Pulsed Laser Heating-induced Surface Rapid Cooling and Amorphization
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1066-A05-06
Pulsed Laser Heating-induced Surface Rapid Cooling and Amorphization Longzhang Tian1, and Xinwei Wang2 1 Department of Mechanical Engineering, University of Nebraska-Lincoln, Lincoln, NE, 68588 2 Department of Mechanical Engineering, Iowa State University, Ames, IA, 50011 ABSTRACT In this work, hybrid atomistic-macroscale simulation is conducted to explore the crystallization process of Si surface in the situation of fast melting and solidification induced by ultrafast laser heating and heat conduction. Using the environment-dependent interatomic potential, samples containing 2,880 and 11,520 Si atoms are modeled to provide accurate details for the relationship between the finial crystal structure and the parameters of laser pulses. An empirical correlation Ec = 448.76 × ( t g )
0.56
is obtained to relate the critical fluence for
amorphization to the laser pulse width. It is found that the final thickness of amorphous layer is related to the fluence of the laser pulse with the same full width at half maximum (FWHM). Employing laser pulses with FWHM = 6.67 ns, the formation and recrystallization processes of a 12 nm thick amorphous layer is further investigated, which may have great potential in laser manufacture techniques for Si-associated structures. I. INTRODUCTION Pulsed-laser annealing techniques are widely used in the fabrication of amorphous semiconductors, which have various applications in solar cells, xerography, and flat-panel displays [1-6]. To understand the microscopics of new melting and solidification characteristics induced by the pulsed laser, much numerical and experimental work has been done in this field. Using nanosecond lasers, Gullis et al. demonstrated that the maximum growth velocities of the principal low index Si surfaces lie in the order of (001) > (011) > (112) > (111) [7]. When the crystal growing rate is beyond the maximum velocity, the crystal growth breaks down and a final amorphous solid phase is produced. This order of growth velocities was also predicted by molecular dynamics (MD) simulations conducted by Landman [8]. The accrual amorphization threshold of interface velocity has been measured by transient conductivity experiments and shown to be 15.8 m/s for (001) Si and 14.6 m/s for (111) Si [9-11]. The MD simulations done in this area mostly focused on the crystallization process of liquid Si driven by a constant cooling. This neither provided the relationship between the laser properties and final structures of Si nor studied the solidification process under the actual cooling rate caused by heat conduction through the solid Si substrate. In this work, hybrid atomisticmacroscale modeling is conducted to explore the solidification/crystallization characteristics of (001) Si under ultrafast laser heating and natural cooling situation. The structure change is captured during laser heating and post-laser solidification. The critical fluence for amorphization, the thickness and the recrystallization process of the final amorphous layer are explored as well.
II. ME
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