Kinetics of Explosive Crystallization Phenomena in Amorphous Silicon and Crystal Structure of the Layers Formed
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KINETICS OF EXPLOSIVE CRYSTALLIZATION PHENOMENA IN AMORPHOUS SILICON AND CRYSTAL STRUCTURE OF THE LAYERS FORMED E. GLASER', H. BARTSCH" and G. ANDRA" "Universitat Jena, Institut ftir Festk6rperphysik, 0-6900 Jena, Germany "lInstitut fuir Festk6rperphysik und Elektronenmikroskopie, 0-4020 Halle, Germany ABSTRACT Systematic relations are shown between the parameters controlling explosive crystallization in silicon (temperature, width of a liquid zone, and gradient of melt undercooling), the kinetics of the crystallization process, the predominance of preferential growth directions, and the crystal structure observed. Three typical regions of crystallization are found: (a) cellular growth of a laminated crystalline layer controlled by the formation of high densities of twins at a nearly plane liquid/solid interface, (b) cellular-dendritic growth of crystal lamellae characterized by branching of a curved interface, and (c) formation of a diffuse "slush zone" due to random nucleation in a-Si and grain growth in the liquid zone. INTRODUCTION Explosive crystallization (EC) of amorphous layers especially during the past decade became a subject of special interest [1,2]. The "explosive" advance of a crystallizing interface is stimulated by the release of free energy (latent heat) which serves to drive the crystallization further. Thus, under suitable conditions a self-sustaining crystallization proceeds across the layer. This phenomenon by then has been observed in metals, elemental and compound semiconductors, metal-semiconductor compounds (e.g. silicides), alloys, and in dielectric layers applying generally short laser pulses with pulse lengths of some milliseconds to some picoseconds. Because of the extensive investigations of laser-induced phase transformations silicon is a promising material to attain deeper understanding of the EC phenomenon. It has been shown that EC in silicon is mediated by a very thin liquid zone [3] and that also during 1100K) velocities of about 16 ms" lateral propagation of the EC front (at temperatures To are reached [4]. Consequently, both solid and liquid phase crystallization will occur and the EC will be controlled both by the heat conduction across the liquid zone (instability of the interface) and by the kinetics of crystallization from the highly undercooled melt. In this paper the specific relations are studied between the parameters controlling lateral EC (TO, gradient of melt undercooling), the kinetics of EC, the shape of the crystallizing front, and the crystal structure observed. EXPERIMENTAL To initiate and to sustain lateral propagation of EC in a layer system 400nm aSi/360nm Si0 2/bulk-Si a combination of two synchronized pulse lasers (Q-switched and freerunning Nd-glass lasers with pulse lengths of 40 ns and 1 ms, respectively) was used. By means of a third synchronized laser pulse (Q-switched ruby laser) the propagating front has been stopped again. In this way and by means of time-resolved reflectivity (TRR) measurements using a focused He-Ne-laser beam the velocity of the f
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