Incubation and nanostructure formation on n- and p-type Si(100) and Si(111) at various doping levels induced by sub-nanojoule femto- and picosecond near-infrared laser pulses

• Published in: Applied surface science 2014-09, Vol.314, p.21-29
• Main Authors: Schüle, M., Afshar, M., Feili, D., Seidel, H., König, K., Straub, M.
• Format: Article
• Language: English
• Subjects: Ablation; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Density; Dopants; Doping; Exact sciences and technology; Incubation; Laser-induced defect; Laser-induced periodic surface structure; Lasers; Nanostructure; Physics; Ripples; Silicon nanostructure; Sub-15 fs laser microscopy; Thresholds; Ultrafast laser ablation
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2014.06.140

•Nanorifts, ripples of period 130nm and randomly nanoporous surface structures were generated.•Such nanostructures emerged on heavily and lightly n- and p-doped Si(100) and Si(111) surfaces.•Strong incubation occurred irrespective of dopant type and concentration or surface orientation.•Incubation is attributed to photoexcitation from laser-induced defect states in the bandgap.•Aggregation of defects results in nanocracks, which turn into nanorift and nanoripple patterns.•Ablation involved predominantly single-photon processes but also multiphoton absorption. N- and p-doped Si(100) and Si(111) surfaces with dopant concentrations of 2×1014–1×1019cm−3 were irradiated by tightly focused 85-MHz repetition rate Ti:sapphire laser light (central wavelength 800nm, bandwidth 120nm) at pulse durations of 12fs to 1.6ps. Dependent on pulse peak intensity and exposure time nanorifts, ripples of period 130nm as well as sponge-like randomly nanoporous surface structures were generated with water immersion and, thereafter, laid bare by etching off aggregated oxide nanoparticles. The same structure types emerged in air or water with transform-limited 100-fs pulses. At a pulse length of 12fs pronounced incubation occurred with incubation coefficients S=0.66–0.85, whereas incubation was diminished for picosecond pulses (S>0.95). The ablation threshold strongly rose with dopant concentration. At similar doping level it was higher for n-type than for p-type samples and for Si(100) compared to Si(111) surfaces. These observations are attributed to laser-induced defect states in the bandgap which participate in photoexcitation, deactivation of dopants by complex formation, and different densities of interface states at the boundary with the ultrathin native silicon dioxide surface layer. The threshold increase with pulse length revealed predominant single-photon excitation as well as multiphoton absorption.