住友 弘二

Recent progress on nanotechnology including nanostructure fabrication and nanometer-scale measurement techniques, and work on biomolecules whose size is equivalent to that covered by nanotechnology, are expected to result in the creation of a new research field called nano-bio science. This article introduces our recent work on the observation of single biomolecule; reconstituted a receptor protein into an artificial lipid membrane using an atomic force microscope (AFM). An AFM is a measurement tool that enables us to observe nanometer-scale objects in a liquid environment. We also examine the orientation of the proteins in proteoliposomes with the dynamic light scattering technique (DLS). Most receptor proteins have orientations in molecules, for example the extracellular and intracellular domains. Determining the protein orientation is essential for nano-biodevice fabrication if we wish to utilize the protein's function. We also introduce our recent attempt to realize a nano-bio device; a very small device obtaining and utilizing biological functions; using our state-of-the-art nanofabrication technique and a technique for handling receptor proteins. Thus, by combining nanotechnology and biotechnology to realize nano-biodevices, we can produce ultra-small biological sensors for implantation. Further improvements are expected. © 2010 The Institute of Electrical Engineers of Japan.

In this study, we observed the topology of a single protein in a stretched lipid bilayer (membrane) suspended over a nanoscale well using a fast-scanning atomic force microscope (AFM). The membrane was located stably enough on the well to prevent the leakage of a liquid placed in the well, and it allowed us to observe membrane stretching using an AFM. We successfully observed the gradual stretching of the suspended membrane. We also observed single bacteriorhodopsin proteins in the stretched membrane, and found that they maintained their trimeric structure, but that the distances between the trimers increased. (C) 2010 The Japan Society of Applied Physics

Production of a self-assembled protein nanotube achieved through engineering of the 11mer ring protein trp RNA-binding attenuation protein is described. The produced mutant protein is able to stack in solution to produce an extremely narrow, uniform nanotube apparently stabilized by a mixture of disulfide bonds and hydrophobic interactions. Assembly is reversible and the length of tube can potentially be controlled. Large quantities of hollow tubes 8.5 nm in overall diameter with lengths varying from 7 nm to over 1 mu m are produced. The structure is analyzed using transmission electron microscopy, atomic force microscopy, mass spectrometry, and single-particle analysis and it is found that component rings stack in a head-to-head fashion. The internal diameter of the tube is 2.5 nm, and the amino acid residues lining the central cavity can be mutated, raising the possibility that the tube can be filled with a variety of conducting or semiconducting materials.

受容体の機能はリガンドが受容体に結合する事でおこる構造変化によって発揮される。例えば、イオンチャネル型受容体の場合はアゴニストが結合すると膜貫通ポアが開き、その中をイオンが通過する事で機能する。多くの場合は電気生理学的に、または指示薬などを用いる事でイオンチャネルの機能を評価するが、実際にその受容体がどのように構造変化して機能を発揮するかについては、溶液中で直接観察する事は困難であった。我々は、原子間力顕微鏡(atomic force microscopy:AFM)と呼ばれる新たな実験手法を用いる事で、水溶液中における一分子のP2×4受容体の表面構造および活性化に伴う構造変化のタイムラプス観察に成功した。また、P2X受容体に特徴的な現象として知られているポアダイレーションに相当する構造変化の観察にも成功した。(著者抄録)

Atomic force microscopy (AFM) imaging of membrane proteins suspended over a nanostructure in liquid is a promising way to understand the structure and function of working proteins, although the membrane deformation that occurs during scanning makes it difficult to obtain a high resolution image. This study proposes an artificial cell system for the AFM observation of functional membrane proteins that consists of a sub-micron well on Si, a biological membrane, and a carbon nanotube (CNT) network. We successfully observed molecular-scale images of a purple membrane suspended over sub-micron well patterns. By using a CNT network to hold the suspended membrane, we suppressed the membrane deformation caused by the "imaging force". The CNT network takes the place of a cytoskeleton in supporting the cell membrane suspended over the well thus improving the spatial resolution of AFM measurement. (C) 2009 The Japan Society of Applied Physics

In this article, we introduce the reconstitution of membrane proteins into lipid bilayers and describe the challenge of creating nanobiodevices based on our fundamental technologies.

