Takahisa Doi

Observation of Si(111) surface topography changes during Si molecular beam epitaxial growth using microprobe reflection high‐energy electron diffraction

Si(111) surface topography changes during Si molecular beam epitaxial growth were observed by reflection electron microscope images using microprobe reflection high‐energy electron diffraction (RHEED). When RHEED intensity oscillations were observed at low substrate temperature (350 °C), it was found that the shape of atomic steps on the substrate was preserved during the growth and the surface topographies changed repeatedly with the period of the oscillations. When almost no oscillations were observed at higher substrate temperature (500 °C), the shape of the atomic steps changed during the growth. These observations provide direct evidence that RHEED intensity oscillations occur as the result of layer‐by‐layer two‐dimensional nucleation growth.

Study of Si(001) 2×1 domain conversion during direct current and radiative heatings

Si(001) 2×1 domain conversions induced by sample electric current are observed using microprobe reflection high‐energy electron diffraction. Samples are heated by using both direct current and a radiative heater. It is found that diffusion anisotropy of Si adatoms exists on 2×1 surfaces and that the domain conversion velocity is proportional to a voltage applied between the sample edges. This indicates that the conversion process is induced by the diffusion anisotropy and the electric force acting on positively charged adatoms. Minor 2×1 domain terraces always spread when the sample is heated radiatively. This preserves narrow 2×1 and 1×2 domain terraces after the conversion process and during Si growth process.

Observation of Surface Micro-Structures by Micro-Probe Reflection High-Energy Electron Diffraction

A new micro-probe reflection high-energy electron diffraction technique has been developed for observing micro-structures on crystal surfaces. In this technique, an electron beam of diameter 20 nm at a beam current of 8 nA and a beam angular divergence of 2 mrad has been achieved by using a field emission gun, making it possible to obtain bright, sharp diffraction patterns from surface micro-areas. A diffraction spot on a fluorescent screen is focused on an apperture by an optical lens; and part of the intensity is used as a signal to produce a scanning electron microscope image. This image is very sensitive to changes in surface crystallographic orientation, and can show areas in which the orientation change is of the order of 0.1 mrad. When the technique was used to observe Si(111) surfaces and metal-deposited Si(111) surfaces, image contrasts caused by screw dislocations, atomic steps and domain structures on the surfaces were obtained.

Observation of Si(111) and gold-deposited Si(111) surfaces using micro-probe reflection high-energy electron diffraction

Abstract Observations of clean Si(111) and gold-deposited Si(111) surfaces have been performed using micro-probe reflection high-energy electron diffraction. It was found that many atomic steps on a Si(111) surface run in nearly the same direction, about 9° off the [112] direction. When gold was deposited on this surface at a substrate temperature of about 800°C, 5 × 1, diffuse √3 × √3 R 30°, sharp √3 × √3 R 30° structures and Au clusters appeared on the surface with continuation of the deposition. During the deposition process, it was found that one kind of Si(111) 5 × 1 Au domain grew selectively along these atomic steps and nearly covered the entire surface. A phenomenon of gold clusters moving during the deposition was also observed. These clusters all moved in nearly the same direction so as to climb the atomic steps.

Diffraction microscopy using 20 kV electron beam for multiwall carbon nanotubes

Diffraction microscopy with iterative phase retrieval using a 20kV electron beam was carried out to explore the possibility of high-resolution imaging for radiation-sensitive materials. Fine, homogeneous, and isolated multiwall carbon nanotubes (MWCNTs) were used as specimens. To avoid lens aberrations, the diffraction patterns were recorded without a postspecimen lens. One- and two-dimensional iterative phase retrievals were executed. Images reconstructed from the diffraction pattern alone showed a characteristic structure of MWCNTs with the finest feature corresponding to a carbon wall spacing of 0.34nm.

