High-temperature Scanning Tunneling Microscopy Research Articles

In situ high-temperature scanning tunneling microscopy studies of two-dimensional TiN island coarsening kinetics on TiN [formula omitted

In situ high-temperature (1023–1223 K) scanning tunneling microscopy was used to study the coarsening/decay kinetics of two-dimensional (2D) TiN adatom islands on TiN(0 0 1) terraces and in single-atom-deep vacancy pits. Island coarsening/decay behavior was found to be non-linear with time and to depend on the local environment (i.e., on adatom concentration gradients on the terrace), the signature of surface-diffusion-limited kinetics. Two simple island geometries––a single 2D adatom island on an atomically smooth terrace and a single 2D adatom island within a vacancy island––were used to extract adatom surface transport parameters. We model diffusion-limited island decay kinetics for these configurations based upon steady-state diffusion equations solved by adaptive finite-element methods with a form of the Gibbs–Thomson equation describing anisotropic islands serving as the boundary condition. Calculated decay rates were compared with the experimental results to obtain an activation energy E a for adatom formation and diffusion on TiN(0 0 1). E a was found to be 2.6±0.6 and 3.1±0.5 eV for adatom islands in vacancy pits and on terraces, respectively. The difference in the two E a values corresponds to the step edge Ehrlich barrier, which for TiN(0 0 1) is less than the experimental uncertainties in the measurements.

Cubic silicon carbide surface reconstructions and Si (C) nanostructures at the atomic scale

In this review article, I present an overview of the latest investigations of atomic scale ordering and properties of cubic silicon carbide surfaces and self-organized nanostructures. These studies are based on room and high temperature scanning tunneling microscopy (STM), photoelectron spectroscopy using synchrotron radiation and ab initio theoretical local density functional (LDF) calculations. I focus on the Si-terminated and C-terminated β-SiC(100) surfaces only. The ordering of the Si-terminated c(4×2) and the C-terminated c(2×2) surface reconstructions is driven by surface stress which, surprisingly, extends far below the surface. Important issues, such as (i) the atomic control and structure of the c(4×2) and c(2×2) surfaces, (ii) the role of stress in surface and sub-surface organization, (iii) the role of defects and antiphase boundaries, (iv) the existence of a reversible semiconducting c(4×2) to metallic 2×1 phase transition with non-Fermi liquid behavior, (v) the self-formation on the Si terminated surfaces of Si atomic lines and dimer vacancy chains having unprecedented characteristics (massively parallel organization, longest atomic lines ever built on a surface ≈1 μm, highest thermal stability for an atomic line >900 °C), (vi) the high temperature dynamics of these nanostructures and dismantling mechanisms, and (vii) the sp–sp 3 diamond-type surface transformation at high temperature are described and discussed. Overall, SiC surfaces and nanostructures are model systems which are especially suitable for nanophysics and nanotechnologies.

Growth of Si and Ge nanostructures on Si substrates using ultrathin SiO/sub 2/ technology

Using scanning reflection electron microscopy and a high-temperature scanning tunneling microscopy (STM), we study the growth processes of Si and Ge nanostructures on Si substrates covered with ultrathin SiO/sub 2/ films. Si windows are formed in the ultrathin SiO/sub 2/ films by irradiating focused electron beams used for SREM or field emission electron beams from STM tips before or during heating samples. Ge nanoislands are grown only at the Si window positions by depositing Ge on the samples and by subsequent annealing of them. Moreover, Ge nanoislands about 7 nm in size and ultrahigh density (>10/sup 12//cm/sup 2/) are grown on the ultrathin SiO/sub 2/ films. These nanoislands can be manipulated by STM when they are separated from Si substrates by the ultrathin SiO/sub 2/ films. Si, Ge, Ge/Si and Si/Ge/Si nanoislands can also be grown on the Si windows by selective growth using Si/sub 2/H/sub 6/ and GeH/sub 4/ gases. These nanoislands are found to be stable on the Si windows during high-temperature annealing. These results indicate that ultrathin SiO/sub 2/ technology is useful for growing Si and Ge nanostructures on given areas.

CO oxidation on Pt(110): scanning tunneling microscopy inside a high-pressure flow reactor.

We have used a novel, high-pressure high-temperature scanning tunneling microscope, which is set up as a flow reactor, to determine simultaneously the surface structure and the reactivity of a Pt(110) model catalyst at semirealistic reaction conditions for CO oxidation. By controlled switching from a CO-rich to an O2-rich flow and vice versa, we can reversibly oxidize and reduce the platinum surface. The formation of the surface oxide has a dramatic effect on the CO2 production rate. Our results show that there is a strict one-to-one correspondence between the surface structure and the catalytic activity, and suggest a reaction mechanism which is not observed at low pressures.

