Identification of tetrahedrally coordinated atoms in supercooled liquid silicon
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Identification of tetrahedrally coordinated atoms in supercooled liquid silicon Luigi Brambilla(1), Massimo Celino(1,2), Fabrizio Cleri(1,3), Luciano Colombo(1,4), Roberto Conversano(2) , Mario Rosati(5) and Vittorio Rosato(1,2) (1) Istituto Nazionale di Fisica della Materia (INFM) (2) ENEA, Casaccia Research Center, HPCN Project, P.O. Box 2400, 00100 Roma (I) (3) ENEA, Casaccia Research Center, New materials Division, P.O. Box 2400, 00100 Roma (I) (4) Physics Dept., Università di Cagliari, Cittadella Universitaria, 09042 Monserrato (Ca) (I) (5) Consorzio Interuniversitario per le Applicazioni del Supercalcolo per Università e Ricerca (CASPUR), Università degli Studi “La Sapienza”, P.le A. Moro 2, 00185 Roma (Italy)
ABSTRACT An atomic-scale model of liquid silicon has been cooled from high temperatures down in the temperature range between the amorphous and the crystalline melting temperatures by nanosecond scale molecular dynamics simulations with the Stillinger-Weber potential. Tetrahedrally coordinated sites have been identified, in the supercooled liquids, by using a few structural order parameters . The local structure and the stability of these crystalline-like regions (c-type sites and clusters) have been characterized. These have been regarded as candidates for crystalline embryos. INTRODUCTION Supercooling a melt, in most cases, creates an amorphous structure. The case of Silicon is particularly interesting. Silicon, in fact, is characterized by two, well distinct, melting temperatures: that of the amorphous phase (occurring at Tm(a-Si)=1415 K) and that of the crystalline phase (occurring at Tm(c-Si)=1685 K) [1]. In this large temperature window, a peculiar sequence of the free energies of the different thermodynamic phases occurs: the free energy of the amorphous phase is higher than that of the liquid (l-Si) one. For this reason, the crystalline phase could be easily attained upon cooling of a stable liquid. This phenomenon has suggested a number of technological application for the production of the c-Si phase: among them, the Czochralsky growth process, which generates a crystalline (c-Si) state from a slow cooling of the liquid and the creation of micro-crystalline structures starting from a melted amorphous phase by using the so-called “super-lateral growth” (see [2]). This is the recipe of a technological process aimed at producing poly-crystalline silicon (poly-Si) starting from a thin aSi film deposited on a substrate. The latter is firstly heated up by a laser pulse of a duration of tens of nanoseconds and then equilibrated at the substrate temperature. According to the various process parameters (substrate temperature, intensity and duration of the laser pulse, energy dose), it is possible to vary the main structural parameters (in particular the grain size) of the resulting poly-Si [2]. The aim of the present work is to simulate the onset of the homogeneous crystallization process, trying to characterize the local thermodynamic/kinetic picture for the growth of the ordered phase. The same model
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