Simulation of SET Operation in Phase-Change Random Access Memories with Heater Addition and Ring-Type Contactor for Low-Power Consumption by Finite Element Modeling

A three-dimensional finite element model for phase change random access memory （PCRAM） is established for comprehensive electrical and thermal analysis during SET operation. The SET behaviours of the heater addition structure （HS） and the ring-type contact in bottom electrode （RIB） structure are compared with each other. There are two ways to reduce the RESET current, applying a high resistivity interracial layer and building a new device structure. The simulation resuIts indicate that the variation of SET current with different power reduction ways is little. This study takes the RESET and SET operation current into consideration, showing that the RIB structure PCRAM cell is suitable for future devices with high heat efficiency and high-density, due to its high heat efficiency in RESET operation.     

Computation of the string tension in four-dimensional Yang–Mills theory using large N reduction

Continuum reduction and Monte Carlo simulation are used to calculate the heavy quark potential and the string tension in large N Yang–Mills theory in four dimensions. The potential is calculated out to a separation of nine lattice units on a lattice with extent six in each direction.     

Improving mixture of grains by using bi-dimensional Galton boards

This paper presents an experimental and numerical study that deals with the problem of mixing grains falling down through a bi-dimensional Galton board (BGB). The special issue addressed here is the influence of the presence of lateral walls in the BGB. Disks of equal diameters but different species are launched from the top of the device. During the fall, disks collide with obstacles (arranged to form a triangular lattice) and with the lateral walls. The exit distribution of particles at the bottom of the board is determined and the incidence of the presence of walls in the mixing quality is studied as a function of W the relative separation between lateral walls. Two types of indexes are evaluated to characterize the efficiency in the obtained mixture. The presence of walls has proven to be crucial to enhance the quality of the mixture of particles.     

Underlying reverse current mechanisms in a-Si:H p-i-n solar cell and compact SPICE modeling

The defect states in bulk of i-layer and at p⁺/i interface have been studied by using dark reverse current-voltage (J-V) measurements. The dark reverse current as a function of voltage has been analyzed on the basis of thermal generation of the carriers from mid-gap states in i-layer. Based on its behavior the thermal generation mechanism has been divided into two types. Thermal generation at lower bias (<5 V) shows V^1/2 behavior, whereas at higher bias follows an exponential dependence of voltage (>5 V). This was explained using a thermal generation zone at lower bias, which is a source of reverse currents, and its evolution towards p⁺/i interface with increasing voltage. The analytical result has shown that at lower reverse bias (V < 5 V) the defect states in the bulk of i-layer and at higher bias (V ∼ 25 V) the defect states at p⁺/i interface are contributing to the reverse currents. Reverse bias annealing (RBA) treatment which has been performed on these cells shows that a reduction of defect states more in the i-region near to the p⁺-layer and at p⁺/i interface as compared to the deep regions in bulk of i-layer. The calculated defect state density (DOS) is varying from its intrinsic value of 2.4 × 10¹⁷ cm⁻³ in the bulk of the i-layer up to 2.1 × 10¹⁹ cm⁻³ near and at p⁺/i interface. These values decrease to 7.1 × 10¹⁶ cm⁻³ and 2.7 × 10¹⁷ cm⁻³, respectively, in the samples annealed under reverse bias at 2 V. The bias dependent leakage current behavior has been modeled and implemented in simulation program with integrated circuit emphasis (SPICE) using simple circuit elements based on voltage controlled current sources (VCCS). Simulated and measured reverse leakage current characteristics are in reasonable agreement.     

