The Mechanism of Solid Nucleation in Excimer-Laser-Quenched Liquid Silicon on Silicon Dioxide

This thesis examines the mechanism of formation of solid nuclei in laser-quenched and deeply supercooled liquid Si films on SiO2. An excimer laser pulse is employed to induce complete melting with subsequent rapid quenching of Si films. The following combination of experimental and numerical techniques are employed: (1) front side and back side in situ transient reflectance measurements, (2) planar and cross-sectional view transmission electron microscopy (TEM) analysis, and (3) a 3-dimensional nucleation simulation (3DNS) program capable of capturing the stochastic nature of nucleation. In order to examine the intrinsic nucleation scenario (i.e., to reduce potential influences arising from native oxide layer and/or from possible reactions taking place at the surface), previously neglected experimental procedures consisting of buffered hydrofluoric acid (BHF) etching and irradiation in vacuum were implemented. This led to the set of experimental results that permitted us, together with the 3DNS analysis, to make definitive conclusions about the mechanism of nucleation. Specifically, we conclude that solid nucleation consistently takes place heterogeneously at, and only at, the bottom liquid Si-SiO2 interface. When interpreted based on classical nucleation theory, this in turn means that homogeneous nucleation in supercooled liquid Si on SiO2 cannot take place in any similar or lower-quench-rate experiments. This conclusion is in direct contrast to the previously established homogeneous nucleation model. When Si films are irradiated in air, a more complicated solidification scenario involving near simultaneous heterogeneous nucleation at the surface was found to take place. This observation, which is not inconsistent with the findings obtained from the vacuum experiments, also explains previous experimental results that might have misled other investigators.