We considered Surface and Microscopic Techniques in Section 3, but only gave a short mention of STM and FIM in sections 3.1 and 3.5. To make definitive statements about the use of STM is almost bound to lead to failure; new applications and new configurations are emerging at such a rate that it is more than a full time job just to keep up with them: so I won't try. At the risk of offending other authors, friends and colleagues recommend Chen's book as an introduction, and refer to multi-author texts and review articles for more specialist information. Books that are helpful in the present context include J. Stroscio and E. Kaiser 'Scanning Tunelling Microscopy' (1993), Guntherodt and Weisendanger, vols 1-3 (1991 onwards), and also R. Weisendanger 'Scanning Probe Microscopy and Spectroscopy' (1994, Cambridge University Press). Despite the immense amount of work described in these books, it is quite difficult to get a perspective on it, since it is all so recent.
An example of the use of non-UHV TEM to study nucleation and growth is shown in diagram E7 from the Robins' group. The deposition (of Au/ NaCl(100)) is done in UHV, but the micrographs are obtained by 1) coating the deposit with carbon in UHV; 2) taking the sample out of the vacuum; 3) dissolving the substrate in water; 4) examining the Au islands on the carbon by TEM. By this means island densities, growth, coalescence and nucleation on defect sites can all be observed. By performing many experiments at different deposition rates R, and temperatures T, as a function of coverage, their group and others have produced quantitative data of island growth and rate equation models have been tested. It is clear that this type of technique is destructive of the sample: it is just as well that NaCl is not too expensive, and that gold/ silver etc are relatively unreactive, or the technique would not be feasible.
The more recent UHV techniques (TEM, SEM, STM, etc, and FIM) examine the deposit/ substrate combination in-situ, without breaking the vacuum. This gives extra power to the experiment, at the cost of rather less convenience to the experimenter; as a result a much wider set of substrates and deposits have been observed. UHV-SEM and related techniques have been used to study Stranski-Krastanov growth systems at elevated T, where, as seen in diagram E8 for Ag/Mo(100), diffusion distances of the adatoms can be many micrometers (Venables et al refs). The SEM is good for very 3D objects, such as the (100) oriented Ag islands seen in this diagram.
At the other end of the length scale, the Field Ion Microscope (FIM) has been used to study individual atomic jumps and the formation and motion of small clusters. These observations are made at low T, with annealing at higher T for given times to effect the jumps. FIM works best for refractory metals, and high quality information has been obtained on diffusion coefficients in systems such as Re, W, Mo, Ir and Rh on tungsten, as shown in diagram E9 for the W(211) surface. Diffusion on this surface is highly anisotropic, essentially moving along 1D channels parallel to the [01bar1] direction, and avoiding jumps between channels. On higher symmetry surfaces, e.g. diffusion on (001) is observed to be isotropic, as it should be (Ehrlich et al refs).
A particularly elegant application of FIM is to distinguish the ‘hopping’ diffusion (pictured schematically in diagram E3 and often assumed implicitly as the adatom diffusion mechanism) from ‘exchange’ diffusion. Draw a (001) surface of an fcc crystal such as Pt(100). We know from problem set #1, question 2(a) where an adatom will sit on this surface. So you can convince yourself that hopping diffusion will proceed in <110> directions. By contrast, the exchange process consists of displacing a nearest neighbor of the adatom, and exchanging the adatom with it. The substrate atom ‘pops out’ and the adatom becomes part of the substrate. In this case you can convince yourself that the direction of motion during diffusion is along <100>, at 45 degrees to hopping diffusion and with root-2 times the jump distance. By repeated observation of adatom diffusion over a single crystal plane, FIM is able to map out the sites which the adatoms visit, and thus to distinguish exchange and hopping diffusion. Such measurements taken at different annealing T can show the cross-over from one mechanism to the other (Kellogg refs).
In the last few years, UHV-STM has become the main technique for studies of this type, with a variable (low) temperature instrument the most powerful for quantitative studies. The sub-ML sensitivity over large fields of view, and the large variations in cluster densities with deposition T, have provided detailed checks of the kinetic models of nucleation and growth described in section E2. An example is shown in diagram E10, from the work of M. Bott et al (Phys. Rev. Lett. 76 (1996) 1304) on Pt/Pt(111). Several other studies on similar systems have now made it possible to do detailed comparisons with (effective medium) theories of metal-metal binding. One illuminating comparison is that of Ag/Ag(111) with Ag on 2ML Ag on Pt(111), and with Ag/Pt(111). The systematic variations that are found reflect small differences in the lattice parameter (strain) and in strength of binding between these closely similar systems (H. Brune et al, Phys. Rev. Lett. 73 (1994) 1955).
Similar studies have been done on Si(100) surfaces, starting with the work of Y. Mo et al (Phys. Rev. Lett 65 (1990) 1020; 66 (1991) 1998). But the complex, anisotropic nature of diffusion on semiconductors, has meant that subtle metastabilities influence the diffusion path and the binding at lower T. In such a situation it may be that the high T behaviour is closer to the simple models described in the last section. Recent work has shown the path of individual atoms can be tracked by an STM tip whose position is locked onto adatoms laterally. This technique reveals preferred paths for the migrating atoms, which often get trapped at surface defects (B. Schwartzentruber, Phys. Rev. Lett. 76 (1996) 459).
An example of a UHV-STM image/ linescan of silicide formation on Si(111) is shown in diagram E11. In this beautiful image, the triangular Co-silicide particles are shown delineating the edges of the Si(111) 7x7 unit cells. Typically, the detail is sufficient to identify the nucleation sites, and the various (early) stages of growth. The scale of diffusion has to be sufficient for Co atoms to reach the nearest islands, in other words at least 10nm in this example. A particularly interesting aspect is that the formation of the 3-layer Si-Co-Si silicide structure proceeds by Co atoms diffusing interstitially beneath the surface (P.A. Bennett et al, Surface Sci. 312 (1994) 377; Phys. Rev. Lett 73 (1994) 452).
There are many other such recent examples, some of which have been written about in review articles and book chapters. The latest one I have been involved with is in ‘Growth and Properties of Ultrathin Epitaxial Layers’, edited by D.A. King and D.P. Woodruff (Elsevier, 1997) Chapter 1 ‘Surface Processes in Epitaxial Growth’, which comments on the following systems, among others. In particular, I find that the comparison between two fairly similar systems can be instructive. The downside is that it can be timeconsuming... And, of course, from your point of view, you may well not need to know the details. Please get in touch if any of these details would be useful in your research.
Latest version of this document: 13 February 1998.
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