Session Tu-A4

Phase Change II

Chair: John Robertson, University of Cambridge

Tu-A4.1 16:10–16:30

Simulation of the Structure and Switching Properties in Chalcogenide Systems

Mihai Popescu, Florinel Sava, Iosif-Daniel Simandan, and Alin Velea

National Institute R&D of Materials Physics, Buchares-Magurele, 077125, Romania

The chalcogenide systems contain usually a well-defined glass formation domain (GFD). The transition from easily obtained glasses to microcrystalline compositions is still not well understood. The border of the GFD is strongly related to the peaking of the switching properties in the system. We have shown that the switching parameters are correlated with the distance from the border of GFD. The rate of switching and the threshold voltage during the electrical switching depend on the material composition in a narrow domain across the border of GFD. We have simulated the structure of the switching composition in the near vicinity of the GFD border. Small structural changes implied by atom movement resulted in a high rate of switching, as shown in the structural model. The simulation was carried out on the case of Ge-Sb-Te system. Calorimetric measurements show a peculiar window in the reversible heat flow behaviour of the material within a narrow composition range across the GFD border. Several other chalcogenide ternary systems were analysed and the conclusion is consistent with a model of mixed nanoamorphous and nanocrystalline phases at the border of the GFD. The paper offers a criterion for choosing the best switching compositions in the complex chalcogenide systems and not only.

Keywords: nanocrystalline, nanoamorphous, switching, glass forming domain, computer simulation

Tu-A4.2 16:30–16:50

Changes in Electrical Transport of Amorphous Phase Change Materials Upon Annealing

D. Krebs (1), T. Bachmann (1,6), J. L. M. Oosthoek (2), P. Jonnalagadda (1), L. Dellmann (1), S. Raoux (3), J. Luckas (4), C. Longeaud (5), B. J. Kooi (2), and R. Spolenak (6)

1. IBM Zurich Research Laboratory, Saeumerstrasse 4, CH-8803 Rueschlikon, Switzerland

2. Zernike Institute for Advanced Materials, Materials innovation institute M2i, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

3. IBM T.J. Watson Research Center, Yorktown Heights, NY, USA

4. I. Physikalisches Institut IA, RWTH Aachen University, 52056 Aachen, Germany

5. Laboratoire de Génie Electrique de Paris (CNRS UMR 8507), Plateau de Moulon, 11 rue Joliot Curie, 91190 Gif sur Yvette, France

6. Laboratory for Nanometallurgy, Department of Materials, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland

While phase change memory technology has become more mature in recent years, fundamental problems linked to the electrical transport properties in the amorphous phase of phase change materials remain to be solved. The increase of resistance over time, called resistance drift, for example prompts a major challenge for the implementation of multilevel storage which eventually is necessary to compete in terms of high storage densities.

A lot of studies have been performed to gain better understanding of resistance drift and the underlying transport mechanism. Thus, the literature on the phenomenological description of resistance drift and its time and temperature dependence [1] on one side and theoretical studies of the structure of amorphous phase change materials on the other side is broad. However, to link structural changes with electrical transport a broader knowledge of (i) changes in the density of states (DoS) upon structural relaxation and (ii) the influence of defects on electrical transport is required.

In this work, we present temperature dependent conductivity and photo-conductivity measurements on GeTe and other phase change materials [2]. It is shown that trap limited band transport at high temperatures (above 165K for GeTe) and variable range hopping at low temperatures is the dominating transport mechanism.

Based on measurements of the temperature dependence of the optical band gap [3], modulated photo-conductivity [4] and photo-thermal deflection spectroscopy [1] a DoS for GeTe has been proposed previously [5]. Using this DoS, the temperature dependence of conductivity and photo-conducticity has been simulated. Our work shows how changes in the DoS (band gap and defect distributions) will affect the electrical transport before and after temperature accelerated drift [2]. The decrease in conductivity upon annealing can entirely be explained by an increase of the band gap by about 12%. However, low temperature photo-conductivity measurements also suggest a change in the defect density.

[1] Krebs, et al., J. of Non-Crystalline Solids 358, 2412 (2012)

[2] Oosthoek, et al., J. of Applied Physics 112, 084506 (2012)

[3] Luckas, et al., J. of Applied Physics 110, 013719 (2011)

[4] Luckas, et al., Physica Status Solidi (c) 7, 852 (2010)

[5] Longeaud, et al., J. of Applied Physics 112, 113714 (2013)

Keywords: phase change materials, phase change memory, amorphous, electrical transport, density of states, resistance drift

Tu-A4.3 16:50–17:10

Phase-Change Materials: Varying Charge Transport through Disorder

Hanno Volker (1), Tobias Schäfer (1), Annika Poitz (1), Karl Simon Siegert (1) and Matthias Wuttig (1,2)

1. Institute of Physics (IA), RWTH Aachen University, 52056 Aachen, Germany

2. JARA – Fundamentals of Information Technology, RWTH Aachen University, 52056 Aachen, Germany

Compounds along the tie-line between GeTe and Sb2Te3 (GST) are classified as phase change materials (PCMs) due to their ability to be reversibly switched between the crystalline and amorphous state. The phase transition occurs on nanosecond timescales and is accompanied by a drastic contrast in optical reflectivity and electrical resistivity, thus making PCMs highly attractive for data storage technology. Beyond established optical data storage media (e.g. DVD-RW) the current development is directed at future electrical devices where the change in resistivity is utilized ("universal memory").

This study focuses on electrical transport properties of PCM, which are critical for several application-related issues such as multi-bit potential or power consumption upon amorphization. We observe a strong dependency of the room temperature resistivity upon annealing in several crystalline GST thin films. This is attributed to a progressing structural transition from an early cubic, rock-salt like phase (c-GST) with a random distribution of Ge, Sb and vacancies on the cation sublattice towards a hexagonal phase (h-GST). The hexagonal phase is formed when a critical number of vacancies is atomically rearranged into ordered layers. Nevertheless, recent DFT calculations underline that the remaining number of unordered vacancies can be sufficient to induce localized states near the Fermi energy. As a consequence the system abides to an overall semiconducting behavior. Further annealing above the structural transition temperature causes these remaining vacancies to order which gives rise to an electronically driven MIT observed experimentally in h-GST. Typically MITs occur as a combination of electronic correlations (Mott MIT) and disorder (e.g. in doped Si:P systems), which complicates the experimental determination of the dominating effect. Therefore, the pronounced impact of disorder in PCMs is not only interesting for technological applications but also for fundamental understanding of transport phenomena in solids.

Keywords: Phase-Change Materials, electrical transport, disorder induced localization effects, MIT