Mark Lutwyche

Interaction of Ga Adsorbates with Dangling Bonds on the Hydrogen Terminated Si(100) Surface

Adsorption of Ga on the hydrogen terminated Si(100)–2×1–H surface has been investigated by scanning tunneling microscopy (STM). We have found that the thermally deposited Ga atoms preferentially adsorb on the hydrogen-missing dangling bonds and on the surface impurities. We desorb hydrogen atoms by the STM current and fabricate atomic-scale dangling-bond wires, in the similar way as was reported by Lyding et al. [Appl. Phys. Lett. 64 (1994) 2010]. In order to fabricate more detailed dangling bond structures, several methods of manipulating (detaching, attaching and moving) the individual hydrogen atoms are tested. We are able to thermally deposit Ga atoms on a dangling-bond wire and fabricate an atomic-scale Ga wire on the Si surface.

A proposal of nanoscale devices based on atom/molecule switching

This paper proposes a very small switching device, called an atom relay, which would supersede present metal‐oxide‐semiconductor devices for the next decade. The basic configuration of an atom relay consists of an atom wire, a switching atom, and a switching gate, with total dimensions below 10 nm, and an operation speed at more than terahertz level. The operation principle of the atom relay is that a switching atom is displaced from the atom wire by the electric field supplied from the switching gate, and the atom relay exhibits an ‘‘off’’ state. The switching characteristics of the atom relay are demonstrated by simulation, and it is shown that the electron propagation is successfully cut if a gap of about 0.4 nm is formed in the atom wire by the displacement of the switching atom. A self‐relay structure, in which the switching atom is displaced by the electric field from the atom wire itself, enables a dynamic memory cell, and the functions are ascertained by simulation. Fundamental logic circuits as N...

SURFACE MODIFICATION MECHANISM OF MATERIALS WITH SCANNING TUNNELING MICROSCOPE

The surface modification mechanism with scanning tunneling microscope (STM) is investigated. Experiments in both ultrahigh vacuum and air are reported, using several kinds of materials to understand the mechanism systematically. Threshold voltages (Vt’s), which are defined as the voltages above which modification is possible under the STM tip, have linear dependence on the binding energies of the materials. Thus, the STM surface modification mechanism is attributed to the local sublimation induced by tunneling electrons. For the modification in air, it is also ascribed to the chemical reaction induced by tunneling electrons with adsorbed water, and the Vt’s also fit on this line by taking the reaction energy into consideration. Therefore, the process is a direct consequence of the high flux of low‐energy electrons incident on the surface from the STM tip.

Estimate of the ultimate performance of the single‐electron transistor

The scaling limit of current semiconductor devices is thought to be about 100 nm. To reduce the size of devices beyond this point will probably require a new device technology. The metal single‐electron transistor, using the Coulomb blockade effect, has been proposed as a replacement for semiconductor devices. Recently devices of this kind with potentially useful properties have been fabricated. The scaling of such devices down to atomic dimensions is investigated to see if they can compete with semiconductor logic or analog devices. It concentrates on the operation of a single device and not on the effects of integration. Until now such models for the single‐electron transistor have assumed that the capacitance and conductance of the various junctions can be chosen independently, but it is demonstrated that the physical geometry causes restrictions on these choices. A second restriction is that as the device is made smaller the capacitance drops. This means that the temperature of operation rises, but so...

Observation of a vacuum tunnel gap in a transmission electron microscope using a micromechanical tunneling microscope

This letter reports the observation of the vacuum tunnel gap between two conductors using a high resolution transmission electron microscope. A 2.5 mm square micromachined tunneling microscope chip has been fabricated with a minimum feature size of 0.4 μm. The chip fits into a modified side‐entry type transmission electron microscope holder. The tunnel gap is controlled by a purpose‐built feedback controller. The micromachines work reliably during observation of the tip apex in a transmission electron microscope, allowing the voltage and current to be changed while the tunnel gap is observed.

Nanofabrication of layered materials with the scanning tunneling microscope

Abstract This paper reports selective etching and deposition (nanofabrication) on the surface of layered materials under the scanning tunneling microscope (STM) tip. Materials such as highly oriented pyrolytic graphite (HOPG), MoS 2 , NbSe 2 and Bi 2 Sr 2 CaCu 2 O x are investigated in air and in ultra-high vacuum. The etching is performed by applying positive substrate bias voltages. Negative bias voltages cause atom clusters to be deposited on the surface from the tip. Threshold bias voltages ( V t ) for etching and deposition are defined as the voltages above which nanofabrication is possible with the STM. Comparison of the V t values with the binding energies of materials leads us to attribute the etching mechanism in UHV to the sublimation of surface atoms induced by the tunneling electrons. On the other hand, oxidation by adsorbed water lowers the energy for etching in air. Furthermore, using the relationship between the V t and the binding energy of the materials, the possibility of identification of atoms with the STM is discussed.

Time-resolved observation of thermal oscillations by transmission electron microscopy

Time-dependent acoustic oscillations driven by stochastic thermal force have been observed by means of transmission electron microscopy. An electron-beam current passing in the vicinity of the edge of the vibrating sample, and thereby modulated, was led through an aperture at the image plane and measured with the electron counting technique as the power spectral density function, allowing the resonant frequency and the Q factor to be found. This enables estimation of the Young’s modulus and the internal friction. The method can be extended to the investigation of the elastic properties of nanoscaled samples.

Atom structures on the Si(100) surface

Abstract We have developed a method of fabricating metal-atom structures on a Si(100)-2 × 1-H surface by scanning tunneling microscopy (STM). The atomic structures can be connected to bulk electrodes formed in situ of the STM. We first fabricated atomic-scale dangling-bond structures by STM manipulation of hydrogen atoms. Using the difference in adsorption energy of Ga atoms on the hydrogen terminated surface area with dangling-bond patterns, we have thermally deposited Ga atoms and fabricated atomic-scale Ga structures on the Si surface.

Evaluation of Thin Silicon Dioxide Layers by Beam Assisted Scanning Tunneling Microscope

The electronic structure of thin thermal silicon dioxide ( SiO2) layers are evaluated by beam assisted scanning tunneling microscope (BASTM). The BASTM operation principle is that an insulator sample is exposed with an electron beam, which excites electron-hole pairs in the insulator and makes the sample conductive. 100 nm wide line and space patterns are delineated by electron beam lithography and dry etching in a 10 nm thick thermally grown silicon dioxide layer, and are observed by BASTM to ascertain the capability. Current-voltage (I-V) characteristics are measured for silicon dioxide layers with thicknesses between 1.8 and 4.5 nm by tunneling spectroscopy. The results indicate that both the band gap and barrier height have no dependence on the silicon dioxide layer thicknesses.

SCANNING TUNNELING MICROSCOPE MEASUREMENT OF INSULATOR SURFACES

The possibility of measuring insulator surfaces with a scanning tunneling microscope (STM) is demonstrated. The mechanism is attributed to the conduction caused by electron beam assisted carrier generation in the insulator. A scanning electron microscope is used as the electron source. A 10‐nm‐thick and 100‐nm‐wide line‐and‐space patterned silicon dioxide layer formed on silicon substrate is observed by the STM only when an electron beam is directed at the sample. Tunneling spectroscopy results indicate a band gap of about 5 eV, which is attributed to that of silicon dioxide of about 8 eV.