Research On Crystallization Of Ge₂Sb₂Te₅ Induced By Pulsed Laser

Information technology is closely related with the progress of human society. With the rapid increase in the amount of information in modern society, the phase change memory storage has obtained continuous attention in the last 20 years based its excellent properties such as high reading and writing speed, lower power consumption and good stability. Despite the rapid development of phase change optical storage, it is very hard to measure the parameters of crystallization in such a fast process with nanosecond range. Moreover, theoretical calculations are not always uniform with experimental results. Therefore, the mechanism of phase change memory storage is still very vague for researchers. It is of great significance to further study the process and mechanism of phase transition for optimizing the phase transformation and revealing the mechanism of phase change induced by pulsed laser radiation. In this paper, the amorphous Ge₂Sb₂Te₅ films were deposited by the magnetron sputtering. First, the crystalline Ge₂Sb₂Te₅ films were characterized under the static isothermal annealing conditions. Then the characteristics of Ge₂Sb₂Te₅ films induced by pulse laser were studied by changing the thickness of the film and energy density of pulsed laser. Finally, a three-dimensional finite element software ANSYS was used to simulate the thermal dynamic process under the pulse laser irradiation and explore the mechanism of crystallization combined with the experimental results.TEM（transmission electron microscopy） observations of Ge₂Sb₂Te₅ films annealed at 180℃ with different annealing time were carried out and the variation of mean grain size in isothermal annealing process was also studied. When the 1000 nm thick Ge₂Sb₂Te₅ film was annealed at 180℃, 6h, AFM（atomic force microscopy） observations were performed and the effect of thickness on the crystallization induced by isothermal annealing was also studied.When the 50 nm, 70 nm and 100 nm thick films were irradiated by the Kr F excimer laser with a pulse width of 30 ns, the crystallization of GST films were investigated by using TEM integrated with SAED. A more complete crystallization in 50 nm thick film was found as compared to films with the thickness of 70 nm and 100 nm. Finally, a heating flow induced by laser was developed to reveal how the film thickness affects the morphologies of crystalline Ge₂Sb₂Te₅ film by pulse laser irradiation.TEM observations SAED measurements on the crystalline morphologies in the thickness as well as were carried out when the Ge₂Sb₂Te₅ films irradiated at the laser fluences of 19.64 m J/cm²,26.67 m J/cm² and 33.14 m J/cm², respectively. It was found that the area of crystalline GST was composed of large columnar grains at the surface and fined equaixed grains inside the film. Additionally, a three dimensional element method simulation was used to calculate the temperature field by single pulse laser with different densities of 19.64 m J/cm²,26.67 m J/cm² and 33.14 m J/cm². The temperature and temperature gradient were further studied under different laser densities. Finally, the reasons for the formation of crystalline morphologies were further analyzed.A heating flow model was established based on the laser parameters and thickness of films, and the evolution of crystallization by laser pulse radiation was explained by studying the characteristics of Ge₂Sb₂Te₅ films induced by laser, combining with three dimensional finite element simulation on the thermal dynamic process. This has profound implications for further optimization and the development of phase change memory.