Observation of Superheating of Si at the Si/SiO 2 Interface in Pulsed-laser irradiated Si Thin Films
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Observation of Superheating of Si at the Si/SiO2 Interface in Pulsed-laser irradiated Si Thin Films J.J. Wang, A.B. Limanov, Ying Wang, and James S. Im Materials Science and Engineering, Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA ABSTRACT Substantial superheating of single-crystal Si films at and near the bottom Si/SiO2 interface was observed. This was accomplished via back-side irradiation of a (100) single-crystal Si film on a quartz substrate using an excimer-laser pulse. The spatiotemporal details of the melting transition were tracked in situ using surface-side and substrate-side transient reflectance measurements, and the one-dimensional thermal profile evolution within the solid film during the heating period was numerically computed using the experimentally extracted temporal profile of the incident beam and temperature-dependent optical and thermal parameters of the materials. A simple lower-bound estimation identifies that superheating in excess of 100 K was attained within Si along the bottom (100)-Si/SiO2 interface even at moderate beam energy densities. INTRODUCTION The extent to which melting proceeds in laser irradiated Si films has long been recognized as the singularly critical factor that determines the ensuing solidification scenario in laser-induced melt-mediated crystallization of the films [1]. For partial-melting-based laser-crystallization approaches [1-5], it is becoming even more evident that the specific details associated with the initial formation and subsequent growth of the liquid phase in the polycrystalline Si films essentially dictate the ensuing microstructural changes that take place in the films; this is the case for excimer laser annealing (ELA) employed in the manufacture of advanced LCDs and OLED displays, as well as for the mixed-phase-solidification (MPS) method [4], which can create largegrained, (100)-surface-textured Si on SiO2 for photovoltaic and microelectronic applications [6]. In other words, we suggest that for these multiple-exposure-based techniques, the melting period corresponds to the all-significant processing phase in that it (1) determines the incremental increase in the average grain size [3] and (2) affects the gradual development of the surface orientation texture [4,7], provided, of course, that nucleation is avoided [1,8]. The established principles of discontinuous phase transitions point to various extrinsic excess free energy sites in polycrystalline Si films (e.g., surfaces, grain boundaries, dislocations, and Si/SiO2 interfaces), along with the Gibbs-Thomson effect [9,10], as the key elements and factors that will dictate spatially and temporally heterogeneous initiation and propagation of the melt in the films [11]. Understanding such details may, therefore, lead to further optimization of, and advances in, these and other partial-melting-based laser crystallization techniques. In this paper, we present preliminary results obtained from experiments on excimer-laserinduced melting of Si films on Si
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