Chapter XII: Morphological Instability and Growth of Phases

Introduction

We will explore the subject of morphological instability in this chapter. There are many manifestations of morphological instability: in solidification, where a planar growth front can degenerate into a dendritic growth morphology; in thin film deposition where the thin film can decompose into an array of globules; in chemical vapor deposition (CVD), where a planar front can degenerate into one containing whiskers, etc. Historically, the first quantitative treatment of morphological instability was due to Mullins and Sekerka1 and their work has since inspired a flood of other studies involving further mathematical development of their linear analysis into the non-linear realm and still other types of morphological instability phenomena.

The instability of a morphology can occur in the presence or absence of a reaction front. At a reaction front, instability is induced by composition or temperature gradients ahead of the interface between parent and product phases and, normally, capillarity forces tend to restore stability in these situations. The morphological instability at a planar reaction front between a parent and a single product phase is that treated by Mullins and Sekerka. The morphological instability at a reaction front between a single metastable parent phase and a duplex product is denoted as an Eckhaus instability. Both these morphological instabilities will be discussed in this chapter. The morphological instability of a dendrite during its growth, treated first by Langer and Muller-Krumbhaar,2 belongs to the first category. Transitions in morphology can occur during growth which are also a consequence of morphological instability, i.e. from...