Absolute orientation-dependent TiN( [formula omitted]) step energies from two-dimensional equilibrium island shape and coarsening measurements on epitaxial TiN( [formula omitted]) layers

In situ high-temperature (1030–1223 K) scanning tunneling microscopy was used to determine the equilibrium shapes of two-dimensional TiN vacancy islands on atomically smooth terraces of epitaxial TiN(0 0 1) layers. Inverse Legendre transformations of the equilibrium island shapes yield relative step energies as a function of step orientation within an orientation-independent scale factor λ, the equilibrium chemical potential of the island per unit TiN molecular area. We then use quantitative TiN(0 0 1) adatom island coarsening measurements to determine λ and, hence, absolute orientation-dependent step energies β and step stiffnesses β ̃ . For 〈1 1 0〉 and 〈1 0 0〉 steps on TiN(0 0 1), we obtain: β 1 1 0 =0.21±0.05 eV/Å, β 1 0 0 =0.25±0.05 eV/Å, β ̃ 1 1 0 =0.9±0.2 eV/Å, and β ̃ 1 0 0 =0.07±0.02 eV/Å. From the β ̃ values, we calculate kink formation energies ε 1 1 0 =0.40±0.2 eV and ε 1 0 0 =0.11±0.1 eV based on the unrestricted terrace-step-kink model.

High-Temperature Scanning Tunneling Microscopy Study of Si(111) Surface Structure Changes Caused by Ga Surface Diffusion

In order to clarify dynamical transition behaviors among Ga-induced superstructures on Si(111) surface, a Ga block was placed on a Si(111) surface elevated at 175°C and 500°C and surface structure changes caused by Ga surface diffusion from the Ga block were in-situ studied by high-temperature scanning tunneling microscopy. Successive structure changes from 7 ×7 dimer adatom stacking-fault (DAS) to 6.3 ×6.3 structure through α′-7×7 were observed in detail on the Si(111) surface at 170°C. No intermediate structure appeared between 7×7 DAS and (\\surd3 ×\\surd3)R30° structures on the Si(111) surface at 500°C.

Absolute TiN(111) step energies from analysis of anisotropic island shape fluctuations.

In situ high-temperature (1165-1248 K) scanning tunneling microscopy was used to measure fluctuations around the equilibrium shape of two-dimensional vacancy islands on TiN(111) terraces. From the equilibrium shape, the ratio of the two <110> step energies was found to be 0.72 +/- 0.02. Combining this with the results of an exact approach for analysis of shape fluctuations, applicable to highly anisotropic islands, we obtain absolute values for step energies and step stiffnesses as a function of orientation.

Formation of anomalously wide Si([formula omitted])[formula omitted] clean surface and its stability

High-temperature scanning tunneling microscope observation has been performed to investigate formation of a Si(111)3×3 structure and its stability on a quenched surface. Formation of anomalously wide Si(111)3×3 domains was observed by increasing adatom density of Si on quenched surface below the transition temperature between disordered “1×1” and ordered simple adatom structures. By adding Si adatoms most of the simple adatom structure transformed into the 3×3 structure. Domain boundaries between adjacent 3×3 domains are straight and along the 〈110〉 direction. The 3×3 structure transforms into the 2×n structure by annealing and Si cluster increases in number to maintain Si adatom density on the surface. This indicates that the 2×n structure is energetically more stable than the 3×3 structure.

Uniform island height selection in the low temperature growth of Pb/Si([formula omitted])-(7×7)

Self-organized, uniform-height, Pb islands with flat tops and steep edges form on Si(111)-(7×7) at low temperatures 120<T<250 K. Islands of heights differing by bilayer height increments are observed depending on growth conditions. The formation of these structures is highly unusual since at low temperatures thermal diffusion is suppressed. The origin of the regular structures is believed to be quantum size effects (i.e. effects related to the quantization of the electron energy levels in the islands). We have studied with two complementary techniques (i.e. high resolution spot profile analysis low energy electron diffraction and variable temperature scanning tunneling microscopy) how the preferred island heights depend on the growth parameters (i.e. temperature, coverage, kinetic pathway etc.). We have constructed a kinetic phase diagram in the coverage–temperature plane which indicates the type of islands formed under different growth conditions. The phase diagram can be used as a guide so the island height can be easily controlled.

