Interface Formation During the Growth of Phase Change Materials Heterostructures Based on Ge-Rich Ge-Sb-Te Alloys

Due to their fast and reversible switching between the high resistive amorphous and low resistive metastable crystalline phases upon the application of SET/RESET current pulses, phase change materials (PCM) based on chalcogenides alloys are currently used for the realization of non-volatile memories. Furthermore, partially crystallized intermediate states can be obtained by tuning the height and width of the pulses, making PCM suitable to emulate a synaptic behavior for in-memory computing applications [1], where reliable and reproducible multilevel resistance values are needed [2].<br/>The ternary (GeTe)_m(Sb₂Te₃)_n alloys (GST) are widely used as active materials for the fabrication of both optical and electrical memories, commonly in the composition Ge₂Sb₂Te₂. In view of applications in neuromorphic devices, improved material properties such as reduced resistance drift and a wide programming window are still mandatory. For this reason, thermally stable PCM with higher crystallization temperature have been identified and intensively investigated [3, 4], among which Ge-Sb-Te with Ge-rich compositions (Ge-rich GST) is one of the most promising materials. The combination of PCM layers with different physical properties might help in tailoring the material requirements for applications in embedded devices finding the best compromise between reducing latency and power consumption and increasing retention and cycling endurance. In this context, we present here experimental investigations on composition, chemical state, electronic and structural properties of different chalcogenides combined in double layered heterostructures. Two heterostructures based on Ge-rich GST are fabricated by physical vapor deposition from solid source Knudsen cells on Silicon substrates in ultra-high vacuum, varying the composition of the first PCM layer upon which a second Ge-rich GST film is grown by successive partial deposition steps. The samples are characterized <i>in-situ </i>by a combination of X-ray and Ultraviolet photoemission spectroscopy and <i>ex-situ</i> by X-ray diffraction. Information on intermixing and composition of the alloys across the heterostructures are obtained and discussed as well as the effects of thermal annealing at increasing temperatures. The evolution of the electronic properties of the heterostructures during the formation of the interface will be compared with Density Functional Theory calculations. The role of the interface on the crystallization of the Ge-rich GST second layer will be discussed.<br/>This project has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 824957 (“BeforeHand:” Boosting Performance of Phase Change Devices by Hetero- and Nanostructure Material Design).<br/>[1] A. Sebastian et al., <i>Nat. Nanotech.</i> 15, 529 (2020)<br/>[2] D.Kuzum et al., <i>Nano Lett.</i> 11, 693 (2012)<br/>[3] P. Zuliani et al., <i>Solid-State Electron.</i> 111, 27 (2015)<br/>[4] N. Saxena et al, <i>Sci. </i><i>Rep.</i> 9, 19251 (2019)