In-Operando Transmission Electron Microscopy Studies on Diffusion Induced Phenomena at the Dielectric-Electrode Interfaces in Ge₂Te₃-Based Memristor Devices

The memristor is a two-terminal device derived from materials that exhibit resistance switching, wherein the resistance of the material is tunable by an applied electric field. This switching process is reversible, and is also non-volatile, so that the change in resistance is maintained for a long period of time even after the applied field is removed. A wide variety of materials are currently being investigated depending on microstructural mechanisms that triggers the switching process, such as those based on defect migration and phase transformation. Independent of the actual mechanism that drives this switching process, the dielectric-electrode interface plays a significant role in determining device properties and performance. In particular, maintaining interface chemistry and morphology during actual operation is critical to realizing devices with higher switching speed, lower power consumption, and higher endurance. Recent advancements in transmission electron microscopy makes it possible for examining interfacial phenomena in such devices in-operando at high spatial resolution and sensitivity.<br/>In this contribution we perform an in-operando cross-sectional TEM study to investigate microstructural phenomena that control the integrity of the dielectric-electrode interface in Ge₂Te₃-based phase change materials. For the purposes of this study we have examined device structures: Pt/ Ge₂Te₃ (50 nm)/Ti/Pt, grown on (100)-Si substrates. Specifically we employ Z-contrast STEM imaging in combination with X-ray energy dispersive spectroscopy (XEDS) to examine the effect of Ti which is widely used as an adhesion layer prior to the deposition of electrode layer atop the dielectric (Ge₂Te₃) layer. Detailed examination of XEDS results upon electrical biasing clear reveal a deleterious effect of Ti, characterized by in-diffusion of Ti to form a reactive telluride layer and out-diffusion of Ge, resulting in its enrichment at beneath the top Pt electrode. Further studies under systematic electrical biasing and heating conditions reveal that this diffusion process can be significant, leading to progressive degradation of the dielectric-electrode interface. Additional studies aimed at combating this process will be presented. Furthermore, the role of residual surface oxide layer generated during the device fabrication process will also be discussed.