Development of an optoelectronic test station for novel phasechange device characterisation and development

This is the final version of the article. Available from E\PCOS via the URL in this record.Optoelectronic applications of phase-change devices are of increasing interest and importance. To enable the proper experimental characterisation of device optoelectronic properties, and to allow for the future development of device designs with improved optoelectronic performance, we have constructed an optoelectronic test station that can simultaneously measure the optical and electrical properties of phase-change devices with high optical resolution and with high electrical bandwidths. The design of this test station, and some preliminary test applications are described

Phase-change computing

PosterThis is the final version. Available from E\PCOS via the URL in this record.Phase-change materials and devices are currently generating much interest for their potential to provide practicable alternatives to traditional von-Neumann computing (i.e. alternatives to computing in which memory and processing functions are carried out at physically separated locations). Indeed, many years after Ovshinsky and colleagues first showed the remarkable computing capabilities of phase-change devices (see for example [1-3]), other researchers have recently experimentally demonstrated the potential of phase-change devices to perform not only arithmetic computing [4], but also to provide hardware mimics of both synapses [5, 6] and neurons [4] (so opening the way to so-called bio-inspired or neuromorphic computing). We ourselves recently demonstrated reliable execution of the four basic arithmetic operations of addition, subtraction, multiplication and division using phase-change materials and micrometrescale optical excitation with (groups of) femtosecond pulses [4]. In this paper however we demonstrate that this arithmetic capability is also accessible via the electrical domain and on the nanoscale. [...

What can polysemy tell us about theories of explanation?

Philosophical accounts of scientific explanation are broadly divided into ontic and epistemic views. This paper explores the idea that the lexical ambiguity of the verb to explain and its nominalisation supports an ontic conception of explanation. I analyse one argument which challenges this strategy by criticising the claim that explanatory talk is lexically ambiguous, 375–394, 2012). I propose that the linguistic mechanism of transfer of meaning, 109–132, 1995) provides a better account of the lexical alternations that figure in the systematic polysemy of explanatory talk, and evaluate the implications of this proposal for the debate between ontic and epistemic conceptions of scientific explanation

A self-resetting phase-change neuron

This is the final version of the article. Available from E\PCOS via the URL in this record.Neuromorphic, or brain-like, computing applications of phase-change devices have to date concentrated primarily on the implementation of phase-change synapses. However, a phase-change device can also mimic the integrative properties of a biological neuron. Here we demonstrate, using both physical and circuit modelling, that by combining a phase-change memory device with a simple external circuit we can readily deliver a self-resetting spiking phase-change neuron

Size Scaling in Phase Change Memory Cells: From Traditional to Emerging Device Structures

This is the final version of the article. Available from E\PCOS via the URL in this record.1. INTRODUCTION AND METHODOLOGY Phase change memory (PCM) based on the reversible phase-transition of chalcogenides (such as Ge2Sb2Te5 (GST)) between a low-resistance crystalline state and high-resistance amorphous state is one of the leading candidates of emerging non-volatile solid-state memories [1]. Scaling is one of the most important aspects for PCM development as it leads to enhanced storage density, reduction in power consumption and improvement in switching speeds [2]. To demonstrate the excellent scalability of PCRAM, switching capability in the sub-10nm region [3-5], programming currents less than 10μA [4], switching speeds in picoseconds [6], and storage densities in Tb/in2 using scanning probe recording and thermal recording [7-8] have all been reported. In this manuscript we combine electro-thermal simulations with the Gillespie Cellular Automata (GCA) phase switching approach to simulate and predict the scaling behaviour (down to sub-10nm dimensions) of three GST-based device structures; (1) mushroom-type PCM cells, (2) trilayer patterned PCM cells, and (3) spherical phase change nanoclusters. The GCA approach is a sophisticated stochastic simulator capable of spatio-temporal modeling in PCM devices, and has previously been described in detail in [9]. This approach is potentially capable of spanning the length scales between atomistic modeling and bulk scale methods such as the JMAK or the classical nucleation and growth methods. Electrical switching is performed by applying trapezoidal Reset and Set pulses of various amplitudes and durations in a test bench consisting of an electrical pulse source, a series load resistance of 10kΩ, and the phase change memory cell itself. [...

