Advances in Scanning Probe Microscopy

The first-principles simulation method of STM images is introduced and applied to various surface systems. It is clarified how atomic resolution can be achieved by STM, and tip effects are discussed. For the case of scanning force microscopy, we have been able to reproduce FFM images of graphite fairly well by use of a simple tip model involving only a single atom. Cantilever dynamics for dynamic-mode AFM are carefully studied using numerical simulations and analysis of the equation of motion. The mechanisms of dynamic force microscopy are discussed in the light of these results.

Recent applications of first-principles electronic structure methods to semiconductor surfaces are reviewed. The detailed atomic structures and stabilities of both As-rich and Ga-rich GaAs(001) surfaces are determined from the standpoint of equilibrium energetics and theoretical simulations of scanning tunneling microscopy (STM) images. Microscopic diffusion processes of Si adatoms on a hydrogen- terminated Si(001) surface are studied and the effect of hydrogen termination on Si homoepitaxial growth is discussed. Diffusion constants, that is, preexponential factors and activation energies, of Ga and Al adatoms on an As-rich GaAs(001) surface are evaluated from first-principle calculations of migration potentials. On the basis of these calculated results, stochastic Monte Carlo simulations for a GaAs—AlAs binary system are performed.

In this paper we present a critical review of the current status of surface studies on 6H-SiC (0001) and (000\(\bar{1}\)). The article is divided into three major sections. The first consists of a summary of surface preparation procedures and the experimental methods used for the work covered by this article. This is followed by a discussion of the progress which has been made in the determination of the atomic structure and composition of the numerous surface phases of clean 6H crystals. Finally we describe work on surface phonons of 6H-SiC (0001) and adsorption of Ga on the SiC surface as related to developments in the use of 6H-SiC as substrate for growth of III-nitrides.

Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and atom manipulation have been used to fabricate and characterize nanoscale and atomic-scale structures on Si(111)−7x7 and hydrogen-terminated Si(100)−2x1−H surfaces. We first fabricate 0.2–0.6 nm deep and 10 nm wide trench lines by applying a high voltage and/or a large tunneling current between the STM tip and the Si(111)−7x7 surface. When the trench lines form a closed figure such as a circle or a square, the apparent height of the surface area inside the closed trench becomes 0.1–0.3 nm lower than outside. This phenomenon is explained by a naturally formed Schottky barrier between metallic surface states and substrate, and by electronic conduction through the surface states. Atom manipulation is then applied on a hydrogen-terminated Si(100)−2x1−H surface to extract hydrogen atoms one by one and to fabricate atomic-scale dangling bond (DB) structures. Electronic structures of DB wires are studied using STS. The wires composed of both unpaired DBs and paired DBs show a finite density of states at the Fermi level and do not show semiconductive band gaps. The results agree well with first-principles theoretical calculations. The hydrogen-terminated Si(100)−2x1−H surface and its interaction with thermally deposited Ga atoms and C₆₀ molecules is investigated. They migrate on the hydrogen-terminated area and preferentially adsorb on DBs. Several methods for manipulating hydrogen atoms (detaching, attaching and moving) are also tested. Atomic-scale Ga wires on the Si surface are fabricated for the first time by thermally depositing Ga atoms on the DB wires.

Theoretical investigations of electronic structures and other characteristics of fullerenes (C₆₀, etc.) using all-electron mixed basis approaches, which are superior to pseudopotential approaches with regard to core treatments, are reviewed in connection with Scanning Tunneling Microscopy (STM) measurements of fullerenes adsorbed on substrates. A universal feature of the Partial Density of States (PDOS) of the HOMO and LUMO levels is discussed and compared successfully with STM images measured by Hashizume et al. When electron charge transfer occurs from substrates, it is pointed out that anomalous intermolecular interactions may appear between fullerenes, due to induced dipoles. This possibility is discussed in detail using a simple analysis. On the other hand, when fullerene samples are irradiated by laser light etc., it is pointed out that, due to electronic excitations, special fullerene orientation may be favored at surfaces. This behavior is explained in terms of an ab initio result for low-temperature simple cubic phases of C₆₀. Some STM experimental evidence concerning these new features of fullerenes is briefly described and compared with theory.

