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. [...

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. [...

Implantation temperature effects on the nanoscale optical pattern fabrication in a-SiC:H films by Ga+ focused ion beams

ArticleProceedings of the IX International Conference on Ion Implantation and Other Applications of Ions and Electrons ION 2012, Kazimierz Dolny, Poland, June 25-28, 2012This work is related to a novel approach of providing some new generation ultrastable (> 50 years), ultrahigh density (> 1 Tbit/sq.in.) data storage for archival applications. We used ion-implantation to write nanoscale data into hydrogenated amorphous silicon carbide (a-SiC:H) films. Wide bandgap a-SiC:H samples, Ga+ focused ion beam implanted, have been prepared. A range of samples has been focused ion beam patterned under different implantation conditions, with emphasis on different substrate temperatures (typically from 0°C temperature to around room temperature). Some of the room temperature implanted samples were further annealed at + 250°C in vacuum. The focused ion beam patterned samples were then analysed using near-field techniques, like atomic force microscopy, to define optimum implantation conditions and the resulting consequences for archival data storage applications. The atomic force microscopy analysis of Ga+ focused ion beam implanted a-Si1 - xCx:H samples at room temperature and at 0°C revealed an increase of both the depth and the width of the individual lines within the focused ion beam written patterns at the lower temperature, as a result of an increased ion beam induced sputtering yield, in good agreement with the previous results for the case of Ga+ broad beam implantation in a-Si1-xCx:H and again suggesting that the best conditions for optical data storage for archival storage applications would be using Ga+ ion implantation in a-SiC:H films with an optimal dose at room temperatures. Similarly, the atomic force microscopy results confirm that no advantage is expected to result from post-implantation annealing treatments.This work has been supported by the European Community as an Integrating Activity “Support of Public and Industrial Research using Ion Beam Technology (SPIRIT)” under EC contract No. 227012. The support of EC funded project BG051PO001/3.3.-05.001 for this publication is gratefully acknowledged. The Marie Curie Fellowship for T. Tsvetkova was also supported by the European Community under the contract PIEF-GA-2009-251845. The help of D. Dimova-Malinovska and O. Angelov with the samples preparation and useful discussions is also gratefully acknowledged

Non-volatile Optoelectronic Phase-Change Meta-Displays

This is the final version of the paper. Available from metaconferences.org via the URL in this record.Phase-change materials have a pronounced contrast between their electrical and optical properties when in the amorphous to crystalline phases, and can be switched between these phases quickly and repeatedly by electrical or optical means. These characteristics have very recently been exploited to produce a novel form of non-volatile optoelectronic display technology. In this paper we combine such phase-change display devices with metamaterial arrays, so as to gain additional control over their spectral properties

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 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

Beyond von-Neumann computing with nanoscale phase-change memory devices

OnlineOpen ArticleThis is the final version of the article. Available from the publisher via the DOI in this record.Historically, the application of phase-change materials and devices has been limited to the provision of non-volatile memories. Recently however the potential has been demonstrated for using phase-change devices as the basis for new forms of brain-like computing, by exploiting their multi-level resistance capability to provide electronic mimics of biological synapses. Here we exploit a different and previously under-explored property also intrinsic to phase-change materials and devices, namely accumulation, to demonstrate that nanoscale electronic phase-change devices can also provide a powerful form of arithmetic computing. We carry out complicated arithmetic operations, including parallel factorization and fractional division, using simple nanoscale phase-change cells that process and store data simultaneously and at the same physical location, promising a most efficient and effective means for implementing 'beyond von-Neumann' computing. We also show that this same accumulation property can be used to provide a particularly simple form phase-change integrate-and-fire 'neuron' which, by combining both phase-change synapse and neuron electronic mimics, potentially opens up a route to the realization of all-phase-change neuromorphic processing.The authors gratefully acknowledge EPSRC for grant funding (EP/ F015046/1). They also would like to thank Dr. A Pauza, formerly of Plasmon Data Systems Ltd, for help in preparation of the GST samples. Professor Peter Ashwin from the University of Exeter is also acknowledged for helpful discussions and guidance in the formulation of the GCA simulator. The authors are also very grateful to Mr. David Anderson of the University of Exeter for valuable assistance with the lithography of the pseudo-devices