Hiroki Minoda

Step bunching, step wandering and faceting: self-organization at Si surfaces

Step bunching, in-phase step wandering and faceting are new morphological evolutions from smooth vicinal surfaces. They are formed by giving changes of externally controlled parameters on surfaces. Two cases on Si surfaces are presented in detail. One is caused by unidirectional drift forces on adatoms on the surfaces, i.e., surface electromigration due to direct current specimen heating. Newly formed step instabilities are due to a change of a kinetic parameter on the surface. The other case is caused by adsorption of foreign metal atoms on the Si surfaces. In this case step instabilities are due to changes of thermodynamic parameters on the surface. Step bunching, in-phase step wandering and faceting processes in two cases are interesting dynamics of self-organization at surfaces and can be useful for microfabrication of surfaces. Recent results of these dynamic processes on vicinal surfaces of Si(1 1 1) and (0 0 1) surfaces and high index Si surfaces studied by real space and reciprocal space observation methods mainly done by our group are reviewed.

Direct demonstration of the cross-bridge recovery stroke in muscle thick filaments in aqueous solution by using the hydration chamber

Despite >50 years of research work since the discovery of sliding filament mechanism in muscle contraction, structural details of the coupling of cyclic cross-bridge movement to ATP hydrolysis are not yet fully understood. An example would be whether lever arm tilting on the myosin filament backbone will occur in the absence of actin. The most direct way to elucidate such movement is to record ATP-induced cross-bridge movement in hydrated thick filaments. Using the hydration chamber, with which biological specimens can be kept in an aqueous environment in an electron microscope, we have succeeded in recording ATP-induced cross-bridge movement in hydrated thick filaments consisting of rabbit skeletal muscle myosin, with gold position markers attached to the cross-bridges. The position of individual cross-bridges did not change appreciably with time in the absence of ATP, indicating stability of time-averaged cross-bridge mean position. On application of ATP, individual cross-bridges moved nearly parallel to the filament long axis. The amplitude of the ATP-induced cross-bridge movement showed a peak at 5–7.5 nm. At both sides of the filament bare region, across which the cross-bridge polarity was reversed, the cross-bridges were found to move away from, but not toward, the bare region. Application of ADP produced no appreciable cross-bridge movement. Because ATP reacts rapidly with the cross-bridges (M) to form complex (M·ADP·Pi) with an average lifetime >10 s, the observed cross-bridge movement is associated with reaction, M + ATP → M·ADP·Pi. The cross-bridges were observed to return to their initial position after exhaustion of ATP. These results constitute direct demonstration of the cross-bridge recovery stroke.

REM study of high index Si(5 5 12) flat surfaces

We found high index Si(5 5 12) flat surfaces on an inner cylindrical surface made in a Si(110) wafer. Fringes of about 5 nm in spacing were seen on the (5 5 12) surface. Assuming a simple structure model of the (5 5 12) surface we calculated an intensity distribution in the RHEED pattern. The result coincided qualitatively with an observed intensity distribution. The (5 5 12) surface is inclined by about 5.3° from a (113) surface, and so-called hill and valley structures of the (5 5 12) and the (113) surfaces were observed at orientations between these two surfaces. It was found that surface morphology on the (5 5 12) surface changed depending on the heating current direction at about 1200°C. It was also found that a surface structural phase transition, considered to be a roughening transition, occurred at about 900°C.

REM observations of Si(hhk) surfaces and their vicinal surfaces

Abstract Clean Si( hhk ) surfaces and their vicinal surfaces, especially surfaces between the (113) and (114) surfaces, were studied by ultra-high vacuum reflection electron microscopy (REM) and diffraction (RHEED). The observations were carried out on inner cylindrical surfaces (0.4 mm in diameter) formed in Si(110) specimens (1 × 7 × 0.4 mm 3 ). On the cylindrical surfaces 13 flat surfaces were recognized. REM images and RHEED patterns showed that vicinal surfaces between the (113) and (114) surfaces were composed of terraces of the (113) and (114) surfaces and the structures depended on the direction of the DC current heating the specimen.