Microscopic observation of Si MBE on Si(001) surface using microprobe RHEED

Abstract Si(001) surface topography changes during Si molecular beam epitaxy (MBE) were observed by reflection electron microscope images using microprobe reflection high-energy electron diffraction (RHEED). When RHEED intensity oscillations were observed at low substrate temperature (450 °C), it was found that alternate growth of 2×1 and 1×2 surface reconstruction domains occurred with the half period of the intensity oscillations, and the shape of the domains was preserved during MBE. When almost no oscillations were observed at higher substrate temperature (>750°C), atomic step migration occurred during MBE. These results provide direct evidence that RHEED intensity oscillations occur by layer-by-layer two-dimensional nucleation growth.

Systematic change in surface structures on Si(111) clean surfaces with temperature

Abstract The temperature dependence of Si(111) surface structures has been studied. Reflection electron diffraction intensities have been measured for Si(111) clean surfaces as a function of temperature. Diffraction intensities change at 830,1110 and 1410 K, corresponding to a low-temperature 7 × 7 phase↔a high-temperature 7 × 7, 7 × 7 ↔ 1 × 1 phase transition, and a surface order-disorder transition respectively. These temperatures are equivalent to 0·49(≍ 3/6)T m, 0·66(≍ 4/6)T m, and 0·84(≍ 5/6)T m respectively, where T m is the melting temperature of bulk Si on the absolute temperature scale. The significance of these temperatures is discussed in relation to the crystal surface sites, for example kinks, ledges and terraces.

Activation energies of Si adsorbate diffusion on a Si(001) surface

Abstract The diffusion of Si adsorbates deposited on a Si(001) surface is studied by a reflection electron microscope (REM). The diffusion constants are obtained from the diffused lengths of the Si adsorbates. The activation energy of diffusion is determined with the Arrhenius plot of the diffusion constants. It is found that the activation energy on both 2 × 1 and 1 × 2 terraces changes at about 750°C. At temperatures below 750°C, the activation energy is about 1.4 eV, while it is about 2.7 eV at temperatures above 750°C. The change in activation energy is caused by the evaporation of the Si adsorbates from a Si(001) surface.

High Dose Rate Effect of Focused-Ion-Beam Boron Implantation into Silicon

The effect of high-dose-rate, 16 keV focused-ion-beam (FIB) B+ implantation into Si has been investigated as a function of current density and beam-scan speed. It is shown by µ-RHEED (micro-probe reflection high-energy electron diffraction) observation that the increase in electrical activation of implanted B atoms at such low temperature annealing as 600°C closely correlated with the increase in amorphous zones produced. It is also found that continuous amorphous layer formation occurs with a 1–2×1015 ions/cm2 (one order lower than for conventional implantation) when both implantation conditions of high current density and slow scan speed (e.g. 20 mA/cm2 and 6×10-3 cm/s) are satisfied. The reason for amorphous zone formation enhancement by FIB implantation is discussed.

High-Temperature MBE Growth of Si-Direct Current Heating Effects on (111) and (001) Vicinal Surfaces

The molecular beam epitaxial (MBE) growth model is extended to account for the electromigration of Si adatoms on both (001) and (111) Si surfaces. Step bunching is predicted to occur at the (111) surface when the heating current has step-down direction. Electromigration-induced formation of a major reconstruction domain at (001) Si is not expected when the growth process is controlled by step kinetics. When the process is controlled by surface transport, the vicinal surface is predicted to be dominated by either the 1×2 or 2×1 domain depending on the direction of the current. Microprobe reflection high-energy electron diffraction (µ-RHEED) observations of MBE growth of the (001) Si surface at temperatures of 750-900°C reveal preferential growth of the SA steps when the heating dc has a step-up direction. This is in contrast with the reported preferential growth of SB steps at lower temperatures (400-600°C). The difference in the surface reconstruction is interpreted to indicate a transition from step-kinetics-controlled growth (at 400-600°C) to surface-diffusion-controlled growth (at 750-900°C).