Field Ion Images Research Articles

FIM observations of W, Ga and Sn structures on W and Mo {001} surfaces

It has been indicated by the LEED studies that the atomic arrangement of the W{001} surface at low temperatures is the c(2 × 2) structure rather than a simple (1 × 1) structure and the different models have been proposed. A field ion microscope has demonstrated that its atomically high resolution can clearly depict the arrangements of Ga and Sn atoms deposited on W and Mo surfaces replicating the arrangements of the substrate atoms. Therefore, it has been realized that the direct observation of the arrangement of the deposited atoms may provide new information on the atomic arrangement of the w{001} planes. Ga showed the (1 × 1) structure on the {001} planes indicating that the arrangement of the deposited Ga atoms is the pseudomorphic layer replicating the underneath W and Mo surface. The (1 × 1) Ga structure was retained even if the field evaporation of the Ga layer proceeded reducing the size of the planes. Sn also formed a pseudomorphic layer on a W surface but the arrangement changed to the c(2 × 2) structure as the field evaporation of the deposited Sn layer proceeded. The observed arrangement change is surprisingly similar to that of W atoms on the {001} planes which has been explained as the result of the preferential field evaporation of alternate surface atoms possibly caused by the periodic perpendicular shifts of the alternate surface atoms. The atomic shifts parallel to the W {001} planes were not resolved in the present field ion images. The stable Ga arrangement and the arrangement change of Sn and W indicate that the arrangement of surface atoms observed by the field ion microscopy may be a faithful reflection of the small difference in a minute atom shift due to the difference in the binding states of the surface atoms.

The shape of field-ion emitters

The determination of the precise geometry of the field-ion emitter from its field-ion image has been a long-standing problem. An analytical method is given which allows the cartesian coordinates (x,y,z) of each imaging site to be determined using data extracted from the micrograph by ring counting techniques. The coordinate data extracted in this way are used to produce profiles of the shape of symmetric and asymmetric iridium and tungsten emitters.

Surface analysis of field-ion samples exposed to the plasma of the impurities studies experiment (ISX-A) tokamak

Measurements of the modifications to the surface of field-emitter samples (tips) exposed to plasma discharges in the impurities studies experiment (ISX-A) tokamak have been carried out for the first time in order to help further identify the physical processes occurring at the tokamak wall during machine operation. The specimen tips were exposed to 40 high-power hydrogen discharges in ISX-A at a position 5.0 cm back from the plasma edge defined by carbon and molybdenum limiters. In contrast to results obtained on silicon samples exposed at a similar location in ISX-A using Rutherford ion backscattering analysis, field-ion images of tungsten tips taken before and after exposure indicate that no structural rearrangement of the specimen near-surface region occurred, even on an atomic scale. Contaminant films 60±10 Å thick, deposited on the specimen surfaces during exposure, were observed in the transmission electron microscope. Control tips, placed in the tokamak during the discharges but shielded from direct plasma exposure, remained free of such deposits. Imaging atom-probe depth profiles into the exposed sample substrates revealed that neither plasma nor impurity species were implanted beyond the deposited surface films. The results from this study are compared with those from earlier analyses of samples exposed in the Princeton large torus (PLT) using similar techniques and also with the results of analyses of macroscopic samples exposed at the same ISX-A location using other surface-sensitive techniques.

Field ion microscopy of silicon

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.

Atom-probe and field ion microscopy of semiconductors

The initial oxidation stages of Ni and Ni-Cu alloys have been studied using the atom-probe field ion microscope (AP-FIM). Oxide layers were formed in situ in the FIM chamber on the clean metal surfaces at 1 × 10−4 Torr (10 −2 Pa) O2 at 870, 970 or 1070 K for less than l min. The atom-probe analysis on the {111} and {100} surfaces of these oxidized alloys showed that only NiO formed on all of the alloys for this relatively low oxygen partial pressure, although incipient Cu2O formation could not be ruled out in a few samples of the Cu-rich alloys. The concentration of oxygen in the NiO fluctuated layer-by-layer in the 〈111〉 direction as expected for its NaCl-type structure. Ne field ion images and atom-probe analyses of the oxidized samples revealed a sharp NiO/metal interface for pure Ni and for the Cu-rich alloys, and a diffuse interface extending over a few atomic layers for the Ni-rich alloys. The NiO and Ni were cube/cube oriented.

The projection geometry of the field-ion image

An expression for the projection geometry of the field-ion microscope is derived and it is shown how a single parameter describing the projection relationship may be obtained from measurements on the micrograph. The relationship of experimentally obtained projections to the stereographic case is discussed, as is the influence of varying emitter radius.

