Wan Soo Yun

Room temperature observation of single electron tunneling effect in self-assembled metal quantum dots on a semiconductor substrate

We report on the observation of room-temperature single electron tunneling phenomena in a metal-insulator-metal-semiconductor double-junction structure. The nanosized Ag dots were self-assembled on a Sb-terminated Si(100) surface, and the Coulomb gap and staircases were observed in the local current–voltage (I–V) measurements using scanning tunneling microscopy. The I–V characteristics exhibiting the single electron tunneling behavior vary significantly with the variation of the measurement position within the same Ag droplet. These phenomena are well described by the tip-dot(Ag)-Si double-junction picture.

Self-organization of uniform Ag nano-clusters on Sb-terminated Si(100) surface

Nanometer sized Ag clusters were found to be uniformly formed in the initial stage of Ag growth on Sb-terminated Si(100) surfaces. Due to the saturation of Si dangling bonds by Sb adatoms, Ag clusters were grown on the Sb-terminated Si(100) surface without a Ag wetting layer. We found that the diameters and heights of Ag clusters were confined to a nanometer scale, and the size distribution was quite uniform compared to Ag growth on Sb-terminated Si(111). Those features are considered to result from the separation of Ag clusters by coherently aligned voids in the underlying Sb-terminated Si(100) surface. Tunneling spectroscopy measurements showed that the local conduction properties of Ag clusters gradually changed from semiconducting to metallic as Ag coverage increased.

FABRICATION OF LATERAL SINGLE-ELECTRON TUNNELING STRUCTURES BY FIELD-INDUCED MANIPULATION OF AG NANOCLUSTERS ON A SILICON SURFACE

Nanostructures composed of Ag clusters on an Sb-terminated Si surface were designed in a highly controlled manner and the electric conduction through Ag nanoclusters to the silicon substrate was investigated by using a scanning tunneling microscopy/spectroscopy. It was found that the lateral conduction between neighboring Ag clusters significantly contributed to the tunneling current–voltage characteristics, and the metallic single-electron tunneling structures employing the lateral conduction channels at room temperature can be fabricated via a field-induced manipulation of Ag clusters.

Coulomb staircases by lateral tunneling between adjacent nanoclusters formed on Si surfaces

We have investigated the oscillatory tunneling current–voltage characteristics on metal nanoclusters formed on Sb-terminated Si(100) surfaces by using scanning tunneling microscopy/spectroscopy. Through the systematic investigation on a variety of cluster configuration environments, we suggest that the lateral tunneling between adjacent clusters dominantly contributes to the occurrence of the single electron tunneling phenomena. In the single clusters formed on Si surfaces, we detected only current oscillations, which must be distinguished from Coulomb staircases. Those results strongly suggest that Coulomb staircases should not originate from the direct conduction of electrons through Schottky junction between the single clusters and Si substrates in contrast to other previous reports.

Scanning tunneling microscope study of Sb/Si(111)-53×53 structure

We have investigated Sb overlayer structures on Si(111) with low energy electron diffraction and scanning tunneling microscope (STM). The Sb/Si(111)-53×53 structure was constructed via the desorption of Sb from the saturated 2×1 surface at elevated temperature (700–750 °C). Its atomic structure and formation process were extensively studied with STM images, considering the structural stability. This Sb structure could be described in terms of the site selective replacement of outermost Si atoms with Sb atoms in the dimer-adatom-stacking fault 5×5 structure, which is generated by both the saturation of dangling bonds and strain due to the large lattice mismatch.

Selective Manipulation of Ag Nanoclusters on a Passivated Silicon Surface Using a Scanning Tunneling Microscope

A precise nanofabrication method using a scanning tunneling microscope (STM) tip as nanoscale tweezers was devised. With the aid of surface passivation by Sb, we could form Ag nanoclusters on a Si(100) surface. It was found that self-organized Ag clusters can be selectively detached and manipulated at room temperature by field application because of the weak bonding strength between the clusters and an Sb-terminated Si surface.

Field-induced manipulation of Ag clusters for tailoring of nanostructures on a silicon surface

We devised new nanofabrication methods using a manipulation of self-organized Ag clusters on Sb-terminated Si(100) surfaces by a scanning tunneling microscope (STM) tip. Various kinds of nanostructures could be manufactured by dot-by-dot manipulation. We found that two methods could be used for those fabrications; (1) Ag clusters could be detached and redeposited by a field-induced manipulation using an STM tip and (2) Ag clusters could be also detached from the surface via the formation of mechanical point contact between the tip and clusters. These fabrication methods were systematically investigated with variation of manipulation conditions such as a bias voltage and a tip-sample distance.

Evolution of surface morphology in the initial stage of nitridation of the Si(111)-7×7 surface by nitrogen ions

The evolution of surface morphology in the initial stage of nitridation of Si(111)-7×7 has been investigated by using a scanning tunneling microscope (STM) and low energy electron diffraction (LEED). The STM and LEED measurements were done on the Si(111) surface nitrided under different experimental conditions including the variations in the nitrogen ion energy, nitrogen ion dose, nitridation temperature, and the postannealing temperature. A growth mechanism of the silicon nitride layer in the initial stage was proposed based upon a comparison of the surface morphology obtained under different nitridation conditions. For the growth of uniform and large silicon nitride islands, it was necessary to have proper heat treatment of the surface. In particular, the surface postannealed at 980 °C after nitridation at 950 °C produced dramatically enlarged flat silicon nitride islands compared to that postannealed at the same temperature after nitridation at room temperature, and is probably due to improved mobiliti...

Thermal Nitridation and Oxygen-induced Etching Reactions: A Comparative Study on Si(100) and (111) Surfaces by Scanning Tunneling Microscopy

Comparative studies of the thermal nitridation and subsequent oxygen-induced etching reactions on Si(100) and Si(111) surfaces were done using a scanning tunneling microscope. Both surfaces were thermally nitrided by exposure to nitrogen gas at 700°C and subsequently reacted with oxygen under an oxygen partial pressure of 1×10-7 Torr. Silicon nano-structures were formed via selective local oxygen etching of silicon using the silicon nitrides as masks against the oxygen exposure. Resultant silicon nano-structures showed distinct differences between the two surfaces. Very narrow size distribution of silicon dots with an average size of ~5 nm was obtained on the Si(100) surface, whereas a broad size distribution of silicon protrusions ranging from 5 to 20 nm was obtained on the Si(111) surface. We discuss the observed differences between Si(111) and (100) surfaces considering the thermal mobility of nitrogen species and the lattice and symmetry mismatches between the silicon nitride layer and Si.

Nanometer scale selective etching of Si(111) surface using silicon nitride islands

Formation of silicon nanopillars via selective oxygen etching of Si(111) surface using silicon nitride islands in the initial stage of nitridation was investigated by scanning tunneling microscopy and low energy electron diffraction. Silicon nitride islands with diameters of 6–15 nm, which were formed by low energy nitrogen ions, were resistive to O2 exposure at high temperatures resulting in silicon nanopillars as high as 2–3 nm. Existence of high density silicon nitride islands is considered to suppress the step flow etching of nearby silicon surfaces, resulting in a spatially nonuniform etching of silicon.