TiN(001) and TiN(111) island coarsening kinetics: in-situ scanning tunneling microscopy studies

In-situ high-temperature scanning tunneling microscopy was used to follow the coarsening (Ostwald ripening) and decay kinetics of two-dimensional TiN islands on atomically-flat TiN(001) and TiN(111) terraces at 750–950°C. The rate-limiting mechanism for island decay was found to be adatom surface-diffusion on (001) and attachment/detachment at step edges on (111) surfaces. We have modeled island decay kinetics based upon the Gibbs–Thomson and steady-state diffusion equations to obtain a 001-step edge energy per unit length of 0.23±0.05 eV/Å with an activation energy of 3.4±0.3 eV for adatom formation and diffusion on TiN(001). The activation energy for adatom formation and attachment/detachment on TiN(111) is 3.5±0.3 eV.

High-temperature real-time observation of surface defects induced by single ion irradiation using scanning-tunneling-microscope/ion-gun combined system

A high-temperature scanning-tunneling-microscope (STM)/low-energy ion-gun combined system has been developed in order to clarify the microscopic aspects of annealing processes of ion-irradiated Si surfaces. This system enables us to perform atom-resolved high-temperature STM observation and ion beam irradiation simultaneously in ultrahigh vacuum conditions. Taking great advantage of this system, we have successfully obtained sequential STM images of high-temperature Si(111) surfaces irradiated with 3 keV Ar+ single ions. The STM results have shown that the surface defects induced by single ion irradiation show various changes in size and shape, which is considered to result from diffusion of vacancies and interstitial atoms in the substrates, diffusion of atoms on the surface, and from an anisotropic character of the surface atomic arrangement.

Illuminating structural transformation of Ir(110): A high-temperature scanning tunneling microscopy study

High-temperature scanning tunneling microscope investigation of Ir(110) reveals that the originally ${331}$ faceted room-temperature surface flattens upon heating to form $(1\ifmmode\times\else\texttimes\fi{}3)+(1\ifmmode\times\else\texttimes\fi{}1)$ and eventually $(1\ifmmode\times\else\texttimes\fi{}2)$ missing row reconstructed terraces at about 800 K, thus finally resolving a long-standing controversy on its equilibrium configuration. The surprising structural transformations can be explained by a surface stress related mechanism that is consistent also with the reconstruction behavior of the other low index planes of the noble metals.

In-Situ High-Temperature Scanning-Tunneling-Microscopy Studies of Two-Dimensional Island-Decay Kinetics on Atomically Smooth TiN(001)

In-situ high-temperature scanning tunneling microscopy was used to follow the coarsening (Ostwald ripening) and decay kinetics of single and multiple two-dimensional TiN islands on atomically flat TiN(001) terraces and in single-atom deep vacancy pits at temperatures of 750–950°C. The rate-limiting mechanism for island decay was found to be surface diffusion rather than adatom attachment/detachment at island edges. We have modeled island-decay kinetics based upon the Gibbs–Thomson and steady state diffusion equations to obtain a step-edge energy per unit length of 0.23±0.05 eV/A and an activation energy for adatom formation and diffusion of 3.4±0.3 eV.

High-temperature scanning tunneling microscopy study of the ‘16×2’⇔(1×1) phase transition on an Si(110) surface

High-temperature scanning tunneling microscopy (HTSTM) has been applied to analyze the mechanism of a reversible phase transition between the ‘16×2’ structure and the 1×1 structure appearing on a clean Si(110) surface. The ‘16×2’ structure is the dominant structure on the clean Si(110) surface at room temperature. It has also been shown that the (17,15,1)2×1 steps, whose length is 5 nm in the 〈111〉 direction, exist in the domain of the ‘16×2’ structure. The transition ‘16×2’+(17,15,1)2×1 step⇔(17,15,1)2×1⇔1×1 is observed upon heating, where the first process starts at 968 K and the second finishes at 1043 K. The temperature width between the phase transition is about 80 K. From taking account of the two processes of the transitions, we can say that the system undergoes at first the Ising transition and, at a higher temperature, a roughening transition. Namely, T c (=968 K) is the temperature for the Ising transition and T R (=1043 K) is that for the roughening transition.

