M. Iwatsuki

Development of a scanning atom probe

In order to extend the applicability of the conventional atom probe (AP), a new AP named ‘‘scanning atom probe (SAP)’’ is proposed. The SAP has a microextraction electrode which scans over a flat specimen surface with many microcusps. Then the electrode and a cusp forms a minute field ion microscope. Calculation of field distribution in the confined small space between the cusp apex and the electrode indicates that the field can be high enough to field evaporate the apex atoms and that the atom‐by‐atom mass analysis by the SAP is highly feasible. Accordingly, a trial SAP is under construction modifying a low‐temperature UHV scanning tunneling microscope.

Observation of surface reconstruction and nano-fabrication on silicon under high temperature using a UHV-STM

Abstract In this paper we discuss three experiments on silicon (100) and (111) surface at high temperature obtained with an ultra-high-vacuum scanning tunneling microscope (UHV-STM). First, the initial stage of the crystalization process has directly been investigated on the Si(111) surface at a phase transition temperature of about 860°C. (7 × 7) domains nucleated from the step edges and expanded towards inner regions of the terraces, and the steps become straight [ 1 ¯ 1 ¯ 2 ] steps. Second, the high-temperature nano-fabrication method has been applied to Si surfaces. We succeeded in creating a hexagonal pyramid and a crater on the Si(111) surface, and a quadrangular pyramid on the Si(100) surface at 600°C. (5 × 5) domains can be observed on narrow terraces due to the relaxation of surface energy. Finally, we attempted to deposit gold (Au) atoms on silicon surfaces. Au atoms deposited on a high-temperature silicon surface migrated to the observation area while forming 5 × 1 structures. Then the Au atoms diffused into the bulk structure of silicon, and silicon (7 × 7) domains covered the surface again.

Atomic level analysis of electron emitter surfaces by the scanning atom probe

Abstract Fine grains of chemical vapor deposition (CVD) diamonds and silicon tip apexes of a microtip array are mass-analyzed at atomic dimension by a scanning atom probe (SAP). The study revealed that the CVD diamonds contain a large amount of hydrogen, which is field-evaporated as carbon–hydrogen cluster ions. Some hydrogen atoms are weakly bound with the carbon–hydrogen clusters by hydrogen bonding and are released from the clusters while flying from the diamond surface to the detector. The depth profile of the hydrogen distribution indicates that hydrogen concentration decreases logarithmically with depth. The analysis of the silicon tip apex also revealed that the tip apex contains a significant amount of carbon and hydrogen. The silicon–carbon ratio of the uppermost surface layer is as high as 1:1 and rapidly decreases with depth and approaches nearly 2% at a depth of 20 nm. This suggests that the carbon might be intermixed or absorbed during the fabrication and/or dry etching process of the microtip array.

Performance of the trial scanning atom probe: New approach to evaluate the microtip apex

In order to examine the feasibility of realizing the scanning atom probe, a trial instrument was constructed by modifying a low‐temperature ultrahigh vacuum scanning tunneling microscope (STM). A scanning electron microscope was mounted on the trial instrument to inspect the relative position of both the open hole at the end of the funnel‐shaped extraction electrode and a cusp apex on a specimen surface. In the preliminary experiment, a single sharp W tip of the STM and two extraction electrodes with respective open hole diameters of 10 and 20 μm were employed. The tip was brought near the electrode by employing a microscrew and a piezoactuator as with a conventional STM. The variation of the field emission current with the position of the center of the hole relative to the cusp apex was found to be significant enough to bring the apex precisely to the center of the hole. An interesting finding was that the field emission was observed from a tip with a radius of more than 2 μm at a relatively low voltage,...

Scanning tunneling microscopy observation of Al‐induced reconstructions of the Si(111) surface: Growth dynamics

The atomic structure of α‐7×7 and γ‐phase structure has been investigated with use of scanning tunneling microscopy. Structural models for both phases are presented where both structures are based on the dimer adatom stacking‐fault structure. Continuously linked boundaries are observed between these phases, in contrast to those between α‐7×7 and √7×√7 phase. It is found that the properties of the boundaries are closely related to the basement structure. Phase transitions appearing at higher or lower temperature are discussed in terms of growth dynamics.

