Etching dislocation pits

Fig. 2. Etch pits produced by point-defect clusters. (A) pits beginning to form at small precipitates (B) pits become flat-bottomed when precipitates are gone (C) small pits caused by precipitates in LiF (D) same as (C) after more etching (large pits are at dislocations).

I have a question concerning dislocations in walls. You find a difference in etch pits at fresh and at aged dislocations. Since some of your walls will be formed as a result of polygoni-zation, would not dislocations in such walls be aged making comparison of dislocation pits more complicated ... 

THE ADVANCES IN the growth, purity, and perfection of crystals has created a demand for etchants to reveal dislocations and to produce specific surface configurations. After a brief review of the etching processes, the various mechanisms by which etch pits form are discussed. Techniques for identifying dislocation pits and the application of pit and sphere studies to mechanism Investigations are pointed out. 

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. 

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... 

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.

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... 

We describe here an experiment which indicates that the dislocation etch pit theory is a useful tool in interpreting formation of... 

Values of r satisfying Equation 3 (corresponding to the minimum and maximum points in Ag) will yield steady state solutions where a pit radius should remain constant, while the rest of the crystal grows or dissolves depending on the chemical affinity (Equation 2). If the term t b2g /2Tt2Y2 > 1, there are no real solutions to Equation 3 and there is no steady state value of r, which indicates that a small pit nucleated at a dislocation core should spontaneously open up to form a macroscopic etch pit. The critical concentration at which this occurs (setting the above term equal to one) is ... 

For C crit t 161"6 ls a double root to the maximization equation, and there is an inflection point in the AG function (curve D on Figure 1). Since there is no activation barrier to opening up the etch pit, any pit nucleated at a dislocation should open up into a macroscopic etch pit. Similarly, for C < Ccr t, there are no real solutions and no maxima and minima in the A G function, and nucleated pits open up into etch pits. At 300°C, the calculated Ccr t for quartz equals 0.6CQ. 

A very accurate measurement of Ccrjt would allow back-calculation of the surface energy for a given crystal. Because Ccrjt is dependent on the square of Y, such a measurement could be a very sensitive method of measuring interfacial energy at dislocation outcrops. The calculated interfacial energy from our experiments is 280+ 90 mJm- for the rhombohedral face of quartz at 300°C. Parks (10) estimated 25°C value of 360 + 30 mJm is well within the experimental error of our measurement. The best way to determine the value of Ccrjt would be to measure etch pit nucleation rate on... 

In general, the shape and character of etch pits may reveal information about the impurity content of the crystal. "Beaked pits (pits with curved apexes, see 12) can indicate impurity haloes. Some forms of the arcuate etching we observed in quartz (16) may be examples of beaking. Very shallow pits can form at aged dislocations while very deep pits form at new dislocations. "Aging" may be related to impurity diffusion in the crystal lattice. 

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