The ATP-gated P2X4 receptor is a cation channel, which is important in various pathophysiological events. The architecture of the P2X 4 receptor in the activated state and how to change its structure in response to ATP binding are not fully understood. Here, we analyze the architecture and ATP-induced structural changes in P2X4 receptors using fast-scanning atomic force microscopy (AFM). AFM images of the membrane-dissociated and membrane-inserted forms of P2X4 receptors and a functional analysis revealed that P2X4 receptors have an upward orientation on mica but lean to one side. Time-lapse imaging of the ATP-induced structural changes in P2X4 receptors revealed two different forms of activated structures under 0 Ca2+ conditions, namely a trimer structure and a pore dilation-like tripartite structure. A dye uptake measurement demonstrated that ATP-activated P2X4 receptors display pore dilation in the absence of Ca2+. With Ca2+, the P2X4 receptors exhibited only a disengaged trimer and no dye uptake was observed. Thus our data provide a new insight into ATP-induced structural changes in P2X4 receptors that correlate with pore dynamics. © 2009 Shinozaki et al.

We have probed the mechanical properties of purple membrane (PM) in a physiological environment using the atomic force microscope (AFM). By suspending PM over nano-trenches, the elastic properties of PM can be evaluated free from the interaction with the substrate. Force-displacement curves were obtained on the suspended membrane and the data was compared to that of a simple model of a thin film over a trench. By fitting the data to the model, the elastic modulus of PM was estimated to be 8 MPa. When the membrane is repeatedly indented, we observed a change in the force-distance data consistent with damage to the two-dimensional crystal of PM. In this paper we demonstrate that the AFM allows us to evaluate the mechanics of biological membranes in their native conditions. (C) 2008 Elsevier B.V. All rights reserved.

Lipid Vesicle fusion is an important reaction in the cell. Calcium ions (Ca2+) participate in various important biological events including the fusion of vesicles with cell membranes in cells. We studied the effect of Ca2+ on the fusion of ego yolk phosphatidylcholine/brain phosphatidylserine (eggPC/brainPS) lipid vesicles on a mica substrate with fast scanning atomic force microscopy (AFM). When unattached and unfused lipid vesicles on mica were rinsed away, discrete patches of fused vesicles were observed under high Ca2+ concentrations. At 0 mM Ca2+, lipid vesicles were fused on mica and formed Continuous Supported lipid bilayers (SLBs) covering almost the entire mica surface. The effect of Ca2+ on SLB fort-nation was offset by a Ca2+ chelating agent. When lipid vesicles were added during AFM observation, vesicles fused oil mica and covered almost all areas even under high Ca2+ concentrations. These results indicate that force between AFM tip and vesicles overcomes the Ca2+-reduced fusion of lipid vesicles.

The experimental creation and annihilation of defects on single-walled carbon nanotubes (SWCNT) with the tip of a scanning tunneling microscope are reported. The technique used to manipulate the wall structure of a nanotube at the atomic scale consists of a voltage ramp applied at constant tunneling current between the tip and the nanotube adsorbed on a gold substrate. While topographic images show an interference pattern at the defect position, spatially resolved tunneling spectroscopy reveals the presence of localized states in the band gap of the nanotube. Removal of the defect by the same procedure demonstrates the reversibility of the process. Such a precise control in the local modification of the nanotube wall opens up new opportunities to tailor SWCNT electronic properties at will.

We report the effect of UV/ozone treatment on nanogap electrodes for molecular devices. Gold nanogap electrodes with a nominal gap of 1-2 nm were fabricated by double oblique deposition and the break-junction technique. Self-assembled monolayers (SAMs) of 4,4'-p-terphenyldithiol (TPDT) were formed on the surfaces of the nanogap electrodes, and the electronic properties of these electrodes were measured. The device characteristics were also measured after repeated UV/ozone treatment and SAM re-formation. Although the resistance of the nanogap electrodes increased with the number of UV/ozone treatments, they could subsequently be used for molecular devices. We also observed Coulomb-diamond (CD) structures in the conductance contour plot with respect to the drain and gate voltages even after UV/ozone treatment. Some of the CDs observed after the treatment were aperiodic, presumably reflecting the discrete energy levels in TPDT.

The direct observation of individual molecules in action is required for a better understanding of the mechanisms of biological reactions. We used a high-speed atomic force microscope (AFM) in solution to visualize short DNA fragments in motion. The technique represents a new approach in analyzing molecular interactions, and it allowed us to observe real-time images of biotinylated DNA binding to/dissociating from streptavidin protein. Our results show that high-speed AFMs have the potential to reveal the mechanisms of molecular interactions, which cannot be determined by analyzing the average value of mass reactions. (c) 2006 Elsevier B.V. All rights reserved.

