Femtosecond laser-controlled self-assembly of amorphous-crystalline nanogratings in silicon

Abstract

Self-assembly (SA) of molecular units to form regular, periodic extended structures is a powerful bottom-up technique for nanopatterning, inspired by nature. SA can be triggered in all classes of solid materials, for instance, by femtosecond laser pulses leading to the formation of laser-induced periodic surface structures (LIPSS) with a period slightly shorter than the laser wavelength. This approach, though, typically involves considerable material ablation, which leads to an unwanted increase of the surface roughness. We present a new strategy to fabricate high-precision nanograting structures in silicon, consisting of alternating amorphous and crystalline lines, with almost no material removal. The strategy can be applied to static irradiation experiments and can be extended into one and two dimensions by scanning the laser beam over the sample surface. We demonstrate that lines and areas with parallel nanofringe patterns can be written by an adequate choice of spot size, repetition rate and scan velocity, keeping a constant effective pulse number (N eff) per area for a given laser wavelength. A deviation from this pulse number leads either to inhomogeneous or ablative structures. Furthermore, we demonstrate that this approach can be used with different laser systems having widely different wavelengths (1030 nm, 800 nm, 400 nm), pulse durations (370 fs, 100 fs) and repetition rates (500 kHz, 100 Hz, single pulse) and that the grating period can also be tuned by changing the angle of laser beam incidence. The grating structures can be erased by irradiation with a single nanosecond laser pulse, triggering recrystallization of the amorphous stripes. Given the large differences in electrical conductivity between the two phases, our structures could find new applications in nanoelectronics.

Coherent scatter-controlled phase-change grating structures in silicon using femtosecond laser pulses

“…1(b)). A similar behavior of amorphous fringe formation and transition to ablation has been reported upon irradiation with 370 fs, 1030 nm laser pulses, although in that case it was at lower N-values, leading to a better fringe homogeneity and alignment 16 . The non-circular shape of the ablation region is a consequence of the directional surface wave triggered at the fringe structures, which is more efficient along the direction of the laser polarization vector, leading to a horizontal enlargement of the ablated region 20 .…”

“…A powerful method to improve fringe homogeneity and alignment, as well as to extend fringes in 2D, is to scan the laser spot over the surface at a well-defined repetition rate and speed 16 . Figure 2(a) shows the best result for a structure written in Si while moving the sample, under similar conditions as those used in Fig.…”

“…Micro-Raman spectroscopy is a suitable high-sensitivity technique to detect the presence of the amorphous phase in laser irradiated crystalline silicon, even if present only in form of a thin surface layer 18 . Yet, the short LIPSS period, close to the laser wavelength for normal incidence, poses a challenge to the spatial resolution of micro-Raman spectroscopy, which effectively averages over fringe and inter-fringe space 16 . Here, taking advantage of the fact that the fringe period can be increased by changing the angle of incidence, we have performed micro-Raman spectroscopy on a single Λ s + fringe, at the position indicated in Fig.…”

“…Regardless of the physical origin, the periodic intensity modulation gradually evolves upon multiple pulse irradiation and an equivalent periodic surface modulation is finally imprinted in the material. While in most cases the process leads to local ablation or material reorganization, we reported recently a proof-of-principle of a fabrication process based on the local amorphization of crystalline silicon, leading to amorphous-crystalline LIPSS using laser fluences below the ablation threshold 15, 16 . Employing laser beam scanning at different wavelengths it was shown that parallel amorphous fringes could be written by a proper adjustment of laser fluence, repetition rate and scan speed.…”

Fabrication of amorphous micro-ring arrays in crystalline silicon using ultrashort laser pulses

“…Using nanosecond laser irradiation, Pena et al produced micrometer-sized amorphous humps with a height of a few nanometers 18 . Recently, it has been shown that Laser-Induced Periodic Surface Structures (LIPSS) in form of alternating amorphous and crystalline fringes can be fabricated by scanning the laser beam over the sample surface, employing an adequate choice of spot size, repetition rate and scan velocity 19,20 . In the present work we present a simple strategy to fabricate surface-depressed annular amorphous rings with a central crystalline disk, temporally resolve their formation process, and show how these structures can be scaled and stitched together to form arrays with different symmetries.…”