Explosive crystallization phenomena in SOI structures
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Using the double pulse technique with two synchronized lasers, we studied the conditions of ignition and evolution of explosive crystallization. The structure of the resulting crystallized layers is analyzed by TEM. Results of calculations are reported describing the development of the two phase fronts: amorphous/molten and molten/crystalline. It is shown that the system takes more than 500 ns to reach the steady state. The experimental results support the model of creating first a melt nucleus in the amorphous layer followed by the formation of the crystalline nucleus in the molten sphere. Competitive solid phase nucleation and growth in the amorphous layer limit the temperature-time interval of melt nucleation. Defined explosively crystallized areas in laterally structured SOI layers are presented.
I. INTRODUCTION From the viewpoint of materials research, fast crystallization processes induced by short laser pulses are of growing interest, because they proceed far from equilibrium and are strongly determined by the kinetic laws of nucleation and growth of the new phase. Under certain conditions of strong undercooling, the release of latent heat during the fast crystallization can accelerate the decay of an amorphous phase. This positive feedback gives rise to an autocatalytic or explosive crystallization process, transforming the amorphous layer. The phenomenon was observed in metals,1 insulators,2 and semiconductors.3'4 Recently, investigation of explosive crystallization in a-Si and a-Ge layers was fostered by the search for new siliconon-insulator (SOI) technologies. It was shown that the crystallization front can proceed explosively both normal to the surface (vertically)5'6 and laterally7'8 in amorphous Si layers on insulators8 and on a crystalline Si substrate.9 Areas of some millimeters in diameter of the Si layer were crystallized with phase front velocities of 15 — 17 m/s, if the conditions of steady state explosive crystallization are fulfilled. Otherwise the process dies out 50 = 80 ns after ignition. Time resolved reflectivity1011 and electrical conductivity measurements5 clearly indicate a fast solidification process of a transient melt. As in a previous paper,8 we will use the term explosive liquid phase crystallization. The model used for the explanation of the lateral explosive liquid phase epitaxy (ELPE) takes into account the feedback between the released heat of crystallization at a sharp crystallization front and a melting front via a thin, molten, and strongly undercooled zone localized between the amorphous and crystalline material.10"12 The autocatalytic behavior is due to the speciality of the metastable amorphous state in Si and Ge which represents lower values of latent heat and melting temperature than the crystalline one.13 In agreement with this model, the steady state of ELPE in Si can be achieved if the substrate temperature is J. Mater. Res., Vol. 4, No. 6, Nov/Dec 1989
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adjusted in the span 1150 — 1350 K, depending on the lo
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