Impact of Stoichiometry on the Structure of van der Waals Layered GeTe/Sb2Te3 Superlattices Used in Interfacial Phase-Change Memory (iPCM) Devices

Van der Waals layered GeTe/Sb2Te3 superlattices (SLs) have demonstrated outstanding performances for use in resistive memories in so-called interfacial phase-change memory (iPCM) devices. GeTe/Sb2Te3 SLs are made by periodically stacking ultrathin GeTe and Sb2Te3 crystalline layers. The mechanism of the resistance change in iPCM devices is still highly debated. Recent experimental studies on SLs grown by molecular beam epitaxy or pulsed laser deposition indicate that the local structure does not correspond to any of the previously proposed structural models. Here, a new insight is given into the complex structure of prototypical GeTe/Sb2Te3 SLs deposited by magnetron sputtering, which is the used industrial technique for SL growth in iPCM devices. X-ray diffraction analysis shows that the structural quality of the SL depends critically on its stoichiometry. Moreover, high-angle annular dark-field-scanning transmission electron microscopy analysis of the local atomic order in a perfectly stoichiometric SL reveals the absence of GeTe layers, and that Ge atoms intermix with Sb atoms in, for instance, Ge2Sb2Te5 blocks. This result shows that an alternative structural model is required to explain the origin of the electrical contrast and the nature of the resistive switching mechanism observed in iPCM devices.