Fumikazu Iwawaki

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.

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.

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.

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.

Correlation between scanning tunneling microscopy/spectroscopy images and apex profiles of scanning tips

Variations of topographic and current images with bias voltages were examined utilizing the current imaging tunneling spectroscopy (CITS) and dual‐polarity tunneling imaging (DPTI) techniques. While the CITS and DPTI images of the Si(111)‐(7×7) reconstructed structure at the specimen voltage of +2 V and the DPTI image at −2 V are reliably reproducible exhibiting bright Si adatoms and dark corner holes, the CITS current image at −2 V often showed reversed contrast grey‐scale images with bright corner holes. The observed reversed image is explained as the result of the difference in the effective tunneling barrier and the surface state density of the tip apex and specimen, and also as the effect of the atomic arrangement and composition and profile of the tip apex.

STM study of geometric and electronic structures of Ge dimers on Si(001)

Abstract The surface topography of an isolated Ge dimer row on the Si(001) surface traced by a scanning tunneling microscope (STM) was examined by calculating the electronic distribution above a cluster of a Ge dimer and several Si atoms with the semi-empirical molecular orbital method. The Ge layer of 0.1 ML thick was deposited on the Si(001) surface at 300°C. Then the STM images of the Ge-deposited Si surface exhibited few symmetric and many asymmetric dimer rows. It was also noticed that the Ge dimers induced the buckling of the substrate Si dimer rows. The observed traces of a scanning tip and the present simplified calculation of the electronic distribution suggest the small tilt angle of the buckled Ge dimers 4–6°. However, the calculated difference in the orbital energies between the Si and Ge dimers on the Si(001) surface clearly suggests a buckling susceptibility of Ge dimers higher than of Si dimers.

Atomic configurations of tip apexes and scanning tunnelling microscopy-spectroscopy

Abstract This paper demonstrates the variation in the images depicted by scanning tunnelling microscopy (STM) with the number and arrangement of apex atoms and in the spectra obtained by scanning tunnelling spectroscopy (STS) with the apex composition and profile of a scanning tip. Thus the clarification of the state of the tip apex is the fundamental requirement for the acquisition of reliable and reproducible STM and STS data. A promising approach which enables us to examine the tip apexes in atomic dimension would be the study of the tip apex with a combined instrument of the atom probe and the field emission electron energy spectrometer.

Scanning tunneling microscopy/scanning tunneling spectroscopy observation of step structures of Si (001) and (111) surfaces

The step‐edge structures of the Si (001) and (111) surfaces roughened by heating at 1300 °C were studied utilizing the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The bunches of 7–8 double layer steps and the 200–300 A wide terraces were observed on the Si (111) surface. An unexpected finding is the interminglement of single and double atomic layer steps on the vicinal Si (001) surfaces tilted to any orientation. The double layer steps split into two single layer steps at the kink sites of the step edge. The dimer rows on the lower terraces shift in the direction of the rotation of the vicinal surface around the normal axis of the surface. The tunneling spectra at the single step exhibits several peaks at the sample bias voltages 0.6, −0.6, −1.4, −1.7, −2.1, and −2.4 V. The peaks at −1.4 and −1.7 V are found to be particularly sharp and high at the step edge.

Tunneling characteristics of silicon covered molybdenum tip apex

The variation of field emission characteristics of Mo tip apexes by the adsorbed Si atoms was investigated with an instrument combined of an atom probe and a field emission electron spectrometer. The Si/Mo surfaces were also examined using the scanning tunneling microscope/spectrometer. The deposited silicon was found to form microclusters on the Mo substrate and exhibit the semiconductive electronic states which are retained even at the Si–Mo interface where Si and Mo atoms coexist. Heating the Si/Mo surfaces results in the formation of silicides with metallic states. While the work functions of the Si clusters are about 10% larger than the Mo substrate, the silicide work functions are nearly 10% smaller than that of Mo.

Elaboration and evaluation of tip manipulation of scanning tunneling microscopy

A set of leaf springs, a micrometer head, a bellows coupling, a ball‐bearing‐type reduction gear, and a stepping motor were coupled in series to realize the quick and fine approach of a specimen surface to a scanning tunneling microscope tip. The minimum tip–surface displacement by the coupled system is in the range of 20–40 A and the maximum approaching speed is 0.25 mm/s. At the final stage of approach a piezoelement brought the specimen to the tunneling region between the Pt–Ir tip and the specimen graphite surface. The material transfer from the specimen to the tip apex was revealed by the atom‐probe mass analysis of the apex after a trial approach at the speed of 0.4 μm/s. However, no overshooting current by a tip–specimen contact was noticed throughout the trial approach. The observed mass transfer without a sign of the contact was attributed to the long response times of the current amplifier and the feedback circuit monitoring the approach. No material transfer was noticed for 0.16 μm/s.