Author: Noemi Casquero Maroto
Thesis submitted in: March 31, 2023
Under the supervision of: Dr. Jan Siegel and Dr. Mario García Lechuga (Instituto de Óptica-CSIC)
Universidad Autónoma de Madrid
Processing materials with short and ultrashort laser pulses is used to modify in a controlled way their morphology, topography and certain physical properties. The surface modification of semiconductors induced by means of laser pulses confers the ability to provide them with new functionalities. This objective requires a precise knowledge of the mechanisms that take place during the interaction of semiconductors with laser pulses.
In this thesis, the melting and solidification dynamics that take place during the irradiation of crystalline germanium and silicon with short and ultrashort laser pulses are studied. Moreover, certain permanent modifications induced after irradiation with these pulses are investigated, such as self-organized structures (LIPSS) and the formation of a superficial amorphous layer on top of crystalline material.
Through pump-probe experiments, the melting and solidification dynamics in crystalline germanium induced upon excitation with single short and ultrashort laser pulses are studied. Combining the experimental results with those of a thermodynamic model, the maximum depth and propagation speed of the solidliquid interface is determined. Also in germanium, different LIPSS structures formed under irradiation with multiple femtosecond and nanosecond pulses are analysed and their formation process discussed.
In crystalline silicon, irradiation studies with ultrashort laser pulses are carried out in a broad spectral range, from ultraviolet to mid-infrared. The ablation and amorphization threshold fluence are determined for each wavelength of this spectral range and possible mechanisms involved for each case are discussed. Finally, with the aim of maximizing the thickness an amorphous surface layer on crystalline material by means of irradiation with femtosecond laser pulses, a systematic study of different wavelengths, number of pulses, fluences and configurations of the sample covered with a thick layer of oxide has been performed, leading to the thickest amorphous layer reported to date.