A theoretical study of scaling behaviour of mushroom PCRAM devices using the Gillespie Cellular Automata Approach

This is the author accepted manuscript.We investigate the scaling characteristics of vertical “mushroom” phase change random access memory (PCRAM) cells down to sub-10nm dimensions using an electro-thermal model combined with the Gillespie cellular automata (GCA) phase-transition approach. The size of the amorphous dome formed during the Reset process decreases linearly with simultaneous reduction of the bottom TiN heater width and Ge2Sb2Te5 (GST) phase change layer volume with re-design of the cell geometry required for sub-15nm dimensions. Re-crystallisation of the amorphous dome is primarily nucleation-dominated, however a transition to growth-dominated crystallisation is observed for dimensions below 20nm. The scaling trend features a resistive window of a factor of 10 even for very small dimensions predicting the scalability and operability of mushroom PCRAM cells in the sub-10nm region

Phase-change band-pass filters for multispectral imaging

This is the author accepted manuscript. The final version is available from SPIE via the DOI in this recordPhase-change materials (PCMs) provide a route to adding dynamic tunability and reconfigurability to many types of photonic devices by changing the phase-state of the PCM itself. In this work we discuss the use of the phase-change alloy GeSbTe (GST) in the design of dynamically tunable filters operating in the infrared. GST is used to manipulate the extraordinary optical transmission of a periodic hole-array in a metallic layer, so creating ultra-thin, tunable band-pass filters. We discuss the use of such filters for multispectral imaging, suggest some approaches to overcome various practical challenges, and, finally, show that through the use of appropriate post processing algorithms this tunable filter could provide a cheap, ultra-thin, real-time, and relatively high performance multispectral imaging device.CDW acknowledges funding via the US Naval Research Laboratories ONRG programme (#N62909-16-1-2174) and the EPSRC ChAMP and WAFT grants (EP/M015130/1 and EP/M015173/1). LT acknowledges funding via the EPSRC CDT in Metamaterials (EP/L015331/1) and via QinetiQ PL

Can conventional phase-change memory devices be scaled down to single-nanometre dimensions?

ArticleThis is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.The scaling potential of “mushroom-type” phase-change memory devices is evaluated, down to singlenanometre dimensions, using physically realistic simulations that combine electro-thermal modelling with a Gillespie Cellular Automata phase-transformation approach. We found that cells with heater contact sizes as small as 6 nm could be successfully amorphized and re-crystallized (RESET and SET) using moderate excitation voltages. However, to enable the efficient formation of amorphous domes during RESET in small cells (heater contact diameters of 10 nm or less), it was necessary to improve the thermal confinement of the cell to reduce heat loss via the electrodes. The resistance window between the SET and RESET states decreased as the cell size reduced, but it was still more than an order of magnitude even for the smallest cells. As expected, the RESET current reduced as the cells got smaller; indeed, RESET current scaled with the inverse of the heater contact diameter and ultra-small RESET currents of only 19 μA were achieved for the smallest cells. Our results show that the conventional mushroom-type phase-change cell architecture is scalable and operable in the sub-10nm region.HH would like to thank the College of Engineering, Mathematics and Physical Sciences at the University of Exeter for PhD studentship funding while carrying out this work. CDW would like to thank the EPSRC for funding via grant EP/M015130/1. The authors would also like to thank Dr. Karthik Nagareddy (University of Exeter) for helpful discussions during this work

Integrated phase-change photonic devices and systems

This is the author accepted manuscript. The final version is available from CUP via the DOI in this recordDriven by the rapid rise of silicon photonics, optical signaling is moving from the realm of long-distance communications to chip-to-chip, and even on-chip domains. If on-chip signaling becomes optical, we should consider what more we might do with light than just communicate. We might, for example, set goals for the storing and processing of information directly in the optical domain. Doing this might enable us to supplement, or even surpass, the performance of electronic processors, by exploiting the ultrahigh bandwidth and wavelength division multiplexing capabilities offered by optics. In this article, we show how, by using an integrated photonics platform that embeds chalcogenide phase-change materials into standard silicon photonics circuits, we can achieve some of these goals. Specifically, we show that a phase-change integrated photonics platform can deliver binary and multilevel memory, arithmetic and logic processing, as well as synaptic and neuronal mimics for use in neuromorphic, or brain-like, computing-all working directly in the optical domain.European Union Horizon 202