Encapsulation Effects on Ge-rich GeSbTe Alloys at High Temperature

Phase–Change Memory (PCM) is a mature technology based on chalcogenide alloys, which has proven its suitability for next generation of non–volatile memory, in particular targeting embedded applications [1]. This result was achieved thanks to the intrinsic features of PCM in terms of scalability, fast programming and high endurance. Enrichment of GeSbTe (GST) alloys by Ge and doping by light elements demonstrated to be a successful solution to enhance the data retention in PCM, opening the possibility for this technology to target automotive applications featuring strict specifications in terms of stability in high temperature environment [2–4].<br/>It has been shown that even a partial oxidation of the chalcogenide layer can affect the properties of the material and its evolution once exposed to high temperature, likely leading to a modification of the final device performances. Indeed, the oxidation phenomena in Ge₂Sb₂Te₅ and GeTe films lead to a reduction of their crystallization temperature [5]. Recently, a comparison between TiN encapsulated and air–exposed N–doped Ge–rich GeSbTe (GGST) layers, highlighted the appearance of heterogeneous interfacial phenomena in the air–exposed sample [6].<br/>In this work, we propose the study of the interfacial effects on GGST alloys at high temperature, comparing different encapsulation materials: Carbon (C), Tantalum Nitride (TaN) and Titanium Nitride (TiN) and specific combination of them. The results are compared to the ones obtained from an air-exposed sample.<br/>Carbon is interesting for its transparency in IR and Raman spectroscopy in the range of frequency of interest. Moreover, it has been used in devices to hinder the electrode/chalcogenide material intermixing [7]. TaN has a high chemical resistance [8] and it has been already integrated in memory devices [9]. Finally, TiN was studied as reference because of its adoption in state of the art PCM devices.<br/>We show how carbon layer faces instability at around 400°C, while TaN encapsulated sample presents stress induced phenomena at high temperature and accelerated crystallization of the chalcogenide layer. Encapsulation by TiN electrode preserves up to high temperature (more than 450°C) the properties of the GGST layer.<br/>We compared the results coming from different techniques in order to highlight the main properties of the samples upon annealing:<br/>● Raman spectroscopy to follow the evolution of Ge atoms vibrations;<br/>● IR spectroscopy to observe the evolution of Ge–N and Ge–O bonds;<br/>● Transmission Electron Microscopy (TEM) after annealing at 500°C;<br/>● X-ray diffraction (XRD) to compare the crystallization dynamics.<br/>The investigations were performed at several increasing temperatures up to 500°C, in order to highlight the intermixing phenomena along with the segregation and crystallization of Ge and GeSbTe phases into the GGST layer. The thickness of TaN layer should be optimized to have the compromise between a good oxidation barrier and low stress effects.<br/>In conclusion, we compared the evolution of different samples in terms of oxidation, interfacial diffusion and stress effects to highlight the advantage and drawbacks of each encapsulating material with respect to GGST alloys evolution in temperature.<br/>[1] P. Cappelletti, et al., JAP 53, 193002 (2020).<br/>[2] P. Zuliani, et al. IEEE TED 60, 4020–4026 (2013).<br/>[3] I. Yang et al., H. J. Electrochem. Soc. 157, H483 (2010).<br/>[4] L. Prazakova et al., JAP 128, 215102 (2020).<br/>[5] P. Noé et al., Acta Mater. 110, 142–148 (2016).<br/>[6] M. Agati et al., A. Appl. Surf. Sci. 518, 146227 (2020).<br/>[7] K. Ren et al. J. Mater. Sci. Mater. Electron. 30, 20037–20042 (2019).<br/>[8] M. Grosser et al., Appl. Surf. Sci. 258, 2894–2900 (2012).<br/>[9] J. Y. Wu, et al., IEEE IEDM 3.2.1-3.2.4 (2011).