We report on the self-spreading behavior of a supported lipid bilayer (SLB) on a silicon surface with various 100 nm nanostructures. SLBs have been successfully grown from a small spot of a lipid molecule source both on a flat surface and uneven surfaces with 100 nm up-and-down nanostructures. After an hour, the self-spreading SLB forms a large circle or an ellipse depending on the nanostructure pattern. The results are explained by a model that shows that a single-layer SLB grows along the nanostructured surfaces. The model is further supported by a quantitative analyses of our data. We also discuss the stability of the SLB on nanostructured surfaces in terms of the balance between its bending and adhesion energies.

The authors show that 100 nm unilamellar thiol-tagged vesicles bind discretely and specifically to Au nanodots formed on a Si surface. An array of such dots, consisting of 20 nm Au-Si three-dimensional islands, is formed by self-assembly on terraces of small-angle-miscut Si(111) after Au deposition. Consequently, both the formation of the nanopattern and the subsequent attachment of the vesicles are self-organized and occur without the need for any "top-down" lithographic processes. This approach has the potential to provide the basis of a low-cost, high-density nanoarray for use in proteomics and drug discovery. (c) 2007 American Institute of Physics.

The combination of biological systems with artificially fabricated nanoscale structures offers the potential for developing new electronic, photonic, and biosensing molecular-scale devices. In this study, we investigated the use of bacteriorhodopsin (bR), the photoreceptor protein in purple membrane, to form the basis of new optoelectronic devices using nanofabricated silicon structures. The periodical hexagonal shape of bR was observed in liquid by atomic force microscopy. Imaging the suspended purple membrane over artificially fabricated cells is an essential first step in developing a device with a functioning protein. We found that slight modification of the silicon surface or difference in the salt concentration of the buffer affected how the bR attached to the surface. We are still at a very early stage on the road toward optoelectronic devices on a silicon surface.

On Si(113) surfaces there exists a peculiar type of reconstructing blocks called pentamers, which are stabilized by self-interstitial atoms at the subsurface. Here, we demonstrate that such interstitial-pentamers (IP) can be boronized with different numbers of B atoms and therefore behave as differently stable blocks. The multistable features of the blocks are presented by images of scanning tunneling microscopy and investigated by ab initio calculations, which revealed the surface energy as a function of the number of B atoms in an IP and a minimum of the surface energy for three B atoms in each block. Copyright (C) 2006 John Wiley & Sons, Ltd.

We report the electrical characteristics of gold nanogap devices modified by conjugated molecules with thiol endgroups. Gold nanogap electrodes with a nominal gap distance of between 1-2 nm were fabricated by double oblique deposition from opposite directions. The electrodes were then immersed in 4,4'-p-terphenyldithiol (TPDT) solutions to enhance conductance. Using the substrate as a gate electrode, we observed a large Coulomb diamond at room temperature with a charging energy as large as 0.25 eV. The characteristics of the device can be explained on the basis of a multi-metallic-island system. It is considered that ultra small gold islands are formed during the metal deposition and that the molecules bridging metallic islands and electrodes work as tunnel junctions.

We implanted B atoms in Si(113) and annealed the samples to make B segregate to the surface. A series of Si(113)-3 x 1:B surfaces with different B-induced features have been observed by scanning tunneling microscopy. It is demonstrated that on a Si(113) surface B is favorable to be self-interstitials underneath a type of surface reconstructing blocks called pentamers and such pentamers can be boronized with different numbers of B atoms and therefore appear at different levels of electronic states. (c) 2005 Elsevier B.V. All rights reserved.

A layered Li-Ni-Ti oxide including divalent nickel and tetravalent titanium with a Ni2+/Ti4+ ratio of almost I was obtained by the ion exchange of a layered Na-Ni-Ti oxide precursor. By using various nickel and titanium sources, we obtained samples with different specific surface areas. Sample with larger specific surface areas had larger first charge capacities. However, a large irreversible capacity and poor cyclability were observed for all the samples when measured at room temperature. However, when we set the ambient temperature at 55 degrees C, the cyclability improved. This suggests that the lithium diffusion rate strongly affects the electrode performance of layered Li-Ni-Ti oxide. We also employed a first-principles calculation of the LiNi0.5Ti0.5O2/Ni0.5Ti0.5O2 system to evaluate its structural and voltage characteristics. (c) 2005 Elsevier B.V. All rights reserved.

