Comparative analysis of different preparation methods of chalcogenide glasses: Molecular dynamics structure simulations

Two different preparation methods (liquid-quenching and evaporation) of chalcogenide glasses have been investigated by molecular dynamics simulations. Our particular aim was to determine how the structural changes occur due to the different preparation methods. We applied a classical empirical three-body potential of selenium to describe the interactions between atoms. Our simulation shows that a significant difference can be observed in the homogeneities

Photo-induced volume changes in selenium. Tight-binding molecular dynamics study

Tight-binding molecular dynamics simulations of photo-excitations in small Se clusters (isolated Se$_8$ ring and helical Se chain) and glassy Se networks (containing 162 atoms) were carried out in order to analyse the photo induced instability inside the amorphous selenium. In the cluster systems after taking an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital a bond breaking occurs. In the glassy networks photoinduced volume expansion was observed and at the same time the number of coordination defects changed significantly due to illumination

Computer simulations of structural and hopping conduction properties of disordered solids

1. Introduction 2. Molecular dynamics simulation of amorphous carbon structures 3. Atomistic simulation of the bombardment process during the BEN phase of diamond CVD 4. Growth of amorphous silicon 5. One-dimensional hopping in disordered organic solidsComment: PhD thesis, 107 pages +45 figures, LaTeX, some color figures availble on the web (see details in the text

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

Response to "comment on 'Threshold switching via electric field induced crystallization in phase change memory devices'" [Appl. Phys. Lett. 102, 236101 (2012)]

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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. [...

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

Ozone chemistry on tidally locked M dwarf planets

This is the final version. Available from Oxford University Press via the DOI in this recordWe use the Met Office Unified Model to explore the potential of a tidally locked M dwarf planet, nominally Proxima Centauri b irradiated by a quiescent version of its host star, to sustain an atmospheric ozone layer. We assume a slab ocean surface layer, and an Earth-like atmosphere of nitrogen and oxygen with trace amounts of ozone and water vapour. We describe ozone chemistry using the Chapman mechanism and the hydrogen oxide (HOx, describing the sum of OH and HO2) catalytic cycle. We find that Proxima Centauri radiates with sufficient UV energy to initialize the Chapman mechanism. The result is a thin but stable ozone layer that peaks at 0.75 parts per million at 25 km. The quasi-stationary distribution of atmospheric ozone is determined by photolysis driven by incoming stellar radiation and by atmospheric transport. Ozone mole fractions are smallest in the lowest 15 km of the atmosphere at the sub-stellar point and largest in the nightside gyres. Above 15 km the ozone distribution is dominated by an equatorial jet stream that circumnavigates the planet. The nightside ozone distribution is dominated by two cyclonic Rossby gyres that result in localized ozone hotspots. On the dayside the atmospheric lifetime is determined by the HOx catalytic cycle and deposition to the surface, with nightside lifetimes due to chemistry much longer than timescales associated with atmospheric transport. Surface UV values peak at the substellar point with values of 0.01 W/m2 , shielded by the overlying atmospheric ozone layer but more importantly by water vapour clouds.Leverhulme TrustScience and Technology Facilities Council (STFC

The Limits of the Primitive Equations of Dynamics for Warm, Slowly Rotating Small Neptunes and Super Earths (article)

This is the author accepted manuscript. The final version is available from American Astronomical Society / IOP Publishing via the DOI in this record.The dataset associated with this article is located in ORE at: https://doi.org/10.24378/exe.1023We present significant differences in the simulated atmospheric flow for warm, tidally-locked small Neptunes and super Earths (based on a nominal GJ 1214b) when solving the simplified, and commonly used, primitive dynamical equations or the full Navier-Stokes equations. The dominant prograde, superrotating zonal jet is markedly different between the simulations which are performed using practically identical numerical setups, within the same model. The differences arise due to the breakdown of the so-called `shallow-fluid' and traditional approximations, which worsens when rotation rates are slowed, and day{night temperature contrasts are increased. The changes in the zonal advection between simulations solving the full and simplified equations, give rise to significant differences in the atmospheric redistribution of heat, altering the position of the hottest part of the atmosphere and temperature contrast between the day and night sides. The implications for the atmospheric chemistry and, therefore, observations need to be studied with a model including a more detailed treatment of the radiative transfer and chemistry. Small Neptunes and super Earths are extremely abundant and important, potentially bridging the structural properties (mass, radius, composition) of terrestrial and gas giant planets. Our results indicate care is required when interpreting the output of models solving the primitive equations of motion for such planets.Leverhulme TrustScience and Technology Facilities CouncilEuropean Research Counci

Ultrahigh Storage Densities via the Scaling of Patterned Probe Phase-Change Memories

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.The scaling potential of patterned probe phase-change memory (PP-PCM) cells is investigated, down to single-nanometer dimensions, using physically realistic simulations that combine electro-thermal modelling with a Gillespie Cellular Automata (GCA) phase-change model. For this study, a trilayer TiN/Ge 2 Sb 2 Te 5 /TiN cell structure (isolated by a SiO 2 insulator) was preferred, due to its good performance and practicability, over previously investigated probe-based structures such as those that used diamond-like carbon capping layers or immersion in an inert liquid to protect the phase-change layer (while still allowing for electrical contact). We found that PP-PCM cells with dimensions as small as 5 nm could be successfully amorphized and re-crystallized (RESET and SET) using moderate voltage pulses. The resistance window between the RESET/SET states decreased with a reduction in cell dimensions, but it was still more than order of magnitude even for the smallest cells, predicting that PP-PCM cells are indeed scalable and operable in the sub-10 nm region. Most importantly, it was found that the storage density could be increased by cell size scaling with storage densities as high as 10 Tb/in 2 being achieved, which is significantly higher than the storage densities previously reported in phase-change probe storage, and other probe-based technologies such as thermomechanical, magnetic and ferroelectric probe storage.Hasan Hayat would like to thank the College of Engineering, Mathematics and Physical Sciences at the University of Exeter for PhD scholarship funding while carrying out this work. C. David Wright would like to thank the EPSRC for funding via grant EP/M015130/1