Microstructure of GeTe-Sb₂Te₃ heterostructures grown by pulsed laser deposition

Abstract text (incl. figure legends and references)

Introduction: Ge-Sb-Te based phase change memory (PCM) is a highly promising technology for e.g., in memory or neuromorphic computing application. Relying on reversible amorphous-crystalline phase change, PCM is non-volatile, scalable, and fast. Heterostructured PCM show optimized performance, especially regarding power consumption and multi-level operation [1,2]. However, further investigations are necessary to understand in-detail the relationship of structure and properties.

Objectives: The influence of systematically varying deposition parameters on the microstructure of GeTe-Sb₂Te₃ heterostructured thin films was investigated to comprehend their outstanding properties.

Materials & methods: Non-periodic (npHS) and periodic (pHS) GeTe-Sb₂Te₃ heterostructures were deposited at room temperature by pulsed laser deposition (PLD). Detailed microstructure analysis was performed by combination of advanced transmission electron microscopy and X-Ray measurement techniques.

Results: For the npHS a decreased laser fluence and number of pulses resulted in a decreased layer thickness which goes along with an increased intermixing and a composition approaching GeSb₂Te₄ (Fig. 1a-d). Unlike amorphous GeTe layers, the Sb₂Te₃ layers are nanocrystalline after PLD growth. Similar to fully crystalline Sb₂Te₃ [3], the nanocrystals consist of different defects like grain boundaries and bi-layer defects (Fig. 2a, b). Moreover, the grain size and crystalline phases vary (Fig. 2a, c). Apart from the known t- and c-phases, a vacancy-ordered (vo) phase was observed [4]. Similar to vo-Ge₂Sb₂Te₅ [5], this structure transforms into the c-phase by electron beam exposure (Fig. 2d). Confirmed by XRD, overall, the Sb₂Te₃ layer mainly consists of {00l}-textured t-Sb₂Te₃ phase. Despite the small layer thickness, the layers of the pHS are still separated with alternating Sb- and Ge-rich layers, respectively (Fig. 1e-h).

Conclusion: Whereas the layers intermixing depends on the deposition parameters and despite the room temperature deposition as well as the use of an amorphous substrate, as-grown Sb₂Te₃ layers are nanocrystalline while GeTe layers are amorphous in the HS. Since for Sb₂Te₃-GeSb₂Te₄-HS already an enhanced operation energy efficiency was reported [6], the results of the present work provide a starting point to establish structure-property relationships for PLD grown GeTe-Sb₂Te₃-HS in addition.

References

[1] T. C. Chong et al., Appl. Phys. Lett. 88, 122114 (2006).

[2] L. Zhou et al., Adv. Electron. Mater. 6, 1900781 (2020).

[3] J.-J. Wang et al., Phys. Status Solidi RRL 13, 1900320 (2019).

[4] Y. Zheng et al., Nano Res. 9, 3453 (2016).

[5] A. Lotnyk et al., Acta Mater. 105, 1 (2016).

[6] J. Feng et al., ACS Appl. Mater. Interfaces 12, 33397 (2020).

Acknowledgements

We thank Mrs. A. Mill for assistance in the FIB preparation. This work was supported by the German Research Foundation (DFG 445693080).


Figure 1: Overview TEM images and EDX maps of npHS deposit with the highest (a,b) and lowest (c,d) laser fluence, respectively, and two pHS (e-h).

Figure 2: Local structure of npHS. TEM images showing Sb₂Te₃ nanocrystallites characterized by multiple grain sizes (a), crystal defects (a, b), and phases (c, d).