Atomic structures field evaporated surfaces

It is a well recognized fact that in field ion microscopy field evaporation does not occur at a constant rate because of the atomic step structures of the tip surface. For the sole purpose of a compositional analysis of a sample, one should try to aim the probe hole at a high index plane where the step height is small and field evaporation occurs more uniformly. But even so, the number of atoms field evaporated per HV pulse or laser pulse within the area covered by the probe-hole will not be the same every time. It is reasonable to assume that the field evaporation events are nearly random even though there has been no systematic study of the nature of such field evaporation events. Let the average number of atoms field evaporated per pulse within the area covered by the probe-hole area be n. The probability that n atoms are field evaporated by a pulse is then given by the Poisson distribution... 

FTM and atom-probe studies of thin films of Ni, Au, Pt, a-Ge H, a-Si H and WO3, etc., on various substrates were reported by Krishna-swamy et a/.81 First, field ion tips each with a field evaporated surface were prepared. They are placed in an MRC model 8502 r.f. sputtering system. Tips were mounted on a recessed and shielded structure behind the sputtering surface which is bored with small holes about 1 to 2 mm in diameter. The very end of the tips came out of the holes to approximately the same level of the sputtering surface. Films were sputtered at about 20 mTorr Ar at an r.f. power of about 50 W. Thickness of a deposited thin film was controlled by both the r.f. power and the deposition time. Film thickness in the range of a few hundred to a few thousand A were studied. These tips were then imaged with Ne in the field ion microscope, or analyzed in the flight-time-focused ToF atom-probe. 

Fig. 4.37 (a) On the Ir (001) surface, for an island of fewer than 6 Ir atoms, a two-dimensional structure is mctastable, whereas the linear chain structure is stable. (/) An island of 6 Ir atoms equilibrated at 460 K ( 7) one atom is field evaporated from the 6-atom island (Hi) upon heating to 450 K, the two-dimensional five-atom island reconfigures to a linear chain, (b) For an island of 6 or more Ir atoms, the linear chair is mctastable whereas the two-dimensional island is stable. (/) 6 Ir atoms in a linear chain, equilibrated at 315 K (if) upon heating to 450 K, the island transforms into a two-dimensional structure. From Schwoebcl ... 

There are of course many other similarities and differences, and some of them are listed in Table 5.1 without further explanations. In general, STM is very versatile and flexible. Especially with the development of the atomic force microscope (AFM), materials of poor electrical conductivity can also be imaged. There is the potential of many important applications. A critically important factor in STM and AFM is the characterization of the probing tip, which can of course be done with the FIM. FIM, with its ability to field evaporate surface atoms and surface layers one by one, and the capability of single atom chemical analysis with the atom-probe FIM (APFIM), also finds many applications, especially in chemical analysis of materials on a sub-nanometer scale. It should be possible to develop an STM-FIM-APFIM system where the sample to be scanned in STM is itself an FIM tip so that the sample can either be thermally treated or be field evaporated to reach into the bulk or to reach to an interface inside the sample. After the emitter surface is scanned for its atomic structure, it can be mass analyzed in the atom-probe for one atomic layer,... 

Atomic structures of field evaporated solid surfaces... 

It is found that for metals, low temperature field evaporation almost always produces surfaces with the (1 x 1) structure, or the structure corresponding to the truncation of a solid. A few such surfaces have already been shown in Fig. 2.32. That this should be so can be easily understood. For metals, field penetration depth is usually less than 0.5 A,1 or much smaller than both the atomic size and the step height of the closely packed planes. Low temperature field evaporation proceeds from plane edges of these closely packed planes where the step height is largest and atoms are also much more exposed to the applied field. Atoms in the middle of the planes are well shielded from the applied field by the itinerant electronic charges which will form a smooth surface to lower the surface free energy, and these atoms will not be field evaporated. Therefore the surfaces produced by low temperature field evaporation should have the same structures as the bulk, or the (lxl) structures, and indeed with a few exceptions most of the surfaces produced by low temperature field evaporation exhibit the (1 x 1) structures. 

Fig. 4.10 (a) (1 X 1) Ir (001) surface prepared by low temperature field evaporation. When the surface is heated to —900 K for 5 ns by a laser pulse, the top layer is reconstructed to the (1x5) structure as shown in (ft). Gradual field evaporation. from (c) to (/). reveals the buckled [110] atomic rows in the horizontal direction. At (c) one can notice that some of the atomic rows at the east and west sides of the top layer are perpendicular to the other atomic rows, indicating formation of mutually perpendicular domains. The domain boundaries line up at 45° to the atomic row directions. If the laser power is properly adjusted, sometimes only a half of the top layer is found to be reconstructed. 

Fig. 4.15 (a) A 15 K He field ion image of a low temperature field evaporated silicon surface. (b) A computer simulation image of Si with (lxl) surfaces, (c) a 60 K Ne field ion image of a 720°C annealed silicon surface where well ordered atomic structures are developed at a few facets of the Si emitter surface, (d) When the Si tip is annealed at 800°C, almost all the facets are well developed. The atomic structures of all these facets are completely reconstructed. 

---