Growth and Microstructure of GeTe-Sb₂Te₃ Heterostructures Deposited by Pulsed Laser Deposition

Characterized by non-volatility, scalability and fast operating speed Ge-Sb-Te based phase change alloys are well suited compounds for memory devices. Nevertheless, high power consumption and resistance drift turn out to be obstacles for big data application. Heterostructured phase change memory (HS-PCM) is shown to be a promising strategy to overcome these issues. However, underlying structure-property relationships are still highly debated. [1-3] As a first step for systematic performance improvement, we investigated how varying deposition parameters influence the microstructure of GeTe-Sb₂Te₃ heterostructured thin films.<br/><br/>The samples were grown at room temperature as non-periodic and periodic HS (npHS and pHS, respectively) using pulsed laser deposition (PLD). Advanced transmission electron microscopy combined with X-ray measurement techniques was applied for in-depth analysis of the microstructure.<br/><br/>For the npHS the system deposited using the highest laser fluence consists of alternating GeTe and Sb₂Te₃ layers. A small degree of interdiffusion of Ge into Sb₂Te₃ layers and vice versa is always present getting more pronounced with decreasing layer thickness. In contrast, the thin film grown using the lowest fluence is a single, completely intermixed layer with a composition close to GeSb₂Te₄. Revealing the deposition rate as a main influence factor for intermixing, the pHS consist of separated layers irrespective of the layer thickness.<br/><br/>GeTe layers within the HS are always amorphous whereas Sb₂Te₃ layers were found to grow nanocrystalline irrespective of laser fluence. As reported for fully crystalline Sb₂Te₃ [4], the nanocrystals are defective consisting of grain boundaries and bi-layer defects. Further characteristics are varying grain sizes and phases. Apart from the main {<i>00l</i>}-textured t-phase, c- and vacancy ordered (vo) phase are present. By electron beam exposure this structure transforms into the c-phase, similar as already reported for vo-Ge₂Sb₂Te₅ [5].<br/><br/>In conclusion, varying deposition parameters were found to mainly effect the intermixing in GeTe-Sb₂Te₃ heterostructured thin films. In contrast, irrespective of the deposition parameters and despite the room temperature deposition PLD grown HS are characterized by nanocrystalline Sb₂Te₃ layers. Considering the correlation of nanocrystallinity and improved operation efficiency reported for Sb₂Te₃-GeSb₂Te₄-HS in literature [6], this work provides insights into the relationship of deposition, structure, and properties of PLD grown GeTe-Sb₂Te₃-HS.<br/><br/>Acknowledgements<br/>We acknowledge the financial support by the German Research Foundation (DFG 445693080). We thank Mrs. A. Mill for assistance in the FIB preparation.<br/><br/>References<br/>[1] A. Lotnyk et al., Appl. Surf. Sci. 536 (2021) 147959<br/>[2] L. Zhou et al., Adv. Electron. Mater. 6 (2020) 1900781<br/>[3] X. Li et al., Adv. Funct. Mater. 28 (2018)1803380.<br/>[4] J.-J. Wang et al., Phys. Status Solidi RRL 13 (2019) 1900320<br/>[5] A. Lotnyk et al., Acta Mater. 105 (2016) 1<br/>[6] J. Feng et al., ACS Appl. Mater. Interfaces 12 (2020) 33397