Quest for a solution to drift in phase change memory devices

The goal of this thesis is to gain new insights into the drift phenomenon and identify strategies to mitigate it. An extensive experimental characterization of PCM devices and in particular drift forms the foundation of each chapter. With respect to time-scales, ambient temperature, device dimensions, and combinations thereof, drift is studied under unprecedented conditions. In three studies, different aspects of drift are examined. (1) The origin of structural relaxation: Drift measurements over 9 orders of magnitude in time reveal the onset of relaxation in a melt-quenched state. The data is used to appraise two models, the Gibbs relaxation model and the collective relaxation model. Additionally, a refined version of the collective relaxation model is introduced and the consequences of a limited number of structural defects are discussed. (2) Exploiting nanoscale effects in phase change memories: Scaling devices to ever-smaller dimensions is incentivized by the requirement to achieve higher storage densities and less power consumption. Eventually, confinement and interfacial effects will govern the device characteristics. Anticipating these consequences, the feasibility to use a single element, Antimony, is assessed for the first time. The power efficiency, stability against crystallization, and drift are characterized under different degrees of confinement. (3) State-dependent drift in a projected memory cell: New device concepts are aiming to reduce drift by decoupling the cell resistance from the electronic properties of the amorphous phase. A shunt resistor scaling with the amount of amorphous material is added. Simulations and the drift characteristics of a projected device put the idealized concept to the test. The contact resistance between the phase change material and the shunt resistor is identified as a decisive parameter to achieve the desired device properties.Comment: PhD thesi

Monatomic phase change memory

Phase change memory has been developed into a mature technology capable of storing information in a fast and non-volatile way, with potential for neuromorphic computing applications. However, its future impact in electronics depends crucially on how the materials at the core of this technology adapt to the requirements arising from continued scaling towards higher device densities. A common strategy to finetune the properties of phase change memory materials, reaching reasonable thermal stability in optical data storage, relies on mixing precise amounts of different dopants, resulting often in quaternary or even more complicated compounds. Here we show how the simplest material imaginable, a single element (in this case, antimony), can become a valid alternative when confined in extremely small volumes. This compositional simplification eliminates problems related to unwanted deviations from the optimized stoichiometry in the switching volume, which become increasingly pressing when devices are aggressively miniaturized. Removing compositional optimization issues may allow one to capitalize on nanosize effects in information storage

Political travel across the ‘Iron Curtain’ and Communist youth identities in West Germany and Greece in the 1970s and 1980s

This article explores tours through the Iron Curtain arranged by West German and Greek pro-Soviet Communist youth groups, in an attempt to shed light on the transformation of European youth cultures beyond the ‘Americanisation’ story. It argues that the concept of the ‘black box’, employed by Rob Kroes to describe the influence of American cultural patterns on Western European youth, also applies to the reception of Eastern Bloc policies and norms by the Communists under study. Such selective reception was part of these groups’ efforts to devise a modernity alternative to the ‘capitalist’ one, an alternative modernity which tours across the Iron Curtain would help establish. Nevertheless, the organisers did not wish such travel to help eliminate American/Western influences on youth lifestyles entirely: the article analyses the excursions’ aims with regard to two core components of youth lifestyles in Western Europe since the 1960s, which have been affected by intra-Western flows, the spirit of ‘doing one’s own thing’ and transformations of sexual practices. The article also addresses the experience of the travellers in question, showing that they felt an unresolved tension: the tours neither served as a means of Sovietisation nor as an impulse to develop an openly anti-Soviet stance.PostprintPeer reviewe

Logic and Learning (Dagstuhl Seminar 19361)

The goal of building truly intelligent systems has forever been a central problem in computer science. While logic-based approaches of yore have had their successes and failures, the era of machine learning, specifically deep learning is also coming upon significant challenges. There is a growing consensus that the inductive reasoning and complex, high-dimensional pattern recognition capabilities of deep learning models need to be combined with symbolic (even programmatic), deductive capabilities traditionally developed in the logic and automated reasoning communities in order to achieve the next step towards building intelligent systems, including making progress at the frontier of hard problems such as explainable AI. However, these communities tend to be quite separate and interact only minimally, often at odds with each other upon the subject of the ``correct approach\u27\u27 to AI. This report documents the efforts of Dagstuhl Seminar 19361 on ``Logic and Learning\u27\u27 to bring these communities together in order to: (i) bridge the research efforts between them and foster an exchange of ideas in order to create unified formalisms and approaches that bear the advantages of both research methodologies; (ii) review and analyse the progress made across both communities; (iii) understand the subtleties and difficulties involved in solving hard problems using both perspectives; (iv) make attempts towards a consensus on what the hard problems are and what the elements of good solutions to these problems would be. The three focal points of the seminar were the strands of ``Logic for Machine Learning\u27\u27, ``Machine Learning for Logic\u27\u27, and ``Logic vs. Machine Learning\u27\u27. The seminar format consisted of long and short talks, as well as breakout sessions. We summarise the motivations and proceedings of the seminar, and report on the abstracts of the talks and the results of the breakout sessions

Structural Assessment of Interfaces in Projected Phase-Change Memory

Non-volatile memories based on phase-change materials have gained ground for applications in analog in-memory computing. Nonetheless, non-idealities inherent to the material result in device resistance variations that impair the achievable numerical precision. Projected-type phase-change memory devices reduce these non-idealities. In a projected phase-change memory, the phase-change storage mechanism is decoupled from the information retrieval process by using projection of the phase-change material’s phase configuration onto a projection liner. It has been suggested that the interface resistance between the phase-change material and the projection liner is an important parameter that dictates the efficacy of the projection. In this work, we establish a metrology framework to assess and understand the relevant structural properties of the interfaces in thin films contained in projected memory devices. Using X-ray reflectivity, X-ray diffraction and transmission electron microscopy, we investigate the quality of the interfaces and the layers’ properties. Using demonstrator examples of Sb and Sb2Te3 phase-change materials, new deposition routes as well as stack designs are proposed to enhance the phase-change material to a projection-liner interface and the robustness of material stacks in the devices