Realization of Multilevel States in Phase‐Change Thin Films by Fast Laser Pulse Irradiation

Multilevel storage techniques are promising for increasing storage density and for reducing energy consumption in the application of phase‐change materials based memory devices. However, accurately controlling the phase transitions as well as understanding the underlying switching mechanisms are still under investigation. In this study, nonvolatile optical multilevel switching in single‐layered GeTe phase‐change films prepared by laser ablation is demonstrated to be feasible and accurately controllable at a time scale of nanoseconds. For this purpose, an ns UV laser pump‐probe setup is adapted for the investigations. It is found that each laser pulse excitation (duration: 20 ns, wavelength: 248 nm) with a fluence of 26 mJ cm⁻² induced a partial crystallization whereas complete crystallization is achieved by superposition of single pulses. In the reverse process, a single pulse excitation at a fluence of 112 mJ cm⁻² leads to reamorphization of GeTe thin films. Regarding the switching times, complete crystallization and reamorphization are determined to be accomplished after ≈300 ns and ≈3 ns, respectively. Furthermore, it is shown that the local microstructure phase transformation processes are correlated to the observed reversible switching of the optical reflectivity, which are important for understanding the underlying mechanisms of the phase‐change‐materials‐based multilevel phase‐change memory.