Direct Current Heating Induced Giant Step Bunching and Wandering on Si(111) and (001) Vicinal Surfaces

Direct current (DC) heating induced giant step bunching and wandering on Si(111) and (001) vicinal surfaces were newly found. The step bunching and wandering are so large that they can easily be studied by optical microscopy. Temperature dependence of the DC heating effects and the dependence of such effects on the crystallographic direction of the current were studied.

Direct-current-induced drift direction of silicon adatoms on Si(111)-(1×1) surfaces

Abstract The direction of the drift of adatoms induced by direct-current (DC) heating of Si(111)-(1×1) surfaces was studied from shape changes of a rectangular groove that was made by a focused ion beam (FIB) apparatus. It was found that the drift direction is to the current direction irrespective of the temperatures of the three ranges where DC-induced step-bunching behavior reverses twice. The result clearly shows that reversals of the current-induced step bunching are not due to reversals of the effective charge on silicon adatoms, but due to changes in the mechanism of step bunching.

Electromigration and gold-induced step bunching on the Si(111) surface

Abstract Step bunching instability induced by adatom electromigration is studied on the Si(111) surface at temperatures of 830–1000°C by ultrahigh vacuum reflection electron microscopy. To highlight the role of the effective charge of adatoms in step bunching, a small amount of gold atoms was deposited onto the Si(111) surface at these temperatures. It was found that in the case of sample heating by DC in the step-up direction, regular steps on the clean surface were unstable after submonolayer gold adsorption, while step bunches on the clean surface were transformed to regular steps on the gold-adsorbed surface during heating by DC in the step-down direction. We conclude that gold adsorption changes the sign of the effective charge of silicon adatoms from positive to negative. In this temperature range, the value of the adatom effective charge was estimated to be 0.004±0.001 in units of the elementary charge.

Electron microscopic evidence for the myosin head lever arm mechanism in hydrated myosin filaments using the gas environmental chamber

Muscle contraction results from an attachment-detachment cycle between the myosin heads extending from myosin filaments and the sites on actin filaments. The myosin head first attaches to actin together with the products of ATP hydrolysis, performs a power stroke associated with release of hydrolysis products, and detaches from actin upon binding with new ATP. The detached myosin head then hydrolyses ATP, and performs a recovery stroke to restore its initial position. The strokes have been suggested to result from rotation of the lever arm domain around the converter domain, while the catalytic domain remains rigid. To ascertain the validity of the lever arm hypothesis in muscle, we recorded ATP-induced movement at different regions within individual myosin heads in hydrated myosin filaments, using the gas environmental chamber attached to the electron microscope. The myosin head were position-marked with gold particles using three different site-directed antibodies. The amplitude of ATP-induced movement at the actin binding site in the catalytic domain was similar to that at the boundary between the catalytic and converter domains, but was definitely larger than that at the regulatory light chain in the lever arm domain. These results are consistent with the myosin head lever arm mechanism in muscle contraction if some assumptions are made.

Direct observation of biological molecules in liquid by environmental phase-plate transmission electron microscopy.

We have been developing a combination method for environmental TEM (E-TEM) and phase-plate TEM (P-TEM) that enables direct observations of the structure of biological molecules in aqueous solution. It is clearly demonstrated that the biological molecules in a water layer can be imaged by the combined method without any stain. The spatial resolution obtained in the present study was about 10nm. This should be improved by using energy filtering. The image contrast of the specimen in water was reduced in comparison with that in vacuum. A model calculation that includes the effects of beam broadening, intensity decrease, and background increase caused by scattering from the water layer around the specimen shows that an increase in the thickness of the water layer reduces the contrast, intensity, and resolution of the image.

STM light emission from Ag/Si(111)

A light detection system combined with UHV scanning tunneling microscopy (STM) was constructed using an ellipsoidal mirror for light collection. The system was applied to the light emission from a flat (111) surface of a silver film with a tungsten tip in the STM. Improvement in the stability and quality of the emission spectra was achieved by cooling the sample to 80 K and decreasing the tunneling current from 500 to 50 nA. We also succeeded in observing a photon map of the silver surface at 80 K. The observed spectra taken under various sample bias voltages were compared with those calculated using the theory. The peaks in the spectra could be assigned as the multipole modes of the local plasmon formed between the tip and the surface.