With the help of scanning tunneling microscopy observations and first-principles calculations, we demonstrate that B preferential occupation at self-interstitial sites of Si(113) induces 3 x 1:B surfaces made up of adatoms and interstitial pentamers. The B atoms may serve as adatoms, while the interstitial pentamers may be boronized with different numbers of B atoms owing to the self-interstitial effects of B atoms. Our findings indicate that a Si(113) surface may be doped with B to an extremely high level. (C) 2004 Elsevier B.V. All rights reserved.

We demonstrate the selective formation of Ge nanostructures on Si(1 1 1) surface with the aid of step-band networks. Step flow growth in the step-band region decreases the critical thickness for transition from 2D to 3D growth. Under the step flow growth, step bunching and faceting are enhanced and the step edge with concentrated strain becomes the nucleation site. The spatial arrangement of Ge islands can be controlled by changing the growth mode locally. (C) 2004 Elsevier B.V. All rights reserved.

Ge growth on stepped Si(111) surfaces misoriented toward <(1) over bar(1) over bar2>, <1 (1) over bar0> and <11 (2) over bar > was investigated with a scanning tunneling microscope (STM) on the same Si(111) substrate, where these different steps exist around the sidewall of a hole fabricated photolithographically. After high temperature (1200 degreesC) annealing for several minutes, the steps on the sidewall of a hole exhibit the threefold symmetry, which consists of debunching <(1) over bar(1) over bar2>-type steps and heavy bunching <1 (1) over bar0> and <11 (2) over bar >-type steps. After Ge deposition of up to three bilayers of Ge at 400 degreesC, the distribution of three-dimensional (3D) islands becomes selectable on the three types of stepped surfaces. The step-flow growth in the step band region decreases the critical thickness of the transition from two-dimensional to three-dimensional growth. The surface energy at the step bunch and Ge islands become more important than the strain energy, and selective growth of Ge islands is realized. (C) 2004 Elsevier B.V. All rights reserved.

It is observed that a Ge wetting layer on Si(113) changes its surface structure from an initial Si(113)-3 x 2 to Ge/ Si(113)-3 x 2 + 3 x 1, Ge/Si(113)-3 x 2 + 3 x 1 + 2 x 2, and finally Ge/Si(113)-2 x 2 for a few-ML epitaxial growth. It is demonstrated that such transition results from the epitaxial stress evolution. Specifically, the 2 x 2 confines the epilayer along the [33 (2) over bar] direction by tension and releases compression in the perpendicular direction of the [(1) over bar 10], forming trenches. Without reconstruction, the trenches are favorable for adatom nucleation and elongated growth of islands owing to dangling bonds and possible relaxation of the structure along the trenches. (C) 2004 Elsevier B.V. All rights reserved.

By performing epitaxial growth of Ge on Si(113) layer by layer, observed using scanning tunnelling microscopy, we show the Ge/Si(113) surface structure and island shape transition with an increase of Ge coverage and we discuss the transition in terms of epitaxial stress evolution and relaxation of the epitaxial morphologies. Nanofeatures of Ge epitaxial islands, such as nanowires and nanodots, are highlighted. Copyright (C) 2004 John Wiley Sons, Ltd.

A unique self-organization of optically active nanoparticles of CdS on a silicon surface forming nanoring structures (see Figure and inside cover) is presented. Tunneling luminescence mapping on these structures show that there is direct correspondence between the morphology and the light emission, suggesting that this novel nanomaterial system could make a significant impact in a wide variety of fields including quantum optics.

We have investigated the atomic structure and electronic states for the Ge/Si(113)-2×2 surface using first-principles total energy calculations. We have found that the model made up of alternating [11̄0]-oriented rows of rebonded atoms and tilted pentamers of five atoms with an interstitial atom has the lowest surface energy of the models employed. Furthermore, the local density of states calculated for this surface provides a satisfactory description of recent scanning tunneling microscope images. © 2003 Elsevier Science B.V. All rights reserved.

We have investigated the atomic structure, electronic states, and stress tensor for a Ge/Si(113)-2×2 surface by using first-principles total energy calculations. We have found that the model made up of tilted pentamers with an interstitial atom and rebonded atoms has the lowest surface energy of the models employed. The local density of states calculated for this surface provides a satisfactory description of recent scanning tunneling microscope images. Furthermore, it has been found that the surface stress is quite anisotropic, resulting in anisotropic growth of Ge films on Si(113).

We have investigated the strain state in Ge nanowires on Si(113) substrate using medium-energy ion scattering. We found that nanowires have negligibly relaxed compressive strain along their length, but the strain across them is almost totally relaxed. Anisotropic strain relaxation plays a role in determining the width of the nanowires.

