Shedding too much light on semiconductors: An investigation on laser-induced damage and ablation in semiconductors

In this thesis, we look at laser induced damage (LID) and laser ablation by studying the interplay of several laser and material parameters. Specifically, we vary the pulse duration, laser wavelength and focusing conditions of the laser and we choose a variety of materials to study, based on the range of band gaps that they span, their (crystal) structure and their industrial relevance. We predominantly study the LID and ablation thresholds and morphology as a function of these parameters. The interplay between these laser and material parameters can provide fundamental physical insights. In Chapter 2 we introduce the experimental setup which we have developed to study LID and laser ablation. This highly automated setup can systematically generate large numbers of ablation craters on a small sample surface area and is equipped with several in-situ microscopy modes. This allows the gathering of large amounts of data and therefore statistics, which is of great value because it allows the exploration of a large parameter space. To validate the functionality of the setup, we investigate the influence of pulse duration on the single-shot ablation threshold of bulk Si. The first parameters which we investigate are the pulse duration, laser fluence and the material band gap. We investigate the interplay between these parameters in Chapter 3. Our setup allows us to easily vary the pulse duration and the laser fluence. We `vary the band gap' by investigating four III-V gallium-based semiconductors: GaSb, GaAs, GaP and GaN. Our main finding is that the ablation threshold varies more strongly as a function of pulse duration for materials with a larger band gap, which indicates that ablation is driven by the peak intensity of the excitation pulse. In Chapter 4, we study the influence of wavelength and laser fluence on laser ablation in several materials. In particular, we investigate the wavelength dependence of the ablation threshold of these materials at photon energies close to integer fractions of the band gap. The widely used Keldysh theory of strong field ionization (indirectly) predicts a stepwise increase in ablation threshold around these integer fractions of the band gap. Currently, there is a shortage of experimental evidence for the validity of this widely used theory. We find that a model, which employs this Keldysh theory and includes avalanche ionization, works reasonably well for direct-band gap materials, but that there is limited to bad agreement for indirect-band gap materials. In Chapter 5, we present our experiments on how the pulse duration and pulse energy affect subsurface laser-induced damage in several wide-band gap semiconductors. We carry out elaborate experiments where we investigate both the damage threshold and damage morphology as a function of pulse duration. This pulse duration dependence can give insights into the fundamental physics behind subsurface laser-induced modification, partly because we can compare the results with those of surface ablation. Aside from the fundamental insights, the experiments can also provide valuable insights for applications in the semiconductor industry as subsurface dicing of semiconductors using ultrashort pulses has proven difficult.