In-Whan Lyo

Field-Induced Nanometer- to Atomic-Scale Manipulation of Silicon Surfaces with the STM

The controlled manipulation of silicon at the nanometer scale will facilitate the fabrication of new types of electronic devices. The scanning tunneling microscope (STM) can be used to manipulate strongly bound silicon atoms or clusters at room temperature. Specifically, by using a combination of electrostatic and chemical forces, surface atoms can be removed and deposited on the STM tip. The tip can then move to a predetermined surface site, and the atom or cluster can be redeposited. The magnitude of such forces and the amount of material removed can be controlled by applying voltage pulses at different tip-surface separations.

Negative differential resistance on the atomic scale: implications for atomic scale devices.

Negative differential resistance (NDR) is the essential property that allows fast switching in certain types of electronic devices. With scanning tunneling microscopy (STM) and scanning tunneling spectroscopy, it is shown that the current-voltage characteristics of a diode configuration consisting of an STM tip over specific sites of a boron-exposed silicon(111) surface exhibit NDR. These NDR-active sites are of atomic dimensions (∼1 nanometer). NDR in this case is the result of tunneling through localized, atomic-like states. Thus, desirable device characteristics can be obtained even on the atomic scale.

Atom‐resolved surface chemistry: The early steps of Si(111)‐7×7 oxidation

The early stages of Si(111)‐7×7 oxidation using scanning tunneling microscopy and scanning tunneling and photoemission spectroscopies have been studied. It has been found that there are at least two different oxygen‐containing sites being formed. Their different surface site distributions and behavior as a function of O2‐ exposure show them to be two distinct early products. By correlating the spectroscopic results and the results of theoretical calculations one is able to identify one of these products as involving an O atom tying up an adatom dangling bond with a second O atom inserted in one of the adatom back bonds, while the other involves a single O atom inserted in an adatom back bond. The preference of these products for the faulted half of the 7×7 unit cell and for corner‐adatom sites is explained in terms of a site‐dependent sticking coefficient involving a process analogous to the gas‐phase ‘‘harpooning’’ processes. It is shown that the majority of the resulting molecular precursors involve O2 ...

Observation of quantum-size effects at room temperature on metal surfaces with STM

Surface steps act as confining barriers for electrons in metal-surface states. Thus, narrow terraces and small single-atom—high metal islands act as low-dimensional, electron-confining structures. In sufficiently small structures, quantum-size effects are observable even at room temperature. Scanning tunneling spectroscopy is used to image the probability amplitude distributions and discrete spectra of the confined states. Examination of the electronic structure of the steps provides evidence for electron-density smoothing and the formation of step-edge states. Estimates of the electron-confining barriers are obtained.

Molecularly adsorbed oxygen species on Si(111)-(7×7) : STM-induced dissociative attachment studies

Scanning tunneling microscope (STM)-induced selective bond breaking in individual molecules and conventional STM imaging are combined to determine the nature of chemisorbed O2 species formed during the initial stages of silicon (111)-(7×7) oxidation. A selective atomic-scale modification mechanism that involves dissociative electron attachment of tip-emitted electrons to empty adsorbate orbitals is introduced. Two molecular species were found: one involves O2 bonded to an already oxidized silicon adatom, and the other involves an O2 molecule that is bonded to a second-layer rest atom and interacting with two silicon adatoms.

Adsorption of boron on Si(111): Physics, chemistry, and atomic‐scale electronic devices

We have used scanning tunneling microscopy, atom‐resolved tunneling spectroscopy, and photoemission to investigate the interaction of B with Si(111). Using decaborane as the source of B, we have followed the structural and electronic modifications of the surface as a function of the annealing temperature. In the stable B/Si(111)‐ 7/8 × 7/8 surface, B occupies a novel configuration where it substitutes for a Si atom in the 3rd atomic layer directly below a Si adatom. Because of a Si‐to‐B charge transfer, the top Si adatom layer has no occupied dangling‐bond states and is insulating. As a result, the chemical properties of Si adatoms on the B/Si(111)‐ 7/8 × 7/8 surface are very different from those of the adatoms on the Si(111)‐7×7 surface. We find evidence for doping effects on chemistry that involve short‐range direct dopant‐reactive site interactions. Finally, we report on the electrical characteristics of localized defect sites on the B‐doped Si surface. We found that I–V curves over such sites may show...

