Mechanisms of Extreme Dynamics in Femtosecond Laser Interaction With Materials; Ultrafast Mass Transport, Desorption, Formation of Periodic Structures, and Oxidation

Abstract

This thesis focused the understanding of the mechanisms responsible for different morphologies that emerge upon ultrafast irradiation. These morphological changes include the formation laser-induced periodic surface structures (LIPSS) on gallium arsenide (GaAs) in both air and vacuum as well as laser-induced oxidation of silicon (Si). Experiments were performed by varying irradiation conditions such as wavelength, pressure, and fluence to form the structures. The structures were then characterized by using a combination of optical, scanning electron (SEM), tunneling electron, and atomic force microscopies.

The first section developed an understanding of the formation of High Spatial Frequency LIPSS (HSFL) at different wavelengths and pressures. It demonstrated that, when the substrate was irradiated with 390 nm light and 780 nm light while in a vacuum environment, the formation of HSFL was caused by material removal and not material reorganization which occurs in ambient atmospheric environments. The mechanism that drove this process was suggested to be the formation and subsequent diffusion of point defects to the substrate surface where they were more easily desorbed. The excited state dielectric function was modeled and was used in a thin film plasmonic SPP model to predict the HSFL wavelength that was experimentally observed after irradiation with 390 nm light in vacuum.

The second section reanalyzed the point defect formation mechanism that has been shown to generate HSFL. Specifically, it roughly quantified the diffusion rates of point defects that eventually form the surface structures. The diffusion coefficient for interstitials upon ultrafast irradiation with 780 nm light was estimated to be 6.5 X 108 at/nm2/s, and compared with experimentally measured diffusion coefficients in the literature. The estimated coefficient was 20 orders of magnitude higher than expected for purely thermal diffusion. A model for an excited state diffusion was then presented which used the point defect formation mechanism as a basis. Lastly, using a discrete Monte Carlo model, it was shown that the excited state mediated diffusion also occurred in the formation of HSFL in GaAs in vacuum with both 390 nm and 780 nm light.

The third section discussed the ultrafast oxidation of silicon. It was shown that the rates for the oxide growth upon ultrafast irradiation was 10 orders of magnitude higher than that for thermal oxidation. Furthermore, cross section SEM showed that the oxide growth occurs at both the oxide-Si interface as well as the oxide-air interface. The silicon atom flux rate from the bulk into the oxide was then calculated to be 1.3 X 108 at/nm2/s, which is on the same order of magnitude for the excited state mediated diffusion mechanism, suggesting that this mechanism occurs in Si as well as GaAs.

Altogether, this thesis reveals that excited state diffusion is a driving factor behind many of the mechanisms observed at sub-melt fluence threshold irradiation with ultrafast laser. Furthermore, it suggests that this is a universal mechanism of ultrafast excitation of semiconductors due to its presence in both the formation of HSFL in GaAs and the oxidation of Si.