IN-SITU HIGH-TEMPERATURE SCANNING-TUNNELING-MICROSCOPY STUDIES OF TWO-DIMENSIONAL ISLAND-DECAY KINETICS ON ATOMICALLY SMOOTH TiN(001)

In-situ high-temperature scanning tunneling microscopy was used to follow the coarsening (Ostwald ripening) and decay kinetics of single and multiple two-dimensional TiN islands on atomically flat TiN (001) terraces and in single-atom deep vacancy pits at temperatures of 750–950°C. The rate-limiting mechanism for island decay was found to be surface diffusion rather than adatom attachment/detachment at island edges. We have modeled island-decay kinetics based upon the Gibbs–Thomson and steady state diffusion equations to obtain a step-edge energy per unit length of 0.23±0.05 eV/Å and an activation energy for adatom formation and diffusion of 3.4±0.3 eV.

Observation of Si(110) Surfaces by High-Temperature Scanning Tunneling Microscopy

The dynamic behavior of Si(110) surface structures at the phase transition temperature has been invesitgated by high-temperature scanning tunneling microscopy (STM). The faceted structures with a (17,15,1) vicinal plane are observed to remain above the phase transition temperature. The STM images reveal that the transition proceeds in units of a pair of up and down terraces at the domain boundary between the “16×2” and 1×1 phases, and that the up-and-down arrangement is found to be an energetically favorable configuration. In addition, rotated double pentagon pairs are observed to hop between the adjacent upper terraces, presumably related to the mass transport of Si across the terraces.

Real time scanning tunneling microscopy study of the initial stages of oxidation of Ni(111) between 400 and 470 K

A real-time in situ study of the initial stages of the interaction of oxygen with Ni(111) in a high temperature scanning tunneling microscope (STM) at temperatures between 400 and 470 K is presented. Oxygen adsorption features, in particular the Ni(111)-p(2×2)O reconstruction, were identified in room temperature STM images. The process of oxide formation is investigated in detail at 470 K. Starting from the initial nucleation at the step edges, a gradual transformation of the terrace structure results in the formation of a three-domain NiO(001) layer finally covering the entire terraces as well as forming bulges at the steps due to surface diffusion of excess Ni. Well-resolved local oxide surface structures are discussed on the basis of a thin (001)-oriented NiO layer model on the Ni(111) substrate. Formation of triangular bulk NiO(111) islands is observed for higher oxygen exposures above 25 L.

High-temperature STM investigation of Au(110), Pt(110) and Ag(110)

Au(110) and Pt(110), which both are (1×2) MR (missing-row) reconstructed at 300 K, are well known to undergo two phase transitions upon heating: an Ising transition, in which the surfaces deconstruct, and a 3D roughening transition. Our real-space investigation, with atomic-scale resolution by high-temperature scanning tunneling microscopy, reveals that Pt(110)(1×2) deconstructs via the formation of 2D island/hole units in accordance with theory. The MR configuration of Au(110) — in contrast to the present theoretical models — is stable close to the roughening temperature. The order–disorder transition seems to be due to the 2D roughening of already-existing step edges. In view of recent findings of tip-assisted movement of single atoms already at moderate tunneling conditions, e.g. on Ag(110), we discuss and eventually exclude the influence of tip/surface interactions on the relevant structural transformations of the two surfaces.

A kinetic scanning tunneling microscopy study of iron silicide growth on Si(113)

High-temperature kinetic scanning tunneling microscopy (STM) studies are used to investigate the surface morphology and growth mode of iron silicide on Si(113) formed by gas-source reactive iron deposition with Fe(CO) 5 as precursor. The first monolayer of silicide on Si(113) forms a (4× n) reconstruction that covers the surface completely before growth proceeds via the formation of strongly anisotropic, three-dimensional silicide islands. After the first monolayer is closed, growth is slowed down by a blocked interdiffusion with the silicon substrate and a reduced sticking probability for the precursor. Lateral spreading of the islands is achieved by a stoichiometric codeposition of iron and silicon using Fe(CO) 5 and Si 2H 6. In this way, nearly closed layers of silicide can be grown.

Facet growth due to attractive step–step interactions on vicinal Si(113)

We have studied the kinetics of facet growth on Si(113) using high temperature scanning tunneling microscopy. At late times in the process, nucleation events are not observed, and the dominant process is zipping up of adjacent step bunches. Facet growth thus results from propagation of the ramified structures consisting of crossing steps trapped during the earlier stages of nucleation and growth. Analysis of the shape of relaxed crossing steps yields a value of the attractive step–step interaction of ∼15 meV Å −1. Crossing steps undergoing zipping maintain a steady-state shape and a constant propagation speed of ∼10 nm 2 s −1. The propagation is quantitatively explained by a simple model which takes into account the line tension and the short-range attractive step–step interactions. The final relaxation of a crossing step to its minimum length configuration is quantitatively described by this model, with no further adjustable parameters.