Electronic transport in phase-change materials
Final Report Abstract
Phase Change Materials can be rapidly and reversibly switched between the amorphous and crystalline state (phase change materials) [Wuttig-2005]. Crystallization is accompanied by a pronounced change of electronic properties such as electrical resistivity as well as optical reflectance. Especially this latter finding is highly unusual. Recently we could demonstrate that the change in optical properties is related to a change of bonding upon crystallization. While amorphous phase change materials are characterized by ordinary covalent bonding, crystalline phase change materials utilize resonant bonding [Shportko-2008]. This bonding mechanism is only possible in solids which employ their p-electrons for bonding and are neither too ionic nor show a strong hybridization of the s- and p-electron valence states [Lencer-2008]. This insight has enabled the first ‘treasure’ map for phase change materials. While this combination of properties is very interesting to improve our understanding of the relationship between chemical bonding, structure and the resulting properties of solids, this unique property portfolio is also very attractive for applications. In particular, phase change materials are attractive as non-volatile electronic memories and might even possess the necessary charcteristics to be employed as a universal memory [Wuttig-2005]. For this application, a profound understanding of the charge transport in the amorphous and crystalline state is a must. It has been the goal of this research project to achieve an in-depth understanding of charge transport in phase change materials. In the course of this project numerous important insights have been obtained by employing a combination of a wide range of different electrical (DC conductivity, Hall, thermopower, impedance spectroscopy) and spectroscopic (FT-IR, THz spectroscopy) methods. A significant number of these experimental techniques have been built within this project. We now have first evidence that transport in the amorphous state of PCMs proceeds via extended states close to the mobility edge. Moreover, we have demonstrated that the puzzling electronic properties of certain crystalline phase change materials are a consequence of electron localization effects arising from the pronounced disorder in the crystalline state [Siegrist-2011]. This outcome provides valuable hints for the optimization of PCRAM devices. In addition, it relates to fundamental questions about localization physics. In particular, we could show that a significant number of crystalline phase change materials are among the most disordered crystalline materials known. Hence they are ideally suited to study the impact of disorder on the transport of charge and heat. This insight has enabled us to apply for an ERC Advanced Grant with the proposal ‚Tuning Disorder in Chalcogenides to realize Advanced Functional Devices‘.
Publications
- Investigation of defect states in the amorphous phase of the phase change alloys GeTe and Ge2Sb2Te5, Phys. Stat. Sol. C 7, 852 (2010)
J. Luckas, D. Krebs, M. Salinga, M. Wuttig, C. Longeaud
- Design Rules for Phase-Change Materials in Data Storage Applications, Advanced Materials 23, 2030 (2011)
D. Lencer, M. Salinga and M. Wuttig
- Disorderinduced localization in crystalline phase-change materials, Nature Materials 10, 202 (2011)
T. Siegrist, P. Jost, H. Volker, M. Woda, P. Merkelbach, C. Schlockermann, M. Wuttig
- Phase-Change Materials: Vibrational softening upon crystallization and its impact on thermal properties, Advanced Functional Materials 21, 2232 (2011)
T. Matsunaga, N. Yamada, R. Koijama, S. Shamoto, M. Sato, H. Tanida, T. Uruga, S. Kohara, M. Takata, P. Zalden, G. Bruns, I. Sergueev, H.C. Wille R. Hermann, M. Wuttig
- On the density of states of germanium telluride, J. Appl. Phys. 112, 113714 (2012)
C. Longeaud, J. Luckas, D. Krebs, R. Carius, J. Klomfass, and M. Wuttig
(See online at https://doi.org/10.1063/1.4768725) - Stoichiometry dependence of resistance drift phenomena in amorphous GeSnTe phasechange alloys, J. Appl. Phys. 113, 023704 (2013)
J. Luckas, A. Piarristeguy, G. Bruns, P. Jost, S. Grothe, R.M. Schmidt, C. Longeaud, and M. Wuttig
(See online at https://doi.org/10.1063/1.4769871)