Scanning tunneling microscopy study of solid‐phase epitaxy processes of argon ion bombarded silicon surface and recovery of crystallinity by annealing

The solid‐phase epitaxy (SPE) processes of amorphized Si surfaces by Ar+‐ion bombardment are investigated in situ using a scanning tunneling microscope. As‐bombarded Si(001) surfaces consist of grains 0.63–1.6 nm in diameter. The grains gradually coalesce and forms clusters 2–3.6 nm in diameter at an annealing temperature of 245 °C. Prolonged annealing at this temperature promotes crystallization of the amorphous layers; (2×1) and (1×2) reconstructed regions surrounded by amorphous regions are partially observed on the surface, which suggests the onset of SPE. Successive observation reveals that the smoothing of the surface occurs layer by layer. As the annealing temperature is raised to 445 °C, the amorphous layer epitaxially crystallizes up to the topmost surface, and a (2×1) reconstructed surface with monatomic height steps is observed. The smoothing of the step edges is observed during annealing at 500 °C. The behavior of the defects during the recovery of crystallinity is also described.

Solid phase epitaxy processes of amorphized silicon surfaces by Ar-ion bombardment observed “in situ” with ultra-high vacuum tunneling microscopy operated at high temperature

Abstract A clean Si(100) surface is amorphized by Ar+-ion bombardment. The amorphous layer consists of grains of 0.75–1.6 nm in diameter. Solid phase epitaxy (SPE) processes of the amorphous surfaces are investigated “in situ” using ultra-high vacuum tunneling microscopy. The surface morphology during thermal annealing is affected by the Ar+-ion dose. For a lightly dosed case, (2 × 1) domains surrounded by amorphous regions consisting of grains are observed at 245°C which indicates the onset of SPE. It is found that the surface crystallizes and is smoothened in a layer-by-layer mode. As the temperature increases, the surface roughness is greatly reduced and the whole surface showed (2 × 1) reconstruction at 450°C. For a high-dose case, both (2 × 2) and c(4 × 4) reconstructions are observed in limited areas surrounded by amorphous regions during annealing at around 620°C. Pyramidal structures are observed on a (2 × 1) reconstructed surface during annealing at 830°C. The step structure of the pyramidal structure is clarified.

A trial scanning atom probe and field distribution at a tip apex of a micro-tip array

Summary form only given, as follows. A trial scanning atom probe (SAP) was constructed by modifying a low temperature UHV scanning tunneling microscope (STM). The specimen holder of the conventional STM was replaced with a holder of an extraction electrode made of a 10 micrometer thick Pt foil. The specimen micro-tip array is mounted at one end of a piezo tube which allows one to move the specimen with subnanometer precision. Silver foils connecting the cold end of the cryogenic refrigerator and the holder of the piezo tube cool the specimen and piezo assembly down to 50 K. Field emitted electrons and field ionized gas ions project images of an individual tip apex of a micro-tip array at atomic resolution and field evaporated apex atoms fly into the flight space of a reflectron mass analyzer through the probe hole at the center of the screen. The results of preliminary experiments are reported. While constructing the SAP, the field distributions around the tip apex were computed in order to examine the variation of field emitted current with the relative position of the tip apex and the open hole of the extraction electrode. The calculated field distribution is presented, comparing the observed variation of emitted current with the relative positions.

Performance of the trial scanning atom-probe-new approach to evaluate micro-tip array

The scanning atom probe (SAP) was proposed to realize atom-by-atom mass analysis of a flat specimen surface with micro-tip arrays or many microcusps. The unique feature of the SAP is in the introduction of a microextraction electrode to the conventional atom-probe which scans over the specimen surface. In this paper a trial SAP was constructed by modifying a low temperature UHV scanning tunneling microscope (STM) with a scanning electron microscope (SEM) in order to inspect the relative position of the tip and the electrode.

Local atomic structures near the domain boundary between the Al‐√3×√3 and the 7×7 phases on Si(111): Substitutional defects

Both Al and Si substitutional defects have been observed in the 7×7 domain and the √3×√3 domain, respectively, on the specimen prepared by higher temperature annealing up to 900 °C by scanning tunneling microscopy. At positive sample bias (+1 V) the protrusions in the √3×√3 domain appear ∼1 A higher on the average than those in the 7×7 domain, though slight modulation in brightness can be observed in each domain. The image taken at negative sample bias (−1 V) reveals clearly the substitutional defects in both domains. The Si adatoms substituting Al in the √3×√3 domain appear in almost the same contrast as Si adatoms in the 7×7 domain. The Al adatoms substituting Si in the 7×7 domain appear ∼1.5 A higher than Al atoms in the √3×√3 domain. These results suggest a distinct difference in the electronic structure between the Al (or Si) adatoms in both domains derived from the surrounding atomic geometry. In addition, Al preferentially substitutes the center adatom site in the 7×7 unit cell.