Nanoprotrusions formed on the silicon specimen surface upon irradiation at , with 1000 pulses. Angle of incidence: . (a) Planar view. Profile below figure is along shorter segment. Notice misalignment along longer segment; (b) 3D image of upper left of (a); and (c) FFT of image in (a) showing regularity of the array spacing between lines spanned in the profile.
AFM image revealing development of nanoripples and their transition to nanoprotrusions. Ripples tend to become thinner and taller as the nanostructure evolves toward protrusions. Notice different axes scales in the profiles from 1 to 5. Specimen irradiated at , with 1000 pulses. Angle of incidence: .
3D image of the same specimen surface shown in planar view in Fig. 2, demonstrating nanoripple evolution and transition to aligned nanoripple structure.
Nanoripple to nanoprotrusion transition region. Planar view of specimen irradiated at , with 1000 pulses. Angle of incidence: . Left profile reveals pronounced increase in height and decrease in width of ripples from left to right, as nanoprotrusion region is approached.
SEM image showing two regular arrays of orthogonally intercepting ripples. Specimen irradiated with 500 laser pulses in air using a Lloyd’s mirror configuration. . Angle of incidence: . Arrow A is normal to the interference pattern [Eq. (5)] , and arrow B is normal to the cosine structure [Eq. (4)] .
SEM image showing nanoprotrusions formed using Lloyd’s mirror. Specimen irradiated with 2000 pulses at the same fluence and angle of incidence as those in Fig. 5. Arrow direction is parallel to the orientation of cosine LIPSS.
Low magnification SEM image of the same specimen as in Fig. 6 showing the extent and regularity of the nanoprotrusion array across a fairly large area. A remarkable alignment can be seen in the array extending from upper left to lower right side of the picture. Spacing of these lines agrees with the grating, Eq. (4).
AFM image of nanoprotrusions emerging from irregular nanoripple structure. Specimen irradiated at , with 1000 pulses. Angle of incidence: . Profile on left taken along line marked in the micrograph. FFT on right with the two circles delineating possible ripple orientations account for apparently chaotic structure. Notice the two bright points at the circles’ intersection, showing the prevalent presence of lines in the direction of the profiling, with spacing obeying the ripple, Eq. (4).
SEM images showing nanoparticle evolution. Irradiation at , UHP . (a) Silicon film redeposited on the surface, 100 pulses. (b) Nanoparticles starting to form by film clustering with additional laser pulses, totaling 150 pulses. (c) After 200 pulses, aligned nanoparticles distributed on the surface, exhibiting long range ordering.
(a) Long-range ordered arrays of aligned nanoparticles after 200 laser pulses. Higher magnification of Fig. 9(c). (b) Same specimen as in (a), showing loss of alignment after annealing treatment in a vacuum chamber (base pressure ) at for .
Schematic representation of the evolution of the nanoprotrusions. The liquid silicon moves along the direction driven by the surface tension gradient, toward the temperature minimum, where it encounters fluid moving in the opposite direction, driven by the same forces. Both liquid fluxes concurring to this region raise the ripple height in the direction.
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