Toshio Sakurai

Field calibration using the energy distribution of a free-space field ionization

Utilizing free‐space field ionization, a new method of field calibration for field ion emitters has been developed. Plotting the differences of relative energy deficits of free‐space ionized H2, D2, or Kr for sets of various applied voltages V against the logarithm of the voltage ratios yields a field factor k from which the surface field F0=V/krtip is obtained with 3% accuracy. Refinement of this method also enabled us to determine the local radii of the specimen tip. In addition to new and more reliable data of evaporation fields of various materials, such as W, Mo, Rh, Ir, Pt, and Ni, it has revealed several significant facts on the geometrical shape of the emitter.

Field ionization within the forbidden zone

Abstract Mass spectra of helium and neon obtained by field ionization in the presence of hydrogen or another low ionization potential gas contain lines indicating noble gas ions with energies exceeding those of the ordinary field ions by 16 or 13 eV, respectively. These ions originate from the apex-adsorbed state within the forbidden zone following excitation by impinging electrons from free-space ionization of the auxiliary gas.

Field‐ion microscopy of liquid‐metal gallium

Ionization of liquid gallium coated on a tungsten field‐ion tip was investigated using a magnetic‐sector atom probe. For the first time, both the energy distribution of ionized Ga and its critical energy deficit with respect to a tip potential were measured as a function of field and tip temperature. Without heating the gallium, a stable Ga+ ion current of up to 10 μA was obtained from a small surface area of a tip with an energy distribution of less than 12 eV FWHM.

Feasibility of ToF atom-probe analysis of silicon

Abstract Data are presented which establish that silicon with electrical resistivity up to at least 10 Ω cm can be field-evaporated and analyzed in a conventional energy-compensated ToF atom-probe instrument.

Optical emission from Ga ionization at a field emitter

When gallium is ionized on the cap of a field emitter, the optical emission emerges from right at the cap of the emitter. This optical emission was spectroscopically studied using an optical multichannel analyzer with a resolution of 1.3 A over the range 1850–6500 A.The dominant emission is due to Ga I radiation and is likely due to the two‐body collision between atomic gallium and electrons.

Atom probe analysis of segregation in Fe–0.15 wt. % Ti

A compositional analysis of a Fe‐Ti alloy was carried out using a time‐of‐flight atom probe. It was found that titanium atoms are segregated to the surface and grain boundaries after heating to 1073 or 1273 K at 10−6 Pa (10−8 Torr). A decreasing titanium concentration profile extends over several atomic distances and even at 20 atomic distances from the surface the Ti concentration is above the nominal bulk value. In the outermost surface layers and in the grain boundaries titanium is present in the form of oxide, mainly TiO and TiO2, whereas in the subsurface atom‐layer region it is present as randomly distributed titanium atoms and to a lesser degree as clusters of Ti around impurities, such as C, N, and O.

Field ion microscopy of silicon

Abstract The 〈111〉 oriented silicon whiskers were investigated using field ion microscopy (FIM) and atom-probe FIM. We studied a strong effect of light illumination on the field ion image. By illuminating the emitter surface with infrared light, the image quality is improved markedly. We show that this observation is likely to result from the increase of the surface electric field provided by the sharp resistivity drop at the surface oxide layer through photoconductivity effect. Another interesting finding is the anomalous field evaporation of a silicon tip in vacuum. Surface Si atoms evaporate randomly forming clusters of Si atoms instead of evaporating one by one from the surface kink sites. We conclude that this is due to the unique bonding geometry of the tetrahedral crystal structure and due to the field penetration in the near-surface bulk.

Field evaporation experiments with a magnetic sector atom-probe FIM

Abstract A magnetic sector atom-probe FIM has been successfully operated for dc field evaporation of tip materials such as Rh, W, Ir, Mo and Ti. A limited number of evaporated metal ions were clearly identified forming a line spectrum. Field evaporation of Rh in the presence of 3 He and 4 He gases showed that the formation of the helium compound (RhHe) 2+ is quite sensitive to He gas pressure; no helium compound were observed below 5 × 10 −7 Torr and all ions detected as helium compound above 5 × 10 −5 Torr at 78 K.

Time‐of‐flight atom‐probe study of a W‐Zr field emitter

Our high‐performance time‐of‐flight atom probe was employed successfully in the composition analysis of zirconiated tungsten field emitters. Surface compositions were analyzed in situ under various emitter surface conditions with a mass resolution m/Δm≳2000. Our study showed that nitrogen, instead of oxygen, may play a dominant role in the surface reaction and causes the buildup of pure tungsten in the (100) plane.