Enrico Piccinini

Electronic, optical and thermal properties of the hexagonal and rocksalt-like Ge2Sb2Te5 chalcogenide from first-principle calculations

We present a comprehensive computational study on the properties of rock salt-like and hexagonal chalcogenide Ge2Sb2Te5 supported by experimental data. We calculate the electronic structure using density functional theory (DFT); the obtained density of states (DOS) compares favorably with experiments, and is suitable for transport analysis. Optical constants including refractive index and absorption coefficient capture major experimental features, aside from an energy shift owed to an underestimate of the bandgap that is typical of DFT calculations. We also compute the phonon DOS for the hexagonal phase, obtaining a speed of sound and thermal conductivity in good agreement with the experimental lattice contribution. The calculated heat capacity reaches ∼1.4 × 106 J/(m3 K) at high temperature, in agreement with experiments, and provides insight into the low-temperature range (<150 K), where data are unavailable.

Monte Carlo simulation of charge transport in amorphous chalcogenides

The most peculiar feature exhibited by the I(V) characteristics of amorphous-chalcogenide materials is undoubtedly its S-shaped behavior. This type of characteristics is very important for the technological application, e.g., in the field of nanoscale solid-state memories. In this paper we give a microscopic particle description of the charge transport across a layer of amorphous Ge2Sb2Te5 sandwiched between two planar metallic contacts. A transport scheme based on the generalization of the variable-range hopping has been implemented in a current-driven Monte Carlo code. This approach allows one to investigate the aspects of the microscopic picture responsible for the electrical properties of the device. The results are compared with experimental data.

Hot-carrier trap-limited transport in switching chalcogenides

Chalcogenide materials have received great attention in the last decade owing to their application in new memory systems. Recently, phase-change memories have, in fact, reached the early stages of production. In spite of the industrial exploitation of such materials, the physical processes governing the switching mechanism are still debated. In this paper, we work out a complete and consistent model for transport in amorphous chalcogenide materials based on trap-limited conduction accompanied by carrier heating. A previous model is here extended to include position-dependent carrier concentration and field, consistently linked by the Poisson equation. The results of the new model reproduce the experimental electrical characteristics and their dependences on the device length and temperature. Furthermore, the model provides a sound physical interpretation of the switching phenomenon and is able to give an estimate of the threshold condition in terms of the material parameters, a piece of information of gre...

Biased Molecular Simulations for Free-Energy Mapping: A Comparison on the KcsA Channel as a Test Case.

The calculation of free-energy landscapes in proteins is a challenge for modern numerical simulations. As to the case of potassium ion channels is concerned, it is particularly interesting because of the nanometric dimensions of the selectivity filter, where the complex electrostatics is highly relevant. The present study aims at comparing three different techniques used to bias molecular dynamics simulations, namely Umbrella Sampling, Steered Molecular Dynamics, and Metadynamics, never applied all together in the past to the same channel protein. Our test case is represented by potassium ions permeating the selectivity filter of the KcsA channel.

Conductive preferential paths of hot carriers in amorphous phase-change materials

We study charge transport properties of amorphous phase-change materials (PCM) using a set of balance equations applied to a three-dimensional random network of sites. In the context of trap-limited conduction, model results are checked against experimental data on PCM devices near the limits of scaling (∼10 nm), explaining the main features of the current-voltage characteristics. The stochastic nature of the network also allows us to investigate the statistical variability of the sub-threshold PCM operation. Simulations of batches of similar samples show a standard deviation for the threshold condition of the order of few percent for the threshold voltage and of ten percent for the threshold current. The analysis of the network at the microscopic level near threshold reveals the formation of high-current paths, connecting the two contacts of the device through network nodes hosting the hottest carriers.

Voltage Snapback in Amorphous-GST Memory Devices: Transport Model and Validation

Charge transport in amorphous chalcogenide-GST used for memory devices is modeled using two contributions: hopping of trapped electrons and motion of band electrons in extended states. The type of feedback that produces the snapback phenomenon is described as a filamentation in energy that is controlled by electron-electron interactions between trapped electrons and band electrons. The model thus derived is implemented within a state-of-the-art simulator. An analytical version of the model is also derived and is useful for discussing the snapback behavior and the scaling properties of the device.

Electrical bistability in amorphous semiconductors: A basic analytical theory

An analytical theory of electrical bistability in amorphous semiconductors is presented, based on some very general and essential assumptions and few equations of simple and clear physical meaning. The theory has been validated by comparisons with experimental transport data in chalcogenide materials and provides predictive analytical expressions for field and current values at the switching point.

Time-dependent transport in amorphous semiconductors: Instability in the field-controlled regime

A time-dependent trap-limited conduction scheme is used to analyze the transient behavior of bistable homogeneous amorphous semiconductors when either the electric field or the current density is prescribed. Numerical outcomes confirm that, for a current-controlled system, the working point is unique and stable in any region of the current-voltage characteristic, while in a field-controlled system the negative differential-resistance region is unstable even in absence of circuit parasitics. The proposed theoretical approach represents a valid tool to grasp the relevant time-dependent features of the Ovonic switching in chalcogenide materials.

Novel 3D random-network model for threshold switching of phase-change memories

The onset of crystallization in phase-change memory devices is studied by simulating an initially amorphous sample through a disordered network of localized states. The transport of charge and electron energy is self-consistently coupled to the Poisson and the Fourier heat equations, so that crystallization sites are found at the nanoscale. Results show how Ovonic switching and crystallization are both correlated to the formation of hot-carrier conduction paths, and the conditions for the occurrence of these phenomena are investigated. The model is then validated against data from ultra-scaled carbon-nanotube-contacted devices. Device-to-device variability of macroscopically identical devices is also analyzed.

A 5th-order method for 1D-device solution

The so-called Numerov process provides a three-point interpolation with an ~η5 accuracy in grids size η, much better than the standard finite-difference scheme that keeps the ~η2 terms. Such a substantial improvement is achieved with a negligible increase in computational cost. As the method is applicable to second-order differential equations in one dimension, it is an ideal tool for solving, e.g., the Poisson and Schrödinger equations in ballistic electron devices, where the longitudinal (that is, along the channel) problem is typically separated from the lateral one and solved over a uniform grid. Despite its advantage, the Numerov process has found limited applications, due to the difficulty of keeping the same precision in the boundary conditions. A method to work out the boundary conditions consistently with the rest of the scheme is presented, and applications are shown.