Tunneling barrier height is one of the most fundamental parameters in STM. It is this barrier height that leads to the exponential gap dependence of the tunneling current and hence enables STM to probe surfaces with atomic resolution. In a simple one-dimensional model of electron tunneling in STM, the barrier height has a definite meaning and its relation to the tunneling current and to the work functions of the tip and sample are both firmly established. In real STM, however, the barrier height derived experimentally from the gap dependence of the tunneling current cannot be simply interpreted as a potential barrier height since it contains complicated contributions from the electronic and atomic structures of the tip and sample. Because of this difficulty in analyzing barrier-height data, STM barrier-height imaging has long remained an auxiliary imaging technique, although it was invented in the early days of STM. However, in recent experiments, barrier-height imaging has been revived to procure unique information which cannot be obtained by other imaging methods. In these experiments, the sensitivity of the barrier height to the sample work function has been utilized for elemental identification on surfaces and for probing local changes in work function induced by adsorption and defect formation. In this article, we will present a brief review of experimental and theoretical studies of the barrier height in STM, together with some recent applications of barrier-height imaging to clean and adsorbed surfaces.

In this paper, after a brief review of the mesoscopic (local) work function, we present our recent studies using Scanning Tunneling Microscopy. We discuss the thickness dependence of the work function for Au/Cu(111) and Pd/Cu(111), the potential profile on Au(111) and Cu(111) steps, and the work function on reconstructed S/Pt (111).

This paper reviews advances towards a comprehensive understanding of the geometric and electronic structures and also the chemical properties of the principal reconstructions found on the (001) surface of III–V compound semiconductors, made during the first decade following the invention of Scanning Tunneling Microscopy in the early 1980s. We consider arsenides, such as GaAs, InAs and AlAs, phosphides, such as GaP and InP, antimonides, such as GaSb, AlSb and InSb, and also nitrides (GaN), with special emphasis on GaAs(001).

We review our systematic investigation of adsorption and film growth of fullerenes on semiconductor and metal substrates using Field-Ion Scanning Tunneling Microscopy (FI-STM). The fullerenes investigated include pristine fullerenes (C₆₀, C₇₀ and C₈₄) and metallofullerenes (Sc@C₇₄/Sc₂@C₇₄, Sc₂@C₈₄, Y@C₈₂ and Gd@C₈₂) and the substrates include Si(100)-2x1, Si(111)-7x7, GaAs (001)-2x4-As, Cu(111)-1x1 and Ag(111)-1x1 surfaces. Fullerene molecules are stable on the Si(100)-2x1 and Si(111)-7x7 surfaces at room temperature because of the strong bonding to substrate dangling bonds. As a result, unique intramolecular structures of the C₆₀ molecule are observed. STM images of the C₈₄, Sc@C₇₄/Sc₂@C₇₄ and Sc₂@C₈₄ molecules are used to obtain information about molecular structures. In contrast to Si surfaces, fullerene molecules are mobile on the terrace of metal surfaces and initially segregate to the step. A well-ordered two dimensional overlayer forms with a close-packed arrangement upon annealing fullerene-covered surfaces. On the Cu(111) surface, the commensurate (4x4) phase forms for the cases of C₆₀, C₇₀ and C_60x C_70(1−x) adsorption, indicating a strong interaction between the Cu substrate and fullerenes. Beautiful intramolecular structures of C₆₀ and C₇₀ molecules are observed and interpreted as local mappings of the electron density of states. One-dimensional cluster formation of C₆₀, Y@C₈₂ and Gd@C₈₂ on the step of the Cu(111)-1x1 surface is analyzed and preferential dimer formation is only observed for the case of Y@C₈₂. This is attributed to the interaction between unpaired electrons of the Y@C₈₂ molecules. On the Ag(111) surface, the C₆₀ and Sc₂@C₈₄ monolayer films exhibit several phases that have an almost identical nearest neighbour distance but are rotated relative to one another. Among the systems studied, C₆₀ on the GaAs(001)-2x4-As surface shows a unique highly-strained fcc(110)-oriented multi-layer film growth, while all other systems show the well known fcc(111)-oriented multi-layer growth.