On the formation of etch grooves at impurity striations and growth sector boundaries in crystals grown from solutions

Abstract

A theory of formation of etch grooves at planar defects like impurity striations and growth sector boundaries by chemical etching is proposed. The theory is based on the concept of the thermodynamic theories of etch-pit formation at dislocation sites, in combination with the concept of the existence of a pseudomorphous deformed layer in the region of impurity microsegregation, the latter concept being taken from epitaxy. After estimates of thickness of the deformed layer, equations are derived for the free energy change involved in the formation of etch grooves at planar defects, and for the slope of the etch grooves. Among important predictions of the theory, the following may be enumerated: (1) Etching media in which a crystal is more soluble (i.e. fast etchants), yield contrasting grooves. (2) The capability of revelation of planar defects on different faces of a crystal by an etchant decreases with increasing morphological importance of the faces. (3) All fast dislocation etchants are also good etchants for revealing etch grooves at planar defects. The theoretical predictions are then compared with the experimental results on the revelation of etch grooves on different faces of KDP and other crystals grown from aqueous solutions. It is found that the theory satisfactorily explains the experimental results.