MRS Spring/Fall Meeting 2020

Towards the Investigation of Electronic Properties of Epitaxial Phase Change Materials

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Phase Change Materials (PCMs) represent an important materials class from both a fundamental and technological point of view. Although much effort is still required to determine all the physical mechanisms underlying their complex physical properties, memory devices based on PCMs, which exploit the resistance contrast between the amorphous and crystalline phases, are already at the production stage. The ternary (GeTe)m(Sb2Te3)n alloys are the prototypical phase change materials commonly used for optical and memory devices in the composition Ge2Sb2Te5 (GST225). Among other fabrication methods, such as pulsed laser deposition and sputtering, which is the main technique used in industry, Molecular Beam Epitaxy (MBE) has recently gained in importance because of the advantage to obtain epitaxial quasi single-crystalline PCM layers. This achievement has made it possible the investigation of material properties for which structural perfection is of crucial importance. For example, we have been able to demonstrate that highly ordered crystalline phases can be deposited by van der Waals (vdW) epitaxy, where adjacent building blocks are weakly bonded among them and to the substrate. Furthermore, epitaxial quasi single-crystalline GST, with ordered stacking of intrinsic vacancies, exhibits a large resistivity range that is promising for the realization of memory cells with large programming windows [1]. In this presentation we will first review our advances in the growth of epitaxial GST alloys by MBE on both Si and InAs substrates. We have carried out a growth engineering of single-crystalline GST to achieve unprecedented control over structural order, phase, and composition. The combined use of X-ray diffraction and Raman spectroscopy allowed us to draw a growth phase diagram for GST [2]. The capability to retain the stored information during the memory lifetime is a fundamental property of PCM, and any spontaneous evolution of the amorphous phase toward the more stable crystalline one is undesired. This is especially true when the device working temperature may be higher than the crystallization temperature, as in automotive applications. In this regards we will present our first results in the search of suitable PCM alloys with improved physical properties in terms of crystallization temperature and crystallization speed. In particular we will show our first crystallization study of Ge-rich GST, aiming to identify thermally stable PCMs with high crystallization temperature, and our first results on the investigation, by X-ray Photoemission Spectroscopy (XPS), of the electronic properties of both epitaxial GST and Ge-rich GST. XPS data will be correlated with X-ray diffraction and Raman characterization. [1] V.Bragaglia, F.Arciprete, W.Zhang, A.M.Mio, E.Zallo, K.Perumal, A.Giussani, S.Cecchi, J.E.Boschker, H.Riechert, S.Privitera, E.Rimini, R.Mazzarello, and R.Calarco, Sci. Rep. 6, 23843 (2016) [2] V.Bragaglia, F.Arciprete , A.M.Mio , and R.Calarco, J. Appl. Phys. 123, 215304 (2018)