Perkolationsphänomene in chalkogeniden Phasenwechsel-Dünnschichten; strukturelle und tomographische Charakterisierung mit dem Transmissionselektronenmikroskop
Final Report Abstract
According to the percolation theory, the crystalline paths within the amorphous PC material exhibit nm-sized dimensions. Therefore, in order to systematically explore the percolation events by TEM, a spatial limitation of the phase transition area is indispensable. For this purpose, in the present studies the Ti-electrodes were designed by low resolution optical lithography and lift-off techniques. The design was chosen in a way that the first electrode has rectangular geometry and the second one a triangular shape. However, the lateral limitation of the switching area amounts to about a few micrometers which makes a detailed investigation of nm-scaled percolation paths by TEM difficult. It is well known, that an intense e-beam causes a crystallization process in an amorphous PC-material. Therefore, we have used a nm-scaled patterning method using an intense e-beam in our SEM. This technique allows us to create crystalline features in 100 nm size areas surrounded by amorphous PC-material. For this purpose, the current induced structural transformation from the as-deposited amorphous to the crystalline phase of GST films in a lateral test structure (Ti/Ge2Sb2Te5/Ti) was systematically studied by transmission electron microscopy (TEM). The microstructures of the switched memory cells were investigated by TEM. For this purpose, lamellae from the switching area (the gap region) of the memory cells were prepared by focused ion beam (FIB) using a Ga ion-source. The FIB-Lamellae were prepared from the middle part of the gap area and perpendicular to the crystalline paths c and d. This procedure ensures to investigate the whole GST-gap influenced by the electrical current. We were able to reveal details about the formation of crystalline channels within the amorphous GST-film, on the basis of percolation theory. Based on these results, the current induced crystalline percolation paths are visualized for the first time by TEM. Additionally, we demonstrate the significant influence of the interface on the creation of percolation paths. Based on our results, we conclude that the nm-sized dimensions of a PC-memory cell is the main reason for the fact that the crystalline percolation paths play a dominant role in the actual switching process. Basically, the creation of a relatively large number of percolation paths in a PC-cell depends on the cell geometry such as the gap length, since the percolation event takes place in a larger gap length. Therefore, the successful visualisation of such crystalline paths in our studies is a major factor in attempts to improve the lateral cell design and the switching behaviour of the amorphous GST-gap. The random spatial dispersion and the different crystallographic orientations of such nm-sized crystalline paths at the interface reveal the stochastic nature of the crystallization mechanism, which is the physical basis for the percolation event. Based on nucleation and growth theory, formation of these nm-sized crystalline channels can be explained by Wiener's percolation model.
