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LASER-MATTER INTERACTION
Modification of Ge2Sb2Te5 Thin Film by Femtosecond Laser Pulses under
Different Irradiation Modes
M. Smayev1, P. Lazarenko2, Yu. Vorobyov3, T. Kunkel4, S. Kozyukhin5
1Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia 2National Research University of Electronic Technology, Zelenograd, Russia 3Ryazan State Radio Engineering University, Ryazan, Russia Moscow Institute of Physics and Technology, Dolgoprudny, Russia 5Kurnakov Institute of General and Inorganic Chemistry, RAS, Moscow, Russia
The simplicity of switching between the crystalline and amorphous phases of the Ge2Sb2Te5 (GST225) thin film, along with the significant differences between the electrical and optical properties for these phases, provides interest in this material for various applications. Multiple reversible phase switching can be observed when GST225 film is subjected to thermal, low-frequency or optical electromagnetic fields. Upon action of ultrashort laser pulses, the formation of a laser-induced periodic surface structure (LIPSS) consisting of alternating lines of a crystalline or amorphous phase is possible [1]. In this work, we study the different types of the laser-induced periodic structures obtained as a result of crystallization and ablation of the initially amorphous Ge2Sb2Te5 thin films as well as the scale them up by a scanning laser beam.
The 130-nm-thick films of amorphous GST225 were produced by a dc magnetron sputtering. The films were deposited on W (200 nm)/TiN (50 nm)/SiO2 (1 ^m)/Si multilayer structures. We used a Yb:KGW laser system with a wavelength of 1030 nm. The pulse duration was t = 185 fs; the repetition rate was in the range R1000 kHz. The films were placed on the translational stage, which made it possible precise positioning and smooth movement of the sample. The light beam diameter on the film surface was about 140 ^m (1/e2). The possibility of scaling up the LIPSS was demonstrated by single-pass scanning of the sample with respect to a laser beam. High-quality periodic structures up to 1 mm long were obtained in this way, consisting of about 50 parallel amorphous-crystalline lines with a period of about 1 ^m, oriented perpendicular to the light polarization [2]. It has been shown that the recorded extended LIPSS are reflective diffraction gratings due to the large optical contrast between the amorphous and crystalline phases.
We demonstrate that laser ablation is also a convenient way to create spatially ordered surface nanostructures. By irradiating of Ge2Sb2Te5 thin films with more energetic femtosecond laser pulses we achieved the formation of organized nanospheres and ripples with pronounced morphology. First pulses generate a surface plasmon-polariton (SPP) wave propagating along the surface. Further irradiation is followed by interference between SPP and incident light; that leads to spatially modulated melting in the interference maxima. The surface represents periodically distributed threads of molten material oriented perpendicular to the polarization vector of laser beam and with a period equal to its wavelength. Plateau-Rayleigh instability modulates the threads into equidistance droplets, which solidify in the form of glassy spheres. Numerical solution of the Fourier equation shows that irradiation of surface covered with regular distributed spheres creates lateral temperature gradient from spheres to film. Due to thermo-capillary (Marangoni) forces arising opposite to the temperature gradient material moves from hot to colder areas, and it leads to the formation of regular structures on the surface.
The study was supported by RFBR (20-03-00379).
[1] S. Kozyukhin, M. Smayev, V. Sigaev, Y. Vorobyov, Y. Zaytseva, A. Sherchenkov, P. Lazarenko, "Specific features of formation of laser-induced periodic surface structures on Ge2Sb2Te5 amorphous thin films under illumination by femtosecond laser pulses" Phys. Stat. Sol., v. 257, art. no. 1900617 (2020).
[2] M.P. Smayev, P.I. Lazarenko, I.A. Budagovsky, A.O. Yakubov, V.N. Borisov, Yu.V. Vorobyov, T.S. Kunkel, S.A. Kozyukhin,
"Direct Single-Pass Writing of Two-Phase Binary Diffraction Gratings in a Ge2Sb2Te5 Thin Film by Femtosecond Laser Pulses" Opt.
Laser Technol., v. 153, art. no. 108212 (2022).
