Periodic Relief Fabrication and Reversible Phase Transitions in Amorphous Ge2Sb2Te5 Thin Films upon Multi-Pulse Femtosecond Irradiation

Abstract

Ge2Sb2Te5 based devices attract the attention of researchers due to wide opportunities in designing phase change memory. Herein, we studied a possibility to fabricate periodic micro- and nanorelief at surfaces of Ge2Sb2Te5 thin films on silicon oxide/silicon substrates under multi-pulse femtosecond laser irradiation with the wavelength of 1250 nm. One-dimensional lattices with periods of 1250 ± 90 and 130 ± 30 nm were obtained depending on the number of acted laser pulses. Emergence of these structures can be explained by plasmon-polariton generation and laser-induced hydrodynamic instabilities, respectively. Additionally, formation of the lattices whose spatial period is close to the impacted laser wavelength can be modelled by considering the free carrier contribution under intensive photoexcitation. Raman spectroscopy revealed both crystallization and re-amorphization of the irradiated films. The obtained results show a possibility to fabricate rewritable all-dielectric data-storage devices based on Ge2Sb2Te5 with the periodic relief.

Artificial Anisotropy in Ge2Sb2Te5 Thin Films after Femtosecond Laser Irradiation

“…The formed LIPSS may indicate surface plasmon polariton (SPP) generation during the intensive photoexcitation of the free charge carriers by high-power ultrashort laser pulses [ 28 ]. The necessary condition of SPP excitation is the metallization of the near-surface layer of the film under laser irradiation [ 18 , 19 ].…”

“…The necessary condition of SPP excitation is the metallization of the near-surface layer of the film under laser irradiation [ 18 , 19 ]. According to our previous estimations, the real part of GST225 dielectric function changes its sign from positive to negative at a carrier concentration of ~4 × 10 19 cm −3 [ 28 ]. The density of charge carriers n e induced by a single laser pulse can be estimated from the total power, duration, and shape of a given pulse, taking into account the absorption and reflection coefficients of the irradiated GST225 film.…”

“…The samples were moved by Standa automized mechanical translators in raster mode, while being irradiated at normal incidence by a linearly polarized laser beam focused by a lens with a focal length of 80 mm to a spot with the diameter D = 100 μm. The step between the raster scanlines Γ was equal to 120 μm to exclude possible intersections of neighbor scanlines and reamorphization effects within irradiated areas due to increased exposure time [ 28 ]. The scanning velocity was V = 5 μm/s.…”

“…As a result, the square areas with the sizes 3 × 3 mm 2 were formed. The pulse number per spot N s = 240 was defined by the formula [ 28 ]: N s = ν · D / V …”

“…This material is well-known as a basic substance for non-volatile and rewritable memory applications, with a large difference between optical or electrical properties in the amorphous and crystalline states [ 25 ]. Recently, the fabrication of LIPSS oriented along or perpendicular to the laser polarization in thin GST255 films under femtosecond laser irradiation was demonstrated, including reversible phase transitions in corresponding regimes [ 26 , 27 , 28 ]. Moreover, prolate ellipsoidal crystalline particles nucleation was theoretically predicted at the crystallization of GST225-like systems in a strong laser field [ 29 ]; the ellipsoid axes are expected to be aligned along the direction of an applied laser radiation polarization.…”