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

Abstract

Van der Waals layered GeTe/Sb Te superlattices (SLs) have demonstrated outstanding performances for use in resistive memories in so-called interfacial phase-change memory (iPCM) devices. GeTe/Sb Te SLs are made by periodically stacking ultrathin GeTe and Sb Te 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/Sb Te 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, Ge Sb Te 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.

Phase‐Change Superlattice Materials toward Low Power Consumption and High Density Data Storage: Microscopic Picture, Working Principles, and Optimization

“…In addition, although the GeTe sublayer in GST‐SL was often regarded as [(GeTe) 2 ] in previous models (Figure ), its thickness in samples/devices have been described as 0.7, 0.8, 0.9, or 1 nm in literatures depending on the publications and groups. Perhaps, this is due to how the GeTe‐sublayer thickness was defined or due to difficulty in the thickness control during material growth of GST‐SL.…”

“…Perhaps, this is due to how the GeTe‐sublayer thickness was defined or due to difficulty in the thickness control during material growth of GST‐SL. In fact, GST‐SLs with varied GeTe‐sublayer thicknesses have been fabricated and studied . Since the GeTe‐sublayer thickness can influence the property of GST‐SL, it is worth to pay more attentions to this issue.…”

“…It should be noted that the IPCM devices were often fabricated by sputtering method . Very recently, Kowalczyk et al argued in favor and definitively stated the intermixing also happened in high quality GST‐SLs deposited by sputtering in industrial physical vapor deposition (PVD) equipment. As such, the switching mechanism of IPCM should be revisited by taking into account the Ge/Sb intermixing.…”

“…Takaura et al fabricated their GST‐SL devices using the PVD methods . Kowalczyk et al also prepared the GST‐SLs on Si substrates by sputtering method at 250 °C . Another technique to fabricate the superlattice is MBE.…”

“…Zhou et al reported that the [(Sb 2 Te 1 ) 2 nm /(GeTe) 1 nm ] 13 superalttice had a lower SET voltage than the prototypical [(Sb 2 Te 3 ) 2 nm /(GeTe) 1 nm ] 13 superalttice . Kowalczyk et al also have demonstrated that the atomic compositions of GST‐SL not only affect the crystal quality but also affect the structures (such as the long‐range order or orientation) and thus its properties . Moreover, they demonstrated that the compositions and periods of GST‐SL can be well controlled by a cosputtering PVD, which is suitable for industrial fabrications.…”