Evidence against surface nucleation following pulsed melting of Si
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Michael O. Thompson Cornell University, Ithaca, New York 14853
(Received 17 October 1989; accepted 5 April 1990) Time-dependent measurements of the melt and solidification behavior of amorphous Si, formed by ion implantation of In, have been obtained following irradiation with ~3 ns ruby laser pulses. The recently observed buried In sheets formed under such conditions are shown to result from internal nucleation of melt, contrary to previous interpretations. Under no conditions was surface nucleation of either amorphous or crystalline Si observed from a surface melt. These results resolve previous inconsistencies in the understanding of phase formation during rapid melting and solidification of Si.
I. INTRODUCTION
Recent studies have led to significant advances in understanding melting and solidification of Si under highly nonequilibrium conditions such as those produced during pulsed laser-induced melting. It is now known that under almost all conditions melting is initiated by nucleation of the liquid phase at the free irradiated surface (air/solid or vacuum/solid).12 Subsequent melting and solidification is then mediated by the internal solid/liquid interface. Although during conventional solidification, surfaces commonly provide heterogeneous nucleation sites for the solid phase, solid phase nucleation has never been unambiguously observed at a free surface following laser-induced melting. Indeed, in pure a-Si, homogeneous bulk nucleation occurs prior to surface nucleation under conditions of substantial supercooling.3'4 The absence of surface nucleation under these extreme conditions requires that models previously proposed5 for laser-induced melting of impurity doped a-Si be reconsidered. Under short pulse laser irradiation of a-Si samples formed by ion implantation of low solubility impurities such as In, Pt, or Sn, solidification into the amorphous phase occurs with the impurity segregated from both the surface and substrate sides into a thin buried sheet. The formation of this buried impurity sheet requires an interface moving from the irradiated surface inward toward the substrate.6 It has been argued that this second moving interface results from heterogeneous nucleation of the amorphous phase at the free surface, perhaps catalyzed by impurities. Since In decreases the melting temperature of a-Si by approximately 100 K/at. %,7 the relatively impurity-free surface can indeed be substantially supercooled during solidification. J. Mater. Res., Vol. 5, No. 7, Jul 1990
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In this paper we report results of experiments which resolve this apparent inconsistency. The melt and solidification dynamics of a-Si implanted with low solubility impurities were measured under a variety of laser irradiation conditions. These studies reveal that, under conditions similar to those of the earlier studies, melting actually occurs internally. The surface remains solid throughout the process and acts as a seed for solidification of a-Si from the surface region. If conditi
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