Field ion image qualities of Ga, In and Sn on W and work functions

It has been found previously that Ga, In and Sn form a pseudomorphic monolayer on W which maximizes the work function of the covered surface. In the present work the bright and sharp field ion images of the pseudomorphic monolayer are shown comparing the images of the substrate tungsten. The interesting finding is that the images of the covering layer are most distinct for Sn which gives rise to the largest increment in the work function, and are least for In which has the smallest work function increment. A simple calculation on the penetration probaility of an electron into the potential barrier between an imaging gas atom and a metal surface indicates that the probability increases significantly with the work function and consequently the covered area would be images brightly. The calculation also suggests that the work function, that is the surface charge distribution, is one of the key factors controlling the quality of the field ion images.

Field ion image formation

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.

Field-ion microscopy of GaAs and GaP

Clean surfaces of GaAs and GaP were studied by field-ion microscope (FIM). Field-ion images with ordered surfaces were first obtained in pure hydrogen, neon-50% hydrogen and pure neon gases at 78 K, by using channeltron electron multiplier arrays (CEMA). The field-ion images of GaAs were quite similar to those of GaP with respect to the surface structure and the image contrast. They showed the anisotropies of the ion emission and the surface structure between the [111] and [ 1 1 1] orientations. Ring steps expected from a spherical surface were observed on the (111) and {100} planes, but not on the [ 1 1 1] and {110} planes. The regional brightness of the FIM patterns was discussed in terms of the Knor and Müller model and the atomic and electronic structures of the surface. The image field of these crystals was much lower than that of metals usually used in FIM. For example, the image field strength for the hydrogen and GaAs system was about 1.1 V/Å. The reduction of the field necessary to image was also discussed in terms of the field penetration effect.

Computer-Simulated Field-Ion Images of Ni4Mo Alloy for Statistically Homogeneous Ordering

Computer-Simulated Field-Ion Images of Ni4Mo Alloy for Statistically Homogeneous Ordering

Surface studies with an imaging atom-probe

The imaging atom-probe (TAP) is an atom-probe field-ion microscope in which the conventional time-of-flight mass spectrometer is replaced by a nanosecond-gated channel-plate image intensifier which is used to record the arrival positions of ions of preselected mass-to-charge ratio which have been field-evaporated from the surface of a field-ion emitter. The evaporated metal ions have the same radial projection that is seen in the normal field-ion image and the recorded image displays the points of origin of the ions with a spatial resolution of 1 nm or better. The field of view covers a selected area of about 0.3 steradian at the curved specimen surface. The mass resolution, limited by the channel-plate gating time, is of order Δm m = 1 15 to 1 25 . The wide field of view allows the rapid determination of the crystallographic distribution of differently-charged ions from those elements which evaporate as a mixture of charge species, and shows that this distribution is frequently a sensitive function both of the local surface geometry and of the presence or absence of field-adsorbed helium or neon at the specimen surface. The wide field of view of the IAP gives it a considerable advantage over the conventional atom-probe in the detection of segregation at grain boundaries.

Quantitative analysis of field-ion micrographs using moiré techniques

It has previously been demonstrated that the configuration of imaged atoms on the surface of a field-ion emitter may be interpreted as a moiré pattern. As a consequence it becomes possible to relate the structure of each plane in a field-ion image to that of a number of other planes, by means of ring counting procedures, and hence to obtain precise quantitative information concerning the emitter geometry. Here it is shown how the analysis may be applied to an understanding of how the various planar facets develop, to the determination of the relative field evaporation rates of different planes, and to the measurement of more accurate values of local radius.

Multilayer field evaporation patterns

Multilayer field evaporation patterns of the fee metals, Ir, Pt, Rh, and the bcc metals, W, Ta, Mo, exhibit sharp networks of dark zones. These patterns are explained as ion-optical effects resulting from deviations of ion-trajectories from an exactly radial projection, as the emitter is a polyhedron rather than a sphere. Step widths on vicinals of major zone lines are retained during progressing evaporation, thereby projecting the ions into fixed directions while keeping the zone image itself dark. The bright [110] zones on fee metals reflect a focusing effect of long atom chains across the zones. Circular intensity features around the basal planes depend upon the step-wise shift of local magnification each time a residual central plane collapses. If as we surmise multilayer desorption image features are predominantly due to tip morphology, they should also be contained in the conventional field ion image, although partially hidden by brightness effects due to varying local gas supply and the large scattering disc of a single atom image. Using multilayer sequences of field ion micrographs we map the location of each atom before it evaporates as a small dot to simultate the sharpness of a single ion spot of a field desorption image. We also use another less subjective photographic superposition method. As these FIM techniques display the three essential features of the multilayer FDM image, the hypothesis of general pre-evaporation local surface migration needs not to be invoked for explaining the FDM image.