The final goal of nanostructure integration based on self-assembly is full-wafer design of whole atomic-level structures. Toward this goal, we must first be able to control atomic steps, reconstructed domains, surface strain, and atomic species. Atomic steps can be rearranged artificially in a large area using lithographic technique and we are now close to achieving complete control of step positions. Patterns of reconstructed domain regions can be ordered by self-organization. In nanostructure self-assembly, such as the coherently grown Ge quantum nanostructures on Si(0 0 1) and Si(1 1 3) surfaces, strain engineering is important for controlling position, shape, and distribution. Ordered Ge-island chains on Si(0 0 1) show that artificial strain distribution design is a powerful tool for nanostructure integration. Surface composition on SiGe mixed surfaces can be reversibly changed by hydrogen adsorption and desorption. These approaches to designing surface structures show that the bottom-up approach is a promising alternative in semiconductor integration technology. (C) 2002 Elsevier Science B.V. All rights reserved.

Based on scanning tunneling microscopy observations of the epitaxial growth of Ge on Si(113) and first-principles total energy and band calculations, we demonstrate that the Ge/Si(113)-(2 X 2) surface is made up of alternating [(1) over bar 10]-oriented rows of rebonded atoms and tilted pentamers of five atoms, where each pentamer is stabilized by an interstitial atom at the subsurface. From the existence of stacking defects in rows of tilted pentamers observed at room temperature, we have deduced that at epitaxial temperatures the pentamers frequently change their tilting orientations between two minimum energy states.

Structure change of Ni(1 ML: 7.83 x 10(14) atoms/cm(2))/Si(1 1 1) by post-annealing was observed by reflection high energy electron diffraction, atomic force microscopy (AFM), medium energy ion scattering (MEIS) and photoelectron spectroscopy. The AFM observation showed dramatic change of the surface morphology after the Ni deposition at room temperature (RT) followed by annealing at 400, 600, 700 and 800 degreesC for 2 min in an ultrahigh vacuum. MEIS using 70 keV He+ ions analyzed the depth profiles of Ni and the crystallographic structure of the Ni-composites formed by annealing. The valence band and the Si-2p and Ni-3p core level analyses using synchrotron-radiation light showed that the NiSi phase appeared by 1 ML-Ni deposition at RT and both NiSi and NiSi, islands were formed by annealing at 400 degreesC. Annealing at 600 and 700 degreesC led to growth of the B-type NiSi2 islands with height of four and six Si-Ni-Si triple layers. After annealing at 800 degreesC three-fourth of the deposited Ni atoms were dissipated from the surface and the dominant surface structure was the I x I-ring clusters accompanied by a small amount of root19 x root19 phase. The present analysis clearly showed the structure change of Ni (1 ML)/Si(1 1 1) by post-annealing and provided the information about the kinetics for the Ni-Si system. (C) 2002 Published by Elsevier Science B.V.

The atomic structures of Ge/Si(113)-(2×2) surface were studied. The study was based on scanning tunneling microscopy observations of the epitaxial growth of Ge on Si(113) and first-principles of total energy and band calculations. It was demonstrated that the Ge/Si(113)-(2×2) surface was made up of alternating [110]-oriented rows of rebonded atoms and tilted pentamers of five atoms. The observations of stacking defects rows of tilted pentamers at room temperatures showed that at epitaxial temperatures the pentamers frequently change their tilting orientations between two minimum energy states.

Based on scanning tunneling microscopy observations, we have investigated the formation and self-stabilization of Ge nanowires on Si(1 1 3) during Ge deposition. Under zero Ge flux, we observed a shape transition from nanowires to dot-like islands by annealing at 430degreesC, which is a favorable temperature for nanowire formation during deposition. The nanowires are, therefore, metastable and are formed Under a kinetically limited growth condition. We find that the strain of the nanowire is relaxed anisotropically. During growth the nanowire shape is effectively self-stabilizing. which leads to elongated growth of the islands. (C) 2002 Elsevier Science B.V. All rights reserved.

Nanofabrication based on the atomic structures of the substrate is called a bottom-up approach. Since individual nanostructures self-assemble in this approach, methods of controlling size, shape, position, and spacing are vitally important. Techniques for atomic step arrangement control allow the steps to be used as a template for controlling the self-assembly of Ge nanostructures on Si(l 11). Strain engineering is particularly important in controlling Ge nanostructures on Si(001). Strain engineering can also be used for shape and spacing control. In realizing the concept of the self-assembled nanoarchitecture, an interconnection technique is equally important to the semiconductor nanostructures themselves. As a self-assembling nanointerconnection material, carbon nanotubes have excellent properties. Bridging Si dots using carbon nanotubes; grown by chemical vapor deposition is demonstrated.

