge layers were developed at step edges and on terraces forming two-dimensional (2D) islands and exhibited c(4 × 2) and p(2 × 2) structures on the Si substrate at 300°C. At a higher coverage of about 2 ML, a new superstructure with a periodicity of about 7 times of the unit cell was observed and the new cells were found to be the cores of the 3D islands. The number of islands increased and the islands grew with Ge coverage. Most Ge islands disappeared by annealing at 500°C and the 2D superstructure appeared again. Formation and growth mechanisms of the 3D Ge islands and the relaxation of the stress due to Si-Ge lattice mismatch will be discussed in analyzing the observed STM images.

Bias dependence of Si 111 7= 7 images observed by noncontact atomic force microscopy

Abstract Noncontact atomic force microscopy (nc-AFM) imaging of a Si(111)7×7 surface has been done in order to examine the bias dependence of the contrast of Si adatoms. While the atomic corrugation depends upon the tip states, the contrast is found to be inverted by increasing the bias voltage at greater frequency shifts. Then, the term of the repulsive force between a Si adatom and a Si atom at the tip apex can play an important role in depicting the topography of the sample surface with atomic resolution. The difference in contrast between faulted and unfaulted halves and peculiar profiles near steps are also presented.

Reproducibility of scanning tunneling spectroscopy of Si(111)7 × 7 using a build-up tip

Abstract A build-up tip, which was made from a [111]-oriented W filament, was used for scanning tunneling spectroscopy (STS) of Si(111)7 × 7 to improve the reproducibility of the tunneling spectra. The apex can be repeatedly sharpened by applying high voltage while heating it. To clean the apex region while keeping the sharp apex, thermal-field evaporation (TFE) techniques were applied for the tip, observing the thermal-field ion evaporation images, FEM images and FIM images with a chevron MCP screen in a subchamber of an FEM-STM vacuum system. The tip was transferred to the STM head and then the change in the FEM images was examined before and after STM/STS experiments using the FEM-STM, which can be rapidly switched between the FEM/TFE and the STM/STS operation mode. The FEM is sensitive to changes in the workfunction and the outline of the tip. As a result, the tunneling spectra with sweeping a sample bias voltage from 2 to −2 eV were reproducible with little change in the FEM images. For 3 to −3 eV, however, the reproducibility was poor with some drastic change in the FEM images. The change was attributed to Si atom transfer from the sample surface to the tip on the STS measurement, which deteriorated the reliability of the tunneling spectra.

STM study of Ge overlayers on Si(001)

Abstract The structures of microscopically small Ge islands on the Si(100) surface were observed bu an ultrahigh vacuum scanning tunneling microscope. After the Ge deposition of 0.1–0.2 monolayer (ML) on the Si substrate at 300°C, Ge islands composed of 1–5 dimer rows were observed. The observed structures indicated that the ordered arrangement of the buckied dimer rows varies with the number of dimer rows and that the position of a buckled Ge dimer has an interesting correlation with the position of a substrate Si dimer. A Ge dimer row separated by a missing dimer alternately was also observed. The stability of the Ge dimer row is discussed.

Sharpening Processes of Scanning Tunneling Microscopy/Scanning Tunneling Spectroscopy Tips by Thermal Field Treatment

In order to prepare a scanning tip suitable for scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) operated in ultrahigh vacuum (UHV) with a reproducible and ideal profile of the [111]-oriented W tip, sharpening and cleaning treatments were carried out by heating the tip while applying high voltage in UHV (thermal field (T-F) treatment). The treated tips were evaluated by field emission microscopy (FEM). The appropriate values to build up the W tip along the [111] axis for a treatment parameter, |V T-F /V base |, range from 2 to 2.6 under fixed conditions of tip heating temperature and time of about 1300K and 60 s, respectively. It has been also realized that the formation of an ideal tip completely surrounded by three {112} facets at its apex requires a successive heat treatment: an initial T-F treatment, simple heating at about 1000 K to develop the {112} facets, and a final T-F treatment.

Visualization of tip-surface geometry at atomic distance by TEM-STM holder

To clarify the tip-surface interaction of the scanning tunneling microscope (STM) by using a UHV transmission electron microscope (TEM), we specially designed a TEM-STM holder. In the present high-resolution electron microscope and diffraction studies, after applying a voltage of 50 V between W-tip and Au-surface at a distance of 70 nm in the feedback control mode of the tunneling current of 0.1 nA, the tip apex which has many protrusions was changed to one with a single protrusion. The tip was found to be cleaned and sharpened to the ideal conical shape. At the tip apex a droplet was generated with the same [110] orientation as the tip. In addition to the sharpening of the tip, a droplet was generated on the Au-specimen surface. Generation of similar protrusions on the tip droplet occurred under applied bias voltage of usual STM operation.

Corrugation of Si surfaces and profiles of tip apexes

Si(111) surfaces heated at 1300 °C show many steps and terraces exhibiting the (√3×√3) R 30°, c(4×2), (5×5), (7×7), and (9×9) reconstructed structures. A detailed study of the surface corrugation indicates that the observed maximum depth of the corner hole is 2.4 A and the trace of a scanning tip is close to the profile of the [111]‐oriented W tip with a single atom at the apex and the most desirable cone angle of ∼120°. Occasionally, anomalous, ordered structures appeared and were explained as the result of the scanning with two apex atoms 4.5 A apart, the distance between the nearest surface atoms of the W(111) plane.