Tien T. Tsong

Pulsed‐laser atom‐probe field‐ion microscopy

A time‐of‐flight atom‐probe field‐ion microscope has been developed which uses nanosecond laser pulses to field evaporate surface species. The ability to operate an atom‐probe without using high‐voltage pulses is advantageous for several reasons. The spread in energy arising from the desorption of surface species prior to the voltage pulse attaining its maximum amplitude is eliminated, resulting in increased mass resolution. Semiconductor and insulator samples, for which the electrical resistivity is too high to transmit a short‐duration voltage pulse, can be examined using pulsed‐laser assisted field desorption. Since the electric field at the surface can be significantly smaller, the dissociation of molecular adsorbates by the field can be reduced or eliminated, permitting well‐defined studies of surface chemical reactions. In addition to atom‐probe operation, pulsed‐laser heating of field emitters can be used to study surface diffusion of adatoms and vacancies over a wide range of temperatures. Example...

Atom‐Probe Field Ion Microscopy

Scientists are always pushing to new frontiers, which often involve questions about phenomena that occur on very large or very small scales. Astronomers search for new stars millions of light‐years away in a quest to learn how these stars and the universe were born, and particle physicists look at elementary particles of size less than 10−17 cm in an effort to understand fundamental interactions. Meanwhile, however, many biologists, chemists and condensed matter physicists are trying to understand natural phenomena that we encounter every day and that occur on some intermediate scale. Questions at this scale are posed in terms of interactions between electrons or atoms and chemical bonds or in terms of atomic theories. The motivation is not only scientific curiosity but also a desire to discover new effects, create new molecules and materials, and develop new technologies that may benefit society. Although a single interaction, electromagnetism, determines the chemical and physical properties of molecules...

Field ion image formation

Abstract Field ion image formation involves three basic physical processes: field evaporation, field adsorption and field ionization. Investigations of these processes cannot only help us to properly interpret the data obtained with field ion microscopy, but can also promote our understanding of the electronic and atomic properties of solid surfaces. Recent progress in this area is reviewed with emphasis on those topics requiring further investigation.

Measurement of the Energy Distribution in Field Ionization

The energy distribution of ions obtained by field ionization of He, Ne, Ar, and H2 near the surface of a tungsten emitter is measured with an improved retarding potential tube. Most ions are found to originate immediately beyond the critical distance determined by the condition that the electron tunnels above the Fermi level. The ion energy is determined by the ionization energy and an image force contribution, while there is no indication of a shift of the ground level of the gas atom due to the proximity of the metal surface. In the case of hydrogen the atomic ion is found to have an additional energy of ½ of the dissociation energy of the molecular ion. The half‐width of the distribution is between 0.5 and 0.7 eV for all the above gases, indicating an extremely narrow ionization zone with a depth of a few tenths of an angstrom. Calculation of the energy distribution by applying the WKB method to a square well gives the proper general features and the temperature effect but a half‐width too wide by a fa...

Direct observation of surface diffusion and atomic interactions on metal surfaces

Abstract The unique attributes of the field ion microscope permit the direct observation of surface diffusion of single atoms and simple atomic clusters on perfect crystal planes. A brief review of the principles and techniques of such studies is presented along with a collection of much of the existing experimental data. Emphasis is placed on more recent work such as the diffusion of adatoms under the influence of a driving force, the kinetics of cluster formation and dissociation, and how the interatomic force between two atoms on a surface is related to the diffusion parameters and the pair distribution function.

Pulsed‐laser time‐of‐flight atom‐probe field ion microscope

A linear‐type pulsed‐laser time‐of‐flight atom probe has been developed. With a flight path length of ∼200 and ∼425 cm, the mass resolution is comparable to the energy focused time‐of‐flight atom probe of much more elaborate mechanical and electrical design. The isotopes of Mo, Xe, and W are completely separated to the roots of the mass lines. We also discuss how the mass resolution of the pulsed‐laser atom probe can be further improved and where this instrument will be useful.

Correlation of real time spectroellipsometry and atomic force microscopy measurements of surface roughness on amorphous semiconductor thin films

We have correlated the results of real time spectroellipsometry (SE) and ex situ atomic force microscopy (AFM) measurements of surface roughness on amorphous semiconductor thin films. Roughness layer thicknesses deduced from real time SE, using a conventional approach based on the Bruggeman effective medium theory, closely obey a relationship of the form: ds(SE)≊1.5 drms(AFM)+4 A, for 10≤dS(SE)<100 A, where drms(AFM) is the root‐mean‐square roughness from AFM. The slope and intercept of this relationship provide insights into the origin and interpretation of the optically deduced roughness layer thicknesses.

Field penetration and band bending near semiconductor surfaces in high electric fields

Abstract The existing theory of band bending in the surface space charge region of semiconductors is adapted to problems in field emission, field ionization and field evaporation. The surface field in the space charge layers of semiconductors appropriate for these phenomena ranges from ~10−2 V/A to a few 10−1 V/A, similar to those encountered in many interface phenomena involving semiconductors. We found that the surface potential resulting from band bending may amount to a few eV. The field penetrates ~10 A into the semiconductor surface for intrinsic cases, and ~200A for an n-type semiconductor in a positive field, or for a p-type semiconductor in a negative field. Both the surface potential and the field penetration will affect significantly the electronic properties of the near surface layers. In particular, the photon adsorption edge will be shifted toward the red by the field penetration effect.

Field Adsorption of Inert‐Gas Atoms

A variational calculation shows that inert‐gas atoms can only be adsorbed above protruding surface atoms, but not on flat metal planes or at surface lattice sites. The calculation also yields the effective polarizability of a helium atom near a metal plane. The inert‐gas atoms are adsorbed at the apex of metal surface atoms by a field‐induced dipole—dipole interaction. A statistical consideration expresses the probability of field adsorption in terms of experimental parameters. Preliminary atom‐probe data on the temperature and field dependence of image gas adsorption agree with the theoretical results.

Measurement of the Polarizabilities and Field Evaporation Rates of Individual Tungsten Atoms

The relative field evaporation rates of tungsten kink site atoms at (110) plane edges and the absolute field evaporation rates of individual tungsten adatoms on tungsten (110) planes have been measured covering nine orders of magnitude. From the experimental data, the effective polarizabilities of the tungsten kink site atoms and the adatoms are found to be 4.6 ± 0.6 and 6.8 ± 1.0 A3, respectively. A theoretical consideration is given to explain why the effective polarizability of metal surface atoms depends on the atom‐to‐surface mirror plane distance, and therefore the atomic sites. The calculation also reveals a first power field dependent energy level shift, which is dependent upon the atom‐to‐surface plane distance. It is further shown that the classical image potential no longer holds at a distance smaller than 1 A from the metal surface.