Title: Melting in Superheated Silicon Films Under Pulsed-Laser Irradiation
Description: This thesis examines melting in superheated silicon ﬁlms in contact with SiO₂ under pulsed laser irradiation. An excimer-laser pulse was employed to induce heating of the ﬁlm by irradiating the ﬁlm through the transparent fused-quartz substrate such that most of the beam energy was deposited near the bottom Si-SiO₂ interface. Melting dynamics were probed via in situ transient reﬂectance measurements. The temperature proﬁle was estimated computationally by incorporating temperature- and phase-dependent physical parameters and the time-dependent intensity proﬁle of the incident excimer-laser beam obtained from the experiments. The results indicate that a signiﬁcant degree of superheating occurred in the subsurface region of the ﬁlm. Surface-initiated melting was observed in spite of the internal heating scheme, which resulted in the ﬁlm being substantially hotter at and near the bottom Si-SiO₂ interface. By considering that the surface melts at the equilibrium melting point, the solid-phase-only heat-ﬂow analysis estimates that the bottom Si-SiO₂ interface can be superheated by at least 220K during excimer-laser irradiation.

It was found that at higher laser ﬂuences (i.e., at higher temperatures), melting can be triggered internally. At heating rates of 10¹⁰ K/s, melting was observed to initiate at or near the (100)-oriented Si-SiO₂ interface at temperatures estimated to be over 300K above the equilibrium melting point. Based on theoretical considerations, it was deduced that melting in the superheated solid initiated via a nucleation and growth process. Nucleation rates were estimated from the experimental data using Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis. Interpretation of the results using classical nucleation theory suggests that nucleation of the liquid phase occurred via the heterogeneous mechanism along the Si-SiO₂ interface.