Solidification of Highly Undercooled Silicon: Bulk Nucleation and Amorphous Phase Formation
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SOLIDIFICATION OF HIGHLY UNDERCOOLED SILICON: BULK NUCLEATION AND AMORPHOUS PHASE FORMATION*
R. F. Wood, D. H. Lowndes, and J. Narayan Solid State Division, Oak Ridge National Laboratory Oak Ridge, Tennessee 37830
ABSTRACT Recent experimental and theoretical work suggests that bulk nucleation occurs in highly undercooled liquid silicon In formed by the pulsed laser melting of amorphous layers. this paper we discuss the dynamics of solidification of undercooled liquid silicon in terms of results from melting model calculations and from the theory of homogeneous bulk The relationship between the occurrence of bulk nucleation. nucleation and the ability to induce a liquid-to-amorphous phase transition by large undercoolings of the liquid is considered. It is concluded that the observed liquid-toamorphous transition in silicon cannot be explained by an equilibrium thermodynamic approach.
INTRODUCTION Developments during the last few years in the area of pulsed laser annealing of semiconductors have refocussed attention on the subject of For example, rapid solidification of highly undercooled molten materials. the laser-induced conversion of crystalline (c) Si to amorphous (a) Si [1-3] by way of ultrarapid formation and solidification of the liquid (2) phase Thus, in Ref. 3 Cullis et al. used 2.5 has generated widespread interest. nsec pulses of a frequency doubled ruby laser (X = 347 nm) to induce the c~a conversion on the and faces of c-Si; calculations indicated that the c+a conversion occurred at a solidification velocity v of 15-20 m/sec. As discussed below, the purely thermodynamic argument generally advanced to explain this phenomenon apparently requires underMore phase transition to occur. coolings of -300 deg in 2-Si for the 2.-÷a recently, work by Lowndes et al., [4,5] and by Narayan and White [6] in which highly undercooled 2-Si was formed by pulsed laser melting of a-Si layers on c-Si substrates, suggests that copious bulk nucleation may occur in the undercooled liquid at temperatures above that for the k+a transition. These two types of results seem contradictory to each other and raise fundamental questions about the mechanisms of ultrarapid solidification and In this paper we discuss some of these questions the role of undercooling. within the context of melting model calculations and nucleation theory.
EXPERIMENTAL BACKGROUND In the studies of Refs. 4 and 5 time-resolved reflectivity measurements were used to determine both the time of onset of melting tm (within the duration of the -18 nsec FWHM laser pulse) and the duration Ts of surface melting of a-Si layers formed by self-ion implantation of c-Si substrates. TEM was used to study the micro-structure of the resolidified Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract W-7405-eng-26 with Union Carbide Corporation.
Mat. Res.Soc.Symp. Proc. Vol. 23 (1984) P1blished by Elsevier Science Publishing Co.,
Inc.
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layers, and modified melting model calculations were used to calculate tm and Ts. It wa
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