Etch-pit

Figure A3.10.8 Depiction of etching on a Si(lOO) surface, (a) A surface exposed to Br2 as well as electrons, ions and photons. Following etching, the surface either becomes highly anisotropic with deep etch pits (b), or more regular (c), depending on the relative desorption energies for different surface sites [28].

Figure A3.10.10 STM image (55 x 55 mn ) of a Si(lOO) surface exposed to molecular bromine at 800 K. The dark areas are etch pits on the terraces, while the bright rows that run perpendicular to the terraces are Si dimer chains. The dimer chains consist of Si atoms released from terraces and step edges during etching [28],...

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

Fig. 9. Corrosion model of silver development. As the haUde ion, X, is removed into solution at the etch pit, the silver ion,, travels interstitiaHy, Ag/ to the site of the latent image where it is converted to silver metal by reaction with the color developer, Dev. Dev represents oxidized developer.

At present, all commercial SFM tips are square pyramids, formed by CVD deposition of Si3N4 on an etch pit in (100) Si. The etch pit is bounded by (111) faces, which means that the resulting tip has an included angle of about 55°. Therefore the edge profiles of all features with sides steeper than 55° will be dominated by the profile of the tip. 

Dislocations, Etch Pits and the Effect of Cold Work on Corrosion... 

Fig. l.S(a) Grain boundary intersecting an etched metallographic surface and (b) etch pit at a dislocation interseaing an etched metallographic surface... 

Videm, K., Pitting Corrosion of Aluminium in Contact with Stainless Steel , Proc. Conf. on Corrosion Reactor Mater., Salzburg, Austria, 1, 391 (1962) C.A., 60, 1412g Lyon, D. H., Salva, S. J. and Shaw, B. C., Etch Pits in Germanium Detection and Effects , J. Electrochem. Soc., 110, 184c (1963)... 

Black, J. R., Etch Pit Formation in Silicon at Al-Si Contacts Due to the Transport of Silicon in Aluminium by Momentum Exchange with Conducting Electrons , J. Electrochem. Soc., 115, 242c (1968)... 

Kang J, Shin Y, Tak Y (2005) Growth of etch pits formed during sonoelectrochemical etching of aluminium. Electrochim Acta 51 1012-1016... 

Figure 19. Schematic drawing of cross sections of two types of pits developing in pitting corrosion of passive metals (a) geometric pit (b) crystallographic or etch pit.

The plastic deformation patterns can be revealed by etch-pit and/or X-ray scattering studies of indentations in crystals. These show that the deformation around indentations (in crystals) consists of heterogeneous rosettes which are qualitatively different from the homogeneous deformation fields expected from the deformation of a continuum (Chaudhri, 2004). This is, of course, because plastic deformation itself is (a) an atomically heterogeneous process mediated by the motion of dislocations and (b) mesoscopically heterogeneous because dislocation motion occurs in bands of plastic shear (Figure 2.2). In other words, plastic deformation is discontinuous at not one, but two, levels of the states of aggregation in solids. It is by no means continuous. And, it is by no means time independent it is a flow process. 

Fig. 11. Capacitive transient spectra of defect states associated with dislocations in ultra-pure germanium (crystal 281 grown along [100] under 1 atmosphere of H2) The micrographs show the etch pits produced by dislocations on a (100) surface. DLTS peak b has an activation energy of Ev + 20 meV. The net-shallow acceptor concentration is 1010 cm-3.

The formation of etch pits and tunnels on n-Si during anodization in HF solutions was reported in the early 1970 s. It was found that the solid surface layer is the remaining substrate silicon left after anodic dissolution. The large current observed on n-Si at an anodic potential was postulated to be due to barrier breakdown.5,6 By early 80 s7"11 it was established that the brown films formed by anodization on silicon substrate of all types are a porous material with the same single crystalline structure as the substrate. 

Gratz, A. J., S. Manne, and P. K. Hansma (1991), "Atomic Force Microscopy of Atomic-Scale Ledges and Etch Pits Formed During Dissolution of Quartz", Science 251,1343-46. 

A common phenomenon in the dissolution of silicate minerals is the formation of etch pits at the surface (90-91.,93-94). When this occurs, the overall rate of mineral dissolution is non-uniform, and dissolution occurs preferentially at dislocations or defects that intercept the crystal surface. Preferential dissolution of the mineral could explain why surface spectroscopic studies have failed... 

Wilson (28) noted the presence of etch pits (crystallographically controlled voids or features of negative relief, or "negative crystals") on some soil feldspars, and reviewed similar observations from earlier studies. Some examples of etch pits on naturally weathered feldspars are shown in Figure 3- Etch pits... 

Figure 3 Scanning electron photomicrographs of feldspar surfaces in various stages of weathering, a) Fresh surface, b) Incipient formation of shallow almond-shaped etch pits, c) Moderate development of prismatic etch pits, d) Extensive penetration of prismatic etch pits into feldspar interiors. Photographs b-d are from naturally weathered materials. All photomicrographs by Alan S. Pooley and the author.

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

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