Based on the scanning tunneling microscopy observations of Ge coherent growth on Si(113), we demonstrate that the anisotropy of substrate stiffness is responsible for the anisotropic relaxation of islands, which leads to island elongation perpendicular to the softer direction of the substrate surface. The transition from wire-like islands to dot-like islands indicates that relaxation of islands tends to become isotropic as the size of the islands increase. Island volume measurements reveal that the material grown on the substrate, including the wetting layer, is continuously rebuilt during island formation and transition. (C) 2001 Elsevier Science B.V. All rights reserved.

The disordering of Si(111) and Si(001) surfaces at high temperatures was investigated using medium-energy ion scattering (MEIS). We clearly observed an increase of MEIS scattering yield on channeling geometry at 1470 K for (111) surface and at 1520 K for (001) surface. These results support the formation of a liquid-like layer at the transition. We also found that there is a distinct difference in the mode of disordering on Si(111) and Si(001) surfaces. The number of disordered atoms increases suddenly at the transition and remains constant above the transition on Si(111) surface. This picture of the disordering is quite similar to the incomplete surface melting of Ge(111) surface. On the other hand, the thickness of disordered layer on Si(001) surface continuously increases with temperature. ©2000 The Japan Society of Applied Physics.

The mechanism of the surface segregation and interdiffusion of Ge on Si(001) during epitaxial growth and annealing was studied using medium-energy ion scattering (MEIS). One monolayer of Ge was grown on Si(001) at 450 degrees C and annealed at 780 degrees C. The Ge was found to redistribute during the very early stages (within 1 min) of the annealing process and remained constant even after annealing at longer periods of time. Therefore, we conclude that the top few layers attain thermal equilibrium in a short period of time, such as less than 1 min. The coverage dependence of Ge distribution in the top few layers was also observed clearly from the surface of Ge deposited on Si(001) at 780 degrees C. This study strongly suggests that the subsurface phenomena should be considered in order to more accurately describe the Si/Ge interface structure. (C) 2000 Elsevier Science S.A. All rights reserved.

We have measured continuum electron yields from Si crystals covered with noncrystalline Si layers of 150-350 angstroms thicknesses using 8.0 MeV/u H+ and He2+. These layers provide unshadowed subsurfaces from which target electrons are recoiled under channeling incidence conditions. The production and escape processes of the electrons can be well understood using a few quantities determined from these experiments. The present experimental approach allows general understanding of the continuum electron spectra from solid targets.

Ion-reduced electron emission has been studied for Si crystal targets bombarded by approximately 100 keV/u H+, He+, and 3.5 MeV/u O8+. Under channeling incidence conditions, we have measured the dependence of the continuum electron yield on the thickness of an overlaid noncrystalline Si layer which increases the number of unshadowed atoms near the crystal surface. The electron yields in the channeling and nonchanneling cases are successfully accounted for using the two parameters determined from the experiments, i.e., the effective target thickness associated with the high-energy shadowing effect mainly on Si L-shells and the effective escape length for the electron yield.

In this paper, we describe the nano-interface engineering of Co/Ge/Si system, involving interface reactions. Controlled annealing causes the Co atoms to diffuse through an already formed quantum dot network fabricated on a silicon substrate and get incorporated as epitaxial CoSi2. Additionally, we performed in situ examination of the early stages of oxide mediated epitaxial growth of silicide at the SiO2/Si interface on a Si surface. Cobalt, deposited at room temperature on an SiO2 layer, diffuses through the oxide layer and forms epitaxial CoSi2 at the interface, preserving the original oxide surface morphology. These results suggest the possibility of an alternative and visible method to introduce functionality into nanostructures.

The Ge segregation mechanism during Si/Ge multilayer growth has been investigated using medium-energy ion scattering (MEIS) and computer simulation with the stochastic Monte Carlo technique. We found that Ge atoms segregated not only in the topmost layer but also in the second and third layers. The segregation probability is found to be quadratic in the Ge concentration at the surface. The exchange between the second and the third layer also occurs readily during growth, even although these layers are buried and therefore are not expected to reduce the surface free energy. We found that the activation energy for the site exchange process at the subsurface is lower than the previously believed value, and flip-flop exchange at the subsurface plays a more important role in the Ge segregation process. (C) 1999 Elsevier Science S.A. All rights reserved.