Basic Research and applied perspectives of chalcogenide phase-change materials grown by molecular beam epitaxy

For twenty years chalcogenide phase-change materials (PCMs) have found their major application in re-writable optical storage media. Nowadays, PCMs are employed also in microelectronics for non-volatile electrical random access memories (PCRAM). The success of these materials is related to the rapid phase-change from the crystalline to the amorphous state, both exhibiting dissimilar optical contrast which is also accompanied by a huge difference in resistivity. Many different compounds exhibit PCM properties; however the most interesting, from both a fundamental and practical perspective for PCRAM application are the alloys along the GeTe-Sb₂Te₃ binary line.
The understanding of the switching process in PCM, dating back to the 1970s, was based upon a picture in which an amorphous phase is generated by a rapid cooling of the melt and the corresponding crystalline phase is created by annealing the amorphous phase above the crystallization temperature. While this simple idea has a strong appeal, the subtle nature of PCM bonding was shown to offer the interpretation to the switching. In fact, from the atomistic perspective the switching process has been suggested to be a reduction in the Ge coordination from octahedral (crystalline phase) to tetrahedral (amorphous phase) symmetry, accompanied by the breaking of the so called resonant bonds.
Our group pioneered the growth of single and multi-layer chalcogenide material by molecular beam epitaxy (MBE). It has to be noted that industrial PCRAM samples are usually grown by sputtering, a technique that does not allow for a perfect interface quality. MBE in contrast, accomplishes this requirement and combines superior thickness control with ultrahigh purity and the possibility of using a variety of in-situ characterization tools. MBE therefore helps in elucidating the fundamental growth mechanisms of a commercially highly relevant material and establishing a material basis for innovative devices, beneficial from both academic and applied perspectives. In this respect, we have recently enlightened the epitaxial rules that apply to this class of materials^1,2,3,4,5. Such understanding has inspired the optimization of the structural properties of chalcogenide materials used for the realization of PCRAM test cells⁶ and allowed detailed photoelectron spectroscopy studies⁷ as well as the investigation of the metal insulator transition^8,9.
References:
¹ J.E. Boschker, J. Momand, V. Bragaglia, R. Wang, K. Perumal, A. Giussani, B.J. Kooi, H. Riechert, and R. Calarco, Nano Lett. 14, 3534 (2014).
² R. Wang, J.E. Boschker, E. Bruyer, D. Di Sante, S. Picozzi, K. Perumal, A. Giussani, H. Riechert, and R. Calarco, J. Phys. Chem. C 118, 29724 (2014).
³ E. Zallo, S. Cecchi, J.E. Boschker, A.M. Mio, F. Arciprete, S. Privitera, and R. Calarco, Sci. Rep. 7, 1466 (2017).
⁴ R. Wang, D. Campi, M. Bernasconi, J. Momand, B.J. Kooi, M.A. Verheijen, M. Wuttig, and R. Calarco, Sci. Rep. 6, 32895 (2016).
⁵ R. Wang, W. Zhang, J. Momand, I. Ronneberger, J.E. Boschker, R. Mazzarello, B.J. Kooi, H. Riechert, M. Wuttig, and R. Calarco, NPG Asia Mater. 9, e396 (2017).
⁶ J.E. Boschker, M. Boniardi, A. Redaelli, H. Riechert, and R. Calarco, Appl. Phys. Lett. 106, 23117 (2015).
⁷ M. Liebmann, C. Rinaldi, D. Di Sante, J. Kellner, C. Pauly, R.N. Wang, J.E. Boschker, A. Giussani, S. Bertoli, M. Cantoni, L. Baldrati, M. Asa, I. Vobornik, G. Panaccione, D. Marchenko, J. Sánchez-Barriga, O. Rader, R. Calarco, S. Picozzi, R. Bertacco, and M. Morgenstern, Adv. Mater. 28, 560 (2016).
⁸ 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).
⁹ V. Bragaglia, K. Holldack, J.E. Boschker, F. Arciprete, E. Zallo, T. Flissikowski, and R. Calarco, Sci. Rep. 6, 28560 (2016).

Bio: Raffaella Calarco is Senior Scientist at the Paul-Drude-Institut (PDI) in Berlin Germany. She received her Master’s degree in Physics in 1996 from the University of Rome Tor Vergata. She holds a Ph.D. in Material Science in 2001 from the University of Rome La Sapienza. From 2000 to 2001 she worked as a Post-Doc at the University of Aachen (RWTH) Germany. From 2001 to 2010 she was with the Research Center Jülich, Germany, at first in the Tenure-track excellence program and then as a Senior Research Scientist, focusing on III-nitride nanowires. In 2010 she received the Habilitation in Physics from the RWTH Aachen and in 2012 from the Humboldt University in Berlin, Germany. In 2013 she obtained the Habilitation to full Professor in Italy. Since September 2010 she is with PDI, her current research interests include on the one hand epitaxy of III-nitride nanostructures and layers on the other hand epitaxial growth of phase-change materials for memory applications. Involved in National and International projects and European networks, she collaborates with several institutions. She is Coordinator of the EU Project 642574 of the Marie Skłodowska-Curie Actions (MSCA) Innovative Training Networks (ITN) H2020. She has been serving as Work Package Leader in the EU-FP7 ICT Project PASTRY. She has been awarded with the Ovshinsky Lectureship Award 2017 for outstanding contribution to Ovonic Science and Technology and with the E/PCOS 2012 excellent presentation Award. Furthermore she participated to Helmholtz-Akademie für Führungskräfte and the “Tenure-track”- excellence program of the Forschungszentrum Jülich.  Since 2012 is member of the International scientific selection committee for proposal of the Synchrotron Radiation Source BESSY II. She is author or coauthor of about 145 publications, 59 invited talks, 4 book chapters, 5 invited review papers, and 2 patents. She has about 3600 citations and her current Hirsch index is 30 (on Google Scholar).