Electron beam induced explosive crystallization of unsupported amorphous germanium thin films

Abstract

Explosive crystallization has been observed in thin, unsupported (free) films of amorphous germanium irradiated with a focussed, moderately intense electron beam. Transmission electron microscopic examination of the crystallized films show radial dendritic growth followed by annularly arranged periodic surface features. Analysis of our results confirm that the heat released during the amorphous to crystalline (a→c) transformation of the irradiated area (∼10 μm) abruptly crystallizes the surrounding area up to ∼120 μm in diameter. Origin of the dendritic growth is consistent with the ‘‘duplex-melting’’ model and confirms that the crystallization mechanism is self-sustaining and proceeds via an intermediate melting step. Periodic morphology in the outer crystalline area is characteristic of the present experiment on unsupported, amorphous films and is analyzed in terms of one-dimensional heat flow. A qualitative model is presented. The observed periodic surface features consisting of alternate large grain (∼1 μm wide) single crystalline and small grain (∼0.2 μm wide) polycrystalline regions in the outer crystallized area are understood in terms of melting and rapid solidification occurring in succession along the direction of propagation of the amorphous to crystalline transformation process. The presence of gaseous bubbles in the single crystalline area supports the existence of the melting step.