Upward penetration of grains through a granular medium

We study experimentally the creeping penetration of guest (percolating) grains through densely packed granular media in two dimensions. The evolution of the system of the guest grains during the penetration is studied by image analysis. To quantify the changes in the internal structure of the packing, we use Voronoï tessellation and a certain shape factor which is a clear indicator of the presence of different underlying substructures (domains). We first consider the impact of the effective gravitational acceleration on upward penetration of grains. It is found that the higher effective gravity increases the resistance to upward penetration and enhances structural organization in the system of the percolating grains. We also focus our attention on the dependence of the structural rearrangements of percolating grains on some parameters like polydispersity and the initial packing fraction of the host granular system. It is found that the anisotropy of penetration is larger in the monodisperse case than in the bidisperse one, for the same value of the packing fraction of the host medium. Compaction of initial host granular packing also increases anisotropy of penetration of guest grains. When a binary mixture of large and small guest grains is penetrated into the host granular medium, we observe size segregation patterns.     

Synthesis and characterization of monolithic Gd2O3 aerogels

In this thesis, we will elaborate on the synthesis and characterization of monolithic Gd₂O₃ aerogel. We conducted the experiment in the following procedure. Use gadolinium nitrate or gadolinium chloride, a kind of inorganic gadolinium salt as raw material, and polymerize it in ethanol with propylene oxide as gelation initiator in the way of sol-gel. After this step, we can obtain the wet gel. Then, dry the wet gel by supercritical CO₂, at last we will get aerogel. The product has strong transparency and also shows some thermal stability. XRD characterization shows that it is amorphous. Nitrogen adsorption/desorption analysis tells clearly its surface area (223 m²/g), average pore diameters (42 nm) and large pore volume (1.83 ml/g). It is also characterized by transmission electron microscopy and high-resolution transmission electron microscopy.     

Baryon masses,chiral extrapolations,and all that<Superscript>⋆</Superscript>

We calculate the baryon octet masses to fourth order in chiral perturbation theory employing dimensional and cut-off regularization. We analyze the pion and kaon mass dependences of the baryon masses based on the MILC data. We show that chiral perturbation theory gives stable chiral extrapolation functions for pion (kaon) masses below 550 (600) MeV. For the pion-nucleon sigma term we find (0) = 39.5...46.7MeV.     

Low-lying dipole strength for weakly bound systems: A simple analytic estimate

A simple analytic expression is proposed for the total low-lying dipole strength observed in the break-up of weakly bound systems. The model assumes pure single-particle transitions from the halo weakly bound state to continuum states treated as plane waves (neutrons) or regular Coulomb functions (protons). The analytic expression for the total strength approximately depends on the inverse of the binding energy and the reduced mass of the halo, with correction factors accounting for the charge of the halo and the finite size of the potential.     

Electromagnetic response and breakup of light weakly bound nuclei in a dicluster model

The light weakly bound nucleus ⁷Li is studied within a dicluster α + t picture. Different observables obtained within our simple model are compared with previous calculations and experiments showing good agreement. In particular, we calculate dipole and quadrupole electromagnetic response to the continuum. The energy distribution of B(Eλ) values are consistent with the energy-weighted molecular sum rule and display a sizable contribution of non-resonant character arising from the weak binding property. The corresponding form factors for excitations to the continuum are used in a semiclassical coupled-channel scheme to get estimates for the breakup cross-section in a heavy-ion reaction. The nuclear contribution is found to play an important role in the process for bombarding energies around the Coulomb barrier. The masses and charges ratios of the two clusters are shown to lead to features of the cluster halo that may significantly differ from the one usually associated with one-nucleon haloes.     

Stress investigations of thin amorphous Ge-Se-Ga films

Thin chalcogenide films from the systems (GeSe₄)_1-xGa_x and (GeSe₅)_1-xGa_x with gallium contents up to 20 at. % have been prepared by vacuum evaporation and their stress has been investigated by a cantilever technique. The addition of gallium to the Ge-Se matrix plays an important role in stress formation in the films: films without gallium possess negligible stress, while all gallium-containing films are under compressive stress. The increase of the gallium content leads to structural changes and an increase in the density, which results in higher stress values. For all films, stress reduction with time is observed due to spontaneous relaxation. Received: 2 October 2002 / Accepted: 22 November 2002 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-561/8044-136, E-mail: popov@schottky.physik.uni-kassel.de