Real space imaging of electron scattering phenomena at metal surfaces

Real space studies of the interaction of the two‐dimensional electron gas provided by metal surface states with localized scatterers are presented. The results involve electron scattering by steps and point defects (adsorbates) at Au(111) and Ag(111) surfaces. These scattering events lead, through interference, to an oscillatory local density of states (LDOS), which is imaged in maps of (dI/dV)/(I/V). Analysis of the LDOS oscillations provides insights into the scattering phenomena involved. We show that the decay of the amplitude of the oscillations as a function of distance from the scatterer can be accounted for by a model that describes the loss of coherence as a result of the wave number (k∥) spread of the states probed by the STM. This model also explains the energy dependence of the amplitude of the oscillations and provides a basis for comparing results from different metal surfaces. Analysis of the properties of the oscillations shows that at low k∥, steps act very much like hard walls isolating ...

STM STUDIES OF THE INTERACTION OF SURFACE STATE ELECTRONS ON METALS WITH STEPS AND ADSORBATES

Abstract Scanning tunneling microscopy (STM) studies have shown that scattering of surface state electrons by steps and adsorbates leads to long-range oscillations in the local density-of-states (LDOS). Here we examine the nature of the states probed in the vicinity of the scatterer. We find that at steps, the LDOS primarily reflects the Smoluchowski charge-redistribution process. Perturbation of the surface state by the step extends more than 10 A from the step-edge and induces bulk-surface state mixing. Unusually narrow spectra and delocalized images are observed for adsorbed sulfur on Ag(111) and are explained in terms of the electronic structure changes induced by chemisorption.

Scanning tunneling microscope tip–sample interactions: Atomic modification of Si and nanometer Si Schottky diodes

In the first part of this article, tip–sample interactions and their use to modify surfaces at the atomic scale are discussed. In particular, a chemically assisted field‐evaporation/desorption process, as a general method for breaking strong chemical bonds and inducing atom‐transfer, is discussed. This capability is demonstrated using the atomic‐scale modification of Si(111). It is further proposed that, due to the elastic coupling between surface atoms, there is a material‐dependent limit to how small and how local a scanning tunneling microscope (STM)‐induced modification can be. Evidence for large‐scale restructuring as a result of ‘‘local’’ modification of the Au(111)‐22×√3 surface is presented. The properties of the actual nanometer‐size contacts of metal tips with Au, Si(111)‐7×7, and Si(100)‐2×1 surfaces, and model nanostructures composed of Si epitaxial islands are then considered. The objective is to investigate the behavior of a simple electronic device, the Schottky diode, at the extreme limit ...

Removal of surface relaxation of Cu(110) by hydrogen adsorption

It is now established that the termination of a metal crystal by a vacuum causes oscillatory relaxation of the interplanar distances near the surface, even when there is no lateral reconstruction. Since the details of the driving forces for this oscillatory relaxation are not fully understood at the present time, we have undertaken low‐energy electron diffraction (LEED)–IV experiments and calculations to study the structure of(1×1) Cu(110) as a function of atomic hydrogen concentration adsorbed at 90 K. At this temperature high‐resolution electron energy‐loss spectroscopy reveals that the hydrogen lies in inhomogeneous sites within the (110) troughs. At most coverages LEED indicates that the hydrogen is disordered. Hydrogen induced changes in the work function and in angle‐resolved ultraviolet photoemission spectroscopy spectra are small, indicating a weak interaction between the hydrogen and copper substrate. This one‐dimensional lattice gas causes a continuous shift in the Cu(110) interplanar distances,...