Markus Ratzke

Genesis of femtosecond-induced nanostructures on solid surfaces

The start and evolution of the formation of laser-induced periodic surface structures (LIPSS, ripples) are investigated. The important role of irradiation dose (fluence×number of pulses) for the properties of the generated structures is demonstrated. It is shown how, with an increasing dose, the structures evolve from random surface modification to regular sub-wavelength ripples, then coalesce to broader LIPSS and finally form more complex shapes when ablation produces deep craters. First experiments are presented following this evolution in one single irradiated spot.

Pulsed Laser Deposition of Hafnium Oxide on Silicon

Hafnium oxide films were prepared by Pulsed Laser Deposition (PLD). The influence of laser wavelength (fundamental, second and third harmonic of a Nd:YAG laser), used for evaporation, and substrate temperature on the film morphology, chemical structure and interfacial quality were investigated yielding the following results: While the laser wavelength exhibits minor influence on layer structure, the substrate temperature plays a critical role regarding morphological and chemical structure of the produced hafnium oxide / silicon stacks. Atomic Force Microscopy (AFM) images show a clear transition from smooth layers consisting of small area crystallites to very rough surfaces characterized by large craters and regular, plane features when the growth temperature was increased. These facts suggest a chemical instability which is confirmed by X-ray Photoelectron Spectroscopy (XPS). Investigations of the hafnium and silicon core level spectra indicate the occurrence of silicon dioxide and hafnium silicide in the case the samples were produced at elevated temperatures.

PLD of high-k dielectric films on silicon

Thin films of high-k materials praseodymium oxide (PrxOy) and hafnium oxide (HfOx) were deposited on silicon (100) surfaces by pulsed laser deposition (PLD), using the third harmonic of a Nd:YAG laser. The two materials are compared with respect to their morphology, in dependence on the substrate temperature during deposition, and their chemical composition and crystalline structure, in particular at the interface. The films of both oxides exhibit a grainy structure when deposited at substrate temperatures below 750°C, with the grain size increasing from ≈ 40 nm at room temperature to ≈ 100 nm at 750°C. However, the PrxOy films are much more uniform than hafnia, the latter exhibiting increasingly larger holes, reaching several nm into the silicon substrate. For a substrate temperature of 900°C, the film morphology for PrxOy completely changes to much larger crystalline areas, while for HfOx the role of holes in the film becomes substantial. Also, the interface chemistry is significantly different for both materials: a silicate formation for PrxOy and a rich abundance of SiO2 and a silicide for hafnia. Finally, our PrxOy-films exert compressive stress on the substrate, for HfOx-films tensile stress is observed which, however, may also result from interfacial SiO2.

Feedback Effect on the Self-Organized Nanostructures Formation on Silicon upon Femtosecond Laser Ablation

The role of multi-pulse feedback in self-organized nanostructure (ripples) formation on silicon surface upon femtosecond laser ablation is investigated. For irradiation at constant intensity and pulse repetition rate, the previously postulated feedback effect of accumulated dose with in¬creasing number of pulses is confirmed and investigated in detail: both the modified surface area as well as the complexity and feature size of generated nanostructures increase with accumulated dose. More interestingly, at constant total incident dose (number of pulses times pulse energy) accumu¬lation and feedback depend strongly on temporal pulse separation. The feedback becomes increas¬ingly weaker with increasing time intervals between successive pulses, involving times up to one second and more before individual pulses act independently. In a first attempt to model this long-lived coupling, we find that conduction band electrons, produced by the preceding laser pulse, can provide, indeed, such feedback by facilitating coupling of subsequent pulses for substantial delays. However, the achieved time span of about a millisecond is still significantly shorter than observed experimentally.