TiTe₂/PCM Heterostructures—Epitaxial Growth, Interface Formation and Stability

Device non-idealities, particularly resistance drift, electromigration, and high reset current, significantly limit the widespread of Phase Change Materials (PCM) -based memory cells. In this context, PCM-based heterostructures (PCHs) have shown the potential to overcome these limitations, where the key role of the interfaces between alternating layers has been lately highlighted [1]. Very recently, PCH composed of PCMs and two-dimensional (2D) chalcogenides as confinement materials (CMs) were realized, addressing several drawbacks of standard phase change memories. Notably, intercalating Sb₂Te₃ with TiTe₂ has shown promising results, producing PCM cells with reduced drift and reset current [2]. However, the stability of such structures is still debated, especially regarding the intermixing and the persistence of the TiTe₂ layers after cycling [3].
In this work, we aim at studying the stability of the interfaces in heterostructures based on TiTe₂ and Sb₂Te₃ or Ge-Sb-Te (GST). In this regard, we have investigated the epitaxial growth of the single layers, in order to produce highly ordered PCH. Samples grown by Molecular Beam Epitaxy (MBE) were analyzed by in situ X-ray Photoemission Spectroscopy (XPS) to characterize their electronic properties, to assess the film composition and to evaluate intermixing. The crystal structure of as-grown, as well as annealed, epitaxial layers was evaluated by X-ray Diffraction (XRD), Raman spectroscopy and High-Resolution Scanning Transmission Electron Microscopy (HR-STEM).The epitaxy of TiTe₂, Sb₂Te₃ and GST was obtained on Mica after a fine tuning of the relevant parameters such as: growth temperature, flux ratios and deposition rate.
The formation of the interface between TiTe₂ and Sb₂Te₃ was evaluated by in situ XPS for increasing thicknesses of TiTe₂ deposited on Sb₂Te₃. The analysis of the Sb, Te, and Ti core levels and their evolution with TiTe₂ coverage suggests the formation of a sharp interface without significant intermixing at the growth temperature. This result is supported by HR-STEM and Raman spectroscopy. Highly ordered epitaxial TiTe₂/Sb₂Te₃ and TiTe₂/GST multilayers where then grown and characterized by XRD, Raman spectroscopy and HR-STEM. The structure and the local chemical composition of as grown and annealed PCHs were investigated by correlated Energy X-ray Dispersive Spectroscopy and Electron Energy Loss Spectroscopy. The stability of the interfaces and TiTe₂ layers upon thermal treatment at temperatures up to 400 °C will be discussed.


REFERENCES
1. A.I.Khan, et al., Nano Letters 22. 6285 (2022).
2. K.Ding, et al., Science 366, 210 (2019).
3. G.Cohen, et al., Phys. Status Solidi RRL 2300426 (2024).