Computer simulation of the contrast of small dislocation loops in field-ion images of f.c.c. crystals

Abstract It is shown that small dislocation loops of diameter 30 Å or less cause field-ion image contrast while wholly inside the tip. During a field-evaporation sequence that brings the loop successively closer to the surface, characteristic changes in the contrast occur which make it possible to distinguish between vacancy-type and interstitial-type loops. For a single photograph, however, unambiguous interpretation of the image contrast observed is not possible for small loops. The contrast effects for small loops are in many cases rather slight distortions of the image rings and, for a single photograph, could occur for other reasons such as tip asymmetry or the presence of impurities. For this reason an entire fieldevaporation sequence is essential. It is pointed out that only sessile loops can be observed in the FIM, since the stresses associated with the electric field will remove glissile loops from the specimen.

Some applications of field-ionization and field-evaporation techniques in the study of surfaces

After a résumé of field electron emission microscopy a brief survey of field-ionization and field-evaporation techniques is presented, with selected illustrations of applications to surface studies. The field-emission microscope is well established as a surface tool, notably in studies of chemisorption. Field-ion microscopy is used to examine the bulk microstructure of materials as well as their surface properties, and has recently been extended to include many of the common non-refractory metals. The atom-probe fieldion microscope provides the ability to chemically-analyse the atoms resolved in a fieldion image. New developments include the discovery of complex and unexpected structures in desorption images formed by field-evaporating ions and the use of desorption imaging to reveal the locations of different atomic species at a surface.

Ion current generation in the field ion microscope: II. quasi-static approach

The field ion current is calculated for helium on tungsten, for various fields, and for various values of tip temperature and gas temperature on the basis of the balance equation developed by Van Eekelen. The expression of ion current by rate constants for ionization and for escape, the total supply and the capture probability is derived. The behaviour of ion current as a function of other parameters is discussed in the light of these equilibrium properties of the system. Field adsorption effects are also considered. The increase of the mass ratio of the gas atom to the metal atom causes the increase of ion current even if ionization probability is decreased by the field adsorption. Anomalous features of the field ion image at 4.2 K are discussed.

Computer simulation of field-ion images from interstitial alloys

Computer simulation has been used as an aid in the interpretation of field-ion patterns from interstitial alloys. Programmes were run for iridium oxide, cementite and tungsten carbide. The small number of spots in experimental images of these materials may be ascribed in part to the larger effective atomic volume of the visible species. In addition there appears to be a small decrease in shell thickness compared to that for a pure metal. This contrasts with the case of substitutional ordered alloys where there is an increase in shell thickness.

Field-Ion Images of Ordered Ni3Mo Alloy and Their Computer Simulation

Field-ion images of ordered Ni3Mo alloy have been successfully obtained and their computer simulation has been made. General features of the images and the regional changes in image brightness are described. The addition of Ne to He imaging gas improved the visibility of dark regions. Double ring images for some planes have been shown clearly. They agree with the simulated images.

Field ion microscopy of germanium: Field ionization and surface states

Field evaporated Ge tips were imaged with Ar and Ne at 80° K. The ions of these gases display identical field ion images. A striking correlation between the regional brightness of FIM and FEM patterns was found. Electron shadow microscopy and the behavior of FIM images above BIV indicate no local field enhancements, thus this correlation must be explained by the electronic structure of the surface. Explanation is furnished by a model, where electrons of the image gas tunnel preferentially into surface states. This model, based on the known fact that surface states of Ge contribute in field emission, is supported by the observation that in regions of low index poles, oxygen adsorption decreases the surface density of centers above which field ionization occurs.

Observations of the fine structure of superdislocations in Ni3Al by field-ion microscopy

Abstract The fine structures of two common types of superdislocation which exist in the intermetallic Ni3Al are experimentally characterized by field-ion microscopy. It is shown how the a<110> type superdislocations and the a/3<211> type superlattice partial dislocations may be distinguished in the field-ion image and the geometrical contrast theory is extended to include these observations. The a<110> type super-dislocations are found to be dissociated, having a fault width of close to 3 nms, and the antiphase boundary energy is hence calculated to lie between 0·25 and 0·35 Jm-2. The a/3<211> type dislocations do not appear to be dissociated in the classical sense. The possibility of distinguishing between an intrinsic and an extrinsic stacking fault in this alloy is demonstrated and an example is given of the contrast from a dislocation pile-up. Finally, the observations of dislocations, and ordering defects in field-ion images from ordered alloys are discussed.